“The next time the world tunes in to watch astronauts fly around the Moon”, NASA boss Jared Isaacman said on May 19, “they will be taikonauts”.

China has not yet announced any such mission, which would likely replicate either Artemis II or Apollo 8. But it is not really in doubt that such a mission is coming, and that it may happen soon. Over the past few months, China has conducted tests of both its crew capsule, Mengzhou, and of its Long March 10A booster, the rocket that will propel astronauts towards the Moon.

For American politicians, the possibility that China might beat America back to the lunar surface is of deep concern. Isaacman has responded to these worries by pushing NASA to move faster and to start building out a base on the surface. America, he has said, “will never again give up the Moon”.

Artemis IV, the mission which might see Americans leave footprints on the Moon for the first time since 1972, is now scheduled to take place in 2028. If it launches on time, or at least before 2030, America has a good chance of beating China to the lunar surface.

Unfortunately, the past month saw two big setbacks in America’s efforts to do this. First was the twelfth launch of SpaceX’s Starship rocket. NASA plans to use this as a landing craft, the vessel that will take astronauts on the final leg of the trip to the surface and back. SpaceX, however, has struggled to get Starship ready in time.

May’s flight was supposed to change things. Over the past few months, SpaceX’s engineers have built an improved version of Starship, one they call V3. It is supposed to fix many of the issues that bedevilled the previous version, and so clear the way towards attempting a Moon landing.

Yet the test flight showed only mixed results: during ascent the rocket suffered from failed engines, and SpaceX was unable to steer the booster back to its target splashdown point. As the upper stage, ‘the ship’, flew on, it too suffered from engine problems. SpaceX abandoned some of their planned tests, though they did manage to demonstrate the ability to deploy dummy satellites. The ship at least made it back through the atmosphere in one piece, and came to a partially controlled ‘landing’ before it toppled into the ocean.

The next test flight may do better. But it is inescapable that things are not going as well as hoped. After twelve flights, SpaceX appears stuck, doomed to endlessly repeat the same suborbital trajectory with mixed results. To get to the Moon they need to break out of this loop. If it wants to get there before China, Starship almost certainly need to reach a proper orbit sometime this year.

In recent months, NASA seemed to be building a backup plan. This would have used a lander built by Blue Origin, a company owned by Amazon founder Jeff Bezos. But last week Blue suffered a major setback of its own. While their New Glenn rocket was undergoing tests in Florida, it suddenly and dramatically exploded in an enormous fireball.

The surrounding launch facilities have been badly damaged. How much this will set things back remains unclear – but it is clearly a serious blow. It would be a surprise if Blue Origin is able to launch again this year. As a result, it may not just be the next voices from the Moon that are Chinese. The next footprints may be too.

The Snowball Earth

Our world has not always been blue and green. For a while it was purple: the earliest photosynthetic life might have used retinal instead of chlorophyll, and this would have given the planet a purplish tinge. Later – during a long stagnant period geologists have nicknamed the Boring Billion – other creatures may have turned the oceans a milky black.

Around six hundred million years ago, some scientists think the Earth became a snowball. Ice sheets would have crept outwards from the poles, and eventually covered almost the entire planet. The equator would have been as cold as Antarctica is today, and even if it wasn’t frozen, the tropics would at least have been covered in an icy slush.

In theory, this kind of frozen world is perfectly stable. Ice reflects sunlight and heat, and this forms a feedback loop that keeps the planet cold. Global ice sheets may have emerged on multiple occasions, stuck around for millions of years, and only receded as something – volcanoes or meteor impacts perhaps – created enough warming to knock the world into a different stable climate.

Since then, however, the ice sheets have either vanished completely – as they did in the Jurassic period – or stuck around the poles, where they are found today. For some reason, Snowball Earths don’t seem to happen any more.

There are two possible explanations as to why not. One theory suggests that something has changed, and thus made global glaciation impossible. But others argue that supporters of the Snowball Earth are simply wrong, and that our planet has never frozen over entirely.

In a recent study, Erica Bisesi of the Astronomical Observatory of Trieste tested the first idea by modelling the conditions that may have prevailed on the Earth during such a snowball scenario. The last time it happened, she points out, the landmasses were fused into a single supercontinent named Rodinia that straddled the equator. Plants and trees had not yet emerged, so this continent was bare and rocky.

That would have made the Earth more reflective. The Sun was also cooler in the past, and half a billion years ago our planet would have received about five percent less heat than it does today. Together, these two factors would have made the world colder, Bisesi says, and more susceptible to falling into a snowball state.

But when that state ended, and the Earth warmed again, the temperature shift may have provided the spark for a more radical change. As the ice sheets melted, the flow of sediments and nutrients they sent into the warming oceans would have allowed life to flourish. Perhaps, some have wondered, the last time this happened it helped spark the Cambrian Explosion, the moment when life rapidly became much more complex.

That, Bisesi says, tipped the scales. As the continents began to be covered in vegetation, they reflected less sunlight. A darker planet absorbed more heat, and that may have been enough to prevent it from ever freezing over again. Once the world turned blue and green, in other words, it may have been just stable enough to stay that way.

Webb Images a Hellish Landscape

A group of astronomers led by Sebastian Zieba of Harvard reported using the James Webb Space Telescope to directly measure the surface of a distant world. Their target, a planet called Kua’kua, lies about fifty light years away. It orbits a red dwarf, a star fainter and cooler than our own, and it is slightly larger than the Earth.

It is not, however, a nice place. Measurements reported in 2019 showed it to be an airless and hot world. Its orbit lies close to its star, and it is tidally locked – which means one side of the planet is forever turned towards the star, while the other perpetually stares out into space. This starward side is extremely hot, soaring to about 1500 degrees Celsius.

Since it has no atmosphere, telescopes like the James Webb can directly measure properties of its surface. Zieba and his team did this, using the telescope to study the rocks and minerals scattered across its terrain. These observations showed it to be a barren place. Its surface is ancient, covered in volcanic regolith, and has likely suffered under billions of years of impacts and stellar radiation.

This means it looks more like Mercury, or perhaps the Moon, than like Earth or Mars. That, given the extreme conditions of Kua’kua, was not unexpected. But other worlds could still offer surprises. The presence of different minerals and rocks can reveal how active a planet’s geology is. And the more dynamic a world is, the more interesting it will be - and the greater the potential for it to host some kind of primitive life.

Psyche Swoops Past Mars

On May 15, NASA’s Psyche probe swung past Mars. The Red Planet is not the spacecraft’s ultimate destination: it is designed to study 16 Psyche, one of the largest asteroids in the belt between Mars and Jupiter. But to get there it needs energy, and it thus used this brief visit to Mars to gain acceleration through a gravitational assist.

Psyche approached Mars from the opposite direction of the Sun. This gave it a view of the planet that we rarely see. In the images it sent back, the Red Planet appears as a crescent, the Sun is almost exactly behind it, and only a sliver of the world is illuminated.

Psyche’s operators used the encounter to test out the instruments and cameras on board the spacecraft. All seems to have gone well, and the team say they captured thousands of observations of Mars. Their next milestone will come in 2029, when Psyche finally reaches its namesake in the asteroid belt.

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The stars are far, almost unimaginably so.

In 1970, as the crew of Apollo 13 fell back towards Earth, they briefly hit a speed of almost twenty-five thousand miles per hour. This is the fastest humans have ever travelled. Even Artemis II, returning from the Moon in April, fell about a thousand miles per hour short of this record.

Had the Apollo 13 capsule been capable of maintaining this speed, it could have looped around the planet in an hour, returned to the Moon in less than half a day, and then gone on to cross the orbit of Mars within a few months. But even at this rate, it would still have taken more than one hundred thousand years to reach Proxima Centauri, the closest star to our own.

As a result, dreams of interstellar travel are often just that. Indeed, there are few technologies that might allow us to span the gulfs of interstellar space. Yet the idea is not completely out of the question. There are a handful of stars within a dozen light-years of Earth. Crossing such a distance is just about feasible, at least according to the team behind Breakthrough Starshot.

They reckon that rather than sending a single big spacecraft, we might be better off sending a swarm of tiny ones. Thousands of these could be launched together. Each would deploy a light-sail, a device that uses the slight pressure of sunlight to create a small but steady acceleration.

This would not normally be fast enough to reach the stars. But Breakthrough Starshot thinks we could accelerate probes faster by using laser beams in place of sunlight. Fire these at small and lightweight spacecraft, they say, and we could get them to a reasonable fraction of the speed of light. Of course such speeds are dangerous - even minor collisions would obliterate a probe - but numbers can compensate for these risks.

To study how such a mission might work in practice, a team led by Marshall Eubanks of Space Initiatives looked at what a swarm of one thousand small spacecraft could do. They imagine accelerating them towards Alpha Centauri at twenty percent of the speed of light. At this rate, they could reach the star system in about twenty-one years.

As they flew through the outer solar system and into interstellar space, however, they would already begin sending back valuable scientific data. At first, they might hunt for planets and dwarf worlds hidden at the edge of the solar system. Later, they could map out the density of interstellar dust, hunt for primordial black holes, and even probe fundamental theories about gravity and about space and time.

The probes would still be travelling at high speed as they reached Alpha Centauri, and would have limited capacity to decelerate. But they would be able to image the planets around the three stars found there, and could look for moons and other objects. Instruments could scan these worlds, image their surfaces, and look for signs of life or other interesting processes.

All this, they conclude, is not only possible to do, but may be the only reasonable prospect for interstellar travel in the coming century. The benefits would be astonishing: swarms could visit half a dozen nearby stars, fly out to rogue planets, and explore the most distant regions of the solar system. But the challenges of making this real should not be understated. At the very least, it will take far more money than anyone has so far seemed willing to venture.

The Clouds of Interstellar Space

Every day, several tons of dust fall upon our planet. Most of it comes from the solar system, in the form of tiny fragments of passing comets, the debris of ancient impacts, and the leftover material from the birth of the planets. But some comes from further afield.

As the solar system moves through the galaxy, it periodically passes through clouds of interstellar dust and gas. About fifteen of these clouds are scattered around us at the moment , and as our system moves through them, it is sweeping up some of the dust they contain.

Bits of this dust eventually make their way to Earth. Although much of it is then simply incorporated into our world, some of it builds up in layers at the bottom of the ocean or in the polar ice sheets. And there, as a recent study found, it can act as a tracer for the history of our star’s long voyage through the galaxy.

The study, led by Dominik Koll of the Australian National University, analysed polar ice formed between forty and eighty thousand years ago. They found that the amount of cosmic dust falling on Earth seem to have increased since that time, possibly because we have recently entered an interstellar cloud.

That would fit well with ideas about our solar system’s recent trajectory. It is currently in the ‘local interstellar cloud’, and probably moved into this cloud sometime in the last hundred thousand years. We are currently heading towards its edge, and at some point in the next few millennia we should enter the nearby ‘G-Cloud’.

Much more work remains to be done. Other sources of dust have been found on the Moon, as well as in oceanic sediments. Together they paint a history of several supernovae erupting in our region of space over the past few million years, and tell a story of how we have moved between the clouds and bubbles sculpted by these violent events.

Getting all of this together into a single cohesive narrative will take time. But once complete, it will reveal the history of our long cosmic journey.

Artemis III

After this year’s successful flight around the Moon, NASA is moving ahead with plans for the third Artemis mission.

Originally, this was supposed to put astronauts on the lunar surface, and would have been the first time anyone had set foot on the Moon in more than five decades. But although NASA now has a rocket and a capsule, it does not have a lander ready to take crews to the surface and back.

NASA was relying on SpaceX to provide this lander. Under contracts awarded in 2020 and 2021, SpaceX was supposed to develop their Starship system to support lunar landings. But progress has been slow, and Starship still isn’t capable of getting anywhere close to the Moon.

With Artemis II complete, NASA has been forced to recognise this reality. Instead of going to the Moon, then, Artemis III will remain in a low orbit around the Earth. To salvage some value from the mission, NASA will practice docking the Orion capsule with Starship. In future missions this step will be crucial, as the two spacecraft are supposed to meet in lunar orbit to allow astronauts to transfer from one vehicle to another.

But in what may be a subtle warning to SpaceX, NASA is also hoping to demonstrate docking with a second lunar lander. This one, being built by Blue Origin, could provide an alternative route to the Moon, and give NASA a backup option if Starship simply is not ready in time.

Saving Swift: A Race Against Time

Since 2004, the orbiting Swift Observatory has been studying the mysterious Gamma Ray Bursts that appear in the distant universe. Although the mission was supposed to last just two years, it is now approaching its twenty-second anniversary in space. In that time, it has carried out more then eight hundred thousand measurements.

Unfortunately, Swift is also on the verge of death. It was originally launched into an orbit six hundred kilometres high, but over the last two decades the atmosphere has dragged it back down. It has now fallen below an altitude of four hundred kilometres and if nothing is done, it will make a fiery re-entry in the next few months.

Last year, however, NASA partnered with a company called Katalyst Space Technologies. Together they have built a spacecraft that could grab Swift and then boost it into a higher orbit. That would save the telescope, and give it another few decades of life.

NASA recently announced this new spacecraft has completed tests on the ground. It will now move towards a launch – possibly as soon as early June – and then start a race against the clock to raise Swift before it falls too low into the atmosphere.

If this works, it would demonstrate an important ability to grasp and raise the orbits of falling satellites. Swift is not the only one of NASA’s telescopes that is in peril of re-entry: Hubble, launched in 1994, is now losing altitude at a rapid rate. If nothing is done, it will burn in the early 2030s. Katalyst’s mission, if it is a success, may offer a way to prolong Hubble into the 2040s and beyond.

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For most of its known history, nobody has cared much about 2002 XV93. It is a small world lying somewhere close to the orbit of Pluto, one presumably covered in a grey and gritty kind of ice, and a place considered so unremarkable that no one has bothered to give it a proper name.

Yet as of last week, 2002 XV93 is suddenly famous. It is, according to a recent discovery, one of the most distant places in the solar system known to host an atmosphere. Truth be told, it is not much of an atmosphere. The small rock measures only a few hundred kilometres across, and the layer of gas surrounding it is at least ten million times more tenuous than the air around our planet.

But that is at least something. And its presence defies our expectations about what the worlds beyond Neptune should look like. All those known to us are icy, frozen objects. Most are small. A few are big enough to be notable: Pluto, for example, was long a member of the exclusive club of planets. Eris is slightly more massive than Pluto, and others – Haumea, Makemake, Sedna – measure more than a thousand kilometres across.

Some of these larger places do have atmospheres. That of Pluto, for example, is a thin layer of nitrogen that appears as a bluish haze stretching over its surface. Traces of methane have been found around Makemake. But others are more elusive. Eris and Sedna, though both large, show no sign of an atmosphere.

That may be because they are much further away – roughly twice as far as Pluto – and thus extremely cold. Any atmospheres these two dwarf planets did possess have surely escaped or condensed into ice. Over the next few centuries, Eris will swing inwards towards the Sun and gently warm. As it does, this ice may sublimate, and the frozen world might regain a thin and temporary atmosphere.

But this should really be the limit. Smaller worlds, those less than about a thousand kilometres across, are not expected to maintain any kind of atmosphere. They lack the gravitational strength to hold on to a layer of gas. If they ever had any, it should thus have long ago escaped into space or frozen to their surfaces.

Yet 2002 XV93 seems to differ. It measures about five hundred kilometres (three hundred miles) across, and so should fall into the category of small, insignificant places unworthy of carrying around a layer of gas. The question is thus twofold. Does it really have an atmosphere? And if it does, how did such a small world manage to keep it?

A Stellar Occultation

The story of 2002 XV93’s atmosphere begins on January 10, 2024. On that day, the small world passed briefly in front of a star in the constellation Auriga, thus creating a kind of mini-eclipse. Astronomers call such events stellar occultations, and they happen whenever a planet or something else in our solar system crosses in front of a more distant star.

Occultations can reveal surprising details about these objects. As a world starts to move across a star, it darkens the light coming from it. If the world lacks an atmosphere, this darkening will be abrupt, caused by the sudden passing of a solid surface between us and the distant star.

But if the body is instead surrounded by gas, the starlight will fade more gradually. The atmosphere may bend and filter the light, changing its colour in response to the gases present there. As the occultation ends, and the world moves on, this pattern will play out in reverse.

Careful study of exactly how the brightness of a star varies during an occultation can thus reveal whether a world has an atmosphere or not. As an example, it was thanks to this technique that astronomers knew Pluto had an atmosphere well before New Horizons arrived and took photographs. By comparing multiple occultations spread over many years, they could even watch this atmosphere expand and contract with the changing seasons.

The 2024 occultation of 2002 XV93 was observed from Japan. A team led by Ko Arimatsu of the National Astronomical Observatory of Japan set up three stations to take measurements of the event. Though they expected to see a sharp pattern, one more resembling that of the airless Eris rather than that of Pluto, the occultation instead showed the brief signs of a layer of gas.

