Planetary Radio • Feb 21, 2024


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Scott Guzewich

Deputy Project Scientist for NASA’s OSIRIS-APEX Mission

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Bruce Betts

Chief Scientist / LightSail Program Manager for The Planetary Society

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Sarah Al-Ahmed

Planetary Radio Host and Producer for The Planetary Society

After NASA's OSIRIS-REx spacecraft's successful sample retrieval from asteroid Bennu, it's onto its next adventure as OSIRIS-APEX, the Apophis Explorer. Scott Guzewich, deputy project scientist for APEX, joins Planetary Radio to discuss the next steps for the mission as we count down to asteroid Apophis’ flyby of Earth in 2029. Then Bruce Betts, The Planetary Society’s chief scientist, pops in for What's Up and a look at other multi-world missions.

OSIRIS-APEX This computer illustration of NASA's OSIRIS-APEX, formerly OSIRIS-REx, shows the spacecraft preparing to fire its thrusters to stir up rocks and dust on the surface of asteroid Apophis. This process will allow scientists to analyze the composition of the material under the surface.Image: NASA's Goddard Space Flight Center Conceptual Image Lab

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Apophis in New York City
Apophis in New York City Asteroid Apophis would fit neatly inside The Battery, a park at the tip of Manhattan in New York City. This image shows an artist's concept of Apophis in the park to illustrate its size in comparison to the rest of the New York City skyline. The image is free for media and educational use with attribution.Image: The Planetary Society
Asteroid Bennu
Asteroid Bennu This mosaic of asteroid Bennu was created using 12 separate images captured in December 2018 by NASA's OSIRIS-REx spacecraft.Image: NASA/Goddard/University of Arizona/Edited by The Planetary Society


Sarah Al-Ahmed: OSIRIS-REx goes APEX, this week on Planetary Radio. I'm Sarah Al-Ahmed of The Planetary Society with more of the human adventure across our solar system and beyond. After NASA's OSIRIS-REx spacecraft successful sample return from Asteroid Bennu, it's onto its next adventure as OSIRIS-APEX, the Apophis Explorer. Scott Guzewich, deputy project scientist for APEX joins us today to discuss the next steps for the mission as we count down to the fly by of Asteroid Apophis in 2029. Then, Bruce Betts, our chief scientist, will pop in for WhatsApp and a look at some other multi world missions. If you love Planetary Radio and want to stay informed about the latest space discoveries, make sure you hit that subscribe button on your favorite podcasting platform. By subscribing you'll never miss an episode filled with new and awe-inspiring ways to know the cosmos and our place within it. On September 24th, 2023 NASA's OSIRIS-REx spacecraft made headlines by delivering precious samples of Asteroid Bennu to Earth. This marked the culmination of a seven-year journey that took the spacecraft over seven billion kilometers or four billion miles. Samples like these are vital to helping us understand the history and future of our solar system and life on our planet. Our solar system is filled with countless small worlds, asteroids and comets. They're made from the leftover material from the disc of gas and dust that formed the planets 4.5 billion years ago. We think some of these bodies may have crashed, landed on Earth, bringing water and organic materials with them, but to piece together this grand story, we need tangible pieces of the past and that's precisely what Asteroid Bennu offered us. There's more to this than just a curiosity about our solar system or our planet's formation. Billions of years ago, objects like Bennu crashing into Earth may have been an asset to the development of life. Nowadays, not so much. Asteroids pose a serious threat to all life on our planet. It's actually part of why The Planetary Society was founded. Defending Earth from objects like these is a crucial but complex task. And thanks to OSIRIS-REx, humanity is now far more prepared to tackle these celestial threats. Now, with the mission to Bennu completed, the OSIRIS-REx spacecraft is setting its sights on another intriguing target, the Asteroid Apophis. Apophis was discovered in 2004 by Roy Tucker, David Tholen and Fabrizio Bernardi at the Kitt Peak National Observatory in Arizona, USA. This asteroid is a stark reminder of the cosmic dangers lurking in our neighborhood. Planetary Society members should be really proud to know that Tucker was a 2002 grant recipient for our shoemaker near Earth object grant program. It funds advanced amateur astronomers who find, track and characterize near-Earth objects. We may have played a key role in defending our planet because Apophis is potentially Hazardous with a capital H. On Friday, April 13th, 2029, Apophis will pass as close as 30,600 kilometers as 19,000 miles above the Earth, but don't panic, it will not hit Earth for at least 100 years, like Asteroid Bennu and other potentially hazardous near Earth objects. With observations, science and ingenuity, we can absolutely make a plan to safeguard our world from future flybys, but it's going to take work. So say goodbye to OSIRIS-REx and say hello to the spacecraft's next adventure as OSIRIS-APEX, the Origins Spectral Interpretation Resource Identification Security Apophis Explorer. Joining us today is APEX Deputy Project Scientist Scott Guzewich from NASA's Goddard Space Flight Center. Scott has a passion for studying our neighboring planet, Mars and near Earth objects like Apophis. He's at the forefront of efforts to uncover the secrets of our solar system, but also, to protect our world from potential threats. Hi Scott.

Scott Guzewich: Hey, how are you?

Sarah Al-Ahmed: Doing really well. I'm really excited because I feel like OSIRIS-REx crushed it with the sample return. We finally got that, tag some container open and got those samples and now, the mission is on to its next phase. How did you get involved with OSIRIS-APEX?

Scott Guzewich: So I got involved, I guess it was probably almost three years ago now when we were still kind of in the proposal stage officially some of the OSIRIS-REx team members, we're kind of rotating off just to focus on the samples themselves and to focus on other work with the primary mission winding down. So there was an opportunity for new team members to join, for the extended mission exclusively, and this was sort of internal to Goddard, NASA's Goddard Space Flight Center here where I am and I applied and I was fortunate enough to be selected.

Sarah Al-Ahmed: Can you give us a brief overview of what OSIRIS-APEX is going to be doing now that it's switched from studying Bennu to Apophis?

