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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Aristotle was the student of Plato; Plato the student of Socrates.

For twenty years, Aristotle studied at Plato’s Academy. By all accounts, the two got along: Plato nicknamed him the “mind of the school”, and seems to have regarded him as the most promising of all his students.

In this academy, Plato taught of abstract realities, of ideal forms and of utopias. He believed our world was messy, but that underneath it lay a perfect reality hidden from casual view. One could probe this world to uncover Truth, but to do so one had to look inwards. Only through logic and reason, Plato argued, could we understand the nature of reality.

His greatest student came to disagree. Loyalty may have kept him at the academy, and perhaps kept his mouth shut too, but after Plato died, Aristotle began speaking of alternative ideas. He doubted the existence of Plato’s hidden world. Instead, he started to believe that the understanding of nature could only begin with observation.

Aristotle thus observed. His range was astonishing, and stretched far beyond what we would call physics or even science. He wrote about logic, explored the nature of reality, studied biology, and opined about happiness. He travelled greatly, roaming across Anatolia and Greece, and eventually was summoned to the court of Philip II in Macedonia.

There he was appointed tutor to Philip’s thirteen-year-old son Alexander. What he taught him remains unknown – the only book on the subject, Rhetoric to Alexander, was almost certainly written by someone else. But it was probably good: within the next two decades Alexander would become king, conquer the empires of Persia and Egypt, and march his armies into India.

While Alexander was off conquering the world, Aristotle returned to Athens. He founded his own school, the Lyceum, and gave the lectures for which he is best known today. Alexander sent back samples of plants and animals from across his empire, and Aristotle collected and sorted them. Yet, as the conqueror grew more powerful and convinced of his own divinity, Aristotle seems to have grown concerned about what his former student was doing.

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The spectacular cloud of dust and debris around Fomalhaut has long been a favourite target of astronomers. After all, the star is one of the brightest and closest of the southern sky, and its debris cloud makes for good photos. Those taken by the James Webb Space Telescope capture glowing rings looping around the star; those from Hubble show a bright outer ring encircling inner debris fields as if to create a cosmic all-seeing Eye of Sauron.

Since the star is around half a billion years old – young, as these things go – this cloud is thought to be a disk of rock and dust slowly coalescing into planets. Its inner rings, imaged by the James Webb, might already be shaped by the passage of large planets sweeping around the star. The outer ones, seen by Hubble, are probably home to countless dwarf worlds known to astronomers as planetesimals.

In 2004, headlines were made when Hubble spotted something that looked like a planet in those outer rings. The discovery was hailed as a big success: indeed, it marked one of the first times we thought we had taken direct images of a world around another star. But things soon got weird. In optical wavelengths the object was far brighter than any planet should be, yet when telescopes examined it in infrared wavelengths they failed to spot anything at all.

How to explain this? Some astronomers clung to the idea it was a planet. But to salvage the big discovery, they had to wrap it in a thick cloud of dust. If this layer existed, they argued, it would reflect starlight and make the world look far brighter than it otherwise would. Others pointed out that the planet itself was unnecessary. Instead, they said, we may simply be seeing a cloud of debris formed by a violent collision between worlds.

In the decades since, the evidence has mostly favoured the second idea. When Hubble revisited Fomalhaut in the early 2010s, it found the object expanding and fading in size, just as a debris cloud might be expected to. Even more convincingly, it seemed to be moving radially away from the star. That would make sense if grains of dust were being blown outwards under the radiation pressure of the star, again as might be expected after a collision.

By 2014, the object had vanished altogether, and it has remained elusive ever since. In 2023 astronomers led by Paul Kalas of the University of California searched for it again. Once again they used Hubble, but, as they outlined in a recent paper, they still couldn’t find any clear sign of the dust cloud. Yet as they searched, they did find something else: a new point-like object had appeared around Fomalhaut.

When Worlds Collide

The idea that small planets might be smashing into each other is not particularly surprising. Fomalhaut is a young star surrounded by a large and chaotic cloud of debris left over from its formation. Within it, chunks of rock and ice are moving, accumulating mass, and trying to sweep their orbits clear. This is how star systems form, and eventually it may well settle down to become a calm and respectable system like our own.