The results, Arimatsu said, came as a surprise. When they analysed the data, they found it showed signs of a thin but clearly present atmosphere. It is tenuous and has a pressure at least five million times lower than that on Earth at sea level. Indeed, to reach a comparable pressure in our atmosphere you would have to ascend hundreds of miles into our faint and rarely visited exosphere.

A World of Ice Volcanoes?

The fact this layer exists at all is hard to explain. 2002 XV93 is so small that even a slight atmosphere should have been lost into space within a few centuries. That we can still see it today suggests something must have recently refilled it, and this tells us this small world is more dynamic than it at first appears.

Some objects beyond Neptune do show signs of atmospheres coming and going. Unlike the Earth, these worlds can follow eccentric paths: Pluto, for example, swings in and out along its long voyage around the Sun. At its closest, it passes within the orbit of Neptune. At its most distant, it is almost twice as far away.

This leads to drastic changes in temperature. Whenever Pluto moves towards the Sun, as it was doing until 1989, it warms up, its ices sublimate, and its atmosphere grows. As it moves away, as it is now doing, it begins to cool again, the gases in the atmosphere condense, and the layer of gas fades away.

Eris, which follows an even more eccentric orbit and is currently much further away, will likely show a similar pattern as it draws closer to the Sun over the next few hundred years. But there is no real sign the same is happening on 2002 XV93: indeed, measurements have found no traces of the kinds of ice on its surface from which an atmosphere could emerge.

Instead, Ko Arimatsu and his team think there are two better possibilities. One is that the world has experienced a recent collision. This would have thrown up a cloud of dust and ice and allowed a temporary atmosphere to form. It will eventually be lost, but it might well stick around for a few decades or centuries.

Alternatively, 2002 XV93 may be home to the phenomenon of cryovolcanism. Though it has a solid and frozen surface, underneath it may have a partially liquid layer. If something is heating this – possibly the tidal force of an unseen satellite, or perhaps a series of small impacts - then this layer could remain liquid. At times, it may be bursting outwards, forming a kind of ice volcano.

We have seen things like this on Pluto. Data sent by New Horizons showed signs of recent eruptions, and its images showed massive features that may have been formed by such volcanic events. We have no clear evidence of anything like this happening on 2002 XV93, but its thin atmosphere offers a hint that volcanoes might be erupting there too.

Whatever the answer is, one thing is clear. The worlds of the outer solar system – from its giants like Pluto and Eris, to its minnows like 2002 XV93 – are far more interesting than they at first appear. They are dynamic worlds, subject to forces and processes so alien and so familiar to us on Earth. They are worthy of attention – and in the case of 2002 XV93, a proper name as well.

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We don’t know for sure what it looked like. Shortly after it was taken, a fire destroyed his laboratory, reducing of his work and early photographs to ashes. But it was enough to prove a point. When the image, and others like it, were shown to French Academy of Sciences, they sparked an excited debate and laid the founding stone of modern astrophotography.

Until that moment, indeed, all astronomy had been done by hand. Since the age of Galileo, astronomers had peered through telescopes and spent painstaking hours sketching what they saw. The results were sometimes beautiful. Galileo’s watercolours of the Moon are impressive both for their simple elegance and for what they revealed about the universe.

Yet, they took time, they invited error, and they relied on the accuracy of the human eye. This was not always to be trusted: Schiaparelli believed he could see artificial lines criss-crossing Mars, the result, he thought, of an advanced engineering project. Others convinced themselves that the Red Planet periodically darkened and lightened, probably, they thought, because plants were growing in the summer and fading in the winter.

In comparison to all this, Daguerre’s technique appeared almost magical. True, it was not the first method of photography: that honour belonged to another Frenchman, Nicéphore Niépce. But Niépce’s process was slow, and sometimes needed a plate to be exposed for days to form an image. Daguerre’s was fast: it could capture an image in seconds.

When the scientists of the French Academy heard what he had done, they were giddy with excitement. Imagine, François Arago said as he announced the work, how this invention could speed up the study of Egyptian hieroglyphics, preserve treasures that would otherwise be lost, and allow the detailed examination of light itself.

For astronomy, the potential seemed great. The camera might, Arago hinted, be the greatest asset to the field since Galileo and his telescope. A revolution had followed that invention, and with Daguerre’s process, science might once again be on the cusp of great and unforeseen discoveries.

Soon after Arago’s speech, the rights to the Daguerreotype process were purchased by the French government. They awarded Daguerre a lifelong pension, elected him as an honorary academician, and later inscribed his name upon the Eiffel Tower. His process was given to the world, published freely, and made available to anyone who wished to use it1.

At the time of the announcement, the American Samuel Morse - he of the telegraph and famous code - happened to be in Paris. He quickly arranged to see the invention in person, and went to meet Daguerre at his residence. The introduction impressed him, and he apparently asked the Frenchman about the possibility of using it to take portraits.

After he returned to the United States, Morse teamed up with a chemist and doctor named John William Draper. The question of portraits still intrigued him, and as an experiment the pair opened a gallery in New York. They charged $5 for a picture - a high sum at the time - yet still attracted many of the city’s elite.

Indeed, the idea quickly proved popular: with a few years everyone from Abraham Lincoln to the Queen of England had posed for the camera. Yet Draper is also remembered for something else: in early March of 1840 he climbed to the roof of New York University, and shot a series of images of the Moon.

Getting it in focus was apparently difficult. Many of the earlier attempts, probably including Daguerre’s lost image, had shown little more than a blur of light against a dark background. But though the details are faint in Draper’s image, the surface of the Moon can be seen, along with its craters, the curves of mountain ranges, and its wide plains or mares.

This is often said to be the oldest surviving photograph of the Moon. There is some debate about that. Draper took many other photos of our satellite, and announced he had succeeded in capturing one several days before this one was taken. But those images have since been lost. This version was likely taken on March 26, 1840, a night upon which the Moon was in its last quarter.

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Where did the comet 3I/Atlas come from?

It was not born in our solar system. Instead, the comet came from the stars: it is passing through our system of planets at high speed, moving so fast that although the Sun has deflected its path, it cannot hold on to the comet. 3I/Atlas will, in the years to come, fly outwards, escape the Sun’s gravity, and then drift on alone through the galaxy for another million years or so.

Based on its trajectory, speed, and direction, astronomers thought 3I/Atlas was probably ancient. It may have come from the halo of stars surrounding the Milky Way. If it did, then the comet could be extremely old indeed, and might have been born around an earlier generation of stars.

At the end of last year, as the comet approached the Sun, astronomers measured the gases streaming off it. Although this showed the object was clearly a comet, it also appeared unlike any other we have seen before. Some of these strange characteristics can be explained by its long and cold voyage through interstellar space. But others are clues pointing to its ultimate origins.

One such clue is the amount of deuterium present in the comet. This is a form of hydrogen. But where normal atoms of hydrogen have a single proton in their nuclei, deuterium atoms have both a proton and a neutron. This makes them heavier than other hydrogen atoms, and subtly influences their chemistry. The ratio of deuterium to hydrogen should be fixed when the comet first forms, and this ratio can thus tell us about the environment in which it was born.

Deuterium is rare. Plenty of hydrogen is present in the Earth’s oceans, especially in the form of water. But for every six thousand atoms of hydrogen in the oceans, there is only one of deuterium. This gives the ratio of deuterium to hydrogen on Earth, and as with the comet, this ratio hints at the origin of our water.

At low temperatures, chemistry tends to favour deuterium over hydrogen when forming water molecules. At higher temperatures, this relationship reverses. This means that when water is created in cold places – for example, at the edge of the young Solar System – it contains a higher fraction of deuterium than that of the gas from which it formed.

This seems to be the case with the Earth’s water. The measured ratio of deuterium to hydrogen is much higher than that found in the primordial material of the solar system. Most likely, then, our water originally formed somewhere very cold – perhaps beyond the orbit of Pluto – and then fell inwards as a long and hard rain of comets and asteroids.

The measurements of 3I/Atlas found an even more extreme case. It has a lot of deuterium, at least forty times the level seen on Earth, and thirty times more than that seen in any other comet. This tells us the comet likely formed somewhere very cold, and probably under very different conditions to those found in our solar system.

But exactly where is still elusive. Astronomers cannot trace its path, at least not for more than a few million years. Comet 3I/Atlas is far older than that. The hints we have, deuterium included, only tell us it came from a very different place, and a very different age, to the one in which we were born.

Big G: Measuring Gravity

In 1797, an Englishman named Henry Cavendish set out to weigh the world.

He did this by suspending a rod from a wire. At each end of the rod he attached small lead balls, and then arranged for a pair of larger spheres to be placed near each. The result was a small deflection in the rod, a twisting motion caused by the slight gravitational attraction between the small and large balls.

By careful measurement, Cavendish was able to calculate the strength of this gravitational force. From this, he could deduce the density of the planet. The result was remarkably accurate: today’s accepted figure is 5.514 g/cm³; Cavendish, at the end of the eighteenth century, found it to be 5.448 g/cm³.

His experiment also gave us a way to measure the strength of gravity itself. Mathematically, this is given by a constant called Big G. It relates the strength of attraction two objects will feel with both their masses and the distances between them. The higher Big G is, the stronger this force will be.

This is a number of immense interest to science. The force of gravity dictates much of the world around us: it determines how fast planets move around their stars, keeps satellites in orbit, and crushes atoms to power the reactions in the core of the Sun. Big G is key to all of this, and the more accurately we can measure it, the better we can understand our world.

Yet doing so is hard. One problem is the weakness of gravity: the deviations it produces are slight, and hard to measure with accuracy. Another is the impossibility of shielding. There is no way to block gravitational influences, and so a passing truck, a nearby herd of sheep, or even a leaf blowing in the wind can all disturb the experiment.

As a result, we don’t know Big G very well. Things have improved since Cavendish’s day, at least slightly, but almost all the other fundamental constants are known with more accuracy. Scientists have tried many ways to change this, but in recent decades experiments have added more confusion than clarity.

The problem is, they find subtly different results. This is probably down to noise, the difficulty of measurement, and other errors. But the overall result is to add uncertainty, especially since we do not always know how big these errors are, and this leaves us in the dark about the true value of Big G.

In an effort to change this, Stephan Schlamminger, a researcher at NIST, dedicated himself to replicating an earlier experiment. If he could get the same answer as they did, then it might settle the question. Unfortunately, he did not. Instead he found several previously unrealised sources of error, and set about removing them.

The result, published in April, was yet another possible value of Big G, in this case, 6.67387x10^-11 meters³/kilogram/second². That doesn’t solve the problem. But it might bring us a little closer. After all, every experiment adds a new data point, and contributes a little to the weight of evidence.

The End of Space Tourism?

A few years ago, it looked as though the long-promised era of space tourism was finally upon us. Two companies – Blue Origin and Virgin Galactic – had begun sending wealthy passengers to the edge of space. Their founders, the billionaires Jeff Bezos and Richard Branson, engaged in an unsightly race to be the first to fly beyond the upper atmosphere. Branson won, but only by a few days.

Since then, Blue Origin has flown seventeen commercial crews beyond the official edge of space. Virgin Galactic has sent seven, each to a slightly lower altitude than Blue Origin. But today, these flights are over.

Blue Origin paused their flights earlier this year. Officially, the company says this is temporary, and it simply wants to devote staff to other projects, including a crewed moon lander. But it also looks unlikely they will resume launches any time soon.

Virgin Galactic, on the other hand, has not flown since 2024. Back then, they retired their spaceplane, and said they were working on a new ‘Delta class’ vehicle. Details have since been limited, but last week they released photos of a new craft outside a hangar.

Virgin Galactic will probably want to fly this spacecraft in at least a few test flights this year. According to the company’s official statements, it should be ready to carry tourists early next year. If it makes it, then the spluttering age of space tourism might get a bit of a boost.

A Hint of New Physics

Over the past decade, researchers at the Large Hadron Collider have been tracking a slight anomaly in the decay of a group of subatomic particles called B mesons.

These particles are formed as the collider smashes together protons. They don’t last for long: within a fraction of a second they decay, or transform, into other particles. There are many possible combinations of particles that can result from this process. But in the one of interest, the B-meson decays into a kaon, a pion, and two muons.

To probe the laws of subatomic physics, researchers study the energies and angles at which particles emerge from collisions like these. The Standard Model of particle physics, our current best understanding of the subatomic world, gives predictions for what should happen.

Yet experiments have persistently shown the reality of this particular B meson decay to differ from these predictions. That could hint at something unknown happening, such at the presence of new types of particles.

But it could also be a statistical fluke. Despite more than a decade of study, the evidence remains weak. Indeed, these kinds of anomaly do occasionally appear, but usually fade away with more data. But, in April, a study found that in this case the more data we get, the stronger the argument seems to become. If that continues to hold true, the B meson could end up being the doorway into a new theory of nature.

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The way NASA tells it, Space Reactor Freedom sounds simple.

This reactor, of course, is a nuclear one. NASA has apparently obtained one, or at least is close to doing so, and has also found a supply of fuel. Fortunately, they also have an electric propulsion system, one they salvaged from a failed project to build a lunar space station. The two, they say, could be bolted together to form a functional spacecraft.

With that done, the whole thing will be fired off to Mars, where it will presumably play a role in spreading Freedom. This is all so easy that the agency thinks it will be ready by the end of 2028, an extraordinary pace for any government program. Afterwards, mission accomplished, Space Reactor Freedom will act as a guide for a future stream of atom-powered probes heading out to the planets.

The catch, however, is that no one has done anything like this before. The United States last launched a nuclear reactor in 1965. It never left Earth orbit, failed after six weeks, and then partially disintegrated fifteen years later. Whatever debris remains is both radioactive and likely to continue circling our planet for the next thousand years or so1.

True, the Soviets did make more serious efforts to develop the technology. Between the 1960s and 1990s, the USSR launched more than two dozen reactors, most of them short test missions. Each was shut down after a few weeks, and they were then boosted into high orbits to allow their radioactivity to decay safely away before re-entry.

Yet in 1977, one of them, a satellite named Kosmos 954, failed within weeks of launch. Finding themselves unable to boost the spacecraft or to jettison its reactor, the Soviets issued secret warnings that the satellite was out of control and falling back to Earth. Four months after launch, it hit the atmosphere over Canada, exploded, and rained radioactive debris across the Northwest Territories.

But even when it worked, the Soviet program restricted itself to niche applications in orbit. No one has ever sent a nuclear reactor into deep space, or sought to unlock the great potential the technology could offer when pushed beyond the Moon.

In many ways, that is a shame. Indeed, space agencies believe that nuclear spacecraft can push aside a lot of the constraints that hold back interplanetary travel. They can go fast, head deep into the Solar System, and carry power-hungry instruments. A nuclear probe could cut the journey time to Mars to mere weeks, carry out a detailed exploration of Saturn or Neptune, and even flit between the icy worlds of the Kuiper Belt.

All we need, if you listen to NASA, is a successful demonstration. And that is what Space Reactor Freedom is all about.

The Reasons To Go Nuclear

To get anywhere in space, you need energy.

Traditionally, this has come from chemical means: rocket engines use various chemicals as fuels, react them to produce gases, and then force them out of a nozzle. The resulting exchange of momentum pushes the spacecraft in the desired direction, rather like a sophisticated balloon carefully releasing its air.

This works, but it can be inefficient. Heavy spacecraft, especially those that head deep into the Solar System, need a lot of energy to propel them. That implies they need to carry a lot of chemical fuel. But the more they carry, the heavier they become; the heavier they get, the more fuel they need. This can quickly turn into a vicious cycle of diminishing returns, one in which every gram of extra fuel simply adds more mass and demands yet more fuel.

Engineers have a few ways around this problem. One is to make use of clever tricks like gravitational assists. These happen when a spacecraft flies past a planet in just the right way to get a kick of momentum, a boost that offers a free acceleration. But they come at the cost of time: Europe’s JUICE mission, for example, is in the midst of a six-year period bouncing between Earth and Venus. Only in 2029, once it has built up enough speed, will it head to its final destination of Jupiter.

A second option is to use more efficient fuels. Hydrogen, for instance, is a good choice: it is lightweight, it reacts strongly and releases plenty of energy, and, in the form of water, it is abundant on Earth. Rocket engines sometimes react it with oxygen, a process that results in hot steam and that can work more efficiently than almost any other chemical engine.

Yet even hydrogen pales in comparison to nuclear fuel. Instead of forging and breaking molecular bonds, nuclear fission splits the atom and unleashes immense energies. A uranium-based reactor produces at least a million times the energy of the hydrogen-oxygen reaction. In spaceflight, where every gram of mass must be carefully accounted for, this is game-changing.

Spacecraft equipped with nuclear engines can avoid the planetary dance taken by missions like JUICE. They can fly direct, and they can take more mass with them. The reactors provide copious power on arrival, and can run day and night. For a rover on Mars, a base on the Moon, or an explorer of Titan, this kind of capability is essential.

