We are going to explore the discovery of a previously hidden “magnetic shadow” on the Moon that could make future space travel safer. New data from China’s Chang’e-4 mission has revealed a patch of space around the Earth, which the Moon crosses over a couple of days each month, where levels of cosmic radiation are much lower than they should be. It could act as a potential natural shield for astronauts and radiation-sensitive lunar-based equipment and experiments. Megan Argo is an astrophysicist at the University of Lancashire and she’s been telling Chris Smith about the study in Science which documents the phenomenon…
Megan – They found what they’re calling a cavity in the distribution of these things called cosmic rays, high energy particles that fly around the Universe. What we thought was that the distribution of these things was fairly uniform and it turns out it looks like it’s not.
Chris – Where specifically have they been looking?
Megan – So this is a result from a spacecraft that’s been sitting on the Moon for a little while now. This is one of the Chinese missions, the first soft landing on the far side of the Moon, which is exciting because we’ve never been there before. One of the instruments that they had on the spacecraft is looking at the distribution of these high energy cosmic ray particles. And as the Moon’s been going around the Earth, they’ve been measuring these things over the last few years and they found a part of the Moon’s orbit around the Earth where there appears to be fewer of these things hitting the detector than in the rest of the Moon’s orbit.
Chris – So it’s almost like some kind of shadow, almost like a cosmic particle shadow there. Do they speculate as to why there is this gap or this cavity where it doesn’t get irradiated to the same extent periodically?
Megan – So they were specifically looking at periods of time over the last few years when the Sun has been less active. These particles are affected by magnetic fields and the Sun has a powerful magnetic field that spreads out throughout the whole Solar System and interacts with the Earth’s magnetic field as well. And these particles tend to follow these magnetic field lines. And when the Sun is active, it’s also pumping out particles that are very similar. So that’s going to affect the readings that you get. So they had to wait for periods where the Sun was less active. And in all of the periods they found during the observations when the Sun was less active, they were looking at and they were expecting a certain rate of these cosmic rays from this background of cosmic rays throughout the Universe. And there was a difference between the numbers that the detector received in a particular part of the Moon’s orbit. And this is a part of the Moon’s orbit where the Moon is between us and the Sun. That’s when it would be up in the sky in daylight. So between sort of when the Moon is a quarter and when it’s a new moon and you can’t see it. And in that range, there is a period of a few days where they see far fewer of these cosmic rays than they were expecting. And the effect seems to be that the Earth’s magnetic field, which we didn’t think was reaching as far as the Moon in this part of its orbit, Earth’s magnetic field is asymmetric. On the sunward side, it’s affected by the Sun’s wind. So it’s pushed backwards and it forms this long sort of tadpole like, it’s called a magnetotail. So it stretches a long way from the Earth in the direction away from the Sun. But in the direction of the Sun, it’s a lot more compressed and the Moon goes outside it, or at least we thought it did. What these results seem to show is that at least for part of that orbit, the Earth’s magnetic field is having much more of an effect on this particle distribution than we thought it had.
Chris – Does the reduction in particles actually make a meaningful difference? In other words, if you look really hard, you can see a tiny difference. It might make almost no difference. It’s statistically there, but makes no difference. Or is it actually a really almost like day and night difference in terms of the particle density of these background cosmic rays coming in?
Megan – It’s certainly a statistical difference. And the thing is, while the effect is certainly there, the Sun’s activity is a bigger effect, if you like, in the data. So finding this involved looking through an awful lot of data to find the bits where they could actually do this statistical comparison.
Chris – The reason for asking that, Megan, is obviously it’s academically extremely interesting and we’ve learned new things about the shape of the Earth’s magnetic field and its influences. But if there is a window being created at certain points in the lunar orbit where radiation is at a minimum, that might be a sort of window of opportunity because we know it is a concern, isn’t it? For humans out there being irradiated, for scientific instruments getting irradiated. So if we know there’s a sort of safe point where they could transfer backwards and forwards or do some experiments, that would be really useful. But if the difference is so tiny, it’s inconsequential, that’s perhaps less useful. So which is it? Do you think this is a useful observation?
Megan – I think it could potentially be, yes. The thing with radiation is it builds up over time. The more exposure that you have, the more risk you have. And it’s the same with humans as it is with electronics. If you send electronics into space, all of the satellites in Earth orbit have to be hardened against these particles because they can damage electronics and that can make your systems fail. And the same thing applies on the Moon, except it’s worse because you’re that much further away. And this is one of the things they’re going to be doing on Artemis. All the astronauts have radiation detectors to track their exposure while they’re outside the Earth’s magnetosphere. The interesting thing is if this is statistical, then it does make a difference because if you’re sending astronauts up to the Moon for the long term, inside their base, they’re going to have a lot more shielding, a lot more protection from cosmic rays and solar winds and all the rest of it. But if you have to have them going outside in spaces which have comparatively little protection, then if you can do that when the risk of radiation is reduced, you reduce their overall lifetime exposure and you reduce their risk of things like cancer for the long term.