But right now, things look far from ordered. Orbits are still in conflict, and sometimes big things crash into each other. On most occasions these are minor events, part of the natural gathering of matter. But at other times they are cataclysmic: two small worlds smash into each other at speed, the violence of the collision shatters them, and a great cloud of dust and rocky fragments takes their place.

This probably describes both events that we have now seen. Surprisingly, however, the objects are unlikely to be large. We are not watching planets like Venus crash into Mars, according to Kalas and his team, but instead seeing collisions between large asteroids or prototype worlds.

The logic behind this is simple: big planets must be few in number, and collisions between them must be rare. It is unlikely we would have witnessed two such events in the past few decades. Instead, it is more reasonable to say we are seeing collisions between dwarf planets or large asteroids. These objects should number in their millions, and a big impact could come roughly once every few dozen years.

But the authors also see hints that more is happening than we currently know. The new object appeared close to the site of the previous one, which seems like an odd coincidence. It is possible, Kalas says, that rings of debris happen to cross at this point, though the observations we have so far seem to contradict this idea. Another option, he writes, is for an unseen planet to be herding smaller worlds into this particular region.

These debris rings may also be shaped by the two small stars that orbit Fomalhaut. Both are quite distant, and will not normally disturb the system. Yet at times they do sweep inwards, and their gravitational pull is bound to have some effect. Indeed, the stars may sometimes direct small planets onto collision courses, and thus help instigate bouts of chaos.

Under Heavy Bombardment

We do not have to look far to find evidence of similar events elsewhere. Craters on our own Moon tell of a cataclysmic period long ago when the Earth and its satellite were bombarded by asteroids and comets. Where these came from is still uncertain, but several theories pin the blame on the outer edges of the solar system.

In one model, the gas giants – Jupiter, Saturn, Uranus, and Neptune – fell into a gravitational resonance. Saturn may have been flung outwards; indeed, some simulations suggest it was born closer to the Sun than Jupiter was, and that the planets swapped place in a moment of violence. The outward swing of Saturn unsettled Uranus and Neptune, and might even have sent a fifth gas giant careening outwards.

If so, that extra planet has long been lost, and now floats alone in the galaxy. But its movements, or those of the other gas giants, pushed the cloud of ice and rock at the edge of the solar system into chaos. Collisions would have been frequent, and a rain of asteroids would have fallen on the inner planets.

Today, of course, things are much calmer. The planets follow regular, well-swept orbits. The asteroids have mostly been herded into groups, and collisions between things are now rare. But this might not last forever. Models show our solar system is still a chaotic system. A small disturbance – perhaps the approach of another star, perhaps the wandering of a stray comet – could throw things off balance.

If that happens, simulations cannot accurately predict the outcome. Models show different catastrophic possibilities: a planet could plunge into the Sun, Mercury could smash into Venus, or Mars could be hurled out into interstellar space. Fortunately, all these scenarios take tens of millions of years to play out, and so we’d get plenty of warning of the impending chaos. And with that, hopefully, we would have time to do something about it.

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Project Mercury was supposed to be fast, and it was supposed to be simple. After all, Sputnik had made it starkly clear the Soviets were ahead, and America feared falling even further behind. The newly created NASA quickly struck back with Explorer 1, launched less than four months after Sputnik, but by then the Soviets had put a dog in space and everyone knew a human would be next. To have any hope of getting there first, Mercury had to be as fast and as simple as possible.

Except, of course, that putting a man into space for the first time was anything but simple. First you needed a rocket, and the ones America had in 1959 still had a nasty habit of blowing up. Then you needed a capsule – and this was supposed to be dumb, just something a person could sit in for a few hours and sail safely through the vacuum of space. And then you needed to bring him back to Earth.

For this last bit, America chose to use the oceans. To return to Earth, an orbiting capsule first needs to fire its thrusters, a burn that slows the capsule and drains it of the energy needed to remain in orbit. After that it will fall, and if you time the burn correctly, it will descend towards a chosen spot on the ground. Put that spot in the ocean, and things are simpler: water is more forgiving than solid rock, at least when it comes to things falling at high speed.

But afterwards things can be complicated. A capsule must float; it must be able to survive waves and whatever else the weather throws at it; and it must be able to be found. And none of that is especially easy. In Project Mercury one of the capsules sank, a disaster which almost took an astronaut with it, and two others came down hundreds of miles off course, each invoking a search over vast stretches of the ocean.

