Frontiers of Space Science

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1:04Roland Pease:Each December, tens of thousands of Earth and planetary scientists from around the globe gather to compare notes and share their latest discoveries at the annual fall meeting of the American Geophysical Union, the world's premier organization for geosciences. And if I can fix it, I, Rel and Pease, try to make sure I can join them to join in the excitement. Well, last month I succeeded. And for the next two episodes of Discovery from the BBC World Service, I'll give you a taste of what I heard, from the fate of the Mississippi to the geochemical origins of life. But this edition takes the big picture, the place of Earth as a planet in the solar system.

1:47Roland Pease:Space science turns out to be a major aspect of geoscience. Stay tuned for solar weather and for moonquakes. But we're starting with planetary defence. The idea that asteroids orbiting close to the sun might cross our path and cause devastating impacts. Not necessarily on the scale of the dinosaur destroying one 66 million years ago, but enough to wipe out, say, a city. The session on how to defend ourselves from that threat returned to a story that hit the headlines a year ago. the discovery of an asteroid dubbed 2024 YR4 that briefly looked like it could smack into us in 2032. And that was very much still in astronomers' thoughts.

2:34Roland Pease:Among the session participants, one of America's top planetary defence experts. I'm Andy Rifkin. I'm a planetary astronomer at the Johns Hopkins University Applied Physics Laboratory. And we've just had a really interesting session on planetary defense. At the beginning of 2025, we had this sort of big excitement about this asteroid YR4, momentarily thought, oh, maybe it's going to come our way a bit too close in a few years' time. Then you did measurements saying, we're okay, actually. So I'm really surprised that there's all this interest still in this object. But first of all, just take us back to those, you know, those first month or so of 2025.

3:15The asteroid was discovered in, I think, December 27th, 2024. So just 2024 YR4. Over the following weeks, as observations started to come in from a lot of the ground-based telescopes and the orbit was refined, it was realized that it was going to pass very close to Earth in 2032. And in fact, given the quality of the orbit at the time, there was this 1 to 3 % chance that it might hit the Earth. We asked JWST to get some time to make a measurement of the size and also get some more positions. As that was going through the system, Other folks were able to show that the Earth was in the clear. We went forward with the measurements to show what JWC could do for potentially hazardous asteroids, measured the size, got some more positions, and it turns out that while Earth was in the clear, the Moon was not, and the Moon still is not quite in the clear.

4:06There's a chance that it could hit the Moon in 2032, you're saying? Yes, there's a 4 % chance right now. Of course, most people were, I think, rightly focused on the hazard to the Earth. there was always a chance it was going to hit the moon. It just wasn't until we were cleared that people started to realize that.

4:21Roland Pease:I think people need to understand, this is an asteroid which is on a four-year orbit, is it, around the sun? Correct. And it goes close to the Earth's orbit sometimes, and at the moment it's sort of looping out beyond Mars or something? Yes, yes. It's on its way out to its furthest point from the sun, which is near the back end of the asteroid belt. Okay. Okay, when you're making these measurements, so this is a four-year orbit, you're just taking a few weeks' data and then you're trying to work out where it's going to be in four years. Yes, exactly, exactly. And so the key to these measurements often is how long it's been between the first measurement and your most recent measurement.

5:02And the longer that time span is, the better you can measure the orbit. What surprised me about this session, there's so many interesting talks here, was that actually you astronomers have not forgotten about it.

5:12Roland Pease:You still think it's a really interesting thought exercise, I guess, about how to defend ourselves against a more dangerous asteroid. Yes, there are some questions about, for instance, if it were to hit the moon, or if something were to hit the moon, if there are bases on the moon, if there are lunar mining rigs, is that part of planetary defense? And there are some people that feel like, look, planetary defense is just the Earth. Other people say, no, no, we have to, if we have activity on the moon, we have to kind of protect that as well. And if you have impacts on the moon that kick up a lot of debris, that debris could make it to the satellites orbiting the Earth, could make it to communications.