The Nuclear Electric Motor

There is no real shortage of ideas of how to make use of this power in space. Indeed, mad scientists have dreamed up a dozen insane variants of the nuclear engine. One concept involved chucking atomic bombs out the back of a spaceship and riding their shock wave out to the stars. In principle it works. In reality this would be impossible to control, hazardous to literally everyone, and a dangerous lunacy to even think about trying.

More sensible is the radioisotope thermoelectric generator. This a device that relies on radioactive decay. It is fuelled by some kind of nuclear material – plutonium-238 is a common choice – and simply uses the heat produced by its decay to generate electricity.

Generators like this are sometimes used for flights into the outer Solar System. Beyond Jupiter, the Sun’s rays fade rapidly, and so solar power cannot keep distant probes running. Radioisotope generators can – and they have been used in missions like Cassini to Saturn, the Voyagers and Pioneers, New Horizons to Pluto and beyond, and on rovers on Mars.

But these are relatively simple nuclear devices, and rarely used for sustained propulsion. Space Reactor Freedom follows an alternative model, that of nuclear electric propulsion. Rather than rely on radioactive decay, it will use a reactor to create a controlled chain reaction. This produces a supply of heat, and that is then converted into a steady current of electrical energy.

The power is then fed into an electrical thrust system. Instead of ejecting gases, these expel charged particles, often in the form of Xenon atoms stripped of an electron, thus creating the desired exchange of momentum. Unlike in chemical rockets, this thrust can be maintained for a long time. Some trials have even produced sustained thrust lasting months at a time.

But until now, electric propulsion in space has always been limited by the power available. Solar is the usual source, and no electric engine has ever exceeded a handful of kilowatts. Nuclear reactors can lift that constraint. They can produce hundreds of kilowatts, even megawatts, and allow the true power of electric propulsion to be unleashed.

A two-hundred-kilowatt electric engine, one ten times more powerful than anything built so far, could send a twenty-ton spacecraft to Jupiter in less than five years. A megawatt reactor, one still easily within the bounds of possibility, could reduce the journey time to Mars to about twelve weeks. More powerful reactors could ship cargo to the moons of Jupiter, to Saturn, and to the very edges of the Solar System.

Space Reactor Freedom

Before all that, NASA needs to demonstrate the concept.

The plans for Space Reactor Freedom are still vague. The details we do have, however, suggest that NASA is prioritising speed over everything else. Nuclear propulsion is not a new idea, they said at a presentation in March, but rather something that has long suffered from poor execution. Over the past six decades, the agency has launched twelve nuclear efforts, yet put nothing more than a single short-lived satellite into orbit.

Their goal is to change this. NASA wants to go fast. They have set a target of launching in 2028, and they want to follow a design based on established technology. Indeed, they think much of the development work is already done, and that the remaining challenge is more about integrating the components than building them.

At the heart of the spacecraft will be a twenty-kilowatt reactor. Details about this are sparse, but NASA boss Jared Isaacman said it was already ‘mostly built’. Yet where it is, and who built it, remain unknown to the public.

This reactor will be mounted on a module originally planned for the Gateway, a space station that was supposed to be placed around the Moon. But that station is now on pause, and the module, which is equipped with electric propulsion systems, could be integrated with a nuclear power source.

Again, details of how this would work are limited, but assuming it can be done, NASA would then have a spacecraft capable of travelling to Mars. Onboard, the agency would put a payload of helicopters, each similar to the small Ingenuity helicopter that flew over the sands of the Red Planet a few years ago.

These helicopters would send back some interesting data, but the science is not really the point of this mission. Instead, Isaacman sees it as a way to prove nuclear technology can work in space. He hopes it would be followed by a series of more powerful missions. These will unlock the full potential of nuclear propulsion, and act, as he puts it, as the ‘transcontinental railroad of the Solar System’.

The deadline of 2028 is still ambitious. Even if the technology is already there, anything nuclear will face regulatory hurdles. The reactor will need to be safe – an accident could be fatal to the dream – and ensuring that will take time. So will the steps of making sure the design actually works. But Isaacman is still right to challenge NASA to move fast.

Indeed, the future direction of space exploration may rest on his success. If the reactor flies – whether that is in 2028 or in the early 2030s – it should inspire future missions to Mars, to Jupiter, and beyond. The road ahead is surely more challenging than NASA will admit now, but once the tech is in place and proven, then the outer Solar System awaits.

After travelling further into space than any human before them, the crew of Artemis II returned safely to Earth on April 10th.

Their voyage lasted ten days, and covered a distance of almost seven hundred thousand miles. At their apex, they were more than a quarter of a million miles from Earth. As they passed over the Moon, they ventured closer to our natural satellite than anyone since 1972.

The moment of greatest peril, though, came in the final minutes of their mission. Their Orion capsule, named Integrity by the crew, hit the outer atmosphere at a speed greater than twenty-four thousand miles per hour. Humans have rarely travelled this fast. Only the Apollo missions, as they too returned from the Moon, have reached similar speeds.

Before launch, some concern had been raised about the ability of the capsule’s heat shield to protect the crew from the dangers of re-entry. In Artemis I, the uncrewed first test flight, this shield did not perform as expected. Later analysis showed it had been damaged during its plunge through the atmosphere.

In an effort to avoid a repeat, NASA engineers redesigned the trajectory the capsule follows during re-entry. They also tested the heat shield on the ground – though, of course, nothing we can do in a lab is able to recreate the reality of falling through the atmosphere at thirty-five times the speed of sound.

Fortunately, the shield seems to have performed as expected. Images taken of it after splashdown appear to show it in good condition, with little of the charring and erosion seen after Artemis I. That is good news. More work, of course, is needed. NASA will now study all the data obtained - including from observation aircraft - and subject the shield to a series of tests to better understand how it behaved.

As they do this, however, attention will surely turn to Artemis III. Until recently, this mission was supposed to take humans to the lunar surface for the first time in decades. For this, NASA was relying on SpaceX to provide a lander in the form of Starship. But progress has been much slower than hoped, and Starship simply won’t be ready to support a landing in time.

Instead, Artemis III will now remain in Earth orbit. Instead of going to the Moon, astronauts will practice docking with the lander. If SpaceX can get Starship into orbit and show sufficient control for a safe approach, the Orion capsule should dock with Starship. That would prove the technical ability of both vehicles to work together. But it would also allow operators and astronauts to get experience in what promises to be a challenging operation.

NASA has also introduced a bit of competition. Blue Origin has been working on its own lunar lander, which Orion could, if the lander is launched in time, use as a docking target as well. That, if successful, would give NASA an alternative way to reach the Moon before the decade is out.

The Darkening Waves of Mars

Until the 1960s, astronomers were convinced that the surface of Mars was periodically swept by ‘waves of darkening’. These would come and go with the seasons, and would spread from the polar regions towards the equator. Nobody knew what was causing these waves, but most theories held that they were somehow linked to seasonal shifts on the red planet.

A paper from 1966 paper summarised the two most common explanations. The first, and favourite, held that the warmth of the Martian spring was melting the polar caps and putting moisture into the air. This sparked waves of plant growth, which from Earth we saw as a wave of darkness spreading around the planet.

The other, more boring suggestion argued that seasonal changes in temperature caused wind patterns to shift. These winds would at times blow clouds of dust off the surface, revealing darker underlying rocks. When winds calmed, the dust would settle and Mars would seem to become brighter again.

When spacecraft first arrived at Mars, it became clear that the first theory was utterly wrong. The Red Planet does not have plant life, and it does not show a seasonal shift in colour as vegetation grows and dies. Indeed, since the dawn of Martian exploration, we have barely noticed any kind of annual change in the colours of Mars.

Yet, over a longer scale, Mars’ surface does show some shifts in colour. Mars’ Utopia Planitia – a vast basin that may once have held a small sea – has gradually become much darker over the past few decades.

A comparison of photographs taken by NASA’s Viking orbiter in 1976 and by ESA’s Mars Express in 2024 shows this change clearly. Places that were once tan in colour are now black, and this darkness seems to be spreading.

Researchers do not know for sure why this is happening. But it is probably caused by dust blowing in Mars’ thin air. Either, ESA says, black volcanic ash is being blown by the wind and covering tan areas with darker material; or winds are blowing tan dust off previously covered areas, and so revealing the underlying darker rocks.

Voyager

After travelling for almost half a century, the Voyager 1 probe now lies more than twenty-five billion kilometres from Earth. It is the most distant man-made object in the universe, and, remarkably, it is still sending data back to us.

Voyager owes this longevity in part to the nuclear batteries that supply it with energy. These are based on a core of plutonium, and convert its radioactive decay into electrical power. But over time the plutonium is decaying, and as it does, the power generated is also decreasing. This will eventually mean the end of Voyager. Sometime in the coming years the power output will fall below the level needed to operate its radios, and contact with the probe will be lost.

Until that moment, however, operators are doing what they can to keep the spacecraft running. NASA has developed a plan for switching off Voyager’s scientific instruments one by one, a process that balances the risk of running short of power with the desire to return valuable data for as long as possible.

At the end of February, however, operators noticed a sudden drop in Voyager’s power output. Out of caution, they decided to switch off a device called the LCEP, an instrument that measures levels of charged particles around the probe.

Only two other scientific devices remain operational on Voyager 1. NASA has plans to reconfigure all of Voyager’s electrical systems sometime next year, which they hope will cut power use and extend the remaining life of the probe. It might even, NASA said, allow mission control to turn the LCEP back on.

Are Quarks Fundamental?

Until 1899, physicists thought atoms were the fundamental building blocks of nature. Their name reflects that: the word atom comes from the Greek atomos, meaning ‘indivisible’. But then J. J. Thomson discovered the electron, and the search for fundamental particles shifted to the next level down: the world of electrons, protons, and neutrons.

Later, in the 1960s, physicists realised yet another layer existed. Protons and neutrons, it turned out, were not fundamental, but were made from quarks. Today, this is where things remain: all atoms, as far as we know, are made from various arrangements of quarks and electrons.

Might there still be another level down? At the Large Hadron Collider at CERN in Switzerland, researchers have probed the structure of the quark to see if it, too, is made of some more fundamental particle.

These experiments collide protons at high speeds. In the aftermath of these impacts, the proton breaks apart into quarks. By studying how these quarks move, physicists can work out whether they act as point-like particles, or whether they have some internal structure.

In a paper published recently, researchers said these experiments have found no signs of any deeper layer of nature. Quarks appear to be point-like, or at least smaller in size than one hundred billionth of a nanometre.

Upgrades to the Large Hadron Collider should soon allow researchers to probe even smaller scales of nature. If there is another layer at the bottom, those experiments may soon spot hints of its presence.

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In March, a chunk of rock and metal appeared over Lake Erie, flew at supersonic speed over northern Ohio, and then blew up in a deafening explosion. Over the next week, at least four other fireballs appeared over North America: one above Texas, two over California, and one that flew across Michigan. None caused any significant damage on the ground, but all were seen and heard by hundreds of people.

This surge of bright meteors did not just hit America. In Europe, an object flew at speed over Luxembourg and Germany on March 8, exploded just before it reached the Rhine, and sent a fragment of rock into someone’s bedroom. A few days later, another bright fireball was seen above France, this time heading towards the border with Spain.

Is something unusual going on? Such a spate of fireballs seems out of the ordinary. And statistics seem to back up the idea that something has changed. According to the American Meteor Society, there were more big meteor strikes in the first quarter of 2026 than in any other recent year.

Most of this increase – the numbers for which are based on witness reports – came in the month of March. Although reports in January and February were slightly higher than normal, the Meteor Society says, it was only towards the end of February that they really started to pick up.

This may partially be explained by a seasonal pattern in appearances. For reasons that aren’t entirely clear, the number of fireballs tends to increase during the spring months of the Northern Hemisphere. Possibly, NASA says, a patch of debris happens to lie in the part of the solar system we traverse at that time of the year.

But it may also be something to do with the solar antiapex, the point in the sky away from which the Sun is travelling. In the spring months this point reaches a zenith in the northern sky, and this seems to be linked to a peak in the number of meteors we see here on Earth.

The real difference, according to the Meteor Society data, is not just the number of meteors seen, but their size. The strikes that came this year seem to have been bigger than average, and large enough to punch deep into the atmosphere and explode with a loud boom.

The Origins of a Fireball

Despite some patterns in how their numbers vary over the year, meteors can come from any direction at any time. All are lumps of rock or ice that were, until they encountered our planet, drifting quietly through the solar system. Most are tiny. Every day about ten metric tons of dust falls upon our world, but others are larger, measuring anything from a few centimetres across to several metres.

Whenever one of these objects draws too close to the Earth, it begins to fall. Since the Earth’s gravitational field extends far into space, anything that encounters the outer atmosphere will already have been accelerating towards the planet for some time. The force of this attraction means it must hit those outer layers of air at a speed greater than twenty-four thousand miles per hour.

At this immense speed, air molecules cannot get out of the way of the incoming meteor. Instead they begin to pile up, forming a region of increasing air pressure directly in front of its passage. As pressure increases, so does temperature, and soon all that heat begins to eat away at the rock and ice.

Small objects disintegrate rapidly. But big ones can survive the initial heat and begin to punch through the thicker, lower layers of the atmosphere. As they do, hot air begins to break into the interior of the rock, starts to attack its internal structure, and eventually rips the object apart. When the end comes, it is sudden. The kinetic energy of the meteor is converted into heat, and it detonates with force.

The result is a hot shock wave. Witnesses to big meteor fireballs – those in Chelyabinsk in 2013, or that in Tunguska in 1908, for example – described flashes of light as bright as the Sun and a wave of intense heat. Some were burned, as if they had stood in bright sunlight for many hours.

The event in Ohio released energy equivalent to almost four hundred tonnes of TNT. The meteor over Chelyabinsk exploded with the force of half a megaton of TNT – dozens of times more powerful than the nuclear explosion in Hiroshima. Tunguska, in 1908, released more than ten megatons, the equivalent of a large thermonuclear bomb.

Is Something Going On?

Fortunately, these kinds of big impact are rare. Events like Tunguska – probably caused by an asteroid or comet measuring about fifty metres across – happen only once every few centuries. Meteors like Chelyabinsk – twenty metres across – arrive once every few decades.

The Earth is big, and most of it is empty. When a big strike does happen, it is far more likely to be over some remote region than over a metropolis. Last year, one of the biggest fireballs occurred over the Pacific Ocean, some four hundred and fifty nautical miles south-west of Mexico. Another exploded over the central Indian Ocean, eight hundred nautical miles south of India.

Both of these events were much bigger than those seen over Ohio or Luxembourg earlier this year, but since there were very few people around to see them, they passed almost entirely without comment. In many cases, indeed, these fireballs are spotted not by people, but by radar systems or by satellites looking for lightning strikes. But these methods are imperfect, and many impacts, even quite big ones, probably pass entirely unnoticed.

This, then, is the main reason why the early part of 2026 has seen an uptick in fireball numbers. By random chance, more strikes have happened over populated areas, and in an age where cameras are now everywhere, more people have captured footage of them streaking through the sky. This randomness probably also explains why this year’s meteors have been larger and more obvious than usual.

Indeed, in the longer run, this year is unlikely to look like much of an outlier. Meteors have always hit the Earth, and people throughout recorded history have reported the occasional flash of light and rain of stones coming from the sky. They rarely hurt people, and those that are big enough to do so are fortunately rare and more likely to explode over the oceans than over land.

This year’s spike in fireballs is nothing to worry about. Instead, if you see one, appreciate you are almost certainly witnessing the demise of an ancient piece of the solar system, one that had survived for billions of years, until it happened to fall upon our blue world.

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For the first time in half a century, humans have flown over the surface of the Moon.

The moment of closest approach came on Monday evening. After flying for six days, the Orion capsule came within four thousand miles of the surface. From inside the capsule, astronauts were able to view the face of the far side of the Moon, and so observed, NASA said, parts of the lunar surface that had never before been seen directly by human eyes.

Shortly before this close approach, the Orion capsule passed a symbolic milestone by becoming the first crewed vehicle to travel more than a quarter of a million miles from Earth. The astronauts on board Orion thus became the most distant travellers from our planet in all of history.

As they broke this record, Jeremy Hansen – the Canadian astronaut on Orion – paid tribute to the ‘extraordinary efforts and feats’ of their predecessors in space exploration. It would, he hoped, be a short-lived record, one that the next generation of explorers should take it upon themselves to far exceed.

While they passed over the Moon, the astronauts studied the lunar surface and relayed comments back to scientists on Earth. Briefly they also experienced a remarkable solar eclipse, during which they were able to see the solar corona extending around the Moon and observe flashes of small meteorites hitting its surface.

Overall, the mission has gone better than most people had expected. The launch, on April 1, came within minutes of the launch window opening and placed Orion almost exactly where mission planners had targeted. The capsule itself has performed well, and aside from a few problems with the onboard toilets – not, sadly, uncommon during spaceflight – the life support systems have apparently functioned well.