This was a time before the navigation systems we rely on today. There was no constellation of GPS satellites, and no network of relay satellites to transmit distress calls. The deep ocean was vast, mysterious, and largely out of reach. If a capsule came down five hundred miles short of its target, as one of the first test flights did, then finding it was akin to looking for the proverbial needle in a haystack.

What was needed was a way to locate capsules amidst the waves. At first, uncertain of what would work, NASA threw almost everything they could at the problem. An armada of ships and aircraft were scattered across the ocean before every flight, and the capsules themselves were fitted with an array of aids to assist in finding them. They scattered chaff as the parachutes opened, hoping it would appear on radar, triggered radio beacons on splashdown, and dropped bombs that would explode deep under the waves.

This last idea, that of the SOFAR bomb, relies on an oddity of the deep oceans. There, roughly a kilometre below the surface, lies a sound channel. If a bomb can be detonated in this channel, the sound of it can be picked up thousands of miles away, and that offers a way to hone in on the position of whatever has dropped it.

The reasons why this works are subtle. First, of course, is the simple fact that sound travels more easily through water than through air. That makes logical sense: sound moves through the vibrations of particles, and those particles are more closely packed in water than in air. This also means sound travels faster through water than it does through air: the speed of sound in fresh water at room temperature, for example, is four times that in air.

The speed of sound in water, however, also depends on both pressure and temperature. In colder water sound travels more slowly. The deeper under the surface of the ocean you go the colder things normally get, and so if all else is equal, sound waves moving deep under the waves should travel more slowly than those near the surface.

But, as always, things are not equal. The deeper into the ocean you go, the more water you have pushing down on you, and the higher the pressure you experience. As pressure increases, sound travels faster. Initially, as you descend under the waves, the change in temperature dominates and sound slows down. But at some point, approximately a thousand meters deep, pressure begins to assert itself, and sound speeds up again.

This creates a minimum, a distinct depth at which sound waves reach their slowest speed. If a sound originates here, it will stay here. If it rises, it will be reflected down. If it falls, it will be pushed back up. The only way it can move is within the layer itself, horizontally under the waves.

This implies two things. First, that sound waves cannot enter this layer. Only what is made within it can be heard by someone else listening inside it. And second, that sound waves cannot leave: any noise originating within this layer can thus be heard at enormous distances. Whale song is a good example of this: it can travel across an ocean, and allow a creature off the coast of Ireland to communicate with another swimming near Virginia.

We have developed two easy ways to create sounds underwater. One is to drop a hollow metallic sphere. At the right depth and pressure it will be crushed and the sound of the implosion will echo through the underwater channel. The other – the method used on Mercury – is to drop a bomb, use a pressure sensor to trigger a detonation, and then wait for the sound wave to travel through the water.

Each of the early Mercury capsules carried two such SOFAR – SOund Fixing And Ranging – bombs on board. One was stowed with the parachutes, and would be thrown out as they deployed. It would fall in the ocean and give any waiting ships a bearing upon which to look for the spacecraft.

The other was stowed within the capsule itself, and would only detonate if the whole thing sank. This would send a second location pulse to ships. But the explosion was also intended to destroy vital components inside the capsule, and thus prevent the Soviets from ever retrieving American designs.

Both bombs had a range of about three thousand miles. Hydrophones installed as part of Cold War efforts to track submarines could listen for them, as could vessels equipped with listening equipment and special buoys dropped by aircraft. All that gave the Navy plenty of points with which to triangulate the signal.

On September 9, 1959, all this was put into practice. Big Joe I, an uncrewed test capsule, was launched from Cape Canaveral. Things did not go to plan. A few minutes into the flight, the booster failed to jettison. That made everything heavier than it should have been, and to compensate the engines burned longer. Yet it wasn’t enough. When they ran out of fuel, the spacecraft was still coming in too steep. It landed five hundred miles short of its target.

But everything worked: the chaff deployed, an automatic radio beacon was picked up by a nearby aircraft, and the bomb exploded deep under the water. It took less than two hours to confirm the capsule’s location via the SOFAR channel, and the recovery operation itself was done within eight.