Scott Guzewich: Yeah, so we're taking the OSIRIS-REx spacecraft, which is an incredibly capable spacecraft with really sophisticated instruments and we're sending it to a new asteroid and one that is going to put on quite a show in about five years time. Something that has not happened in a long time, probably for Earth's history, for human history and probably won't happen again for a long time hopefully, which is to have an asteroid of this size pass so incredibly close to Earth. And so this is a really unique opportunity and we're fortunate enough to be able to get the spacecraft there just after it passes by Earth.

Sarah Al-Ahmed: That's really cool and a great target because this is an opportunity that does not come by very often. From a scientific perspective, it's very exciting, but I'm sure a lot of people are going to be a little scared when this thing comes closer to Earth than our geosynchronous satellites.

Scott Guzewich: Yeah.

Sarah Al-Ahmed: How was the decision made to redirect this spacecraft to its new target?

Scott Guzewich: So after the primary mission, we were sort of on a set trajectory. The primary goal was to get that sample dropped off to Utah and so the spacecraft was targeted at Earth to make sure it got there and it did and that all went wonderfully. And of course, the spacecraft started with a certain amount of fuel on board, when it was launched and it used some of that fuel, not as much as it needed to ultimately, as was possibly the case. So it has only a certain amount of fuel left, and so once we kind of diverted past Earth, after dropping off the sample from Bennu, it sort of limited the options of what was available. We had this trajectory that we're starting on and we only have so much fuel, so what can we get to with those conditions? And it ended up being but there was really only two targets available and one of those was Apophis and because of the fascinating case of Apophis being so close to Earth in about five years time, it made that decision, I think really easy. So Apophis was the one that we chose to target.

Sarah Al-Ahmed: Well, I'm glad you decided to go after Apophis because this is going to be a really good moment to educate the public, not just about asteroids but also about planetary defense and why these studies are so important. We've kind of touched on this a little bit, but what makes Asteroid Apophis such an interesting target, particularly considering this flyby in 2029?

Scott Guzewich: Yeah, so of course, when Apophis was first discovered, there was a non-zero chance of it impacting Earth in five years time and if not then, then maybe kind of in the next time it approaches Earth later this century and next century. Further observations show that that was not really a threat, but it's still passing so incredibly close to Earth and it's this really unique opportunity. So it makes it really compelling to study and to see how an asteroid is going to be changed in potentially significant ways by this really close approach to Earth. I mean this is something that we think happens all the time through the history of the solar system, and this probably is a really important factor in how asteroids are distributed around the solar system. So seeing this happen in real time I think is going to be really, really informative.

Sarah Al-Ahmed: Understanding how these kinds of asteroids are changed by their pass by Earth is going to be really interesting, especially considering the type of asteroid. I'm sure different asteroids might react differently to that interaction.

Scott Guzewich: Yeah, for sure. I mean Apophis is what we call a stony asteroid or an S-type asteroid, which is the more common type kind of in the near inner solar system, near where we are, not the type that Bennu was, which is a little bit more common farther out. So yeah, it's sort of a very, in some ways, standard case of how these types of asteroids that are approaching Earth may pass by. So it should be a really good case study for that reason.

Sarah Al-Ahmed: As we're exploring these different asteroids, we are finding more and more interesting types of asteroids. I always assume that asteroids would be more rigid body type objects, but we are finding more and more of these rubble pile type asteroids.

Scott Guzewich: Yeah.

Sarah Al-Ahmed: Do we know whether or not Apophis is going to be more solid or is it going to be as flaky as Bennu was?

Scott Guzewich: We think Apophis is probably a rubble pile of some type. We don't quite know maybe how directly we'll compare to Bennu. Hopefully, we'll learn about that when we get there. Yeah, we don't really think from what we can tell from observations from the ground that Apophis is that sort of rigid solid body, one big giant stone. It's probably a lot of kind of loose pieces. We think maybe there's sort of two lobes kind of like, that maybe it was two asteroids, that kind of merged together or slowly combined or perhaps broke apart and re-coalesced at some point. It seems that this is a really common thing for asteroids. The closest analogy to Apophis of an asteroid that spacecraft have visited is probably Itokawa, which is the asteroid that was visited by the Japanese Hayabusa spacecraft and it was covered two lobes as well. So it might be somewhat of an analogy. Then, very recently, the Lucy spacecraft flew by Dinkinesh and it's moon, which we didn't even know it was there before. It turned out to be what's called a contact binary, which looks like two small asteroids just glued together by the force of their very weak gravity. So this seems to be a really common thing for asteroids, and so Apophis probably is going to be another example of that.

Sarah Al-Ahmed: That's really interesting because I was speaking with the Lucy team right after that Dinkinesh flyby, and it was really interesting to see that the two lobes on this moonlet were very similar in size. There's got to be some really interesting mechanics going on here that we really don't understand that are causing these moonlets to form in this way. So it'll be cool to get another closeup view of one of these contact binary objects.

Scott Guzewich: Yeah, seeing how they link together and merge and hopefully, by understanding Apophysis' gravity, we can get a sense, a bit of a picture of what the inside might look like, how that's distributed. So yeah, it would be very interesting.

Sarah Al-Ahmed: The spacecraft is actually going to rendezvous with this asteroid after it's fly by Earth, right?

Scott Guzewich: Yes, correct.

Sarah Al-Ahmed: So what kind of orbit are we going to go into around Apophis or is it going to be kind of like a trailing behind the asteroid kind of motion?

Scott Guzewich: It's a few different ways. I mean, these objects are so small that orbit is almost a loose term compared to orbiting Earth, orbiting Mars orbiting Jupiter or whatever the case may be, where it's almost like station keeping. We're just sort of moving around the asteroid in sort of a relative frame, but we're kind going to approach slowly and sort of study it from a bit of a distance before we get really close. And there's a few different kind of reasons for that. One is to get a good sense of its shape so we kind of know what it looks like and can kind of study and optimize our observations and our approach because we know precisely how Apophis is going to be shaped. And then to understand its gravity field, it's not a regular gravity field like Earth, a round object where that's big and massive. This is a lumpy, probably irregularly distributed rubble pile. So the gravity field is really wonky probably. So we want to study that. So there's a few phases where we kind of move in closely and study these different aspects of the mission and then, get into something that maybe is a little bit closer to what an orbit would be like, where you're just going around and letting the asteroid rotate or tumble beneath us.

