Gravity Assist: This Asteroid Is Metal, With Lindy Elkins-Tanton

What’s inside a planet? We can’t drill into the center of Earth. But with the upcoming Psyche mission, scientists will have the opportunity to visit a unique object in the asteroid belt called Psyche, which may be the exposed metallic core of a planetary body that stopped growing before it became a big planet like Earth. Dr. Lindy Elkins-Tanton describes her fascination with Psyche as well as the rock record here on Earth.  

Jim Green: A mission to the most mysterious asteroid in the asteroid belt is getting ready to fly. What is that object? And why is it important for us to explore it? 

Lindy Elkins-Tanton: It’s probably the first metallic object we humans will ever go visit. 

Jim Green:  Hi, I’m Jim Green. And this is a new season of Gravity Assist. We’re going to explore the inside workings of NASA in making these fabulous missions happen. 

Jim Green: I’m here with Dr. Lindy Elkins-Tanton and Lindy is the vice president of the Arizona State University’s interplanetary initiative. And she is also the principal investigator of the Psyche mission. Now, Psyche was selected in 2017 when I was head of planetary science.  

Lindy Elkins-Tanton:(laughs) 

Jim Green: Yes, that’s right. And it has a window to launch beginning in August 2022. So Lindy, welcome to Gravity Assist.  

Lindy Elkins-Tanton: Thank you so much, Jim. It’s really great to join you here.  

Jim Green: Well, you know, your background in geology and geochemistry is really fascinating, you know, because you specialized in the formation of terrestrial planets. So how does the rock record really give us insight into how our terrestrial planets form? 

Lindy Elkins-Tanton: The question is, what rock record are we talking about? Because for centuries, for millennia, we tried to understand our Earth by looking at the rock record we have here on Earth. And it turns out, it doesn’t go back far enough. Our oldest rocks on Earth are really about 4 billion years old.

But that is several hundred million years after the formation of our planets, it turns out, because of our weathering, and our plate tectonics and all the things that happen on the surface of the Earth, we don’t have the record of the very early formation of planets. So for that, we have to go to meteorites, the remnants, the shrapnel of planet-forming, and we need to leave the Earth and look at other planets and asteroids that have earlier surfaces.  

Jim Green: So indeed, we had to brin rocks back from the Moon and find out that they were older than the oldest rocks here on Earth to really tease that out. 

Lindy Elkins-Tanton: Exactly. That’s exactly right. And so then we were able to get some information about when the Moon formed, but then to find out when the majority of the Earth formed before that, and also when Mars formed really, really early.

And then the very earliest bodies in the solar system, the ones that I really love these days, planetesimals, meaning little planets, these little planets that formed in just the equivalent of if our solar system was 24 hours long, they would have formed in the first 10-20 seconds. 

And they’re really the materials like the raw materials like the eggs and the flour from which our big planets were formed. And so it turns out, there are stages and stages of earlier and earlier information we can get. So starting with the Moon earlier, then the, then the Earth’s surface that we have today, and then stepping backward in time even further. 

Jim Green: Well, you know, one of the things that we recognize as humans here on Earth, is that species come and go. Is there evidence of that in our rock record? 

Lindy Elkins-Tanton: Yes. All right. So here’s what our rock record on Earth is amazing at, is telling us what has happened on our Earth in the last 3 to 4 billion years. But even better at doing it in the last, say, 700 million years, just the most recent, say, one-fifth of our Earth’s age. 

And by looking at those rocks, we can see the record of species developing, you can see their fossils in the rocks, and then fossils disappear with younger rocks. And we know that species went extinct, and then new species show up. We can track the branching and the extinctions of life on Earth by looking at the rocks.  

Jim Green: Now, you did some fieldwork and collected samples when you were looking at extinction events.  

Lindy Elkins-Tanton: Mhmm.  

Jim Green: What was your most memorable field experience? 

Lindy Elkins-Tanton: Oh, my goodness, those experiences were unbelievable. I was just, I was just chatting about this with my husband last night. Amazingly, I spent five field seasons in Siberia in very remote places, finding samples of rocks with a big team, eight nations, 30 scientists, all different disciplines, because you need everybody at the table to answer these questions. Trying to understand if the world’s largest ever volcanic event, the Siberian flood basalts, caused the world’s largest ever extinction event, the end Permian extinction. And just when I say it like that, probably your response is, “Well, duh, like that seems like it would be pretty obvious that that could happen.”  