Overall, however, SOFAR proved less useful than hoped. The Navy often took hours to process the data from its microphones, and even then the results would reflect the point at which the parachutes had opened rather than the actual location of the capsule. When Mercury-Atlas 7 splashed down two hundred miles off-target, it was the radio beacons that first alerted search crews, not the bomb.

These difficulties, along with the effectiveness of radio beacons and water dyes, spelled the eventual end of the SOFAR bomb. After the fourth manned flight, NASA decided to drop both the bombs and the chaff. They were not used again on Mercury, nor on Gemini or Apollo. The Shuttle, of course, had no need for them, and by the time American capsules started splashing down in the oceans again, global positioning satellites had rendered all other techniques unnecessary.

Yet the bomb is a testament to a time when spaceflight was new and engineers were unafraid to think creatively. Project Mercury had to be fast, and there was no time to run years of analysis to prove what would work or what wouldn’t. Engineers were pragmatic, they tried new things, and if they proved unnecessary they weren’t afraid to move on.

True, not everything went right. There were near misses, close calls, and moments that could easily have ended in tragedy. But Mercury did succeed in putting a man into space, even if the Soviets were first by a month. And for a brief time, the sounds of those bold missions echoed through the depths of the oceans, accompanied by the songs of the whales and the mechanical whirr of the submarines.

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For the past decade, NASA has had a large mirror sitting in a warehouse. In principle, it could one day form the centrepiece of a new Hubble-class telescope. But in reality, money is short. NASA has the technology to build such an observatory, and plenty of ideas for what to do with one, yet sadly the political desire to fund such a project has long been missing.

Could America’s billionaires fill that gap? They have the cash, the rockets, and a few, it seems, also have the will. Last week two of them, Eric and Wendy Schmidt, announced they would fund a set of powerful astronomical observatories. The centrepiece of their investment is a space telescope they call Lazuli, an observatory of greater power than Hubble they hope to place in a high orbit over the Earth.

Eric Schmidt was an early CEO of Google. He is now fantastically wealthy, and he and his wife have set up a series of foundations intended to encourage scientific work. It is one of these, Schmidt Sciences, that has decided to fund the observatories, a project that seems likely to cost more than a billion dollars in total.

Four astronomical centres will be funded. The first, the Argus Array, will study the sky in optical frequencies – those visible to the human eye – and will be suited for spotting rare events like supernovae. Another, called the Deep Synoptic Array, will observe radio frequencies. A third, LFAST, will act as a scalable prototype that could, if expanded enough, gather as much light as some of the largest telescopes now under construction.

It is the space telescope, however, that has captured the most attention. Schmidt Sciences propose to build and operate an observatory with a bigger mirror than Hubble’s. It will be equipped with more advanced instruments, and will orbit at a much higher altitude. It is, Schmidt Sciences said, a modern realisation of Hubble-like capabilities.

If this announcement were coming from NASA or ESA, it would not be surprising to hear that operations will not start until the late 2030s. But Schmidt Sciences reckon the telescope can be in orbit by 2029. That is fast. And if it can be done, it will be a major boost to the world’s astronomical abilities.

Although it is rare today, historically many observatories were funded by wealthy patrons. A few examples may still be found, but nothing of this scale has ever been attempted by private organisations. Yet in a world where government money can be scarce and in which budgets can be held hostage by politicians, new funding sources and alternative models for astronomy are to be welcomed.

Isaacman Takes Charge at NASA

After a tumultuous nomination process, Jared Isaacman is now in charge of NASA. The billionaire and private astronaut was first proposed for the job at the end of 2024. But he was brushed aside after Donald Trump and Elon Musk had a spectacular, if rather childish, falling out. After they repaired their relationship his name was raised again, and he was confirmed by the US Senate on 17 December.

His start at NASA has been eventful. Last week, as astronauts on the space station prepared for a space walk, one of them made an urgent call to ground controllers in Houston. Exactly what happened is not yet clear, but it appears some kind of medical situation developed. NASA has said it was not an injury and that the astronaut is stable, yet it was serious enough to postpone the planned excursion.

After conferring for a few days, NASA then decided to return the astronaut and crew to Earth early. Details of the medical situation, and the astronaut in question, have not been released – indeed, NASA typically goes to great lengths to protect the privacy of their astronauts, and it may not ever give the full details of what exactly has happened.