Sarah Al-Ahmed: So these different types of motion near the asteroid, do these represent different mission phases that we'll be going through with this close approach?

Scott Guzewich: Yeah, we have heard names for these different phases and they last different durations of time, and probably those may be a little bit flexible based on what we discover as we approach and we'll refine that over time, over the next few years, but we kind of have it marked off a few months at a time for these different phases and extending over the course of our time around Apophis.

Sarah Al-Ahmed: Are there any other critical mission phases that you're most looking forward to?

Scott Guzewich: So of course, what I guess you could call in some ways the grand finale is going to be really exciting. So somewhat like the final sample collection on Bennu where the spacecraft got down super close to the surface and took the sample and then, blasted away. Of course, we can't sample Apophis. Our sample container is back here on Earth in the clean room full of Bennu rocks and Bennu dust, but we can still get down close in a very similar approach just without sampling, and we're going to approach the asteroid very slowly, get down really close, potentially just a few meters above the surface. And then, sort of blast our thrusters to kind of back up really fast and that thruster blast will sort of excavate or somehow disturb the surface. Of course, we don't know precisely how it will, right? On Bennu, it did a lot more than I think was expected, right? It almost created a series of craters around the sampling location because the thruster force was strong enough to really reshape the surface and blast a lot of material off of it. Is Apophis going to be something like that or is it going to be maybe a little bit more cohesive and just sort of expose a thin layer of the subsurface? We don't really know, but yeah, that's kind of why that's at the end of the mission, right? Because it'll be a little bit hairy.

Sarah Al-Ahmed: I remember seeing those videos of all these pieces of Bennu, just flying off everywhere and the spacecraft basically almost got eaten by the asteroid at some point. So it'll be really interesting to see how this works out. Are we concerned in any way that we might not understand the properties enough and the asteroid might just swallow the spacecraft whole?

Scott Guzewich: I'm not worried about it swallowing the spacecraft. We'll keep our distance above the surface, but I mean, I think there's certainly a chance at least that we get showered with some amount of dust or pebbles or whatever other material as happened on Bennu, right? Certainly there was a lot of stuff that flew back from Bennu and hit the spacecraft but didn't cause any damage that we're aware of. So certainly that's possible on Apophis as well. We won't know, but that's kind of why it's at the end of the mission because it's sort of accepting a bit more risk, so we'll just see how that goes.

Sarah Al-Ahmed: Yeah. I know I read as well that there's a potential that near the end of the mission we might actually try to land the spacecraft on this asteroid. Is that something that we're actually thinking of doing?

Scott Guzewich: It's not in any plan right now.

Sarah Al-Ahmed: Okay.

Scott Guzewich: I guess it's sort of in a ... that would be at the end of a second extended mission perhaps or something like that. So there's certainly things that I think we would probably want to do from, quote-unquote, orbit or from a standoff position before landing. I mean you could refine the measurement of something called the Yarkovsky force, basically the way the asteroid rotation is changing by the force of sunlight, we could probably get a bit more coverage of Apophis with our instruments, maybe even attempt a second one of these sort of thruster blast maneuvers. So there's some other options. I wouldn't say landing is excluded, but that's probably again the end of a second mission, second extended mission before we think about doing that.

Sarah Al-Ahmed: It'll be really interesting to get some more information on this Yarkovsky effect, but also on the other things that might be disturbing this asteroid's orbit and its spin rate, particularly as it passes by the Earth, the tidal interactions might do some really interesting things, but the spacecraft isn't actually going to be that close to Apophis during this flyby.

Scott Guzewich: Yeah.

Sarah Al-Ahmed: So how are we going to be able to figure out how the asteroid was changed during this event?

Scott Guzewich: Right now, kind of ahead of the Earth flyby, we have a really pretty precise measurement of Apophis' rotation just from ground-based, Earth ground-based observations, and we as in the Apex spacecraft, we'll be able to see Apophis ahead of when it passes by Earth, but only kind of as a point source, like a star in the distance. Even from that, and that's kind of how a lot of Earth ground-based observations work, you can get a pretty good measurement of the rotation rate and properties of the asteroid from that alone. So we'll be able to monitor that and so we'll be able to see how that changes ourselves. And of course, ground-based telescopes will be watching that very closely as well. Soon after the asteroid passes by Earth, I mean really soon, within a few hours, it's going to get too close to the sun for Earth ground-based telescopes to watch anymore. So it'll really be down to us, to APEX to watch this change as it happens somewhat in real time. And then, of course, when we really rendezvous with the spacecraft a bit later, we'll get a very, very precise measurement of its rotation rate and properties. You mentioned the Yarkovsky effect, so what's going to be exciting is we're going to be able to watch that force change over time, which is the first time we'll have a spacecraft really measure that change in that force, in that Yarkovsky forcing versus just sort of measure it in steady state.

Sarah Al-Ahmed: That'd be really interesting to know as well because I've heard people suggest that this Yarkovsky effect, the way that these things are spun up or interact with sunlight is part of why they might spin up over time and be flinging pieces off that might actually form these lobed moonlets. So this will be interesting not just for that, but also for understanding long-term projections of these things, orbits around the sun, especially for Apophis because we know it's not going to hit Earth for about 100 years, but that could get a little weird a few years out.

Scott Guzewich: Yeah. I mean, just like at Bennu, being kind of at the spacecraft and knowing its orbit around the sun, much more precisely, because we have a spacecraft there that's radioing with Earth, and so we get a really, really precise measurement of its orbit. That was able to kind of reduce the threat level that Bennu was projected to be. Bennu, as of all the asteroids we know right now, Bennu is still the most threatening even though it's a very, very low chance of impacting Earth anytime in the next 100 years. The same thing will happen for Apophis. We'll get a very, very precise measurement of its orbit and that'll help us project out into the future where it will be. The longer you go on the future, the more uncertain that knowledge can be, just because of the gravity effects of the planets and other asteroids and things like that, but it'll give us the best estimate of that by being at the asteroid itself.