Engineers did notice issues with helium leaking from propulsion systems at a higher than expected rate. This helium is used to keep the spacecraft’s fuel at the correct pressure, and is normally confined by a set of valves. Though the leak is not an issue for the success of Artemis II, it will likely trigger a redesign for future versions of the capsule.

The next big challenge will be the re-entry. Orion will hit the upper atmosphere at twenty-four thousand miles per hour. It will need to lose almost all that speed in less than fifteen minutes, during which time the capsule will be exposed to a furious heat and pressure. To keep the crew safe, Orion is fitted with a heat shield.

Yet this shield has been a matter of some controversy. During the previous flight of Orion, the shield did not behave as expected. Engineers thus modified the re-entry trajectory of Orion to try to avoid any risk – but this does mean the crew are relying on a shield that has not been fully tested and that may not perform as well as it should.

The risk to the crew, NASA said, is low. But all will no doubt be breathing a sigh of relief as soon as Orion is spotted descending gently towards the waves under a canopy of parachutes.

Ignition For a New Golden Age?

After Artemis II, what comes next?

At an event at the end of March, NASA laid out an ambitious vision of its future. Key to this is a series of planned missions to the Moon, together involving robotic explorers, crews of astronauts, and the building of a lunar base. But NASA also wants to build nuclear-powered spacecraft and find a new future for the International Space Station.

Under the new timeline, the next Artemis mission will launch in 2027. It will not go to the Moon, but will instead stay in orbit around the Earth. NASA will use the mission to practice docking with the proposed landers – one, Starship, built by SpaceX, and the other, Blue Moon, built by Blue Origin.

This, NASA boss Jared Isaacman says, will be followed by two missions to the lunar surface in 2028. Both will take crews to the surface, and will be the first of a long series of missions taking place every six months. Alongside these, NASA will send twenty robotic missions to the Moon, including the previously cancelled VIPER rover.

NASA’s Artemis plans had, until now, included a lunar space station called the Gateway. Work on this was proceeding slowly, and it was unlikely to be built until the 2030s. NASA has now called time on it. In place of the Gateway, Isaacman wants to direct efforts towards building a lunar base. It is this project that will be supported by many of the crewed and robotic missions he foresees over the next decade.

Closer to home, NASA has also rethought the future of the ISS. At the moment this is supposed to be retired sometime in the early 2030s. But Isaacman now proposes attaching a new module to the station. This – a core module – will contain the key functions needed to support a space station.

Commercial partners could attach their own modules to it, allowing NASA to build a kind of hybrid commercial station. But it also seems possible the module will help extend the life of the station, and perhaps make it less reliant on the Russian segment. The end result could be an extension of the ISS, or at least a core that could support a new, smaller station in the future.

All this is exciting. But it is also expensive, and far from fully funded. Congress will need to approve the plans and agree to pay for them. That, in a week when President Trump proposed cutting NASA’s budget by a quarter, promises to be the biggest challenge of all.

Hubble Watches A Comet Die

A year ago, astronomers spotted a comet heading towards the inner solar system. It was named C/2025 K1, and measurements of its orbit soon showed that it was probably a fresh comet, one that had fallen from the distant Oort Cloud and that had never before passed close to the Sun.

Despite its size – the comet measures about five miles across – astronomers thought it was unlikely to survive its encounter with our star. Indeed, at its closest the comet passed closer to the Sun than the orbit of Mercury, and the heat of this close approach placed immense strain on the icy object.

About a month after the comet had passed the Sun, the Hubble telescope captured some images of it. They show the comet breaking up – instead of a single lump of ice, several large fragments were visible. These fragments were not themselves stable. Indeed, even as Hubble watched, one of them broke apart into several smaller pieces.

Thanks to Hubble’s images, astronomers were able to trace back the timeline of just how the comet had disintegrated. Each event in its break-up exposed fresh, pristine ice from its interior, and astronomers now hope to study how this ice behaves as it is exposed to sunlight for the first time.

The surviving fragments of the comet are now heading back towards the outer solar system. A million years from now they may exit it entirely, and then go on to drift through interstellar space. But at least one fragment might remain bound to the Sun. If it does, it may yet return to the inner solar system one day in the far distant future.

A Flood of New Asteroids

Astronomers working at the Vera Rubin Observatory in Chile announced the telescope had discovered over eleven thousand new asteroids. This haul came even though the observatory has not yet started its main mission, and was based on observations taken during work to optimise its performance.

Among the thousands of new asteroids are thirty-three Near-Earth Objects. As these can potentially threaten Earth – one need measure only a few metres across to cause an impact – astronomers are keen to find and track as many as possible. Fortunately, none of those spotted by Rubin are likely to hit Earth any time soon.

Looking further out, Rubin also saw hundreds of comets and asteroids lying beyond the orbit of Neptune. Only about five thousand such objects are currently known. With the aid of Rubin, astronomers hope to increase that number at least tenfold in the next few years.

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Unfortunately, the American moths he tried proved unsuitable. They were susceptible to disease, and none could produce silk fast enough to make commercial sense. But Trouvelot thought he had a solution to this too. A hardier species of moth, he reckoned, could be bred by importing the gypsy moth from his native France.

By 1868, he had acquired some eggs and started rearing them on his farm in Massachusetts. Unfortunately, the next year there was a sudden storm, his caterpillars were blown into a nearby woodland, and soon they began to multiply.

Within a few decades of that fateful night, the gypsy moth was everywhere. Today, its caterpillars consume a million acres of foliage every year and cost the economy about a billion dollars annually in lost timber. Efforts at eradicating them have mostly proved unsuccessful.

Trouvelot abandoned his silk farming efforts some time after that disaster. Instead he turned his mind towards art and astronomy. At first, it seems to have been a hobby: he sketched the aurora and a meteor shower. But eventually his work was seen by astronomers at Harvard, and they invited him to make use of their great telescopes.

This was a time before photography was widespread. Early efforts at photographing the stars in the 1840s had used daguerreotypes, but they had proved unsuitable for anything but the brightest objects. By the 1880s, astronomers had photographed the Moon and the Sun and precious little else.

Recording observations was thus mostly done by hand, and this was the art at which Trouvelot excelled. He made over seven thousand astronomical sketches. He drew stars and nebulae, the planets, the rings of Saturn, and the appearance, in 1881, of a comet.

In 1876, some of his sketches were displayed in Philadelphia as part of America’s Centennial celebrations. After this, he set to work preparing a book of his work, and though he had returned to Paris before it was in print, this book - The Trouvelot Astronomical Drawings Manual - was published in 1882. The drawings below are taken from that work, and were made in pastel.

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The hottest stars are blue. These are the beasts of the galaxy, massive stars that can be seen from incredible distance. But they do not live long: they burn their fuel furiously, and die young. Scattered all around them, and far outnumbering them, are smaller, redder stars. These, the red dwarfs, are almost immortal. They burn their fuel so slowly that they will endure for trillions of years to come, and will only face death in that future epoch when the cosmos begins to fade into darkness.

Somewhere between these two extremes lie stars like our Sun. It is a G-type star, a class that puts it alongside stars like Alpha Centauri, Tau Ceti, and 51 Pegasi. These stars are not especially big or bright, though they are considered to be stable and most will survive for about ten billion years before they run short on fuel.

Studying other G-type stars can tell us about the past and future of our Sun. Those that are most like our Sun – especially in terms of mass, colour, and chemical makeup – are known as solar twins. Though they were probably not born together with our star, they will follow a similar path through life.

Until recently, only a dozen or so solar twins were known. But thanks to the Gaia Space Telescope, we now have a map charting the positions and properties of more than a billion stars in the Milky Way. By searching this map, a team of researchers based in Japan and France say they have identified more than six thousand solar twins.

Each burns with a temperature within two hundred degrees of that of our Sun. Each also has a similar surface gravity, a detail which implies they have more or less the same mass and density. And each is made of a similar set of chemical elements, and so emits light with the same characteristics as our star does.

This catalogue has a lot of potential. If we know where the solar twins are, we can watch them and see how they behave. This can offer insight into rare events: for example, there are hints that the Sun occasionally unleashes gigantic flares. Since this happens once every few hundred years, our odds of catching one are small. But if we can watch many Sun-like stars, those odds suddenly become far better.

It can also allow us to trace the history of the galaxy and of our solar system. Analysis of the ages of these solar twins shows two separate groups. One is relatively young, and formed about two billion years ago. But the other is older, and matches the age of the Sun: these stars formed between four and six billion years ago.

Many of the stars in this second group appeared to have moved outwards from the centre of the galaxy. The Sun was probably part of this vast stellar migration: it seems to have formed about ten thousand light-years closer to the galactic core than its current distance, and then drifted outwards at some point.

The reasons why this migration took place are still unknown. But the researchers behind the analysis suspect it has something to do with the bar that stretches across the centre of the Milky Way. The formation of this bar could have sparked the birth of new stars, they say, including our own. Afterwards, its presence could have helped push these new stars outwards.

The centre of the galaxy is probably hostile to life: it is crowded, chaotic, and often hit by powerful events like supernovae. Our star’s outward migration brought it into a calmer neighbourhood. We might owe our very existence to that long-ago voyage across the galaxy.

The Great Silence in The Heavens

It has been almost one hundred and twenty years since the first human words were transmitted by radio. In theory, those words – a short speech, a song, and a wish for a Merry Christmas – are still out there somewhere, marking the forefront of a bubble of sound coming from our planet. In practice, of course, that early signal is now so distant and dilute that even the most sensitive receiver would have trouble hearing it.

Yet the idea remains. Our radio signals have been leaking out into space for more than a century, and anyone out there listening could pick up the sounds of our world. The same may be true in reverse: one day we could pick up a radio message coming from another intelligence, another civilization out there in the stars.

This formed the basis for SETI, a long search for artificial radio signals coming from space. But, despite a decades-long hunt, this search has found nothing. There appears to be a Great Silence in the heavens, a dearth of noise that makes some wonder if we are alone.

But there may, according to a new paper, be a good reason for this silence. It points out that any radio signal leaving Earth must first pass through a region of space shaped by the Sun. This can be a turbulent place: it has shifting magnetic fields, the everlasting solar wind, and the occasional passage of a big solar storm. All this alters the radio signals passing through it.

The result, the authors say, is a broadening of narrow radio signals. Those focused around a single frequency – as artificial ones tend to be – will be weakened, and the ratio of signal to noise will fall. Any observer would then struggle to distinguish these signals from natural ones.

This is not just a theoretical possibility. The study also looked at the radio signals coming from distant probes, including those sent by Pioneer and Voyager. All showed signs of distortion, and saw this worsen when the Sun was more active than normal.

Such effects do not make it impossible to detect radio signals coming from other star systems. But it does mean we might have been looking for the wrong thing – and that by modifying our search parameters, we might stand a better chance of spotting the long-sought-after message from the stars.

A Charming Proton

Every atom is made of a collection of quarks and electrons. The electrons fly alone, forming a hazy cloud around a central nucleus. The quarks cluster in this nucleus, and combine to form protons and neutrons.

There are six kinds of quark, though only two of them – the up and down quarks – appear in atoms. Protons are made of a group of two up quarks and a single down quark; neutrons are the reverse, containing a single up paired with two downs.

The other kinds of quark – the charm, the strange, the top, and the bottom – are seen more rarely. Indeed, the last of them, the top quark, was only discovered in 1995. But as with the up and down quarks, these can also combine into larger particles. When these particles contain three quarks, as protons and neutrons do, they are called baryons.

About eighty such baryons are now known. But almost all are still dominated by up or down quarks, and have only one of the more exotic flavours. Last week, however, CERN announced the discovery of a new baryon, one made of two charm quarks and one down quark.

This makes it look something like a proton, just with the up quarks replaced by the heavier charms. The discovery was not unexpected – its existence is allowed for by the known laws of physics, although it is not thought to be particularly stable. But this is at least an opportunity to study another particle, and to see if it acts as our laws of physics say it should.

Nucleobases on Asteroids

Last week, researchers announced they had found all the essential components of DNA in a sample brought back from the asteroid Ryugu. This includes the five nucleobases found in all DNA and RNA molecules, each of which is thought to have played a key role in the origins of life on Earth.

These kinds of molecules have been found on asteroids before – samples of Bennu collected by Osiris-REx contained some of them, as did some meteorites that were examined after they fell to Earth. This discovery is not therefore something completely new, but it does strengthen the picture we already had.

Because these molecules seem to be widespread on asteroids, it suggests they can form easily through chemical reactions taking place in space. After the Earth formed, it may have been impacts of asteroids like Ryugu that brought the ingredients of life to our planet. And if that was the case here, it should have been the same on many other worlds.

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Yet every now and then, astronomers notice that a big star has simply vanished. This is quite remarkable. Rather than exploding, it would be as if Betelgeuse switched off one day and calmly blinked out of existence. Instead of shining with the light of a billion suns, it would quietly cease to be.

These disappearances are hard to spot. Unlike supernovae, which are bright and all but impossible to miss, vanishing stars don’t announce themselves. We spot them only by chance, usually when astronomers happen to notice that a bright star is no longer shining where it once did.

One such case came in 2024, when researchers realised a bright star in the Andromeda Galaxy had disappeared. There was no supernova, and instead only a faint glow coming from the place where this star used to lie. A decade earlier it had seemed to be a yellow supergiant, one on the verge of a dramatic death. Yet despite a small flare-up in 2014, nothing else of note has been seen since. The star seems to have quietly faded away.

The only other convincing example was spotted in 2015. It, as before, seems to have been a supergiant star, probably one twenty-five times the mass of the Sun. In 2009 it brightened, but in the years after it too faded away without a supernova. Today it is gone. Like the more recent example, the star seems to have silently slipped away into retirement.

Where are these giant stars going? That they have quietly turned off seems hard to believe, if not physically impossible. Without the radiation pressure coming from within, these stars would collapse. And that implosion should trigger a shock wave that must blow the star apart. A supernova, according to our current understanding of dying stars, would be inevitable.

But there is a possibility they are directly collapsing into black holes. Some theorists think that in a big enough star, one with a compact core, the shock wave could stall. If this happens, the centre of the star forms a black hole. The outer layers are sucked in, and whatever remains slowly fades away. No supernova takes place, and, if one is watching from a distance, the star quietly seems to vanish.

The Problem With Red Supergiants

The fate of a star depends on its mass. Those the size of our Sun will, after a few billion years of life, swell up into red giants, gradually shed their outer layers, and slowly fade into darkness. This happens because they are powered by fusing hydrogen, and when this hydrogen starts to run out they begin to cool and expand, and then – once this process reaches an end – gradually dim.

But when a star is above about eight solar masses, another process kicks in. These stars are big enough to fuse heavier elements, and they can burn helium and carbon, and other elements up the periodic table until they reach iron. Iron is not well-suited for nuclear fusion: the reaction takes in more energy than it releases, and when a star reaches this point, the end is not far off.

But before that, the star expands. As it begins to fuse helium, it swells into a red supergiant – a bubbling cauldron of a star similar to the current state of Betelgeuse or Antares A. The most massive stars can briefly become yellow supergiants. These are extremely bright, very unstable, and rare – indeed, only a few dozen of them are known in the Milky Way.

All big stars become a delicate balancing act. On one side they are massive, and all that mass tries to fall inwards and compress the star. But this force sustains immense nuclear reactions in their cores, and this creates a pressure that pushes out and tries to expand the star. The more a star compresses, the more furious the nuclear reactions become, and the stronger the expansion pressure that results.

This can sometimes result in pulsing stars, in which the balancing forces oscillate. At times gravity seems to be winning the battle, and the star shrinks. But this triggers a response from the core, and strengthens the opposing pressure. The star brightens, and then expands again. This cycle can repeat over and over, following patterns that last many years at a time.

Eventually, gravity always wins. At some point, the core of the star fills with iron, the nuclear reactions slow, and the internal pressure stops. The outer layers implode at enormous speed, crash into the core, and trigger a powerful shock wave. This blows the star apart. The inner core collapses, forming a black hole or neutron star, and around it a supernova detonates.

When we spot the flash of light this creates, astronomers swing into action. We cannot yet predict exactly when a giant star might explode. But we do have automated telescopes that detect them almost as soon as they appear. Astronomers then look back at previous observations to work out which star exploded – this is called the progenitor star.

Progenitors are valuable, since they can sometimes tell us what a star was doing in its final years. Yet these studies have raised an odd puzzle: though red supergiants range in mass from eight to twenty-five solar masses, almost all the ones seen to explode are smaller than about twenty solar masses.

The biggest stars, it appears, do not die as they should. This is known as the red supergiant problem.

Failed Supernova and Dust Clouds

Are the biggest stars simply vanishing? If they collapse directly into black holes, with no corresponding supernova, it stands to reason we would not find any progenitors bigger than about twenty solar masses. Above this point the core might become compact enough, the shock waves could stall, and the whole thing could be swallowed by a fast-forming black hole.