Lindy Elkins-Tanton: But exactly how the volcano could cause the extinction really wasn’t clear. So we spent lots and lots of time selecting those rocks and bringing them back and analyzing them and getting our answers. But so memorable. And I was just, what I was saying to my husband last night is that is that an adventure is what you call it when it wasn’t a tragedy. (laughs)  

Lindy Elkins-Tanton: And we had so many adventures going down these big Arctic rivers in like ridiculous inflatable boats and flying in planes that turned out the seats weren’t bolted to the plane, and being out, you know, hundreds of kilometers from any person, hundreds of kilometers from any railroad or road. And really, they were some of the most amazing experiences of my life.  

Jim Green: Well, I’m so glad you made it through that, so that it wasn’t a tragedy. But indeed wasn’t the end of the Permian a big tragedy? 

Lindy Elkins-Tanton: I guess it was a big tragedy. That was the biggest extinction that we have recorded. So far in Earth history. We lost, maybe above 95% of ocean species all went extinct, leaving just a few percent left. And at least 70% of land species went extinct. It was pretty close to the end of multicellular life on Earth for a while. Recovery came very quickly. But it was a very, very dramatic event.  

Lindy Elkins-Tanton:And one of the things that went extinct people often ask me, well, Wasn’t this the dinosaurs? No, not the dinosaurs. Before the dinosaurs. dinosaurs hadn’t even come to exist yet when this happened. So what exactly went extinct? Well, one thing that went extinct, which is one of my favorite animals of the past, was trilobites. They look a little bit like horseshoe crabs, but they’re quite different organisms. And there were so many of them, and they all went extinct. That was the end of trilobites. 

Jim Green: So in the last 500 million years, there’s been, what? Five extinctions? Where’s the Permian in that set? 

Lindy Elkins-Tanton: Right? The five big extinctions? Well, the famous dinosaur one is about 66 million years ago. And the Permian was 252 million years ago. And an interesting thing about it is that these five extinctions when you think about it always being an asteroid strike, which was certainly a big contributor with the dinosaurs. The other ones all seem to be more related to big volcanic eruptions, and in fact, the global climate change they caused by changing the chemistry of the atmosphere.  

Jim Green: Well, I tell you, you know, we are the first species on this planet that recognize we can become extinct.  

Lindy Elkins-Tanton: Yeah. 

Jim Green: We’re also the first species that actually could do something about it.  

Jim Green: Everyone associates, you know, the extinction of the dinosaurs with a major impact event.  

Lindy Elkins-Tanton: Mmm.  

Jim Green: You know, an asteroid that’s come to us. So this brings us to, I think, another really super topic that connects well, and that’s your Psyche mission.  

Lindy Elkins-Tanton: Psyche!  

Jim Green: Yeah. So what exactly is Psyche? And what do we know about it so far? 

Lindy Elkins-Tanton:Oh, my goodness. This is so fun to talk about. Psyche is the name of an asteroid that orbits out past Mars, in the outer main asteroid belt between Mars and Jupiter. And it is also the name of our robotic spacecraft that is going to visit this asteroid Psyche. 

Psyche was named after the goddess Psyche, by the man who discovered this asteroid back in the 1800s. Now, what do we know about Psyche? Surprisingly little, which makes it an incredibly fun kind of exploration to do. We’re so privileged as planetary explorers to send robots to Mars and learn more about Mars to send robots and soon humans to the Moon, again, to learn about the Moon.  

Lindy Elkins-Tanton: But what about going to a place where we’ve never been, and where we’ve actually never had a close-up photograph? We don’t even really know what it looks like. So we’re exploring a whole new kind of object where we don’t have any answers yet. What we know so far is that it’s unusually dense. It’s so dense that it can’t be just made of rock. It has to also be made of metal and so it makes it probably the first metallic object we humans will ever go visit.  

Jim Green: Well, Lindy, when was the first time you recognized that Psyche was so special, and that we needed to go out and visit it? 

Lindy Elkins-Tanton: You know, it was after we started planning the mission. And I think a lot of people think of that as backwards. You know, that the principal investigator and the team of people. At one point we were 800 people, by the ways, big teams. Think that you start with this idea.

I’m going to the Moon, I’m going to Mars, I’m going to Ceres, but actually, we started with the science question. People were interested in a paper that we’d written in a hypothesis we had about how planetesimals form and it took us a few months to decide that the very best place in our whole solar system we could go to learn about this science question was Psyche. So the science question came first. And then Psyche came second. 