Sarah Al-Ahmed: That's part of why I'm so grateful for all the asteroid hunters and all of these upcoming satellites, they're going to be helping us track NEOs and other objects because the more information we have about what's actually in our solar system, the easier it's going to be to do these gravity simulations to figure out where these things are going. Because right now, it's like the worst extreme of a million body problem.

Scott Guzewich: Exactly right. Yeah, for sure. We want to know where all of them are at, all the time, preferably.

Sarah Al-Ahmed: Yeah, because it's been a while since the Chicxulub impact, but we have situations like Tunguska and these things happen all the time, so it's really important that we know how these objects are up close, because this could potentially be make or break for us when we're trying to deflect one in the future.

Scott Guzewich: Sure. Yeah, I mean even just I think a month ago or so, there was a very small asteroid that was discovered, I think just a day or two before it burned up in the atmosphere above Germany. I think that was only maybe a small couple of meters size object, but we found it just a little bit right before ... a little bit before impact, but it was small enough it burned up in the atmosphere and was just a pretty impressive bolide from Germany.

Sarah Al-Ahmed: We also saw what happened in Russia during that-

Scott Guzewich: Sure.

Sarah Al-Ahmed: That was a terrifying situation, and it wasn't that big of an asteroid or a rock. It was maybe the size of a small-ish car or something. Thankfully, it exploded before it hit the ground, but even that can damage so much property and hurt so many people.

Scott Guzewich: Absolutely.

Sarah Al-Ahmed: Thankfully, we're going to get to know more about this one before it comes close to hurting us, which is great. We're going to be doing this thing where we stir up some of the regolith and see what's underneath. What is it that we can learn about the material properties of this asteroid through this process that we couldn't learn from just looking at it through our observations above it or through spectra?

Scott Guzewich: Yeah, it'll help us tell how cohesive it is. Is it a rubble pile even on the surface? And that's what Bennu appeared to be, which was not even expected despite studying Bennu for a year or more before the sampling. So we can really understand its material properties of the surface, which is really helpful from a planetary defense point of view for a kinetic impactor like the dart spacecraft was, understanding how that force would impact on the surface, how that force would then get distributed through an object and then, hopefully change its orbit if that was needed. So that's really informative and it also will excavate the subsurface material so we get a better understanding of what the material is like without being exposed to the vacuum of space. Being exposed to the really hard radiation of the sun and the galaxy, which kind of weathers the surface of asteroids. So it can throw us off a little bit perhaps about what the real chemical composition and structure of the asteroid is. So digging down a little bit gives us a bit more of a pristine sample, so to speak, of what the material is like and its chemical composition.

Sarah Al-Ahmed: You brought up the DART mission and it just occurs to me that I bet as with the DART mission where asteroid hunters all over the world just redirected all their telescopes to observe that thing. This is probably going to be one of those moments too, where-

Scott Guzewich: Absolutely, yeah.

Sarah Al-Ahmed: People around the world are going to be trying to observe this. Is there any way that we can coordinate people's observations or are there any plans to gather people's data after the fact?

Scott Guzewich: I think there are already plans and progress to coordinate a lot of the ground-based, both visible and infrared and radio telescopes to study Apophis as it approaches Earth. And as I said, the way the geometry works out to my understanding is it's really going to be on that approach to Earth, that ground-based telescopes can observe it because really soon afterwards it'll not be in a very favorable position because it'll be too close to the sun. Yeah, I think there's already a coordination happening to make that really a big effort and study Apophis as it gets close. And then, really it'll kind of be on us on APEX to study it afterwards. So I think that collaboration is going to be really fruitful.

Sarah Al-Ahmed: You've just made another great point about why it's so necessary for us to have these space-based ways of observing these objects because on Earth, we're very limited by what we can see that's near the sun, and we can have techniques in space that allow us to block out the sunlight so we can see things better, which is a great thing for protecting us, because you never know when something is going to come from the sun's direction right in our faces.

Scott Guzewich: Yeah, absolutely.

Sarah Al-Ahmed: Are there any big mysteries about Apophis that you're personally hoping that we can solve through this mission?

Scott Guzewich: I'm particularly interested in the planetary defense aspects of it. I think this is because Apophis is something that's approaching so close because it's a very common type of near Earth asteroid. In some ways, it's the most typical case of an asteroid that would threaten Earth perhaps in the future sometime. So I think kind of understanding those properties and kind of helping to provide a way or different ways that we could divert an asteroid that's threatening Earth, that's like Apophis in a lot of ways in the future. I think that's where I'm perfectly excited to see the data that we collect and what we can do to inform those strategies.

Sarah Al-Ahmed: I'm also really excited about being able to compare what's going on in this asteroid with a lot of the others because there are so few of them that we've been able to get close to. There are several over our history, some asteroids and comets, but we're finding with a lot of these, as with Bennu that they're very carbon rich maybe with a lot of interactions with water. We haven't got all the results from the samples from OSIRIS-REx yet, but this is sounding like it might have less interaction with water. It might be a very different kind of substance because it's so stony. So it'll be interesting to compare all these objects and where they're located and where they're from in our solar system to kind of get a better sense of how our solar system evolved.

Scott Guzewich: Yeah, absolutely. It seems we've visited now ... I can't remember off the top of my head, but a dozen or 20 some different small objects around the solar system and every single one seems to be different, even if they're sort of the same class as observed from Earth, but they all seem to be very unique and that makes it really fascinating. So even though we've been to asteroids of the same type of spacecraft before, I don't think there's any doubt that Apophis will surprise us in some way and be different than those other ones.

Sarah Al-Ahmed: They always surprise us though, don't they? I mean, as you said with Lucy, they did not see that moonlet coming, but I believe that mission is going to be visiting 11 different asteroids, most of them Trojan asteroids. We've got some other missions coming up from some other nations as well that are going to be visiting, so the number of these is about to jump up dramatically in the next decade or so, which is really exciting.