5:52Roland Pease:So you just have a projector all over the place. Yes, and some of it could make its way, some people are thinking, all the way to the communication satellites on Earth. I don't think people imagine that it would take them out, but it might be kind of short in their lifetime. Yeah. But I hadn't even thought about that. Yeah, there was a paper that recently came out by Paul Wiegert that started to look at that. If YR4 is found to be on an impact trajectory toward the moon, more sophisticated models will be done. But there is this philosophical question of, is defending the moon part of planetary defense?

6:26Roland Pease:Well, I mean, just on that moon impact bit, there was also this talk here that, in principle, if I had a pair of binoculars on the right day, in fact, not the right day, the right minute or the right second in 2032, there would be a small flash. Yeah, I think, I honestly think, I mean, that was great work by Patrick King. I think he is being appropriately conservative, but I think that undersells it. I think anyone who sees, is on the right side of the Earth and has a clear view would see this intensely bright flash. And it would be, I think, unprecedented in human history that you'd see a flash that is almost as bright as the full moon all by itself as a point against the moon.

7:03I mean, that would be fantastic. It would be.

7:05Roland Pease:I'm in favor of a moon impact myself. You know, honestly, a lot of basically everyone that we talk about it with also is. It's hard to hard to root against. I think the obviously people would need to make sure that the consequences on Earth are are minimal. I think every every indication is that we would be 100 percent safe. So I think that's why people are rooting, rooting to be able to see this. As I understand, even though it is now quite a long way away, you are hoping to get another view in the next few months with the JWST. So you do think you're going to get another chance to refine the orbit and know where it's going to be in five or seven years' time?

7:42Yes. When we got our last data in May, our most recent data in May, and realized that, okay, now there's this 4 % chance of hitting the moon, we immediately began to say, okay, when is it next observable with JWST? And we put in another proposal. They approved it. We were expecting to have two different observing shots at it in February a couple days apart.

8:03Roland Pease:At the moment, no one's seen it since last May. Correct. Am I right that basically you're going to be pointing the telescope at a patch of the sky where you expect it to be? And if it is there, you'll know then exactly that you'll get a much better idea of its future trajectory. Yes, the field of view for the camera is big enough that it will be in the field of view, no matter where it's going to be in 2032. The trick then will be the sort of thing where if it's in this pixel, it means it's going to pass by the moon on this side. If it's in this other pixel, it's going to pass the moon on that side.

8:36If it's in this pixel, maybe it's going to hit the moon still. So it's going to be at that sort of, you know, refining the orbit to that level. We know it well enough that we know we'll see it. So it's just a matter of where exactly, which pixel it's in, will tell us what the fate of that asteroid is. I want to know which pixel to bet on for a good moon, in fact.

8:56Roland Pease:Yeah, everything's gambling these days, isn't it? If Andy Rifkin and the James Webb Telescope succeed in updating 2024 YR4's trajectory this February, and it does turn out to be heading for the Moon, there'd be just a few years to prepare for a science bonanza as well as a public spectacle. The other celestial spectacle we've enjoyed this past year has been a series of fantastic auroral displays as solar storms have buffeted our outer atmosphere during an extended peak of solar activity. NASA has its Parker Solar Probe swooping into the sun's corona to study the turbulent region where the storms and the solar wind are born.

9:42Roland Pease:At AGU, we journalists were introduced to six other missions launched in the past year to give a broader picture of the solar-terrestrial connection, including PUNCH, a suite of all-sky telescopes to shine a light on the solar weather between the Sun and us. Hi, I'm Craig DeForest. I'm the principal investigator for the PUNCH mission, which stands for Polarimeter to Unify the Corona and Heliosphere. We are embedded in the outer reaches of a stellar atmosphere. The solar wind streaming off the Sun fills our solar system, and it really is a wind. And it's not just a stream of particles. There are flows, there's turbulence, there are storms.