Jim Green: Wow, that’s interesting. (laughs)  

Lindy Elkins-Tanton: Back in 2011, I published a paper with my friends Ben Weiss and Maria Zuber, thinking about what the structure of these little planetesimals these tiny planets could be. And it had been generally thought that they either melted entirely and had a rocky exterior and a metal core, or they didn’t melt at all. And we started working on could they melt partly, could they have a metal core inside, but then an unmelted lid on the outside, which matches a bunch of observations of meteorites. And this is the kind of thing, this cracks me up to talk about.  

Lindy Elkins-Tanton: Because, you know, it seems like a very, very niche topic to discuss, doesn’t it? But to those of us who are in planetary science, it was kind of a big idea. And so we presented at conference and we have people lined up at the microphones before we even started talking, it was standing room only there were hundreds of people crammed in the room. Because for the 200 people in the world who care about this, it was a really big and controversial idea. And so that’s what got us thinking, how do we look inside a planetesimal? And that’s what eventually led us to Psyche. 

Jim Green: So how does one get all this metal together? 

Lindy Elkins-Tanton: Yeah. Why is there a big chunk of metal out there? Well, I can tell you with some confidence, it’s not the Death Star because those who know the Death Star say the size is not correct. So how do we collect metal? Well, that is a natural process of making rocky planets.

It turns out that we know this from meteorites, that the most primitive material just like the building blocks of planets has little bits of metal and little bits of rock all mixed together really intimately like, centimeters, millimeter size grains.

And when you put all that material clumped together into a planetesimal, to return to that tiny planet idea, the little bodies that formed early, I mean things that are 10s or hundreds of kilometers in diameter, like the size of a state, maybe the size of Australia, they’re heated up by those early short-lived radioisotopes. 

Turns out there was enough of one of the aluminum 26 to actually melt those planetesimals. So when you melt the mixed-up rock and metal, in a body the size of Australia, the metal sinks to the middle because it’s denser.

And that’s how you get a big clump of metal. We’ve got a big clump of metal, our core, inside of the Earth. There’s one inside of the Moon, amazingly, inside of Mars inside of Mercury, inside of Venus, but we never, ever get to see them. It’s too hot. It’s too pressurized. Like, no matter what Jules Verne says, we are never going to go to the Earth’s core. So Psyche gives us, we think, a way to see the core of a planetesimal, maybe the only way humans will ever see a core if our ideas are right. 

Jim Green: But that tells us that the metal must be exposed for us to be able to… 

Lindy Elkins-Tanton: Mhmm.  

Jim Green: … when we get there see it on the surface. How did that happen? 

Lindy Elkins-Tanton: Oh, okay, so they’re so Psyche went through something that really went beyond an adventure a bit into a tragedy. And we think this is our best idea. We think that that’s Psyche was part of a planetesimal.

And it had that metalcore and that rocky outside, and that as it collided with other planetesimals rather than clumping up into bigger and bigger things like the Earth, instead, it got bashed into pieces. And so its rock was bashed off of it and some of its metal was exposed. Maybe it’s all-metal, it could be all metal, it could be half metal, we really are not sure. But we think it must be a fragment of a larger body that finally had its metal revealed on the surface through impacts that were destructive to it. 

Jim Green: Well, you know, this really makes sense to me. Because, you know, when I was in grade school a long time ago, I was told that the asteroid belt was where two planets collided, and here’s the debris. But isn’t it true that the asteroid belt is actually trying to become a planet… 

Lindy Elkins-Tanton: (laughs) Right. 

Jim Green: But Jupiter is not letting it. 

Lindy Elkins-Tanton: Yes. 

Jim Green: It’s pulling those pieces apart? 

Lindy Elkins-Tanton: That’s exactly right. And you know, that idea that there was a planet there has been around for centuries. And it turns out that if you clumped up everything in the asteroid belt, it would be just really tiny, it wouldn’t even make a planet even if you get it together. But Jupiter is the great disrupter, you’re right. Its gravity interacts with the asteroid belt objects and keeps them apart. And so in fact, there’s really lovely dynamical work done by theorists who can show that Jupiter actually starved Mars. It disrupted the material in that area so much that Mars could not grow beyond its you know, we don’t like Mars to feel bad, but small size. 

Lindy Elkins-Tanton: And this idea, you know, what is the asteroid belt and where is the planet that belongs there is something that has been around for centuries and, and in the early 1800s.

This is just my favorite story about asteroids, that… that Franz Xavier von Zach in Germany, organized all these astronomers all over Europe into a team to find the missing planet. And they started searching because at that time, they didn’t know about asteroids they hadn’t been seen yet.