Scott Guzewich: Yeah, for sure, for sure.

Sarah Al-Ahmed: There's enough differences between these objects that I imagine you might need to be using the instruments on OSIRIS-REx, now OSIRIS-APEX a little differently. Are there any calibrations or any changes to the ways that you're going to be using these instruments?

Scott Guzewich: Yeah, a couple ways. So first of all, because when we sampled Bennu, we blasted a lot of dust up. So the spacecraft is kind of coated in a thin layer of Bennu and including on a lot of the instruments, and this was studied and already kind of calibrated to some degree while at Bennu because we knew, "Okay, we're kind of looking through Bennu material to see Bennu," so what does that do to change the data and how can we sort of subtract that? So we're just studying the true surface of the asteroid. So, that's something that's already ongoing and it will continue between now and when we arrive at Apophis, and that's something that is understood pretty well. The second aspect is Apophis is just a much brighter asteroid than Bennu. Bennu was very dark, this sort of cold black surface and Apophis both by the nature of where it is and just its material is going to be a lot brighter for the instrument. So all things being equal, we would want to look at Apophis for a shorter amount of time than we did at Bennu to get the same data. So imagining you're looking at a bright flashlight versus a dim one or something like that, but that's again, pretty easy to understand and to change, and actually, that's where having the Bennu dust covering the instruments actually helps a little bit because it attenuates that light a little bit as it comes into the instruments.

Sarah Al-Ahmed: That's a really cool thing to know, because I was concerned given how bright this object is, it might limit your observation time or you might accidentally burn out your CCD chips or something if it's too bright. I've seen people do that on telescopes and it's brutal.

Scott Guzewich: Yeah, we have a good idea of the changes that'll be necessary, but it'll be something again that's sort of probably fine-tuned a little bit once we get to Apophis.

Sarah Al-Ahmed: How is this mission going to tell us more about the relationship between stony asteroids and meteorites?

Scott Guzewich: Yeah, so we think that the S-type asteroids of which Apophis is a member of that sort of asteroid class is very similar to one of the most common meteorite types that we have on Earth. These what we call ordinary chondrites. And in fact, the samples that were returned from the Itokawa asteroid by the Hayabusa spacecraft, their study basically finds that the materials are very similar to ordinary. So we think Apophis is going to be ... yeah, Apophis is going to something similar to that, but again, when you observe something in space versus when it's passed through the atmosphere and it's been changed in some way by that experience, and again, everything's a little bit different and having it in space, there's that weathering effect I mentioned. So being able to observe a substance in its sort of natural environment, which is the vacuum of space versus on the ground and comparing those I think is valuable, and so, we'll be able to do that with APEX.

Sarah Al-Ahmed: We'll be right back with the rest of my interview with Scott Guzewich after the short break.

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Sarah Al-Ahmed: It's really important that we understand where these meteorites come from, much of the material that we have to actually gauge what space is like. It's literally just meteorites. It's very rare that we get a sample back from somewhere and can be very challenging to figure out where these meteorites come from, given that we don't know the chemical properties of things like Apophis.

Scott Guzewich: Exactly right. Yeah, for sure. I mean, there's a wide diversity in the meteorite catalog and then, we observe these asteroids in space and also see diversity of spectral types and the way that they absorb and reflect light in different portions of the spectrum. And so again, being able to kind of space truth that data is something that is one of our goals with the mission.

Sarah Al-Ahmed: If this is some kind of rubble pile stony asteroid, do we think that pieces might actually be disturbed off the surface during its flyby Earth? Could the tidal effects do that?

Scott Guzewich: It's possible perhaps for some of the smallest grains. We don't think half the asteroid will sort of rip off or anything like that, but one of the most surprising things at Bennu was that there were these small particles being ejected from the surface, and that's one of the things we will actually look for when we arrive at Apophis, both for the scientific interest in that, and also just for the spacecraft safety point of view. We don't want to be in range of these little pebbles being shot off the surface, if that's happening. When Apophis does the close flyby of Earth, we will be pretty far back, so there's no risk to us at that particular time. Yeah, studying that change will be really interesting and certainly the gravitational torque of Earth could disturb the surface in some way, and that's one of the things we're going to look for after we arrive.

Sarah Al-Ahmed: What other things do we think these tidal interactions might do to this asteroid?

Scott Guzewich: So two things that we know very well just from sort of modeling is that it should change the rotation rate of the asteroid and it's also going to change the orbit very significantly. So both Apophis and the Apex spacecraft are essentially doing a Earth gravity assist. Even a number of our spacecraft do all the time and they want to go out to Jupiter or Saturn or anywhere else. So Apophis' orbit is really going to change dramatically by this close passage to Earth and then again, its rotation rate, basically the length of its day is going to change as well. So both of those we know will happen very well, and again, we'll refine that with the spacecraft observations when we get there. And then, we're kind of looking to see is the surface modified in some way? Is there seismic shaking that has sort of smoothed out regions of the surface or disturbed boulders and that sort of thing? We're going to look as closely as possible to see if that has happened.

Sarah Al-Ahmed: How quickly does it currently spin?

Scott Guzewich: It is a pretty slow rotating, but it's what we call in technical terms non-principal axis rotation, so it's sort of tumbling. It doesn't have one fixed axis like Earth or Mars or even Bennu. It's sort of tumbling in this complicated rotation rate and that rate is going to change yet again when it passes by Earth.

Sarah Al-Ahmed: I'm wondering whether or not the asteroids' actual orientation as it flies by Earth is going to have any real effect on how the Earth will affect it, because if it's some kind of blurby double lobe object that's facing one lobe toward Earth versus sideways, that could make a difference.

Scott Guzewich: Yeah, I think there's some uncertainty obviously, because of yeah, that orientation aspect that you're mentioning as to how its final state will end up being. So yeah, it'll be really interesting to see.