Yet, this should still leave some signs. As the debris of the dead star circles the black hole, it should release a lot of energy. We ought to see x-rays flaring out, and this should mark the spot where the star once stood. But surveys have not found any clear signs of this actually happening.

Neither have efforts to monitor big stars revealed much. In one study, astronomers watched millions of big red supergiants over more than a decade. Almost all of them were still there by the end, and only one disappeared in a way that could hint at direct collapse into a black hole. And even in that case, the x-ray signature was missing, and still is, more than a decade after the vanishing act.

Some astronomers wonder if there is something more complicated going on. It could be that red supergiants shed mass before they die. Strong winds blow from within them, and these send clouds of gas streaming out into space. Eruptions can sometimes eject big chunks of stellar material – something like this seems to have happened to Betelgeuse in recent years.

This could create two effects. One is that the biggest stars might shrink drastically before they die. If they lose mass fast enough, then all progenitors might be expected to be below some upper limit, just as observations have seen. But second, is that the dust and gas they expel could make them look fainter than they really are.

If this is the case, then it may confound our measurements. Astronomers normally assume a bigger star shines more brightly than a small one. But if big stars on the verge of death happen to be dusty, then they will be darker, and astronomers will think they are smaller than they really are. Big progenitors could be missing simply because we don’t detect them properly.

As for the two stars that we thought we saw vanish, there might be another story that fits the facts. Emma Beasor, an astronomer based in Liverpool, England, thinks these are better explained as collisions between stars, rather than collapsing ones. In the years leading up to the collision, the two stars would look like a single very bright one. The collision itself would cause a flare, and then in the years afterwards the resulting object would darken, and possibly be surrounded by dust.

There is, then, no shortage of possibilities. The idea that giant stars might collapse directly into black holes is a neat solution, and a seductive one, but we need more data to understand if it is right or not. But even if it turns out to be wrong, the search will still reveal the workings of some of the most violent events in the cosmos.

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Of all the known stars, WOH G64 is surely one of the most extreme. It is enormous. Were it placed at the centre of the solar system, it would consume all the planets out to the orbit of Jupiter. If it were instead at the distance of Alpha Centauri, it would shine in the sky almost as brightly as the full moon. Even across such a span of space, its light would still be strong enough to cast shadows at night.

Such big stars do not live long. WOH G64 is probably about five million years old. Some time in the past few hundred thousand years it became a red supergiant: that is, it ran short of hydrogen fuel, switched to burning heavier elements such as helium, and swelled to its present brobdingnagian size.

Over the last decade, however, it seems to have undergone another change. Around 2011, the star started to dim. It then reversed course, and dramatically brightened between 2013 and 2014. At the same time, it became hotter, shrank, and changed colour, shifting from red to yellow.

That was a potentially significant change. Very big stars like this one can sometimes evolve into what astronomers call a yellow supergiant. These are rare: in our galaxy there are only about two dozen of them. Partly this is because big stars are uncommon. But it is also because they don’t live long – yellow supergiants are hot, bright, and highly unstable.

Yet over the past year, WOH G64 seems to have calmed down. It has cooled, and its colour has shifted back to a more reddish hue. It has also become less bright, and might have once again swelled in size. If it was a yellow supergiant, it doesn’t seem to have been one for long.

In an attempt to understand what is going on, Gonzalo Muñoz-Sanchez at the University of Athens has studied the data we have on this star. He reckons we might have been fooled in the past. Instead of a single star, he says, we might be looking at a binary system in which a small hidden star is orbiting a much larger giant.

These two stars are close, and this means the smaller one can pull material out from the larger one. That could be creating a thick blanket of gas around the partners, something that might glow red and look rather like a vast red supergiant. But at times – and this may have happened over the past two decades – some kind of disturbance may hit this blanket.

When that happens, the true nature of the star is revealed to our telescopes. In the place of a vast supergiant, we see a smaller and yellower star. Its brightness fluctuates as its partner moves around it, and for a few years the star thus seems to radically alter in nature. Afterwards, as the gas blanket recovers, the system goes back to its previous reddish appearance.

Astronomers are uncertain about what the future holds for this system. It is certainly dying. Within the next few thousand years it might explode in a dramatic supernova. That, since the aftermath would be easily visible to the naked eye, would be a spectacular event. But it is also possible that a stranger fate might await it: one day the star might simply vanish, and implode silently into a black hole.

NASA Rethinks Artemis

NASA has shaken up its approach for returning to the Moon. The move comes at the behest of Jared Isaacman, the newly appointed head of NASA, who is keen to reform and accelerate the work of the space agency.

He has identified NASA’s moon rocket, the SLS, as a key issue. It costs a lot of money and is hard to get off the ground. Last time it flew, in 2022, engineers spent months troubleshooting problems. The issues seem to be repeating this year: NASA wants to launch the SLS to send humans around the Moon, yet it is once again struggling with fluid leaks as the rocket prepares for lift-off.

To solve this, he says, NASA needs to fly the rocket more often. That would give engineers more experience working with it, and would allow for more opportunities to perfect the complexities of launching it. To that end, he wants to launch the rocket four times over the coming three years.

The next launch should come in the next few weeks. That will be Artemis II, a mission that will send astronauts on a loop around the Moon. Nothing has changed there. But Artemis III – which was supposed to support the first lunar landing in five decades – will now only reach Earth orbit. Engineers will use the mission to practice docking the Orion capsule with either SpaceX’s or Blue Origin’s lunar lander, a step that will be needed before crews can head to the surface.

In 2028, Isaacman wants the agency to try for two Moon landings, both of which would put human boots on the regolith. Yet it remains to be seen if America’s space industry can meet the challenge. The SLS has been delayed many times before, and neither SpaceX nor Blue Origin have yet shown they can get a human lander anywhere close to the Moon.

Even so, Isaacman’s approach is probably the best chance America has of landing an astronaut on the Moon this decade. 2028 may still be too ambitious a goal. But his revised plans signal a willingness to consider other options, and to put industry – including SpaceX – on notice.

JUNO Spots 3I/ATLAS

Back in November, the interstellar comet 3I/Atlas reached its closest point to the Sun. Unfortunately for us, the comet then lay on the far side of our star and so was hidden from view of our telescopes.

Luckily, however, Europe’s JUICE spacecraft was positioned for a better view. It is nearly three years into its eight-year-long voyage to Jupiter, and in November it also lay behind the Sun. Operators thus took the opportunity to direct its cameras to study the comet.

But as JUICE was behind the Sun, it was unable to immediately send the data it collected back to Earth. Instead it stored it onboard. When it emerged, earlier this year, it transmitted the photos and measurements it collected back to mission control.

ESA has now released the first of these images. They show the comet as seen from a distance of sixty-six million kilometres, and reveal it to be surrounded by an egg-shaped halo of gas. Behind it streams a faint tail of particles, making it look rather like any other comet approaching the Sun would.

Researchers will now study the rest of the data JUICE sent back. Later in March they will meet to discuss what they have found – and what the observations can tell us about what the comet is made of and where it might have come from.

3I/Atlas itself is now approaching Jupiter. It should make its closest approach to that planet on March 16. Afterwards it will fly onwards and begin to fade from view. But the simple fact it has traversed the distance from Mars to Jupiter in mere months hints at its impressive speed: JUICE will take another five years to cover the same distance.

Life and Asteroid Impacts

Sixty-six million years ago, an asteroid at least six miles wide smashed into a shallow sea close to what would one day become North America. It left a crater eighteen miles deep, and blasted a trillion tons of rock and dust upwards. Much of that fell back to Earth, set everything on fire, and contributed to the extinction of the dinosaurs.

Yet some of that rock escaped our planet. Over the next few million years, pieces of Earth fell on Mars, or burned in the thick atmosphere of Venus. A fraction of it even reached the moons of Jupiter and Saturn, and there it likely plunged into the deep oceans that lie under their icy surfaces.

It is all but certain that some living creatures were blasted into space along with all that debris. Since experiments on the International Space Station have shown that some hardy bacteria can be revived after spending years exposed to the vacuum of space, it seems likely that some lifeforms survived this initial ordeal.

Now, an experiment has found that some of those same bacteria are also capable of surviving intense shocks and pressure waves. That came as a bit of a surprise, the scientists conducting the experiment said. The shocks were expected to rupture the cell walls of these creatures and so kill them. Instead the experiment found that almost all of them survived.

This lends even more weight to theories of panspermia, the idea that life has emigrated between the planets. And since Earth rocks surely lie at the bottom of oceans on Titan, Europa, and Enceladus, it strengthens the idea that we may soon find evidence of thriving colonies of bacteria elsewhere in the solar system.

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America’s billionaires are suddenly obsessed with the idea of building data centres in orbit. Elon Musk has, as usual, taken the lead. In the past month he has merged his AI company with his rocket company, and applied for permission to launch a million satellites into orbit. Within the decade, he says, this company will build a city on the Moon dedicated to pumping out an endless stream of AI spacecraft.

He is not alone. Google is pursuing an AI moonshot, and wants to put prototypes in orbit as soon as possible. Starcloud, a start-up funded by Y-Combinator and backed by NVIDIA, has already put AI chips in orbit, and wants to build a constellation of eighty thousand satellites. Amazon founder Jeff Bezos thinks the first gigawatt-scale data centres will be in orbit by the 2040s.

The question, though, is why anyone thinks this is a serious idea. Working in space is hard. Very few things make economic sense in orbit. Those that do - communications, navigation, and observation – are feasible only because their advantages outweigh the tremendous downsides of launching equipment into the vacuum of space.

Much of the current noise is coming from the rapid build-out of data centres for AI. Training the models behind apps like ChatGPT takes a lot of processing power – we’re taking hundreds of megawatts here, or the demand of a small city – and the difficulty of getting this power supply fast enough is threatening to curtail the race towards ever more intelligent machines.

Building more power stations is the obvious answer, but this takes time and getting the necessary permission to build new plants is a lot of work. For a while it looked like nuclear fusion might be the answer: OpenAI and Google both poured millions into start-ups claiming they could build a working reactor. But progress, as always, has been slower than the hype, and the AI companies have impatient investors to satisfy.

Why not, then, turn to the giant fusion reactor we call the Sun? It has provided us with a steady stream of energy for the past few billion years. The most direct way to access that power is with solar panels, and on paper these can be deployed in such numbers as to provide us and our data centres with all the power we could ever need.

But doing this on the Earth’s surface has some drawbacks. The most obvious is the night – after the Sun sets, solar panels are useless. Yet even when the Sun is shining, its light can be blocked by clouds, and since the Earth wobbles as it spins the intensity of the Sun’s light varies over the course of a year.

So why not put the panels in space? With the right orbit you can ensure they are almost always illuminated. There is no atmosphere to weaken the Sun’s rays, and neither are there any clouds to block them out. Panels can easily be oriented to maximise their exposure, and as a result solar panels can in theory produce much more energy in space than they can on Earth.

Even better, at first glance this energy looks cheap. Solar power is free and almost endless, and once the satellites are in orbit they require little in the way of maintenance. Their numbers can scale almost without limit, with no neighbours to complain or landowners to bargain with.

Into The Great Wide Open

So what’s not to like about endless free energy? Musk in particular seems enamoured with the idea. His constellation, he claims, will propel consciousness into a new age, one in which we convert the entire energy output of the Sun into the musings of an artificial intelligence. Humanity might not survive that – who knows what such a being might think of us – but that does not matter, just as long as the spark of consciousness lives on in this wondrous creation.

Yet there are basic problems with the idea. Much ink has already been spilt on the topic. One is a basic question of thermodynamics: every watt of energy a spacecraft takes in must somehow be later expelled. If you don’t or can’t do that, then it will heat up, and heat quickly becomes a problem for computers. Indeed, most data centres on Earth put a lot of effort into keeping their processors cool.

Fortunately space is cold, enthusiasts say. What they really mean is that the vacuum of space is cold. Astrophysicists have measured its temperature, and found it to be only a few degrees above absolute zero. But the vacuum is also a good insulator, and so heat does not conduct away from spacecraft as it would on Earth. Instead, a spacecraft can only lose heat by radiation, and this is not an efficient process.

Engineers thus put a lot of effort into controlling heat flow in and out of spacecraft. Some deploy enormous heat shields to reflect the light of the Sun away; others are fitted with special devices to radiate heat as efficiently as possible. Inside, they run heat pipes and pump fluids to cool computing devices and to push heat to places where it can be expelled.

Simply capturing enough solar power is another problem. Each megawatt you need demands at least eight hundred square metres of solar panels1. Such enormous arrays quickly become a challenge to manoeuvre and position. Even worse, the good orbits are full of old satellites, pieces of debris, and micrometeorites. Avoiding collisions becomes imperative, and will become a nightmare task even the smartest AI will struggle to solve.

Yet even if you can solve these fundamental questions – and with enough ingenuity and equipment you can probably get close – you still face issues of radiation, of degrading solar panels, of replacing failed processors, and of keeping your whole system up-to-date with the latest AI chips developed back on Earth.

These are all, in truth, engineering problems rather than showstoppers. But they are each a downside to the dream of unlimited free energy. Addressing them costs money, lofting them into orbit costs more, and pretty soon all that free energy starts to look rather expensive after all.

One Day, But Not Today

In 2012, Popular Science ran an article titled “Why Mining an Asteroid for Water and Precious Metals Isn’t as Crazy as it Sounds”. Back then, asteroid mining was all the rage. The usual suspects rushed to invest, hoping to stake a claim to the minerals of the future. Eric Schmidt and Larry Page of Google poured in cash, as did the director James Cameron. Even countries got involved – in 2016, the Grand Duchy of Luxembourg invested millions of dollars into the concept.

Of course, it went nowhere. The over-hyped start-ups were sold off – one smoothly transitioned into blockchain services – and the idea of mining asteroids now lingers as a faintly embarrassing episode in the history of spaceflight. In many ways the concept was doomed by the technology needed. It could one day be done, perhaps, but it would take a solid decade or two of consistently pushing the frontiers of spaceflight.

Nor did the sums really add up. The minerals investors dreamed of are all already available on Earth. If the demand for them were there, we could extract them more easily and cheaply from the bottom of the sea, or from deep in the Earth’s crust. Both would be far simpler than venturing into the depths of space and landing on an asteroid.

Is history repeating itself? Last week, The Economist ran an article entitled “Data Centres in Space: Less Crazy Than You Think”. It lays out a possible path to building orbiting facilities at lower cost than on Earth. But look closely and the argument is flawed2. It relies on technologies that haven’t been developed, on contradictory assumptions, and on unrealistic input costs.

The only real difference this time may be the deep pockets of Elon Musk. He could, if he wishes to, build orbiting data centres at a loss. With enough money, many of the engineering obstacles might be overcome. Prototypes could be launched. But it still won’t be cheaper than doing it back here on Earth. The sums simply don’t add up.

One day, decades from now, things might look different. Asteroid mining would be viable if we had a use for the minerals in orbit. The hard part is often the transfer between Earth and space – if you never bring the rocks back to the surface, then perhaps a profit is to be had. The same might be true for computing power in orbit. For certain applications, it may one day become cheaper to keep everything in space.

Yet until that day comes, the idea really is crazy. And if you have to keep reassuring people it isn’t, you are probably wrong.

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In the 1950s, the American government poured millions of dollars into studying something called nuclear pulse propulsion. The name only hints at the insanity of the idea: the pulse came from the detonation of atomic bombs, and the propulsion came from placing a spacecraft in just the right spot to surf the nuclear shock wave out towards the stars.

Papers on the subject painted exciting visions of the future. If only the technology were pursued, they said, Mars could be reached within a decade and the moons of Saturn would be settled by the 1970s. A chain of a million fusion bombs could blast a spacecraft towards Alpha Centauri and humans – travelling at a decent fraction of the speed of light – could then cross the interstellar gulf in mere decades.

Of course, the lethal doses of radiation involved were problematic, as were the bone-shattering accelerations astronauts would experience. No one really found a solution to those concerns, and estimates anyway showed the whole thing was fantastically expensive. After almost a decade of study, the idea was quietly shelved in 1965.

But in recent months, nuclear-powered spaceflight has once again come into fashion. Today’s ideas are, thankfully, more realistic. Instead of atomic explosions propelling interstellar travellers, dreamers imagine reactors powering electric motors. In place of violent pulses, probes would be gently accelerated by the flow of ions streaming behind them.

On paper, these dreams look pretty good. Electric propulsion systems have been in use for decades now, and though low-powered, they have already sent spacecraft to the Moon and kept interplanetary probes on course. They are lightweight - especially when compared to chemical rockets - and can provide a small but steady thrust over long periods of time.

A nuclear reactor could supercharge them. With it as a power source, an electric system would have more power and run for far longer, allowing to us to propel big spacecraft deep into the solar system. If we want to get serious about exploring the outer planets, then nuclear electric propulsion might be the key technology with which to do it.