And so they just saw a big blank space between Mars and Jupiter. And the reason I love this story twofold. One is that their nickname was Die Himmels Polizei, the the celestial police is how people translate it, they were going to set the heavens straight, they were going to set them to order by finding the missing planet. And then they started looking at one of the things they found was Psyche. So thank them for looking. 

Jim Green:Yeah, in fact, the first set of things they found like Ceres and Vesta,  

Lindy Elkins-Tanton:Exactly. 

Jim Green:They thought they were planets. 

Lindy Elkins-Tanton: Right? Right. Maybe they’re just really far away. And they’re big planets. But now they’re just little sparks of light. So, asteroids. 

Jim Green: Right. And that led them to believe that there’s a field of smaller bodies.  

Lindy Elkins-Tanton: Mhmm.  

Jim Green: Well, how many of these smaller bodies do we expect out there? In the asteroid belt? 

Lindy Elkins-Tanton: Well, I think that we’re, we’re expecting in excess of a million. And as much as as those numbers just boggle us to think of, a million objects orbiting there, the fact is, it’s really mostly empty space. Space is really, really empty. Even finding another asteroid that we could look at from a distance as we sent our spacecraft to Psyche, it turns out there was just nothing nearby, even with, you know, a million objects, it’s just space. 

Jim Green: So we really have to reorient ourselves from the Millennium Falcon flying through the asteroid belt, to then be able to realize if we can get to one, that’s the that’s the goal of the Psyche mission. 

Lindy Elkins-Tanton: Exactly, exactly. And that Millennium Falcon image is just in our brains permanently. It’s very hard to get away from that. 

Jim Green: Well, what instruments are onboard the spacecraft and really be able to look at the body and tease out what it’s all about? 

Lindy Elkins-Tanton: It was such an interesting challenge to go through the the process of trying to figure out what instruments would allow us to measure what we needed to measure no matter what Psyche ended up being. Because there’s a there’s a wide range of possibilities.

Here, we have our favorite hypothesis, but there’s other ideas. So in the end, what we settled on was, first of all, magnetometers on Psyche, because if it was a core, and if that core made what’s called a dynamo, and created a magnetic field, like we have on Earth, created by our core, then we should be able to measure its record. Won’t have one now.  

Lindy Elkins-Tanton: Very cold, nothing happening on Psyche now. But the record of the past. We could use the magnetometers for that. Then we’re sending imagers, of course, because we always want to know, what does it look like. And the imagers also allow us to create a topographic map of all the hills and faults and impact craters and all the things we hope to see there. And again, we don’t really know.

And then our third instrument is what’s called a gamma-ray and neutron spectrometer. And this is an instrument that I did not have a lot of experience with before this planning process started. And I think it is just such a miracle of ingenuity. And should I take us just a second and describe what this instrument does, because it’s so incredible?  

Lindy Elkins-Tanton: So there are these things called galactic cosmic rays. And we think they are created in the middle of galaxies in their black holes, and they go shooting out through all the star systems in between galaxies and two other galaxies. They’re these little tiny, super energetic particles. And sometimes they hit a body that has no air on it, like Psyche. They hit the surface of this airless body.

And what they do with the galactic cosmic ray, when it hits Psyche, it hits an atom on Psyche’s surface. And that atom then gives off to other pieces of radiation: a neutron and another gamma-ray. And we have our spacecraft orbiting fortuitously around Psyche with a special crystal to intercept the gamma-ray and a special tube to intercept the neutron. 

Lindy Elkins-Tanton: And it turns out that those gamma rays have exactly the energy of the atom that gave them off. And so by counting those gamma rays, produced by cosmic rays that come possibly from other galaxies We are able to count up and know exactly what the atoms are that make up the surface of Psyche and, and figure out its composition. And to me, that is just astonishing innovation.  

Jim Green:That’s a great instrument. Now, Psyche is at least a 30-light minute trip away from Earth. So that means it’s very far out there. So we got to have enough fuel one propellant to get there. What are some of the really creative ideas that came out that allowed Psyche to get out to the asteroid Psyche, way out in the asteroid belt? 

Lindy Elkins-Tanton: Right, right, so, so very often of spacecraft are trying to go far away from the Earth, they might have a radioisotope power source. But that was not an option for us. Too expensive, not something we were going to create for this mission. And so, and so how are we going to do it? How are we going to be able to have enough propellant to get all the way out there? Well, we rely on this beautiful, I sort of think of it is as like as like the sustainability option for space travel. It’s called solar electric propulsion.  