Sarah Al-Ahmed: Right now, we have a solid idea of where this object's going to be for about the next 100 years. We can say it's not going to hit Earth, but do we have any idea of how much this flyby is going to help us refine that understanding of its long-term hazardous nature?

Scott Guzewich: I can't put a number on how much farther out with whatever accuracy we can predict once we get our data, but with what we know already, we think Apophis is going to pass through what's called a keyhole of some size and it's pretty small now, right? It's only maybe a few kilometers in size. We'll get that down to even finer precision when Apophis passes by both from the ground and from APEX observations, and then we'll be able to track it closely over the time that we are at the asteroid. So that will refine that prediction of the future orbit significantly, but I can't tell you how much farther out into the future we'll be able to predict. The close flybys to Earth I think again happen in another 50 or 60 years and then, another 50 or 60 years after that, I can't remember quite the cadence, but it's a ways out there still, afterwards

Sarah Al-Ahmed: I'll imagine what our capabilities will be then. 50 years ago we were just landing the first humans on the moon and look where we are now. I'm sure we'll figure out a lot in the meantime. How do you think that this mission and its study of Apophis is going to help us with our future planetary defense strategies?

Scott Guzewich: I think it's going to help a lot. I mean, I think there's a lot of possible defense mechanisms that have been proposed the Armageddon way, right? Send a nuke to it or the DART kinetic impact way, and there's been even some more sort of subtle ways that have been proposed utilizing the Yarkovsky effect, for example, almost painting an asteroid and having it reflect more light and doing a very kind of slow modification of its orbit over years and decades and centuries. So I think by having APEX there, understanding its physical properties both its sort of cohesiveness, its material properties, its bulk density, and then, just the physical chemistry and geology of the surface will inform a lot of what effect these different mechanisms these defense mechanisms have on an asteroid like Apophis. And again, like I said, apophysis is a probably pretty typical example of the type of asteroid that could be threatening to Earth at some point in the future. So I think that really, excellent data that we're going to collect is going to help tell us which one of those mechanisms might be better in different circumstances or how far in advance will we need to know which mechanism to choose. That type of information I think is going to be really valuable.

Sarah Al-Ahmed: I like the idea of trying to use the Yarkovsky effect to alter these asteroids' trajectory because we can do something like DART it, blow the thing up, but who knows how long it will be before that thing reforms. If we have these rubble pile objects just going around the solar system, we're going to need some extra data on how hard you have to explode it so the pieces don't come back together again. Whereas we could just send up a spacecraft to just spray paint one side of the asteroid.

Scott Guzewich: Yeah, yeah, there's a lot of different ways and it's the brute force method versus these really sort of gentler methods so to speak, that maybe just take a lot longer to have the effect, but when you're talking about an object that's maybe threatening us in decades or centuries time, you just need a tiny, tiny little nudge at the right point in time and that can be enough to kind of shift it off the path, so it's not going to be a threat, if it's in that last final days or weeks, then we're a little bit limited in our options, right? Because the spacecraft has a lot of inertia when it's coming on its orbit and to move it or disturb it in some way takes a lot more force.

Sarah Al-Ahmed: You mentioned earlier that cosmic rays and other radiation from space can alter the chemical makeup, but also, the coloration and stuff on the outside of these asteroids. Is there enough difference from the inside to the outside that we could perhaps use this Yarkovsky effect by simply brushing dust off the surface of one side?

Scott Guzewich: That's a really interesting idea. Yeah, I don't know. It's worth thinking about.

Sarah Al-Ahmed: Yeah, and it probably depends too on the chemical makeup, right? The other side might not be bright enough for ...

Scott Guzewich: One of the things we'll hopefully be able to figure out is how deep do you have to go? So the regolith on asteroids is the term people use as gardened, because these small micro meteorites and tiny little bits of other asteroids are constantly hitting and churning up the surface of any object in space, whether it's an asteroid or the moon or whatever. So how deep do you have to go to get a bit more pristine sample that's not been exposed to the vacuum and radiation of space and has not been affected by this micro meteorite gardening over whatever timescale you're speaking about. So hopefully, kind of doing that maneuver at the end with our thrusters will help tell us how deep do you have to get to really see that different coloration, that different makeup.

Sarah Al-Ahmed: That'd be cool to know too because who knows if these rubble pile objects are kind of churning over time, and that could be fascinating. You talked a little bit about this earlier that we might maybe at an extended, extended mission try to land on the surface of this thing, but there are some other activities that we could do after this November, 2030, end of missions. Are there any activities that you think we should definitely try to accomplish if we get that extended, extended mission?

Scott Guzewich: So the one thing that's really kind of low effort and we've talked about already was extending the observations to refine that estimate of the Yarkovsky effect. So it's one of those things where the longer you're sort of at the spacecraft and can observe that effect, it's a really, really tiny force. So the longer you can sort of integrate that observation, lets you refine that precision down to smaller and smaller numbers. So just adding a few months or a year to that observation, again, which really doesn't require much more than the spacecraft just sort of sitting next to the asteroid, would really kind of beat down that noise and let us have a much more precise estimate of the Yarkovsky effect on Apophis. Some other things we could do is just kind of continue to extend the coverage with our instruments, our cameras, and our spectrometers because Apophis is in this tumbling sort of orientation, it's not going to be like Bennu or even our observations of Earth or Mars where a spacecraft can just sit back and let the object or the planet rotate underneath of us and you're going to see the whole thing. Because Apophis is tumbling. There could be these really kind weird effects where we just keep missing one spot, right? Because where we are and where the asteroid rotates just don't link up properly. So if we're ... hopefully by November of 2030, like you said, we'll have a good understanding of, "Hey, we've got this coverage in this part of the asteroid, but we're missing this one place, or maybe we want to go back to this one place and get a bit more data." So those sort of options would also be kind of top of our list, I think.

Sarah Al-Ahmed: I'm flashing back to the European Space Agency's Rosetta mission and when they were looking at Comet 67P, Churyumov-Gerasimenko and they were trying to find the Philae lander for ages, that little lander was lost and then, right near the end of that mission, they finally found it on a place that they hadn't been able to observe before. Who knows what we're going to find in those last days and weeks of the mission, especially if there's places that are hidden from us. That's really cool. This is way far out, but if you in your dreams could come up with the final landing place or the final resting place for OSIRIS-APEX after all this time and space, after all that it's done, what do you hope would be the final resting place for the spacecraft?