Though plans for medical evacuations have been in place since crews first set foot on the station, this is the first time they have been put into action. The early departure of the four astronauts, which should take place on Wednesday, will leave a minimal crew of three onboard the International Space Station.

Alongside that, NASA is moving ahead with plans to launch Artemis II, the first crewed flight beyond low earth orbit since the days of Apollo. Four astronauts will travel in the Orion capsule over a ten day mission that will take them around the Moon. They will not set foot on the lunar surface, but they will become the closest visitors to our natural satellite since 1972.

There are, however, questions about the safety of Orion’s heat shield. Examination of the shield after Artemis I, an uncrewed test flight, found it had behaved in unexpected ways.

Engineers have modified the planned re-entry trajectory to minimise the risk, but some are still concerned about whether it – and the crew – will survive their fiery plunge back to Earth. After examining the data, however, Isaacman declared he was satisfied with the proposed approach. NASA is now working towards a launch window opening on 6 February.

Does Siwarha Really Exist?

Betelgeuse is one of the brightest stars in the sky. At first glance it seems to be alone, but for a long time we have suspected it has a hidden companion. Fluctuations in the star’s brightness show a regular pattern, as if something is tugging on it in a periodic way, just as another star in orbit around it might do.

Yet observations have so far failed to spot any companion. Some measurements have hinted at one – a study last year, for example, claimed to have seen it – but all have been inconclusive. Now, for the first time, astronomers looking at data from the Hubble Space Telescope claim to have found evidence of a wake left behind this companion star.

Models suggest the star, which has been named Siwarha, orbits so close to Betelgeuse that it should lie within the giant star’s outer atmosphere. As it moves through this layer it ought, therefore, to leave a trail of disturbance behind it.

After examining Betelgeuse carefully over the past decade, the astronomers say they found multiple signs of such a trail rising and falling over a six year cycle. That matches well with other observations about the periodic nature of Betelgeuse, and should be taken as new evidence supporting the existence of this companion star.

As always, caution is needed. Light from this star has not yet been definitely spotted, and right now the models say it is behind Betelgeuse and cannot be seen. It should re-emerge in 2027. When it does, telescopes are bound to be watching for it. If the growing pile of evidence is correct, someone may be able to claim a discovery by the end of next year.

The Roman Telescope

NASA announced the successful completion of construction work on the Nancy Grace Roman Space Telescope. The final step came at the Goddard Space Centre in Maryland, where engineers connected the inner and outer segments of the new observatory.

Like the James Webb Space Telescope, Roman is designed to study the sky in infrared light. But unlike the James Webb – which often looks deep into small areas of the cosmos - Roman is equipped to study wide areas simultaneously. This will allow astronomers to study swathes of the sky, and so to look for things like supernova, exoplanets, and the effects of dark matter.

The telescope will now go through a period of final testing. By the summer it will be delivered to the launch site, and there the final preparations for lift-off will take place. The launch date itself is still to be determined, but NASA and SpaceX say it could come as soon as October.

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First, I want to take the opportunity to say thank you to all of you for reading, for subscribing, and for your kind feedback.

I set out at the start of last year to write more often, while still focusing on quality over quantity. In practice, that has meant devoting more time to both researching and writing. Luckily, this has been enjoyable. As I wrote last year, it is easier to write about things I find interesting, and this year I have been fortunate to cover everything from the rusting of our Moon to the lost sisters of the Pleiades.

In the coming year I hope to keep that up, and so I also want to take this opportunity to renew my commitment to writing about interesting things, and to taking the time to make that writing good.

The Year Ahead

Aside from that, I have a few other things to announce:

1. My aim remains to send out about one email per week. I don’t always meet this target, so in practice it will probably be a bit less.

2. For the past year, paid subscribers have been getting additional articles. This comes to about one extra article each month, though again, I will try to increase this rate. Most of these articles are “deep dives”, that is, they explore a topic in greater detail. Examples from the past year include the Gaia telescope, the evidence for recent supernovae close to Earth, and the ways in which we make progress in science.

3. For the next week, I will offer a discount on subscriptions: $30/€25 per year, or $3/€2.50 for a month. Please do consider taking one.