Jared Isaacman, the new head of NASA, is a big fan of the idea. In Project Athena, his leaked plan for reshaping the agency, he argued that NASA should go all in on developing it. This would start by flying a one hundred kilowatt demonstration mission, perhaps one that heads to Mars. A more powerful version would follow, and this would prove the concept for human crews and eventual missions to other worlds.

If he can pull all this off, it would be something well worth doing. NASA would be investing in a new technology, one that opens doors for future exploration. Armed with nuclear-powered probes, we could put orbiters around Pluto and Uranus, send a big spacecraft to Jupiter and its moons, fly closer to the Sun than ever before, and even venture beyond the edges of the solar system. It would, in other words, help create a new vision for space exploration. And that is exactly the kind of thinking NASA needs.

The Mysterious Disappearance of a Star

One of the brightest stars in the Andromeda Galaxy seems to have vanished. Until 2014 it was probably a red or yellow supergiant, perhaps something like the nearby star Betelgeuse. But in that year it suddenly flared up, and then, in the years after, quietly faded away. Today it no longer seems to shine where it once did.

What happened to it? Big stars don’t typically blink out of existence. Instead they burn brilliantly, and when they run out of fuel they explode dramatically. If this star, named M31-2014-DS1, had died, we would expect it to have done so in a supernova, and that would have been so bright it would have been impossible to miss.

Perhaps, a team of researchers at Columbia University suggested, it had simply collapsed into a black hole. Yet that too would be unusual. When big stars exhaust their fuel they can indeed implode into a black hole. But that implosion almost inevitably triggers a sudden burst of neutrinos and a shockwave that sparks a supernova.

Some models do suggest this process could occasionally fail. Instead of exploding outward, in this case the star would simply collapse. Its core would become a black hole and whatever remains would swirl dangerously close to its outer edge. From a distance the star would simply seem to vanish, just as we have seen in the Andromeda Galaxy.

The problem, though, is that this process should still release a lot of X-rays, and none of these have been seen. It is possible, says Emma Beasor of Liverpool John Moores University, that they are being hidden by a cloud of gas and dust. But it is also possible that something else has happened here.

She thinks the event is better explained by a collision between two stars. As they spiralled towards one another they would have appeared deceptively bright, and after the collision the resulting star would have dimmed. Clouds of dust might have been thrown up too, and this would have further darkened the object.

As usual, more data will be needed to settle the dispute. If we spot X-rays coming from the site of this vanished star then the failed supernova idea is probably the true one. But if we don’t, and we see signs that a faint star still lingers, then perhaps the collision theory is closer to the truth.

The Shape of Jupiter

No planet is perfectly round. Some deviations are obvious: the Earth has mountains and troughs; the surface of Mars is pockmarked by craters. But others are more subtle. When precisely measured, most planets are wider than they are tall, a detail known in geometry as their oblateness.

This happens because planets spin, and as they do some of their mass is pushed farther out at their equators than at their poles. The faster a planet spins, the greater this degree of flattening. Earth, as a result, is about forty kilometres wider than it is tall. That isn’t much, yet it is enough for the point furthest from the Earth’s centre to lie not at the peak of Mount Everest, as logic might dictate, but rather at the top of Mount Chimborazo, a mountain much closer to the equator.

Jupiter – made mostly of gas – has no mountains. But it does spin once every ten hours, and this is fast enough for its equator to bulge significantly. Measurements taken in the 1970s by the Voyager and Pioneer probes found it to be about seven per cent wider than it is tall, a difference that adds up to more than a thousand kilometres and that creates a noticeable flatness.

Since those observations are now decades old and were based on few readings – the two probes collected just six measurements – researchers recently used another probe, Juno, to repeat the study. They found Jupiter to be slightly smaller than previously thought, with a radius about twenty kilometres less. It is, though, still flat: the planet measures 71,488 kilometres from centre to equator, but only 69,886 kilometres from centre to pole.

Artemis II: Delayed Again

NASA has once more delayed the lift-off of Artemis II. On Friday last week, things had looked good. Operators had spent the week running through a dress rehearsal of the launch countdown, and had successfully reached a point twenty-nine seconds before lift-off. The hydrogen leaks that had forced the abandonment of a previous rehearsal seemed to have been solved.

But on Friday night a new issue emerged. According to NASA administrator Jared Isaacman, engineers could not get helium to flow through the rocket as intended. This is not an easy thing to fix – indeed, the rocket will need to be removed from the launchpad, and rolled back into an assembly area.

This rules out any launch attempt in March. The position of the Moon means lift-off can only come in the first half of the month, and NASA can’t get the rocket back on the pad before then. The next opportunity will only come in early April.

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In 1768, the Royal Society of London dispatched the vessel Endeavour on a voyage of discovery. The mission was one of exploration. Cook was to boldly go into the unknown Pacific Ocean, explore the strange lands of Terra Australis, and stop on the isle of Tahiti to make observations of the Sun and of Venus.

His target was the transit of Venus, a rare event in which the planet crosses the disk of the Sun and for a few hours appears as a dark shadow against the brilliant background of our star. Accurate measurements of this transit would allow astronomers to determine the distance to the Sun. But to get the best data, observers had to be scattered across the world.

The results were good: with them in hand, scientists calculated the solar distance to within one percent of the actual value. Cook, meanwhile, went on to achieve daring feats: he became the second European to reach New Zealand, the first to land on the eastern coast of Australia, and then narrowly escaped a wreck on the Great Barrier Reef on his way back north.

Today, transits of Venus are little more than an astronomical curiosity. The last occurred in 2012, the next will not come until 2117. But transits around other stars are pursued with vigour. Indeed, today we have telescopes in orbit dedicated to searching for them, and for the resulting discovery of an exoplanet.

These telescopes are not powerful enough to directly see the shadow of these distant worlds. But they can detect a slight darkening in a star’s light, and since this follows a characteristic pattern – a sharp dip, followed some hours later by an equally sharp rise - it has proved a reliable method of hunting for new planets.

The easiest planets to spot in this way are the biggest - those the size of Jupiter, say. And since repeated transits help confirm that a planet is really there, the method also favours worlds that orbit close to their stars and frequently pass in front of them. Hot Jupiters – big planets swinging around their stars every few days – were thus an early discovery of the transit method.

But the most interesting exoplanets are those that look like Earth, that orbit stars that shine like the Sun, and that sit at roughly the same distance from them as we do. Few planets like this have been found. For such orbits transits come at best once a year, and it can take half a decade to gather enough data for a discovery. Even worse, such planets are small, especially when seen at a distance of several dozen light-years, and the slight darkening of their transits is hard to detect.

Nevertheless, a team of Australian astronomers recently claimed a discovery. Their world, named HD 137010 b, is slightly bigger than ours and orbits slightly closer to its star. That star itself is cooler and older than ours, and as a result this planet is probably cold. But it may be warm enough for life to exist on its surface, and that, as always, is a fascinating discovery.

The team have seen only one transit of this planet, and so the data they have remains unconfirmed. But they seem sure the world really is there. Indeed, they wrote that there is nothing “other than a planetary transit that can reasonably explain the event”.

More data will surely come in time. And it is, perhaps, worth remembering that the years can bring a lot of progress. In the transit of 1769, the Pacific Ocean and the lands of Australia remained unknown to men of science. By the transit of 2012 we had satellites in orbit around Venus. Where might we be the next time that planet passes in front of the Sun?

Black Hole Suns?

The James Webb’s little red dots continue to puzzle astronomers. Hundreds of them have been spotted in the early universe. All appear to be ancient, existing only in the first two billion years of cosmic history. And as far as we can tell, they are small objects, each measuring no more than a few hundred light-years across. Yet all somehow shine with an intense brightness.

No theory had predicted them, but most astronomers reckon they must be some kind of black hole. Unfortunately the facts there don’t quite add up: the matter swirling around a black hole should emit energetic radio and X-rays, but none have been observed coming from the little red dots.

A recent discovery, however, seems to point to a possible solution. In a paper led by Raphael Hviding of Heidelberg, a team of astronomers describe a strange object that looks something like a little red dot. It is, however, emitting X-rays, and so it also matches our expectations about what a black hole should be doing.

Does this object represent a late-stage red dot? Astronomers have paired the idea with another that suggests early black holes were surrounded by dense clouds of dust. These clouds might have been thick enough to obscure the X-rays coming from within, and would instead have glowed hot.

That would have made them look a bit like gigantic stars. But instead of being powered by nuclear fusion, they would have been heated by gas and dust falling into a inner black hole. That could have allowed them to grow far bigger than any stars we see today, to the point where they may have spanned dozens of light years.

Black hole stars like these could not have lasted forever. At some point the black hole would have emerged from the cocoon of surrounding gas, and when this happened they might have looked something like the object studied by Hviding and his colleagues.

All this is still very speculative. Astronomers are only tentatively sketching out the story of these objects and how they might have evolved over time. Hviding’s object could still turn out to be just another black hole, albeit one with a strange set of properties. And if so, the little red dots might one day be revealed as something else entirely.

A Million Orbiting Data Centres?

SpaceX requested permission to launch and operate a million satellites in Earth orbit. The application came with grandiose language. SpaceX, the application said, wants to build a constellation of data centres that together will be the “first step towards becoming a Kardashev II-level civilization”.

Such a civilisation, according to a scale devised in the 1960s, is one capable of using the full power output of its star. Humanity, needless to say, is nowhere close to such a thing, and even this new constellation will not bring us much closer.

Elon Musk instead sees the project as an opportunity to dominate the future of artificial intelligence. The constellation, he said in a press release, would be built by launching a Starship every hour, would enable factories on the Moon, and help create a “sentient sun”.

Alongside the constellation, SpaceX also announced a merger with Musk’s xAI, the company behind the controversial Grok chatbot. Musk now values the combined company at over one trillion dollars – a valuation he seems certain to pursue in a stock market offering later this year.

Indeed, the problems with orbiting data centres are so big – they are tough to cool, expensive to launch, and impossible to maintain – that it is hard to imagine this constellation will ever be fully built. Instead, it is probably better seen as an effort to capitalise on the bubble of AI spending and investment as SpaceX prepares to go public.

Artemis Delayed

The launch of Artemis II has been delayed until March. Last week NASA went through a two-day-long Wet Dress Rehearsal, a test in which the rocket was fully loaded with fuel and operators proceeded through simulated countdowns to check their readiness.

Prior to the test, NASA was hoping to proceed to the point thirty seconds before launch. In reality they fell short of that goal, with the simulated count terminating five minutes before lift-off. At blame were leaks in the rocket’s hydrogen fuel supply, which suddenly spiked during the countdown and forced a halt.

Orbital mechanics mean the next launch attempt can only come in early March. Before then, NASA will need to do another wet dress rehearsal – so far, however, there has been no official decision about when that might happen.

Don’t be surprised if further delays happen. Artemis I faced multiple issues over the eight months it took to get off the ground. That was an uncrewed mission. This time, with four humans sitting on top of the rocket, NASA is bound to be even more cautious.

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Last month, I finished reading Astronomy in Minutes, a brilliant guidebook by Giles Sparrow. Page 78 and the accompanying picture caught my attention the most. The topic was Trojan Asteroids. Despite being a physics post-grad, I didn’t know about them!

I had always pictured asteroids as rocks drifting aimlessly in the belt between Mars and Jupiter, or perhaps as those near-Earth objects that occasionally make headlines. Trojan asteroids don’t fit that image at all. They are remarkably “disciplined” objects that share a planet’s orbit without ever colliding with it. Instead of being swept up by a planet’s massive gravity, they maintain stable, independent orbits around the Sun.

For billions of years, they have occupied the same celestial neighborhoods with remarkable stability. This orbital harmony poses a very natural question: How does a relatively tiny body resist the overwhelming gravitational pull of a nearby planet to maintain its own path?

Then I remembered the classical mechanics lectures from my good old undergraduate days. I revisited my notes and ended up binge-reading online. Not only that, I created my own Python simulation!

What Are Trojan Asteroids?

Formally,

Trojan asteroids are minor celestial bodies that share a planet’s orbit, clustering around two specific regions known as the L4 and L5 Lagrange points. These gravitational sweet spots are located 60° ahead of and 60° behind a planet along its orbital path.

The most famous Trojans belong to Jupiter, who hosts several thousand of them. They are divided into two camps:

• The Greek Camp: Occupying the leading L4 point.

• The Trojan Camp: Occupying the trailing L5 point.

This naming convention comes from Homer’s Iliad, and it stuck. The individual asteroids are themselves named after the heroes in the story of the Trojan War.

While Jupiter dominates the conversation, Trojans have also been identified orbiting Mars, Uranus, Neptune, and even Earth. Saturn is a special case; we will return to it later.

The Physics of Lagrange Points

To understand their strategic location, we must examine the Restricted Three-Body Problem through two different lenses. In this scenario, we have a massive primary (the Sun), a secondary mass (a planet), and a third body of negligible mass (the asteroid).

A Bird’s-Eye View of the Inertial Frame

Imagine hovering far above the Solar System against a frozen backdrop of stars. In this frame, the physics is straightforward:

• Jupiter and its two Trojan swarms revolve around the Sun in massive, nearly circular orbits.

• The L4 and L5 regions act as “dynamic parking spots” that rotate in lockstep with Jupiter, perpetually maintaining their 60° offset.

• From this perspective, an asteroid performs a triple-action: it orbits the Sun, wobbles within its Lagrange region, and experiences slight fluctuations in its orbital radius.

A Stationary View of the Rotating Frame

The asteroid’s true relative motion emerges when we switch to a Rotating Reference Frame. So imagine standing on a carousel spinning at the exact rate of Jupiter’s orbit. In this frame, the 12-year orbital transit of Jupiter and asteroids is cancelled out.

• Jupiter appears stationary.

• Tadpole Orbits: Most Trojans trace small, kidney-bean-shaped paths around L4 or L5.

• Horseshoe Orbits: If an asteroid possesses enough energy, it may embark on a much longer journey, traveling the long way around the Sun to visit the opposite Lagrange point before swinging back.

Why are L4 and L5 Special?

In the figure above, you can see five equilibrium points. In the Restricted Three-Body Problem, these Lagrange points are where the gravitational pull of the Sun and the planet perfectly balance the centrifugal force of the asteroid’s motion.

Not all these points are created equal:

• L1, L2, and L3: Discovered by Leonhard Euler, these are unstable. Occupying them is like trying to balance a marble on the tip of a needle; the slightest perturbation sends the object drifting away.

• L4 and L5: Discovered by Joseph-Louis Lagrange, these can be stable. They act like wide, shallow bowls. If an asteroid wanders away from the center, the Coriolis force (an apparent force in our rotating frame) deflects it back into a loop, keeping it trapped in a stable Tadpole orbit.

The Routh Stability Criterion

Stability at L4 and L5 is not a universal guarantee; it depends entirely on the mass ratio between the two large bodies. According to the Routh stability criterion, the Trojan points are only stable if the secondary body (the planet) is significantly smaller than the primary (the Sun). Specifically:

(Or, the planet must be less than approximately 4% of the Sun’s mass).

Jupiter, Earth, and the other planets easily satisfy this condition, allowing them to shepherd long-lived Trojan populations. Only two Earth trojans have so far been discovered: (706765) 2010 TK7 and (614689) 2020 XL5.

Our Moon’s relatively low mass and the overwhelming, chaotic gravitational influence of the Sun and Earth make it a case where the Routh criterion is not satisfied.

The Saturn Mystery

Interestingly, while Saturn satisfies the Routh criterion, it lacks a significant Trojan population around the Sun. This is due to gravitational interference from Jupiter. The “Great Purge” caused by the nearby gas giant’s massive pull destabilizes Saturn’s Sun-centered Trojans over long timescales. However, Saturn demonstrates the same physics on a smaller scale: it has Trojan moons (like Telesto and Calypso) that sit at the L4 and L5 points of the Saturn-Tethys system.

The Lucy Mission: Visiting Fossils of Planetary Migration

Launched by NASA in October 2021, Lucy is a high-stakes reconnaissance mission to Jupiter’s Trojan population. Over its 12-year journey, it has already visited two main-belt asteroids (Dinkinesh-Selam and Donaldjohanson) and is now headed for eight Jupiter Trojans (Eurybates-Queta, Polymele-Shaun, Leucus, Orus, Patroclus, and Menoetius) across both the L4 and L5 swarms.

Why the Trojans?

Unlike main-belt asteroids, Trojans have remained dynamically isolated for billions of years. Because many have avoided significant heating or collisional evolution, they serve as the most primitive surviving bodies in the Solar System.

The primary scientific motivation rests on the Nice Model of planetary migration. If the early Solar System was as chaotic as this model suggests, Jupiter’s Trojans should contain material from the inner disk, the outer disk, and even the Kuiper Belt. By comparing objects in both the L4 and L5 swarms, Lucy will test whether these asteroids are a heterogeneous mixture of the early Solar System or a uniform population that formed in situ.

