The Week in Space and Physics
On nuclear fusion, the cosmic optical background, rocks from the Oort Cloud, and an accident on the Soyuz
Centuries ago alchemists sought the lost secrets of transmutation. This search, inspired by tales of ancient mystics, centred on turning lead into gold: an art they believed would bestow fantastic wealth and eternal life. Of course, the search was futile - lead and gold, we now know, are different elements. You cannot, simply by mixing potions and chemicals, transform one into the other.
For that the alchemists would have needed to probe deeper, to a world far stranger than even they imagined: to that of quantum physics and nuclear science. Only then could they have mastered the techniques known as nuclear fusion and fission: the dark arts of meddling in atoms themselves, of splitting and combining the building blocks of nature.
With these secrets we have, it is true, finally transformed lead into gold. Yet it is the conversion of other elements - of uranium into krypton, of plutonium into radium - that gave us the extraordinary power promised by transmutation. Humanity has, thanks to nuclear fission, powered its cities, reached the edge of the solar system and - it is equally true - unleashed the spectre of nuclear Armageddon.
The flip side of nuclear fission, which refers to the art of splitting atoms, is nuclear fusion. Here we build up atoms, combining light ones, such as hydrogen and helium, into heavier ones, such as carbon and oxygen. As Einstein released, this produces vast amounts of energy: enough to power a star for billions of years, or to vaporise a city in an instant.
The latter ability, in the form of the hydrogen bomb, has been in our grasp for decades. Yet controlled nuclear fusion - the kind with which we could run a power station - has turned out to be fiendishly complicated. Despite half a century of work, we have never even got close to building a useful fusion reactor.
For nuclear fusion to work, we need two things. First, a way to trigger the reaction by forcing atoms together; and second, a way to keep the reaction running long enough to start giving out useful energy. That, if we can do it, would open the door to a self-sustaining reaction; one that keeps on running and generating power for as long as we need it.
The first step, that of triggering the reaction, is well covered. Scientists have found many ways to do this, and today labs routinely spark fusion in their experiments. Yet reaching the point where those experiments give out more energy than they take to start - a moment known as ignition - has proven challenging.
Last week, for the first time, researchers at the US National Ignition Facility announced they had, finally, achieved ignition. The experiment, which involves firing high power lasers at a pellet of hydrogen, gave out slightly more energy than it took in. That is a big milestone: proof, if it were needed, that controlled nuclear fusion is possible.
Yet it also falls far short of being something we can use to power our civilization. The output of the experiment was small, and the reaction short-lived. Building this up to something that generates more useful and continuous energy will take a lot more research. Still, the age of nuclear fusion seems to be drawing closer. Later this decade an experimental large-scale reactor will open in France. That, if it works, could demonstrate nuclear fusion on a far more practical level.
A Hint of Missing Galaxies - or of Dark Matter?
In 2015, New Horizons flew past Pluto, giving us the first close-up view of that distant world. Now, after flying a billion miles further from Earth, New Horizons has revealed another unique view: that of the cosmos beyond our solar system.
We can, of course, see the stars and galaxies around us from Earth. But the Inner Solar System is also full of dust, which tends to obscure some of the finer details of the universe. New Horizons, perched far beyond the planets, thus has a clearer view of the stars and galaxies that we do here on Earth.
That turns out to be crucial in estimating something known as the “Cosmic Optical Background”. This, to put it simply, is the combined light coming from all the galaxies and stars beyond our own Milky Way. Since most of those galaxies are very far away, it is naturally faint and, therefore, hard to measure accurately on Earth.
In recent years observations from New Horizons have given astronomers a more accurate way to view this background. Surprisingly, the data suggests it is somewhere between two and three times brighter than expected; a measurement that a recent study has once again confirmed.
Where this extra light is coming from is not yet clear. The most likely explanation is simply that more galaxies exist than we think, making the universe brighter than thought. If so, most of these galaxies must lie far from Earth, beyond the reach of our current telescopes. The James Webb should be able to help here: it has, for the first time, the power to spot galaxies close to the edge of the visible universe.
Yet it is far from certain that the James Webb will find enough galaxies to explain the brightness. If not, astronomers are already idea the existence of glowing ancient black holes, or of objects known as mini-quasars. The Webb should be able to help find these, too, if they exist.
If it does not, then astronomers do have one more exotic proposal. The glow could, according to some theories, come from particles of dark matter. If so, New Horizons may have discovered a crucial clue to a puzzle that has long haunted physics.
A Piece of the Oort Cloud Falls to Earth
In February last year telescopes captured a meteor striking the Earth somewhere north of Edmonton, Canada. They saw the meteor, which weighed two kilograms, hitting the atmosphere at over sixty kilometres a second, before exploding forty-five kilometres above the ground.
Such events are common. Meteors of a similar size hit the atmosphere every day, yet rarely make it within a few dozen kilometres of the surface. However, further analysis showed that this event was rather unusual. The meteor, it turned out, had come from the Oort Cloud: a cloud of rock and ice that surrounds the solar system.
The Oort Cloud is thought to extend for vast distances, up to two light years from the Sun. But, given this huge distance, and the small size of the comets and asteroids that should exist there, little hard evidence of its existence has ever been found.
Models had suggested the Oort Cloud is mostly made up of icy comets. Yet this meteor, and the analysis of its path, suggests that there is far more rock scattered among them than once thought. Perhaps five percent of all objects there, the study estimates, could be rocky asteroids.
If so, that would lend support to models suggesting the early Solar System was a chaotic place. Studies hint that Jupiter and Saturn were once much closer to the Sun. When they suddenly swung outwards, billions of years ago, they would have thrown a vast cloud of debris far into space. That, simulations show, would have created a rockier Oort Cloud than expected.
Soyuz Accident
The announcement from NASA was rather calm: stating simply that a leak had occurred on the Soyuz spacecraft docked to the International Space Station. In reality, the situation appears to be serious, with images showing large amounts of liquid spewing from the spacecraft.
The leak occurred as a pair of Russian cosmonauts were preparing for a spacewalk. As alarms started going off the walk was cancelled, and space station operators switched their attention to the problem. One further spacewalk, this time by American astronauts, was also cancelled.
The leak seems to have knocked out the cooling system on the Soyuz. Fortunately, temperatures on the spacecraft do not seem to have risen by much. However, operators now need to understand if the Soyuz is safe enough to carry cosmonauts back to Earth as planned, or even if it can leave the space station safely. The cause of the leak is still unknown, but speculation centres on a possible impact from a small meteorite.