4. That said, I am not fully happy with the subscription model Substack employs. I am currently working on re-editing and updating articles I’ve written over the past year, including those that were for paid subscribers only. My aim is to collect these in an e-book or PDF format and make this available for a one-time purchase (i.e. with no subscription necessary). Paid subscribers will get early access to the articles and free access to the booklet. If this trial works well, I plan to offer more such booklets in future for those who want to support me without signing up for a subscription.

5. I’d like to thank all those who have written to me over the past year to ask questions or just to give some kind words! Please don’t hesitate to get in contact with any feedback or questions. I’d be especially happy to hear about any articles you liked (or hated!), any topics you’d like to hear more or less about, or any questions you have.

Highlights from 2025

I spent some time, this summer, in the valley of the Vézère in southwestern France. People have lived there for a long time. Caves, of which there are dozens in the rocks and cliffs of this valley, show the traces of inhabitants stretching back over four hundred thousand years. In many there are ancient works of art: paintings of mammoths, of constellations, and, in a few rare cases, of humans themselves.

As I stood there, looking up at the cliffs and caves, trying to imagine the long-gone world these people lived in, another story came to my mind. In his book Wind, Sand and Stars1, Antoine Saint-Exupéry wrote of plateaux he had flown over and landed on in the Sahara Desert. These, he says, were made of hard limestone, formed from the slow accumulation of sea shells over the ages.

That, amid the dry sands of the desert, was strange enough. But having landed on them, he noticed something else. Scattered across the brilliant white surface of these plateaux were hard black pebbles. Each was heavy and cast in the shape of a tear drop. They had, he realised, fallen from the heavens: a long slow rain of rock; each plateau a sheet stretched under the stars and slowly gathering their dust.

“We are living on a wandering planet”, he notes in that book. And so we are. Things are vaster than we imagine, tied together across spans of space and time that humble our self-importance. The discoveries of science and the work of engineers cannot protect us from this; our only defense, indeed, is to look up and wonder.

Life Beyond Earth

“Year after year”, H.G. Wells wrote in 1896, “...there recurs the question as to the existence of intelligent, sentient life on the planet Mars”. Things have not changed. More than a century later we still speculate about life on Mars, though admittedly few today think that sentient beings might one day be found there.

Back then, scientists thought Mars might be rather Earth-like, with changing seasons, vegetation, and regular rainfall. Today we know it is not – Mars is a dry, dead place, one that has seen little change in the past three billion years. Yet we also know this was not always the case. Once Mars really was Earth-like, and had flowing water, seas, and a thick warm atmosphere.

This means it could also have been alive. And this year, researchers announced the discovery of a tantalising hint of this long-gone world. A rock examined by the Perseverance rover shows plausible signs of ancient biology, they said, and contains a mix of minerals that on Earth would be a sure sign of life.

For now, though, this remains speculation. The researchers called the signs a “potential biosignature”, and cautioned that a closer look would be needed to confirm their origin. That means we either need to get the rock back to Earth – for which budgets are sadly tight – or send people to Mars to look at it directly. Neither seems likely to happen any time soon.

Other researchers have focused their attention on worlds far beyond the solar system. The James Webb telescope has been studying the Trappist system, a collection of seven rocky planets a few dozen light years away. In theory some of them could have the right conditions for life.

So far, however, Webb’s data has been discouraging. Its studies of the innermost planets of the system have found them to barren worlds more akin to Mercury than Earth. Yet data from the outer planets – including those thought to be most habitable – is still to be released. Next year might, then, bring an exciting discovery of a world with a thick atmosphere and warm temperatures.

Studies are also ongoing of K2-18b , a world that shows hints of life. Its atmosphere, according to studies released last year, contains traces of dimethyl sulphide. On Earth, this gas is made by life – and so its presence on an alien world is intriguing. But other researchers again urge caution: the evidence is still weak, and even if the gas is there, we have no clear proof it is being made by living creatures.

Vera Rubin and Interstellar Comets

The second half of the year brought a rare visitor to the solar system. Comet 3I/Atlas was spotted by telescopes on July 1st, heading rapidly towards the inner planets. Astronomers soon realised it had come from interstellar space, and so this comet was distinguished as only the third object known to have visited from the stars.