What Lucy is Measuring

While Lucy is not a sample-return mission, its strength lies in comparative planetary science. Its instrument suite is designed to dissect these fossils from a distance:

• Spectroscopy (Visible & Infrared): To map minerals, organics, and ices.

• High-Resolution Imaging: To study surface geology and cratering histories (which act as a clock for the asteroid’s age).

• Thermal Radiometry: To determine the physical properties of the surface regolith.

• Radio Science: To calculate the mass and density of these bodies via gravitational perturbations on the spacecraft’s path.

If the Trojans turn out to be compositionally diverse, it would provide smoking gun evidence for large-scale planetary migration. If they are unexpectedly uniform, it will force a fundamental revision of our models of how the giant planets found their current homes. Either way, the results will rewrite the history of our celestial neighborhood.

The Bigger Picture

There is something quietly profound about the Trojan asteroids. They are not dominant or destructive. They reveal how structure emerges from gravity and how stability can thrive within nonlinear systems. They show that the Solar System is far more than a collection of isolated orbits. It is a deeply interconnected, dynamical whole.

These asteroids sit at the precise intersection of celestial mechanics, planetary science, and chaos theory. They are elegant solutions to a gravitational puzzle, written across astronomical time. If you want to understand how the Solar System remembers its past while remaining dynamically alive, Trojan asteroids are a very good place to start.

On this page I have gathered a list of free resources for learning physics. My intention is to cover the main areas of modern physics for the interested reader, stretching from Newton’s laws to quantum field theory and beyond.

To really learn physics you should, of course, enrol in a university degree. But for those who are simply curious or who lack the time for full time studies, I hope this guide will be of benefit. For those who do decide to study physics more seriously, it may also act as a companion; a set of resources to aid you in your study.

How to Use This Guide

Please don’t let the perceived difficulty of physics and maths put you off. Like anything else, these are subjects that can be learned, and can be done so by pretty much anybody. It may take time, it may be frustrating at times, it may occasionally go slower than you would like. But it can be done.

Below I have divided modern physics into its more or less accepted subdivisions. The biggest of these, of course, is the split between the “classical” physics of Newton, and the modern physics of Einstein and Quantum Theory. Within each are further sections: mechanics, thermodynamics and electromagnetism for classical physics, and quantum theory and relativity for modern physics.

For each I’ve provided links to free online textbooks and other resources. Where possible I have linked to material prepared by well-respected physicists, and I have tried to offer a range of options for each subject. If one book is not working out for you, then feel free to abandon it and try a different one from another author.

The links also differ in difficulty and scope. Those marked in bold should be considered as “essential” material, that alone is sufficient to give you a general understanding of physics. The rest is optional, but studying it will give you deeper insights into the topic.

You should work through the material in roughly the order presented. Later textbooks will often assume knowledge taught before, and trying to leap into them too early will be more challenging than necessary. To get the best results you should also attempt to complete the problem sets included in many of the textbooks.

This list is (and always will be) incomplete. I have preferred online resources to off-line texts (though some “essential” texts are listed under General Resources). Over time I hope to expand it. Should you know of resources not listed here or find anything that does not work, please feel free to contact me. I will be grateful for your help.

General Resources

To begin, here are several good resources that provide a broad overview of physics. Many of these are also suitable for the more casual reader who is less ready or less interested in going deeper into sub-fields.

Similar Lists

• How to Become a Good Theoretical Physicist: A list of resources compiled by Gerard ‘t Hooft, winner of the 1999 Nobel Prize. Broad, with a focus on theoretical physics. Unfortunately it is not yet complete and several links no longer work.

• So You Want to Learn Physics…: A guide to learning physics from journalist and former physicist Susan Rigetti. A decent guide, but one more focused on purchasing physical textbooks than taking advantage of online resources.

• How to Learn Math and Physics: Another list of resources, this time from Joan Baez, a mathematical physicist at the University of California. Again, heavily based on textbooks, although links are provided where these are freely available online. As you might expect, Baez provides many resources on learning maths as well as physics.

The Feynman Lectures

• The Feynman Lectures: Collected in three volumes, Feynman’s lectures on physics make for an excellent starting point and later reference. Feynman, who was famously called “The Great Explainer”, covers his topics in an intuitive and easy-to-read manner.

Textbooks

As opposed to most other resources in this list, the three textbooks mentioned here are not freely available online. I have provided links to Amazon for each - note that these are affiliate links, and you can of course find these textbooks from other bookshops.

• University Physics with Modern Physics: By Young and Freedman, this is the standard undergraduate textbook for many courses in physics. Because of that you can often pick up a second hand copy.

• Mathematical Methods for Physics and Engineering: Again, a standard undergraduate textbook for university physics courses. This one focuses on the mathematics you’ll meet along the way.

• Course of Theoretical Physics: A legendary ten volume series by two Soviet physicists, Lev Landau and Evgeny Lifshitz. More advanced than University Physics, these textbooks are better suited for those readers looking for a deeper understanding and who already have completed the basics.

Pre-Newtonian Physics

Pre-Newtonian physics (i.e., that of Ancient Greece, India and the Arab world) is rarely taught in universities today. Many of the ideas from this time are out of date, incomplete or just plain wrong. I include it here for those who are interested in understanding the path towards a more modern form of physics.

• Galileo and Einstein: Despite the title, this course traces our understanding of space, time and forces from Ancient Babylonia and Greece to the modern day, giving a nice overview of how our view of the physical world has changed over the centuries.

Mathematics

Mathematics lies at the heart of physics, and thus forms an essential prerequisite to going further. Here I have gathered material covering much of the maths key to later study. However, the field of mathematics related to physics is vast in its own right. Should you wish for more, see the further resources section below.

Fundamentals of Mathematics

The fundamentals are essential to all later subject areas. Make sure you are familiar with basic algebra, geometry, trigonometry as well as imaginary and complex numbers.

• Beginning Algebra: Tutorials on basic algebra provided by West Texas A&M University, covering the essentials from fractions to simple geometry.

• Intermediate Algebra: Continue the tutorials from West Texas A&M by moving onto linear equations, more complex fractions and roots.

• Dave’s Short Trig Course: Get a deeper look at trigonometry with this short course by David Joyce at Clark University.

• Dave’s Short Course on Complex Numbers: David Joyce continues with this quick introduction to imaginary and complex numbers.

• Linear Algebra: A free online textbook by Jim Hefferon, covering linear algebra as needed for a standard undergraduate course in physics.

Calculus

Calculus is the mathematics of continuously changing variables. It is what allows us to deal mathematically with velocities (changes in position), accelerations (changes in velocity) and other changing quantities. Historically it was developed by Leibniz and Newton, and is vital to understanding the problems addressed by Newtonian physics.

• Calculus Made Easy: An old textbook, but a good one that will give you a friendly introduction to calculus. 

• Calculus: From MIT, this free book will take you from basics of calculus through to partial derivatives and vector calculus. Working through it should give you the tools to go further into Newtonian physics.

• Notes on Differential Equations: After you grasp the essentials of calculus, this compilation of course notes will take you deeper into differential equations and their physical applications.

• Multivariable Calculus: This course gives an introduction to multivariable calculus from vectors to Gauss’ theorem. Much of this will have been covered by other resources already, but it is a useful summary of the topic and will find applications from mechanics to electromagnetism.

• Advanced Calculus: For a deeper view of calculus and its applications, this textbook provides an extensive overview of the subject. 

• Primer on Partial Differential Equations for Physicists: An introduction to mathematical techniques for physics. You should already be familiar with partial differential equations before starting this textbook.

• Mathematical Tools for Physics: After going through the above, this free online textbook will give you a good grounding in the general mathematics needed for physics.

Statistics

Lies, damn lies and statistics. People are famously bad at understanding statistics; studying it will help you critically evaluate many studies and experiments. It is also crucial to many fields of physics, and especially so for thermodynamics.

• Concepts & Applications of Inferential Statistics: Don’t be afraid of the title: this resource is a friendly introduction to the statistical tools needed to do physics, especially of the experimental kind.

• Introduction to Probability: From Grinstead and Snell, this textbook takes you through the ideas and techniques of probability. To get the most of this book, you should already have some understanding of calculus, multiple integrals and matrices. 

Further Resources On Mathematics

Mathematics is an entire subject by itself, and extends far, far beyond what I have covered here. Rather than devote this page to maths, I suggest checking out the following resources for pointers on how to proceed.

• Mathematics for Theoretical Physics: An extensive online textbook covering the mathematics useful for pursuing and understanding theoretical physics. I recommend you to take a look, even if you just treat it as a reference.

• Basic Concepts of Mathematics: Despite the title, this book actually provides an introduction to a more rigorous study of mathematics than so far presented. Useful for going deeper into theory.

• How to Learn Math: Again from Baez, this list of resources provides a comprehensive path into the field of mathematics.

• Online Maths Books: Georgia Institute of Technology professor George Cain has collected this list of freely available online maths books. Unfortunately many of the links are out of date and no longer work.

Classical Physics

Classical physics was developed between the work of Newton in the 17th Century and that of Einstein in the 20th. Though today it has been superseded by quantum theory and relativity, classical physics is still relevant in describing much of the world that occurs at our scale - when, in other words, things are not too small and not too fast.

Mechanics

Classical mechanics begins with Newton’s famous laws of motion and deals with the movements of matter through space. It is a world of forces and accelerations; momentums and energies. Einstein’s theories have superseded it, though mechanics still remains relevant and mostly accurate when working with our everyday world.

• MIT OpenCourseware - Classical Mechanics: An online textbook and course provided through MIT OpenCourseware. It provides a solid introduction to classical mechanics.

• Newtonian Dynamics: This textbook covers classical mechanics from Newton’s Laws onwards, placing a particular focus on using them to describe the motions of planets and other objects in the Solar System. Worth going through to go a bit deeper into the main areas of classical mechanics.

• Classical Dynamics: From Cambridge University, David Tong provides these lecture notes on classical mechanics at an intermediate to advanced level.

• Graduate Classical Mechanics: A short online course covering advanced classical mechanics at a graduate level, written by Michael Fowler. A good mathematical background is essential here.

• Theoretical Mechanics: Another graduate level course on classical mechanics, this time from the Perimeter Institute.

Thermodynamics

The study of how heat moves. Its development helped us build and improve the steam engine; its consequences, spelt out in the fundamental laws of thermodynamics, predict the ultimate fate of life, the universe and everything. If you think you have built a perpetual motion machine, check here first.

“The law that entropy always increases holds, I think, the supreme position among the laws of Nature. If someone points out to you that your pet theory of the universe is in disagreement with Maxwell's equations - then so much the worse for Maxwell's equations. If it is found to be contradicted by observation - well, these experimentalists do bungle things sometimes. But if your theory is found to be against the Second Law of Thermodynamics I can give you no hope; there is nothing for it to collapse in deepest humiliation.”

~ Arthur Eddington

• Thermal and Statistical Physics: By Harvey Gould and Jan Tabochnik, these book chapters, notes and problems cover the main concepts of thermodynamics. 

• Thermodynamics and Statistical Mechanics: This course by Richard Fitzpatrick introduces the ideas behind thermodynamics and statistical mechanics. Recommended reading to get a good overview of the subject. 

• Thermodynamics - Fundamentals and Application in Science: This online textbook introduces thermodynamics and then looks at its application to other areas of science, including biology and chemistry.

• Statistical Physics: An advanced level course on statistical physics from the Perimeter Institute.

• What is Entropy?: A short online book (by Joan Baez) explaining the concept of entropy for non experts.

Electromagnetism and Optics

The mathematical description of electromagnetism was one of the crowning achievements of 19th Century physics. Maxwell’s Equations, first written down in 1861, showed how electricity and magnetism were intimately related; thus opening the door to mastery over those two forces. This, in turn, gave rise to some of the most transformative technologies of the 20th Century: electrical power, radio, television and radar.

• Optics: The field of optics was developed far earlier than other areas of electromagnetism and thus can be studied without much of the mathematics needed for the rest of the subject. These lecture notes offer a nice introduction to the topic.

• Electromagnetism and Optics: Richard Fitzpatrick provides an introductory course into both electromagnetism and optics, covering the essential basics of the field.

• Classical Electromagnetism: Fitzpatrick goes deeper into electromagnetism in this course, beginning with Maxwell’s equations and ending with relativistic electromagnetism.

• Electromagnetism: A Cambridge University course on electromagnetism by David Tong. More intermediate level than beginner but worth following.

• MIT Electromagnetism: Lecture notes from an MIT course on electromagnetism. You should already have familiarity with Maxwell’s Equations before tackling this course.

• Electromagnetic Field Theory: An advanced textbook going deeper into the properties of electromagnetic fields and radiation phenomena.

The Rest of Classical Physics

Mechanics, thermodynamics and electromagnetism are the three primary branches of classical physics. Beyond that, however, classical physics includes many subfields that are either too specific or too advanced to cover above. Below I have collected some resources that fall into these categories:

• Solid State Physics: Solid state physics is the physics of materials, and particularly how electrons move through solids and how atoms arrange themselves into crystals and lattices.

• Lecture Notes in Fluid Mechanics: Fluid mechanics deals with the motion of liquids and gases, as opposed to solid objects. Technically a subfield of mechanics, but often taught as a separate course. These notes assume a knowledge of Newtonian mechanics.

• Fluid Mechanics: A second introductory course on fluid mechanics, this one from the University of Cambridge.

• Plasma Physics: Fitzpatrick provides an introduction to plasma physics and then to magnetohydrodynamics, the study of magnetic fluids. An essential topic to study if you are interested in nuclear fusion or the intricacies of stars though, truth be told, this subject is simply fascinating in its own right.

• Applications of Classical Physics: A textbook written by Nobel Prize winner Kip Thorne and astrophysicist Roger Blandford. This book covers much of classical physics, with the exception of mechanics, electromagnetism and basic thermodynamics. Both Thorne and Blandford are known for their work on black holes, so it should come as no surprise that the final chapters take the reader into the realm of general relativity.

• Richard Fitzpatrick’s Courses: I have linked to several of Fitzpatrick’s courses here. The rest, which do not easily fit into other sections, can be found here.

Relativity

Einstein’s revolutionary theories of relativity reshaped our view of the universe, forever changing the way we thought about time and space. His theory of special relativity arose from efforts to close contradictions in the laws of electromagnetism. General relativity, which came later, radically changed our conceptions of space and time.

Special Relativity

Einstein’s theory of special relativity tackles the motion of light throughout the cosmos. It was born out of an effort to resolve problems with Maxwell’s theory of electromagnetism, and ended with the abandonment of old ideas about time and space. Special relativity, discovered first, addresses the “special” case in which gravity can be ignored.

• Notes on Special Relativity: Michael Fowler introduces the ideas of special relativity in these lecture notes, starting from the experiments and theories that motivated its discovery.

• Special Relativity: Benjamin Crowell covers the theory of special relativity in this online textbook.

• Relativity: The Special and General Theory: Written by Einstein, this book introduces the theories of relativity from the master himself.

General Relativity

The theory of general relativity is one of the most elegant and beautiful ever written down. It speaks of a cosmos in which space and time are intimately linked; in which the presence of matter warps the fabric of both; and from which arises the phenomenon we perceive as gravity. Einstein’s work predicted many strange things, from black holes to wormholes, and reshaped our view of the universe.

• Lecture Notes on General Relativity: Sean M. Carroll provides this introductory course on general relativity. It makes for a decent starting point.

• Introduction to General Relativity: From Nobel Prize winner Gerald ‘t Hooft, these lecture notes provide a readable introduction to the topic of general relativity.

• General Relativity: An Introduction for Undergraduates: If neither of the other introductory courses worked for you, this brief introduction to general relativity may help.

• Cambridge General Relativity: Another course on general relativity, this time from David Tong.

• General Relativity: By Benjamin Crowell, this textbook covers the theory and geometry of general relativity. Problem sets with solutions are included.

• Light Rays, Singularities, and All That: From Edward Witten, this book takes a more advanced look at the implications of general relativity.

Quantum Theory

The quantum world is famously bizarre. Particles can vanish and reappear without warning, cross seemingly impenetrable barriers and somehow exist in two states at once. Even physicists are not immune to this strangeness - few, indeed, would claim to really understand what is happening in the quantum realm. Yet the mathematics works, and the theory provides, so far at least, our deepest understanding of nature.

Quantum Mechanics

Quantum mechanics provides the tools to describe the quantum world. These tools are heavy on mathematics and limited on conceptual understanding, but delving into them will give you a grasp of how things work in the quantum realm.

• The Physics of Quantum Mechanics: By James Binney and David Skinner, this book was born out of the second year undergraduate course on quantum mechanics at Oxford University.

• Quantum Mechanics: An alternative introduction to the subject, this time from Richard Fitzpatrick.

• Cambridge Quantum Mechanics: A first course on quantum mechanics, from David Tong at Cambridge University.