10:24And for the first time, we can see those routinely with this mission. And it's because you're looking at as much of the sky as you can. That's right. With a telescope, you look in as much detail as you can at a small object in one place, usually far away. In this case, we're looking over a 90-degree field of view. That's from the horizon to the zenith, right? It's an immense field because we're looking at things that are right around us, that are happening here and now in our stellar environment.

10:52Roland Pease:I talked about the beauty of the aurorae, but what I see as a beautiful thing is actually a nightmare. Well, certainly for some satellites, but a GPS and things like that. So space weather is important to us largely because of our technology. And it affects us in a number of ways. The first is any long wire that's present on the Earth during a geomagnetic storm or a solar storm becomes an electrical generator. And so if you have equipment on either end of that wire, be it a telegraph, a phone line, a power transfer line, that equipment is going to be affected in a major way. And it has taken out power for whole cities before to have a solar storm impact.

11:34More immediately, if you've ever used a cell phone or GPS, those are affected by space weather. GPS becomes less effective. And if you're, for example, using a GPS tractor to plant seeds, you will miss your target. If you're trying to navigate a city, you might miss your turn. If you're trying to use a cell phone to transfer data or call a friend, you might lose contact unless the provider is able to prepare for the solar storm.

12:02Roland Pease:When I talk to solar physicists and people who worry about space weather, I'm always getting this sense that you know that an interruption has happened and then you're waiting and you're waiting until it arrives at Earth. And it seems to be a sense that we don't know what's actually going to happen until pretty late. Is that going to change with Punch? Absolutely. So in the video I showed, there's a dark circle that's the inner edge of our wide field image or field of view. That dark circle is also the outer limit of what we have been able to see routinely until now from the sun. And so space weather forecasting at the Earth is done using about one to two hours of data near the sun.

12:43We see an eruption come off the surface of the sun, travel through that field of view, and then it's gone. And we extrapolate for the next day to two days to determine whether it will hit us and when it will hit us. But with imagers like Punch, we can see them in three dimensions all the way across the solar system. It's like the innovation when we got weather satellites and we could suddenly see hurricanes moving across the face of the Earth instead of trying to forecast them from a couple of point measurements of wind.

13:16Roland Pease:What is the physics ultimately you're looking for? And I don't even know if, for a scientist as yourself, these eruptions are the most important thing, or whether there are sort of more gentle processes going on all the time that are actually more important in understanding this sort of solar Earth environment. We're interested in two major topics. One is the storms themselves, the transients that we see crossing the solar system. The other is what we call the ambient solar wind. It's sort of the background process, which is surprisingly poorly understood. We've known that the solar wind exists since the 1960s and have been studying it avidly since then.

13:55But there's a region right at the top of the solar corona and the bottom of the solar system, if you will, called the Alfvén zone that's just very poorly measured. It's very hard to measure. And there's a lot of unknown physics happening in that region. Parker Solar Probe is flying through there. They can sample what's happening in minute detail, but they can't measure the large-scale structure of it, the anatomy, if you will. But the two of you together... The two together can give us a complete picture.

14:24Roland Pease:A reminder, this is Discovery from the BBC World Service, focusing on the Earth's place in space.

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15:31Roland Pease:The other end of that solar terrestrial connection I've long understood is the Earth's ionosphere. high up above the atmosphere that we breathe and fly in, which wobbles and contorts as solar storms blast by, and which is where the aurorae lights up. But at the press conference, I learned of another, higher layer, the exosphere, extremely tenuous, and discovered during the first space race by George Carruthers, but otherwise largely ignored. It's transparent to incoming solar storms, but dampens auroral displays in complex ways, which another mission called Carruthers plans to reveal, as principal investigator Laura Waldrop told me.

