1 00:00:04,440 --> 00:00:07,520 This is Geology Bites with Oliver Strimpel. 2 00:00:08,960 --> 00:00:12,840 In several previous episodes of Geology Bites, such as the ones 3 00:00:12,840 --> 00:00:16,079 with Sarah Russell and Harold Connolly, I've talked to 4 00:00:16,079 --> 00:00:19,120 researchers seeking to understand the early history of 5 00:00:19,120 --> 00:00:22,440 the solar system through studies of asteroids. 6 00:00:23,520 --> 00:00:26,880 Asteroids are especially valuable in this regard because 7 00:00:26,880 --> 00:00:29,760 they preserve a record of the early solar system. 8 00:00:31,560 --> 00:00:34,080 There are many unanswered questions about the early 9 00:00:34,080 --> 00:00:38,680 history of the solar system. In particular, did the giant 10 00:00:38,680 --> 00:00:41,560 planets form roughly at the distance from the sun they 11 00:00:41,560 --> 00:00:45,680 presently occupy? Or, as some theories predict, 12 00:00:46,120 --> 00:00:48,880 have they migrated to their present positions after they 13 00:00:48,880 --> 00:00:51,840 formed? The discovery of other solar 14 00:00:51,840 --> 00:00:55,560 systems with radically different configurations of planets has 15 00:00:55,560 --> 00:00:58,840 made this question more pressing, since it appears that 16 00:00:58,840 --> 00:01:02,000 the configuration of the solar system is highly atypical. 17 00:01:03,040 --> 00:01:06,960 It turns out that certain asteroids called the Trojans 18 00:01:07,280 --> 00:01:11,080 that are locked into the same orbit as Jupiter offer us the 19 00:01:11,080 --> 00:01:14,360 best opportunity to discriminate between the various models of 20 00:01:14,360 --> 00:01:18,240 solar system evolution. And that is why a spacecraft 21 00:01:18,240 --> 00:01:22,200 called Lucy is now well on its way to a rendezvous with these 22 00:01:22,200 --> 00:01:26,440 asteroids. Hal Leveson is principal 23 00:01:26,440 --> 00:01:28,480 investigator of the Lucy mission. 24 00:01:29,520 --> 00:01:33,040 He studies the dynamics of astronomical objects and in 25 00:01:33,040 --> 00:01:36,480 particular the formation and long term behaviour of solar 26 00:01:36,480 --> 00:01:39,280 system bodies. He is one of the original 27 00:01:39,280 --> 00:01:43,560 proponents of the Nice model, a scenario that proposes the 28 00:01:43,560 --> 00:01:47,080 migration of the giant planets from an initial compact 29 00:01:47,080 --> 00:01:50,520 configuration closer to the Sun to their present positions. 30 00:01:51,520 --> 00:01:54,800 He is chief scientist in the Department of Space Sciences at 31 00:01:54,800 --> 00:01:57,800 the Southwest Research Institute in Boulder, Co. 32 00:01:58,920 --> 00:02:01,400 Hal Leveson, welcome to Geology Bites. 33 00:02:01,640 --> 00:02:04,040 Thanks for inviting me, it's a pleasure to be here. 34 00:02:04,560 --> 00:02:08,199 Can you explain how it is that the small bodies in the solar 35 00:02:08,199 --> 00:02:11,960 system have survived as fossils of the very early solar system? 36 00:02:12,720 --> 00:02:16,040 Let's take a step back. Our understanding of how planets 37 00:02:16,040 --> 00:02:21,240 form is that originally the solar system consisted of gas 38 00:02:21,240 --> 00:02:26,960 and a bunch of dust, and it does slowly build up over time to 39 00:02:26,960 --> 00:02:32,440 make first pebbles, then rocks, the mountains, then asteroid 40 00:02:32,440 --> 00:02:35,560 sides, things all the way up to the planets. 41 00:02:36,000 --> 00:02:40,840 So the planets are made of these things, but planet formation is 42 00:02:40,840 --> 00:02:45,560 not 100% efficient. And as a result, we have a 43 00:02:45,600 --> 00:02:49,440 population of small bodies that are basically leftovers, the 44 00:02:49,440 --> 00:02:53,400 dregs of planet formation. And that's what asteroids and 45 00:02:53,400 --> 00:02:56,080 comets are. And the reason why they're 46 00:02:56,080 --> 00:02:59,840 interesting for somebody like me who's interested in how the 47 00:02:59,840 --> 00:03:05,040 solar system formed is that while planets have done their 48 00:03:05,040 --> 00:03:08,040 own thing, they have internal geology or they have weather 49 00:03:08,040 --> 00:03:10,800 that have changed what the planets look like. 50 00:03:11,120 --> 00:03:16,360 These small bodies are basically fossilized at the time that the 51 00:03:16,360 --> 00:03:18,360 planets formed. And so if you really want to 52 00:03:18,360 --> 00:03:21,200 understand how the planets formed, these are the best 53 00:03:21,200 --> 00:03:25,760 places to go, I think. OK, so the Trojans are 54 00:03:25,760 --> 00:03:29,520 asteroids, but they're not in the asteroid belt that lies 55 00:03:29,520 --> 00:03:33,400 between Mars and Jupiter, but further out and actually in the 56 00:03:33,400 --> 00:03:37,160 same orbit as Jupiter, with one group of Trojans leading Jupiter 57 00:03:37,160 --> 00:03:41,440 in its orbit by about 60° and the other trailing Jupiter by 58 00:03:41,440 --> 00:03:45,200 the same amount. How come they're herded into 59 00:03:45,200 --> 00:03:49,240 that space, and how do they compare to the asteroid belt in 60 00:03:49,240 --> 00:03:51,600 terms of their numbers and the volume they occupy? 