Scott Guzewich: I mean, I think it'd be so cool if we could have the fuel to send it to another asteroid, but that won't be the case. We won't have enough fuel to really leave and go anywhere after our mission, so the spacecraft is going to end near or on Apophis one way or the other, right? It's just the effect of limited gas in the tank and gravity will ensure that. So we'll end up near Apophis one way or the other, and I think hopefully doing science as long as the spacecraft allows us would be my hope, and we'll see what happens. We've got to first extend the mission to accomplish and I think it'll be great and then, we will go from there afterwards.

Sarah Al-Ahmed: It'd be really funny if it managed to stay close enough to this asteroid that eventually someday when the asteroid swings back around, we can go snag OSIRIS-APEX out of there and-

Scott Guzewich: There you go. [inaudible 00:42:04].

Sarah Al-Ahmed: Yeah, it would deserve it. I was just speaking with Matt Golombek who worked on all the Mars exploration rovers recently and talking about the ways that we could potentially memorialize these things years and years, even maybe centuries in the future. These artifacts of human space exploration are all over our solar system and we're going to have to treat them lovingly. Otherwise, I bet space pirates are going to get them. Do you have any plans for the actual Apophis flyby? Are you going to fly out to Europe or Northern Africa to go see this thing or are you going to be in mission control?

Scott Guzewich: Yeah, I have not thought about that yet. So just for the listeners, I mean, unfortunately for those of us in North America and South America, the western hemisphere, it's going to be daytime when Apophis does its close flyby, so it's not going to be visible to us. It's only going to be visible in the nighttime sky to Europe, Africa, a good portion of Western Asia. So that's going to be the place to be if you want to look up in the sky and see Apophis flying by. I floated the idea of us going on a Nile cruise that day.

Sarah Al-Ahmed: Yeah.

Scott Guzewich: I don't think that has gone anywhere yet, but we'll see what happens. I think most likely I'll be back here at the office getting our data and preparing for rendezvous a few weeks and months later, but it's fun to dream.

Sarah Al-Ahmed: Yeah, I've been lucky enough to actually do that cruise on the Nile, and I'm just imagining what that would be like at night with this potentially terrifying, but also absolutely awe-inspiring event going on overhead. I hope you guys get to do that, because that would be amazing. Either way, I'm really looking forward to this and I am really hoping that this is going to be a moment that really makes us all reflect about our relationship with asteroids and near Earth objects because we can pretend that this isn't something we have to be concerned about. We can even laugh at movies like Don't Look Up, but it's only inevitable that one of these days, one of these things is going to come our way, and these are the missions that are going to really help save humanity.

Scott Guzewich: Yeah, absolutely. Yeah, I think it's going to be a really great opportunity to communicate with the public and understand our place on the solar system and that we live in an active and dynamic solar system, and it can be a dangerous place, but it also can be a really exciting place to visit and study.

Sarah Al-Ahmed: Well, thanks for helping us do this kind of study, Scott and for everything that this mission is going to do, it's going to be a few years until we get to give the really cool updates, but I'm looking forward to those images.

Scott Guzewich: Yeah, me too. Thanks, Sarah.

Sarah Al-Ahmed: When I was a child watching movies like Armageddon and Deep Impact, I was absolutely terrified that a rock from space was going to take away everything that we love. It makes me feel a lot safer knowing that people around the world are working together to track, characterize and potentially, deflect objects like Apophis. Life is scary enough without the threat of asteroids and comets taking us out like the non-avian dinosaurs. So I hope it helps you sleep at night. There are scientists around the world working to keep you and everyone you love safe. Now let's check in with Bruce Betts, our chief scientist for WhatsApp. Hey, Bruce.

Bruce Betts: Hello, Sarah. How are you today?

Sarah Al-Ahmed: I'm doing fine.

Bruce Betts: I don't know why I'm talking like this

Sarah Al-Ahmed: Part of the human experience, Bruce, being weird.

Bruce Betts: Wow.

Sarah Al-Ahmed: You're a weird one.

Bruce Betts: It's just part of the human experience. Don't be alarmed

Sarah Al-Ahmed: Speaking of things that are outside of the human experience, nice segue.

Bruce Betts: Segue.

Sarah Al-Ahmed: I think it's really, really cool that OSIRIS-REx is now OSIRIS-APEX. It's really smart that we already have this spacecraft. It's already done its thing, and then, they figured out a way to continue to use it and then, send it off to another world in this case, Apophis.

Bruce Betts: Yeah, those orbital dynamics people, they're crafty, to get things done, and this was great because we'll see what else, if anything, we can get to Apophis, but at least, we have this a very well-equipped spacecraft. We'll be checking it out shortly after encounter, and that's great. And it's what few other spacecraft have done, of course, lots of ... planned such things, but Stardust did this after encountering their comet, they brought back samples, dropped them in Utah because that's where we dropped samples and then, are they ... US anyway. Then, they went off and checked out the Deep Impact Missions Comet and saw the crater that was formed by that years before, and they redirected that after their main mission was done. So it's super cool and they have enough reserve fuel to be able to do this along with the crafty redirects. I'm a big fan of such clever use of existing functional hardware and teams that are together to get more use out of things since the tough parts, getting it up there and getting it working. So if you can keep it going, you're likely to keep it going. That's profound.

Sarah Al-Ahmed: That is profound. The fact that we can have spacecraft that can strategically maneuver to actually visit more than one world is really cool to me. The only one I can think of that was designed to go into orbit around multiple worlds purposefully was the Dawn Mission to Ceres and Vesta because it had those cool ion thrusters, but we've still managed to do things like New Horizons flying by Pluto and then out to Arrokoth. That's-

Bruce Betts: Yeah, another analog where they completed their primary mission and then thought, "Hey, what can we do with the fuel we've gotten?" Theirs was trickier because they had to tie up to a whole campaign to try to find something in the cone that they could hit with the fuel they had and they found the perfect snowman.