Analysis showed 3I/Atlas was probably billions of years old, and likely older than the solar system itself. Although its origins remain unknown, we believe it ultimately came from a far distant and very ancient star system. At some point it was hurled out towards the stars, and it has since spent several billion years wandering the galaxy alone.

Its encounter with the solar system was a rare event for both the comet – at least a million years have passed since it last came so close to a star – and for us. Astronomers wasted no time in studying 3I/Atlas. Telescopes watched as it flew through the solar system, swung past Mars, and then passed on the other side of the Sun. It is now heading back out into interstellar space, and should once again cross the orbit of Jupiter next year.

After that, however, interstellar comets may become a more regular sight. Dozens of them are probably transiting through the solar system right now, but since most are faint and hard to see, we simply don’t spot them. The new Vera Rubin observatory in Chile will change that: it is designed to survey the whole southern sky in unprecedented detail, and will allow astronomers to find millions of objects that have so far been missed.

This bonanza of discovery may open the possibility of getting a close-up view of an interstellar comet. Europe is working on the Comet Interceptor, a spacecraft that will launch by the end of the decade and that will then wait for a suitable target to come along. An interstellar comet, ESA has said, would be a tempting object for it to examine. And with Vera Rubin, we should have plenty of options to choose from.

A New Race to the Moon?

Once again, NASA failed to land humans on the Moon.

No surprise, you might think – after all, were they really trying? But as recently as 2023, NASA’s stated goal was to return to the lunar surface by the end of 2025. Deadlines have, of course, slipped. NASA’s new goal is to reach the surface by 2027, though even this relaxed target seems hard to meet.

This much delayed schedule has raised concerns that China will get there first. Indeed, they might. In recent years China has made rapid strides in its space program. It has built a space station, tested reusable rockets, and mastered the art of landing and taking off from the surface of the Moon. It still needs a Moon rocket and a lander capable of carrying astronauts, but work on both seems to be well underway.

With all this, China hopes to put an astronaut on the lunar surface by 2030. That gives NASA some room to delay further – but fears are rising in Washington that America simply won’t make it in time. There is, acting NASA administrator Sean Duffy said this year, a new race afoot, and America needs to move faster if it wants to win.

The coming year should make the state of things clearer. NASA wants to launch Artemis II in February. That mission, if it succeeds, will carry astronauts around the Moon for the first time since the 1970s. Alongside that, SpaceX hopes to make progress with Starship, the spacecraft that the crew of Artemis III will use to descend to the surface.

If both projects go well, America has a good chance of reaching the Moon this decade. But if not, then China will look like the favourite, and the wisdom of declaring a new space race will surely be questioned.

The Shifting Balance of Science

It is not only in space that the balance of power seems to be shifting. In recent years China has invested in scientific facilities. The world’s largest single-dish radio telescope is nestled in the hills of Guizhou, one of its most advanced X-ray lasers is in Shanghai, and there is now also a powerful neutrino observatory buried under a mountain in Guangdong.

Although metrics of scientific output are hard to compare, the investment in new world-class facilities shows that China is serious about its scientific research. And it comes as a stark contrast to the current policies in the United States, which have led to sharp reductions in research funding.

This is not, to be clear, a new trend. The collapse of the famed Arecibo telescope in 2020 was preceded by cuts in its budget for maintenance. Lack of funds also ruled out the construction of any replacement observatory. American scientists have been trying to find money for two large new telescopes for years – and now, with cash still short, it seems likely that one of them will move to Europe instead.

The result of all this will not be a sudden halt to American science. Indeed, in many ways America is still the world’s scientific superpower: there is nothing else like the James Webb Space Telescope, and much of the research behind the AI revolution took place in the United States. But the balance of where scientific progress is made is shifting.

This is a matter of consequence. Physics is not just a subject of academic curiosity. It is the field from which much of the modern world has emerged: without the work of Carnot and Kelvin we would not have perfected the steam engine; without the discoveries of Maxwell we would not have mastered electricity; without the theories of Einstein we would not have built the atomic bomb.

This, very deliberately, is not a political newsletter. But whether we like it or not, science does rely on politics, and in its turn dictates the future shape of the world. The breakthroughs awaiting in the next few decades – from quantum computing to nuclear fusion – will in all probability be just as significant as those of centuries past. Basic research may not always have an obvious payoff, but history shows that civilizations which abandon it will inevitably fall behind.

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