The Week in Space and Physics: Telescopes on the Moon
The Week in Space and Physics: Telescopes on the Moon
On lunar radio telescopes, space debris, the cosmic web and a growing threat to astronomy
Nine decades ago an engineer in New Jersey started noting down the patterns of static his radio was picking up. Thunderstorms, he realised, were creating a lot of it, with the exact type of static depending on how distant the storm was. But storms couldn’t account for everything: even after they passed, a faint hiss of static remained.
This hiss seemed to follow a daily rhythm, a detail that led him to suspect the Sun was to blame. Yet on closer inspection he realised this could not be the case - though the signal followed a twenty-four hour cycle, it did not match the rising and setting of the Sun. Instead he found another celestial object was at blame: the heart of the Milky Way.
This chance discovery gave birth to the field of radio astronomy. In the years since, astronomers have found radio waves coming from all sorts of cosmic objects, from quasars to black holes. Indeed, some of the most symbolic telescopes of astronomy - Arecibo perhaps the most famous of all of them - have been dedicated to searching the skies for radio waves.
On the whole, however, Earth has proven to be a rather poor location for radio astronomy. Its atmosphere, especially its ionosphere, absorbs many low frequency radio waves. Even worse, it is noisy: its skies are full of loud signals from radio stations, cellular phone systems and orbiting satellites.
Could the Moon be a better option? It has no atmosphere or ionosphere, so there is nothing to block even the lowest frequency radio waves. And its dark side, the side perpetually facing away from Earth, is shielded from all the human-made interference that plagues terrestrial radio telescopes.
Placing a radio telescope on the far side of the Moon, then, sounds like a good idea. Yet it is also a challenging one. The Moon is a harsh environment; suffering freezing cold nights that last two weeks at a time, followed by baking hot days. The far side, too, cannot be directly observed. Any radio telescope there would be forced to rely on a relay satellite to communicate with Earth.
Still, a team at Brookhaven National Laboratory in New York state thinks the benefits are enticing enough to make it worth trying. They are developing LuSEE-Night, a small radio telescope they hope to place on the Moon’s surface by 2025. LuSEE; they say, could give us an unprecedented view of faint low frequency radio waves lingering from the “Dark Ages”, an early period of cosmic history from which no visible light remains.
Larger telescopes, some hope, would follow in LuSEE’s footsteps. Yet the window for lunar astronomy may end up being rather short. Plans are already afoot to begin ringing the Moon with satellites, a step that would forever end the radio silence of the dark side of the Moon.
Time for a Clean Up?
As the number of satellites orbiting our planet continues to rise, so does the risk of a collision between them. Such events have, mercifully, been rare in the past, partly because space is big and satellites are usually far apart. Yet some orbits are now becoming dangerously crowded. At certain points - primarily over the Earth’s poles - satellites are regularly coming close enough together to raise serious concern.
An example of this came on March 6, when the International Space Station took action to evade a satellite, firing its thrusters for six minutes in order to raise its orbit. Events like this are still relatively rare, with around eight incidents occurring since 2020.
Companies and governments are beginning to think about how to address the problem. One option is to find ways to deorbit old, dead satellites, allowing them to burn up in the atmosphere instead of leaving them to clutter up orbits. Ideally, of course, satellite operators would already be doing this, but many do not and even when they do, satellites can fail unexpectedly.
Instead, satellite operators could pay for others to help deorbit their dead satellites. One company, Astroscale, recently raised $76 million for building a spacecraft capable of doing exactly that. If all goes to plan, they will use a robotic arm to grab defunct satellites and pull them down into the atmosphere.
Yet this approach is expensive and tricky, requiring Astroscale to make a close approach to another satellite. Even worse, the situation is mired in legal concerns. Astroscale have, in this case, permission to remove two old British satellites. But many dead satellites belonged to the former Soviet Union, or to countries like North Korea. Getting permission to take them down is tough, and even if it is granted, questions still linger about who should pay.
Whatever happens, the scale of the problem will make it hard to fix. NASA is already tracking tens of thousands of objects in orbit around our planet, many of them far too smaller to be removed with robotic arms. Many more near misses are likely to come.
Shockwaves Hint at Cosmic Magnetism
The cosmic web spans the cosmos; connecting vast clusters of galaxies in a network of filaments. In terms of scale it is extraordinary. The Perseus-Pegasus filament, for example, seems to measure over a billion light years in length.
These filaments, though widespread, are hard to see. Models predict they are made mostly out of dark matter and cold hydrogen gas, neither of which shines particularly brightly. Indeed, some observations suggest they glow only slightly brighter than the background glow of the cosmos itself.
As astronomers have gradually mapped out the cosmic web, attention has started to turn to the magnetic fields that may lace through them. Magnetism, some researchers believe, could play an important role in sculpting the large scale universe. Recent findings, indeed, have shown how magnetic fields shape galaxies, and how they may even be responsible for the spiral arms of our galaxy.
If magnetic fields are actually present in the cosmic web, then they should reveal their presence as shock waves ripple through them. These waves - created by collisions between galaxies and clusters - result in surges of particles moving through the web. As they do so they should interact with any magnetic fields present, creating a burst of radio waves.
So at least, goes the theory. But such bursts of radio energy have proven hard to find, mostly because any individual shock wave glows only faintly. In a new study, however, researchers combined images of thousands of pairs of galaxies. Since filaments likely connect many of these pairs, the combined images, they hoped, would amplify the weak radio signals.
Their results did indeed show traces of the expected radio waves. These, they say, are the first evidence that shockwaves really are shaking the filaments of the cosmic web. That, if the findings hold up, might be one our first glimpses of the magnetic fields spanning the universe.
A Growing Problem for Hubble
SpaceX now owns more than three thousand satellites around our planet. OneWeb controls hundreds more, even as nations from China to Europe plan vast constellations of their own.
This is contributing to a growing space debris problem, as covered above, but it is also hindering the work of astronomers. As satellites fly over telescopes, they leave streaks of light across sensitive measurements. This problem, a paper revealed this week, is not limited to Earth-based observatories.
Analysis shows satellite streaks present in around three percent of images taken by the Hubble Space Telescope. Unfortunately that rate appears to be growing over time: roughly six percent of images taken in 2020 suffered from the problem. The next decade, sadly for astronomical research, is only likely to see the disruption get worse.