Lindy Elkins-Tanton: So what we’re doing is we’re flying huge solar arrays that will unfold from the spacecraft after launch into things that look like huge wings. They would cover a whole singles tennis court, 20-odd meters across. And at Earth, these solar panels will create 20 kilowatts of energy, out of Psyche about a 10th of that.

And so what do we do with all that power, what we do is we power little thrusters called Hall thrusters. And the propellant that we use is the noble gas xenon. We’re going to bring over 1000 kilograms of xenon with us in a big tank. And we take that electricity from the solar arrays, and we ionize individuals, xenon atoms, we pull an electron off of them.

And then we shoot them out the back of the spacecraft through these Hall thrusters in what’s called a little potential, in other words, that the charge of the atom will drive itself out of the Hall thruster, and that little tiny atom gives a little tiny push to the spacecraft. And so we do that over and over again.

And we’re going to go very slowly and at very high efficiency, all the way out to Psyche using solar electric propulsion, and the noble gas, xenon. And I just want to tag on to this, that this is all made possible for this mission by our fantastic industry partner Maxar because they have vast experience in building just this kind of power system and chassis for Earth orbiters. Only this time, we’re going to send it all the way out to Psyche. 

Jim Green:Well, I also know that you’re going to use another technique, which is flying by Mars. 

Lindy Elkins-Tanton: Oh! 

Jim Green:And now why do you do that? 

Lindy Elkins-Tanton: We do it so that we can shout out “gravity assist!” (laughs) 

Jim Green: Absolutely. (laughs) 

Lindy Elkins-Tanton: That’s the whole point here. Yes. So great. So Mars is in fact going to give us a gravity assist. You know, people always say, and it’s a beautiful metaphor, if you’re holding hands and ice skating and one person stops and the other person slings around them and then speeds up tremendously — that’s what Mars is going to do for us. And so we’re going to fly by Mars, and it’s going to give us a gravity assist and send us on our way to Psyche. 

Jim Green: Yeah, that’s fantastic. Well, I can’t wait for the launch window to open up. August of 2022. Oh, my gosh. 

Lindy Elkins-Tanton: I know that it’s over a year away. But it feels like tomorrow. And you know better than I do, how this goes every single day as scheduled. You know, hour by hour between now and launch. There’s so much still to do. But we’re going strong. 

Jim Green: Well, we know you’re in for a whole series of surprises when you get there. But what’s the top thing that you would like to learn about Psyche?  

Lindy Elkins-Tanton: Well, our number one science objective is to figure out whether or not Psyche is a core. Is it part of that metal middle of a planetesimal? But here’s my secret favorite thing. My secret favorite thing is that all our ideas about Psyche are wrong. And when we get there, we’re going to discover a kind of material and a kind of body that we had not anticipated. And it’ll teach us something entirely new about how planets are formed. That’s my secret wish. 

Jim Green: You know, I think you’ll be right. Well, Lindy, you know, I always like to ask my guests to tell me what that event or person, place ,or thing that got them so excited about being the scientists they are today. And I call that a gravity assist, of course. So Lindy, what was your gravity assist? 

Lindy Elkins-Tanton: Oh, this is a really lovely question to answer and I don’t, I’m going to tell you right now I have a little bit of a narrative about it. I don’t have just a single answer. So many people have told me their gravity assist was seeing Saturn or seeing Jupiter through a telescope when they were 10, 11, 12 years old. Like, so formative. I saw Saturn when I was 10 I think and I still wanted to be a veterinarian.  

Lindy Elkins-Tanton: So when did my gravity assist come along? It wasn’t until much later in my life when I realized that the thing that transformed my own life and gave me huge impetus to keep working in this direction was the ability to work on a team of people, where the thing we were producing was so much more than what any one of us could do alone, and where we had a big aspirational goal.

And those things together add up to just making me want to jump out of bed in the morning and get the thing done and feel like we really have meaning and purpose. And it’s hard to imagine, for me, a bigger and more motivating goal than expanding human knowledge by visiting a place we’ve never been. And so that’s, that’s my gravity assist, is working with the team after I realized in my 20s that that was what really made me happy. 

Jim Green:Well, Lindy, thanks so much for joining me in discussing this fantastic topic, the asteroid belt, and the history of the Earth and our rock record and what we can learn by visiting one of the most mysterious asteroids in the asteroid belt. Thank you. 

Lindy Elkins-Tanton: Jim, thank you so much.  

Jim Green:You’re very welcome. Well join me next time as we continue our journey to look under the hood at NASA and see how we do what we do. I’m Jim Green, and this is your Gravity Assist.  

Credits

Lead producer: Elizabeth Landau

Audio engineer: Manny Cooper