• Advanced Quantum Mechanics: A more advanced level course from Freeman Dyson. An old course, but still relevant.

• Classical and Quantum Mechanics via Lie Algebras: This book offers an alternative approach to both classical and quantum mechanics from an algebraic perspective. Certainly not useful as an introduction, though it may be of interest to those who wish to get a different mathematical view of quantum mechanics.

• Quantum Theory, Groups and Representations: Again going far beyond the essentials, this book approaches quantum theory from a mathematical perspective. Useful if you wish to gain a deeper understanding of the maths and symmetries behind quantum physics.

• Introduction to the Theory of Open Quantum Systems: A graduate level course on open quantum systems. Make sure you are comfortable with the basics of quantum mechanics before attempting this - though it is far from essential reading.

Particle Physics and the Standard Model

Our world is made from atoms; atoms are made of electrons, protons and neutrons; protons and neutrons are made from quarks. The standard model describes the subatomic world as we know it today, covering everything from quarks and electrons to neutrinos and the Higgs Boson, as well as the forces between them.

• Particle Physics: Based on lectures given at CERN summer schools, this course on particle physics is designed to be accessible to students with high school mathematics.

• Elementary Particle Physics: Lecture notes from the University of Rome introducing the subject of particle physics. A good starting point for the subject.

• Introduction to High Energy Physics: A second introduction to particle physics, though one that is slightly harder to follow than the course from Rome.

• Nuclear & Particle Physics: Another introduction to subatomic physics, beginning with the discovery of the electron, working through to the standard model and ending with an introduction to quantum field theory.

• The Standard Model: A Primer: A more advanced textbook introducing quantum field theory through the standard model of particle physics. 

Quantum Field Theory

The quantum field theory emerged in the 1920s and was largely developed by the 1960s and 70s. It describes particles in terms of underlying quantum fields, and combines classical field theory, special relativity and quantum mechanics. 

• Introduction to Quantum Field Theory: A solid introduction to quantum field theory, intended for final year undergraduates or early graduate students. 

• Quantum Field Theory: A textbook intended for newcomers to the field. The prerequisites are tough, so it may be advisable to revisit some of the material above if you run into difficulty.

• Quantum Field Theory by Sidney Coleman: Lecture notes from Coleman’s 1986 course at Harvard. Videos of his course are also available, though the quality is often rather poor.

• The Conceptual Basis of Quantum Field Theory: For those ready to take a deeper look at the Quantum Field Theory, Gerald ‘t Hooft reviews some of the conceptual issues with the theory.

• Quantum Field Theory I: From the Perimeter Institute, this online course introduces quantum field theory.

• Quantum Field Theory II: The Perimeter Institute goes deeper into quantum field theory in this follow-on course.

Beyond the Standard Model

Quantum theory, along with the Standard Model, is perhaps the most successful theory of physics ever conceived. Yet in one key area of physics - gravity - it is silent. We cannot, therefore, use it to tell us what happens inside a black hole, or in the earliest moments of the Big Bang. For that, physicists will need to move beyond the standard model, into the realm of quantum gravity, supersymmetry and strings. Be aware, much of what you will encounter here is highly speculative, theoretical and, above all, unproven.

• Quantum Gravity in Everyday Life: A short look at reconciling general relativity and quantum physics into a theory of quantum gravity.

• Supersymmetric Quantum Mechanics: A short course on supersymmetry by David Tong.

• Superspace: On supersymmetry. Once highly fashionable, today much less so - especially since the Large Hadron Collider failed to find any evidence to support it.

• Introduction to String Theory: String theory may be the future of physics, or it may be a load of unprovable nonsense. Gerald ‘t Hooft offers an introduction to the topic here.

• String Field Theory - A Modern Introduction: A second text introducing the ideas behind string theory.

• String Theory: From Cambridge University, David Tong provides this course for beginners in string theory.

Astronomy

So far I have focused on the key areas of physics. There are, of course, many branches and subfields of physics, each of which you can spend a lifetime studying in depth. Astronomy is perhaps the oldest of these, as the rhythms of the stars and planets were studied long before the Greeks dreamed up the foundations of what would later become physics. 

• Astronomy: A free textbook that covers much of our modern understanding of astronomy without a lot of mathematics. A good introduction for those looking for an overview of the subject.

• A Practical Guide to Observational Astronomy: This book covers the way in which we actually observe the universe, from charting the positions of the stars to recording the photons coming from them. Good reading for those interested in how we do astronomy.

• Introduction to Celestial Mechanics: Celestial mechanics concerns the movements of stars and planets, typically as seen from Earth. It is an ancient subject, once studied to divine messages from the Gods. Today it has found application in the orbits of satellites and spacecraft.

• The Fundamentals of Stellar Astrophysics: A slightly old textbook, but one that addresses our understanding of how stars burn and how they are structured.

• Principles of Heliophysics: Heliophysics covers the interaction between stars and planets. This textbook delves into how space weather affects planets, and how, in turn, that affects the potential for life on worlds across the galaxy.

• High Energy Cosmic Rays: Lecture notes on the origins and types of cosmic rays that bombard our planet from deep space.

• Extrasolar Planets: Compiled lecture notes from ETH Zurich covering exoplanets, how they form, and how we can detect them.

Cosmology

Astronomy and astrophysics study the objects that fill the universe: the planets, the stars and the galaxies. Cosmology, by contrast, is the study of the universe itself - how it began, how it evolved, how it is shaped and, indeed, how it will all end. 

• Introduction to Cosmology: Lecture notes providing an introductory course on cosmology. It is useful to have at least some knowledge of general relativity before starting.

• TASI Lectures: Introduction to Cosmology: A somewhat more advanced “introduction” to cosmology. Requires a deeper understanding of general relativity to get the most from it.

• Cosmology: From the Perimeter Institute, this online course includes 16 lectures on modern cosmology.

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If NASA keeps the schedule, the moment of lift-off will come in early February. Six immense engines will roar to life, each belching out a tremendous cloud of fire and smoke. Eight point eight million pounds of thrust will be the result, and all of it will force a towering structure upwards, hurling two thousand tonnes of metal and fuel up, up into the sky.

What an acceleration the four people atop this tower will experience! In sixty seconds the engines will have forced them past the sound barrier. A minute later the solid boosters have gone, peeling away and falling back towards the ocean thirty miles below. The rocket burns on, the speedometer races higher, and then – eight minutes and three seconds on the clock – the whole thing falls silent.

That will be enough. By then the capsule will be in orbit, moving fast enough to reach an altitude greater than any astronaut has ventured in the past fifty years. The rocket – price tag two billion dollars – will be spent. It separates, falls back, and plummets towards the floor of the Pacific Ocean.

The capsule flies on, and within an hour it and its crew will reach their record-setting apex. At that point, if all has gone well, another set of engines will fire, this time raising its orbit to an altitude above that of the geostationary belt of satellites. From there the astronauts will be able to view the Earth as a solitary disk hanging in the sky, and they will become the first humans to do this since 1972.

From then on, things slow down. The burn towards the Moon takes place about twenty-four hours after launch. The journey towards it takes another four days. On the sixth day of the mission, the crew will travel beyond the record set by Apollo 13, and become the most distant travellers from Earth in all of history. A few hours later, the capsule will fly slowly over the lunar surface, offering its crew a view from the height of a few thousand kilometres.

Then, the irresistible laws of Newton will pull them home. After another four days of flight they will hit the atmosphere, briefly bounce off it, and then plunge, hurtling through its upper layers at a terrifying four hundred miles per minute. Fortunately, the capsule should be able to take it. The air will slow them, the heat shield will protect them, and within a few minutes they will be descending gently towards the ocean under a canopy of parachutes.

The whole thing is quite mad.

The Shadow of Apollo

The closest precedent to this mission is Apollo 8. Launched in December 1968, that flight took three astronauts – Frank Borman, Jim Lovell, and William Anders – around the Moon. They became the first men to fly into deep space, and the first to see the far side of the Moon with their own eyes.

Artemis II will not be an exact replica of that mission. Apollo 8 stayed and orbited the Moon ten times. The crew of Artemis II are following a simpler trajectory, and will merely loop around the Moon without trying to enter orbit. But the objectives are similar: like Apollo 8, Artemis II will test the ability of the Orion capsule to support life and to carry astronauts far beyond the Earth.

Of the four astronauts on board, three are American – Reid Wiseman, Victor Glover, and Christina Koch – and one is Canadian – Jeremy Hansen. The Americans are all experienced astronauts, having each flown to the International Space Station. But for Hansen it will be the debut of a lifetime: his prior experience is as a fighter pilot in the Royal Canadian Air Force, and he has never been to space before.

During the flight, these astronauts will conduct tests to prepare for future missions. While still in orbit around the Earth they will take the capsule through a series of manoeuvres called “proximity operations”. These will replicate actions the spacecraft would need to do to dock with another object – such as a lunar lander – and will allow engineers to gather data and experience for future missions.

Throughout the rest of the flight, the crew will evaluate the life support systems built into Orion, test its communications systems, and practice taking shelter from solar storms. All of this will be vital for the safety of future crews heading towards the Moon.

Heat Shields and Quarantines

One final test will come as the capsule falls back through the atmosphere. After the re-entry of Artemis I, engineers noticed alarming holes had formed in the capsule’s heat shield. Analysis showed that gases had built up within it, but then hadn’t been able to escape as models had predicted. Instead they had expanded under the heat of re-entry, and the resulting pressure had cracked and broken the shield.

To prevent this happening again, engineers have modified the re-entry trajectory. In Artemis I, Orion first “skipped” off the atmosphere, making a shallow plunge that slowed the capsule but wasn’t enough to bring it all the way down. It was during this period that gas built up. When the capsule then did make its final plunge, this gas heated and expanded.

In Artemis II, the capsule will still make this first shallow dive. But in an effort to reduce the gas build-up, engineers have made this portion of the flight shorter. That means Orion will have more speed to lose in its final fall, and that implies a faster and hotter re-entry. Briefly, indeed, the crew will reach the fastest speed ever attained by humans. But it should, engineers say, be safer this way.

If all goes well, Artemis II will pave the way for more ambitious flights in the future. The Artemis program, as it currently stands, envisions a moon landing before 2028, a space station in lunar orbit by 2030, and a series of missions to the lunar surface throughout the following decade. Much work remains to be done for any of this to happen, but Artemis II is an essential step towards these dreams of future exploration.

The launch window for lift-off opens on February 6. A handful of dates are available in early February, but if the rocket hasn’t gotten off the ground by February 11, the window closes until March 6.

Delays are likely1. Artemis I was delayed multiple times as engineers worked issues on the rocket. This time, with astronauts sitting on top, they are likely to be more cautious still. The crew themselves add constraints: they cannot wait in the capsule for more than a few hours, and they must take time to rest and prepare themselves for flight.

But whenever the lift-off comes, the march towards the Moon has already begun. In mid-January the rocket was rolled out to Launch Pad 39B at the Kennedy Space Center. On the 23rd, the crew entered a pre-flight quarantine. After years of training, their moment is almost upon them.

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Just how massive can a star get?

The Sun makes for a logical starting point. It is big, weighing in at over three hundred thousand times the mass of the Earth. But by cosmic standards, that is merely average. Betelgeuse is about fifteen times more massive. Eta Carinae – a star so unstable it occasionally erupts in spectacular detonations – is five times more massive than even that.

But the most massive star known with reasonable certainty is RMC 136a1, a star of almost three hundred solar masses. Located in the nearby Large Magellanic Cloud, it burns with an astonishing luminosity. Were it located at the distance of Alpha Centauri, it would shine as bright as the full moon in our sky; fortunately, or sadly, it is more than a hundred thousand light-years away and visible only through telescopes.

RMC 136a1 is so big that we struggle to explain how it could possibly have formed. It is about twice the expected mass limit for stars, and so some astronomers think it might be the result of other giant stars colliding and merging. Whatever it was, the star is unlikely to be around for long. At such a mass it must be burning fuel at an incredible pace, and it will probably exhaust its reserves within the next two million years.

This seems to be about the limit of what is possible, at least for the current universe. But astronomers have long speculated that the first stars could have been far larger. Back then, the cosmos contained almost nothing but hydrogen, and the first generation of stars would have been very pure. That could have allowed them to swell to enormous sizes.

But just how big is a matter of debate. Entering the debate recently, Harvard astronomer Avi Loeb – he of the alien interstellar comet theory – suggested they could have exceeded a million solar masses. That would make them by far the largest stars ever to have existed, and in essence would be like combining an entire dwarf galaxy into a single star.

Each would have begun with a core weighing about ten times the mass of the Sun. This would have pulled in gas and grown at an extreme rate. At some point, which Loeb fixes at about a million solar masses, this accumulation of matter would have come to an end. Why is unclear - Loeb merely says they would have run out of gas to feed on – but at this point the star would cease defying reality and simply collapse into a black hole.

Still, there is some evidence for these stars, he says. The James Webb telescope has spotted mysterious red dots scattered across the early cosmos. All seem to be bright and fairly compact. They might, he argues, be the first generation of stars, each so massive and burning so brightly that their photons have survived a thirteen-billion-year-long trip across the universe.

But there are plenty of other explanations too. They might be giant black holes surrounded by clouds of fast-moving gas and dust. Or they could be “black hole stars”, strange objects powered by a central black hole but still shining immensely brightly. For now, astronomers are still free to speculate. So why not let some imagine they are the biggest and brightest stars ever made?

Artemis II Prepares For Launch

In the past half century, no one has ventured deeper into space than Jared Isaacman. In September 2024, he reached an altitude of fourteen hundred kilometres (almost nine hundred miles) above the surface of the Earth. That, though it does not seem like much, is as far as anyone has gone since the days of Apollo.

Perhaps it is fitting, then, that as head of NASA, Isaacman is now overseeing an effort to break that record. If things go to plan, a rocket will soon launch from the Kennedy Space Center in Florida, blast a capsule towards the Moon, and send four astronauts on one of the most daring voyages of the past few decades.

The crew, which is made up of three Americans and one Canadian, will not set foot on the lunar surface. Instead they will loop around the Moon, passing about six thousand kilometres above its craters and mountains. They will then fly back towards the Earth, and re-enter our atmosphere at the astonishing speed of forty thousand kilometres per hour.

All this should prove the ability of the Artemis rocket and capsule to send astronauts to the Moon. The next step, to be done sometime in the coming years, is to prove the viability of a lunar lander. For this NASA has chosen to use Starship, a spacecraft under development by SpaceX.

NASA says Starship should demonstrate this ability before 2027, in the form of an uncrewed touchdown on the Moon. If that works, then astronauts will once again set off to the Moon on Artemis III. Once there, they will dock with Starship, transfer to its cabin, and then descend to the surface. NASA, and America’s politicians, are hoping all that can be done by 2028.

A Failed Galaxy

Modern models of cosmology say the universe should be full of dark matter halos. In many cases these surround galaxies: the dark matter came first, astronomers say, and then through the force of gravity it attracted normal matter in the form of hydrogen and helium. Later, that matter collapsed into stars, and so the first galaxies were born.

But these same models also say small halos of dark matter should be scattered around the universe. These would still attract normal matter, but not in the volumes needed to form galaxies. In theory, these halos should look like clouds of hydrogen gas, perhaps illuminated by a few scattered stars.

In practice, detecting such things is hard. But in recent years the FAST radio telescope in China has found signals coming from a few isolated clouds of hydrogen. One, located about fourteen million light-years away and named Cloud-9, looks especially interesting.

When it was first spotted, in 2023, astronomers noted it as a possible dark matter halo. But their telescopes lacked the power to spot faint stars, and so they could not rule out the possibility that this was just a small galaxy full of stars we could not see.

Now Hubble has taken a closer look. Its observations have made it clear that this is not a galaxy. There are few stars within it, and instead Cloud-9 looks rather like the long-imagined cloud of gas surrounded by a halo of dark matter.

Within it are about a million solar masses of gas – far too little to spontaneously collapse into stars. This is far outweighed by the mass of dark matter. Astronomers estimate there is about five billion solar masses of the dark stuff holding the cloud together.

The First Private Space Station

Last week Vast Space, an American startup, said it hopes to launch its prototype space station early in 2027. Though in effect a demonstration mission, the space station will be capable of hosting small crews of astronauts for two weeks at a time.

That is not much, especially when compared to the International Space Station. But as Max Haot, the CEO of Vast, said in an interview with Ars Technica, it offers an opportunity for the company to prove it can safely host astronauts in orbit.

That may help them win future contracts from NASA. As the ISS is scheduled to deorbit in the early 2030s, NASA wants commercial partners to manage future stations in orbit. If that works out, NASA would simply pay them to put their astronauts on board, much as they now pay SpaceX to launch them into space.

How well this will work in practice remains to be seen. Though several companies have talked about building private stations, actual progress has been slow. If Vast can launch next year, and if their station can host crews of astronauts, they will place themselves well ahead of the competition.

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