Sarah Al-Ahmed: Do you have a favorite multi world mission?

Bruce Betts: I love them all, just like my sons, I love them all. I mean, there aren't that many and clearly Voyager 1 and 2, although all flybys were ... I mean, Voyager 2 did the master stroke of doing four worlds and an orbital opportunity that only happens once every 175 years, I believe, to be able to use Gravity Assist from one to the next. So that alone is pretty awesome, and it still gives us our only closeup views of Uranus and Neptune. So that would be pretty high on the list. Obviously, Dawn doing clever orbiting of two different worlds and being able to study them in detail. You can go back to the first uses of gravity assistant going world to world. Mariner 10 actually was I believe first, and it did by Venus, did a little science, a little testing and then, gave us our first and only views for many decades of Mercury up close. So also, kind of a throwback favorite, and I'm sure I've missed a few and they're all awesome. Stardust, now that it's good. One planetary body is good, and two, great. Three, awesome. Four, we're looking at you Voyager 2. Actually, we've also got a larger, much larger number of things coming up with Lucy

Sarah Al-Ahmed: Lucy the Trojan asteroid?

Bruce Betts: Lucy is on its way, and they actually upped the number along the way, so they threw in another flyby that they did and found the cute little binary asteroid system.

Sarah Al-Ahmed: Dinkinesh.

Bruce Betts: So, I believe they're 11. Right, because they cleverly put on their website that now their mission goes up to 11, so they have 11 objects schedule.

Sarah Al-Ahmed: That's so cool. We got to dial it up to 11 on all these worlds, because there's so much left, so much left to explore.

Bruce Betts: Yeah, it goes up to 11.

Sarah Al-Ahmed: This is a bit weird and spicy question, but how do you think the world is going to respond when Apophis flies by Earth?

Bruce Betts: Well, this is not my expertise, but I'd say it's safe to assume, not the same. Not all few billion humans will respond the same. So you'll get people, including all of our listeners and members who are just super psyched on it and learning about it and drinking up as much science information. You'll have people who'll just go, "Whoa, that's so cool." That happens, and we're hoping, turn this into a lesson of, "Hey, that came close. We need to keep working so we can actually prevent impact of an asteroid doing major catastrophic damage," which is one of the big foci of the planetary society, is planetary defense and doing that. Then, you'll have the stuff that I will find annoying, and that is the people who don't believe it's going to hit Earth, the people who think it's a conspiracy, the people who think that I was the one who directed the asteroid, and it actually will change at the last minute and hit Earth.

Sarah Al-Ahmed: Now, I'm already mentally prepping myself for all the articles that are like Apophis is coming. Panic. It's Bruce Bett's fault.

Bruce Betts: My job is done.

Sarah Al-Ahmed: All right, what is our random space fact for this week?

Bruce Betts: Random space fact.

Sarah Al-Ahmed: Beautiful.

Bruce Betts: So sample return, moon astronauts, you probably wanted to get a feel for how much rock and dirt was brought back by all the Apollo missions from the moon. And of course, the way to do that is to turn to one of our favorite sports. So the mass of rocks and dirt brought back from the moon by astronauts in the Apollo program is about equivalent to 21 granite rocks or stones used in the sport of curling.

Sarah Al-Ahmed: That's more than I thought it would be.

Bruce Betts: Those things, they slide down the ice, which depending on where you live, are called rocks or stones. They range from 17 and a quarter to about 20 kilograms or 38, 44 pounds, and you'd have to throw together about 21 of those on average.

Sarah Al-Ahmed: That's so awesome. I had a really good time when I was working at Griffith Observatory. We had a piece of moon rock there. Of course, I didn't get to touch it or anything, because it was behind several layers of encased glass, but it's still cool being that close to a moon rock.

Bruce Betts: It's true. Way back in the dark ages when I did lab spectroscopy, I played with lunar soils. They're not actually soils. Any live person on Earth will tell you that. Lunar regolith, lunar dirt, and it's just pretty profoundly weird.

Sarah Al-Ahmed: Whenever I talk to geologists and stuff like that about rocks, it's usually some question about, did you lick the rock? I would not want to lick a moon rock. I'm sure the regolith is sharp.

Bruce Betts: Yeah, the particles are actually a problem for exploration because they are small and sharp and get in everything.

Sarah Al-Ahmed: Yeah, weird.

Bruce Betts: That's why I only have a geology minor on my PhD.

Sarah Al-Ahmed: You didn't lick enough rocks, Bruce. All right, let's take this out.

Bruce Betts: All right, everybody go up with air. Look up, look up, look up in the night sky, and think about dolphins as they move through, just beyond the surf at the beautiful beach, relaxing setting, and occasional as they blow air out and exhale. Think of that. Thank you. Good night.

Sarah Al-Ahmed: We've reached the end of this week's episode of Planetary Radio, but we'll be back next week to commemorate the 20th anniversary of the Red Rover Goes to Mars program. We'll be joined by two former student astronauts who experienced life-changing opportunities when they applied to the program. Love the show? You can get Planetary Radio T-shirts at, along with lots of other cool spacey merchandise. Help others discover the passion, beauty and joy of space science and exploration by leaving a review and a rating on platforms like Apple Podcasts and Spotify. Your feedback not only brightens our day, but helps other curious minds find their place in space through Planetary Radio. You can also send us your space, thoughts, questions, and poetry at our email, at [email protected], or if you're a Planetary Society member, leave a comment in the Planetary Radio space in our member community app. By the way, we've now hit 10,000 people in our member community. Please join us. Planetary Radio is produced by The Planetary Society in Pasadena, California, and is made possible members around the world who love asteroids way more than we fear them. You can join us and become a Planetary defender today at Mark Hilverda and Rae Paoletta are our associate producers. Andrew Lucas is our audio editor, Josh Doyle composed our theme, which is arranged and reformed by Pieter Schlosser and until next week, ad astra.