16:14The analogy I like to use is that if you imagine Earth the size of an apple, the four layers that most people would think of as being the atmosphere, this would be the, including the stratosphere and the mesosphere and thermosphere and so on. They're about the thickness of the skin of that apple. And then the region we're interested in is the exosphere, which would start there and then extend all the way out. You know, again, with an apple-sized Earth, it would be much bigger than a beach ball. This region is huge. And it's because the constituents are atomic hydrogen. They're very lightweight.

16:48So they're able to float off. They're not as pulled down by Earth's gravity, the way molecular nitrogen is, for example, that's held close to the Earth.

16:56Roland Pease:I mean, so it's distinct still from what I might call interplanetary space then. It's a thing. Is this what I'm saying? Yes, very much so. Yeah. So, you know, more than 99 % of Earth's mass in its atmosphere is held below the mesosphere and below, essentially. So there's not a lot of mass there. But the fact that it glows when the sun shines on it, that's what allows us to see how big it is and how far away these atoms, they're still gravitationally bound to the Earth. They're just on these huge orbits, you know, slow, meandering, you know, days long. So in a way, they're orbiting a bit like a satellite.

17:35That's right, but atomic-sized satellite.

17:37Roland Pease:I mean, it's fantastic that it's there, but does it matter? It does. So that's one of the reasons that most people have never heard of it is because we didn't know how important it was until just the last decade or so. The reason it matters is it dissipates geomagnetic storm energy. When you take one of these hydrogen atoms and you bring it close to an ion, and these ions exist in Earth's near space environment, they're trapped by Earth's magnetic field, they get energized significantly when a geomagnetic storm is occurring, you know, such as a coronal mass ejection. So those are the aurora we see and so on?

18:14Sure, yes. That's one of the effects that they cause. So when one of these hydrogen atoms in Earth's exosphere, this extended cloud of upper atmosphere, gets close to one of these ions, what happens is the electron or the hydrogen atom jumps to the ion. Effectively, what you've done is you've switched the place. One is a cold hydrogen and one is a hot ion. And now you end up with a hot hydrogen and a cold ion. And so the hot hydrogen flies off.

18:41Roland Pease:It's taking energy out. It takes energy out of the system. That's correct. The density of the exosphere directly governs how fast the Earth recovers from these storms. And is that what you're going to look at? Because, I mean, there's all these other missions which are trying to work out how solar activity impacts the magnetosphere and the ionosphere and so on that we have, causes problems with the satellites. And you're looking at that sort of, it sounds like it's a damping mechanism, a shock mechanism. That's exactly what it is. And, you know, what's so interesting to me is that not only is this awareness of its significance relatively new, it's so new that right now our models of geomagnetic storm recovery just assume, for example, a spherically symmetric, time-invariant exosphere with some nominal density because nobody's been able to measure it.

19:29It's too big to measure. There have only ever been, from George Carruthers' first image of the exosphere 50 years ago. And that was from the moon, I think. And that was from the lunar surface. Since then, there's only been three others ever acquired. Single images. We have no idea how it changes over time, whether or not it has any spatial asymmetries, you know, that you can't tell from just a single image. That's the knowledge that Carruthers will provide. And eventually we'll end up with a three-dimensional, global-scale, time-varying model of the exosphere that we can then put into our storm forecasting models and do a much better job of storm recovery predictions.

20:06Roland Pease:Space scientist Lara Waldrop. Still in outer space but on more solid ground, there was a session at the 2025 AGU on lunar earthquakes, or should I call them moonquakes, and plans, very imminent, to put seismometers on the Moon to measure them. The first one will land next year. It will be a Chinese mission, Shange 7, with a seismometer on board. Next one will land in 27 from NASA on the far side of the Moon, and then we hope to have two seismometers on each of the Artemis 3 and Artemis 4 missions. So this is on the way. This is a good question. Why? Why? Because we still do not know what is the interior structure of the Moon and that is very important to understand the mechanics of the impact which occurred almost 4.5 billion years ago when a big planet hit the Earth.