61 00:03:52,320 --> 00:03:58,520 So Trojans are in the special balance points gravitationally. 62 00:03:58,960 --> 00:04:01,320 And the way to think about it, although it's technically not 63 00:04:01,320 --> 00:04:06,760 really right, is that if you put an object in front of a planet 64 00:04:06,760 --> 00:04:11,720 by 60° in its orbit, the gravitational attraction of the 65 00:04:11,720 --> 00:04:16,399 planet and the sun plus the centripetal force of the orbit 66 00:04:17,000 --> 00:04:21,560 balances out in such a way that the object put there stays there 67 00:04:21,560 --> 00:04:24,040 forever. And that's how come we know 68 00:04:24,040 --> 00:04:27,360 these objects are actually very ancient objects. 69 00:04:27,680 --> 00:04:32,280 And what makes them interesting from a solar system formation 70 00:04:32,280 --> 00:04:36,920 point of view is they're leftover from the outer planets. 71 00:04:37,080 --> 00:04:41,920 While we think most asteroids in the asteroid belt are leftovers 72 00:04:41,920 --> 00:04:46,600 from the rocky terrestrial planets, these guys come from 73 00:04:46,600 --> 00:04:48,920 the outer solar system and that's what makes them 74 00:04:48,920 --> 00:04:51,720 interesting. Now the population is 75 00:04:52,240 --> 00:04:56,480 comparable, somewhat less, but comparable to what is in the 76 00:04:56,480 --> 00:05:00,640 main asteroid belt today. And they occupy actually a 77 00:05:00,640 --> 00:05:04,320 really small region of orbital element space. 78 00:05:04,520 --> 00:05:08,200 They're actually more concentrated in a way than the 79 00:05:08,200 --> 00:05:12,080 main asteroid belt is. I said in the introduction that 80 00:05:12,360 --> 00:05:16,040 the Trojans offer us the best opportunity to distinguish the 81 00:05:16,040 --> 00:05:20,000 various models of solar system evolution and in particular, to 82 00:05:20,000 --> 00:05:23,680 test the NICE model. You were one of the original 83 00:05:23,680 --> 00:05:28,680 proponents of that model. What motivated the development 84 00:05:28,680 --> 00:05:32,480 of this model and can you explain how the solar system 85 00:05:32,680 --> 00:05:35,320 evolved according to the model? Sure. 86 00:05:35,320 --> 00:05:37,600 What motivated me to work on the model? 87 00:05:37,600 --> 00:05:41,240 And there's a lot of reasons why people started questioning 88 00:05:41,240 --> 00:05:45,360 whether the planets that we see in the outer solar system today 89 00:05:45,720 --> 00:05:48,800 formed where we see them. So there's a lot going on there. 90 00:05:49,000 --> 00:05:53,160 But the thing that motivated me is the realization that Uranus 91 00:05:53,160 --> 00:05:57,080 and Neptune, which are the ice giants in the outer parts of the 92 00:05:57,080 --> 00:06:00,920 solar system, could not have formed where we see them today 93 00:06:00,920 --> 00:06:05,200 after Jupiter and Saturn formed. So the classical thinking of 94 00:06:05,200 --> 00:06:09,680 planet formation is that the planets in the outer part of the 95 00:06:09,680 --> 00:06:14,360 solar system formed more slowly because the orbital periods are 96 00:06:14,360 --> 00:06:16,800 longer. So just things just take a lot 97 00:06:16,800 --> 00:06:21,560 longer to get going out there. And we realized that that's not 98 00:06:21,560 --> 00:06:23,520 true. Uranus and Neptune can't have 99 00:06:23,520 --> 00:06:25,640 formed after Jupiter. Saturn formed. 100 00:06:26,080 --> 00:06:31,560 So we looked at a way to allow the four giant planets to form 101 00:06:31,560 --> 00:06:36,080 at the same time. And that led us to the idea that 102 00:06:36,080 --> 00:06:40,160 the solar system formed in a much more compact configuration, 103 00:06:40,720 --> 00:06:45,240 and that some dynamical event moved the planets around to 104 00:06:45,240 --> 00:06:49,840 where we see them today. So the NICE model basically 105 00:06:49,840 --> 00:06:54,520 hypothesizes that all the four giant planets formed in a much 106 00:06:54,520 --> 00:07:01,520 more compact configuration, let's say within 12/13/14 107 00:07:01,760 --> 00:07:06,600 astronomical units from the sun. So an astronomical unit is the 108 00:07:06,600 --> 00:07:09,280 average distance between the Earth and the sun. 109 00:07:09,760 --> 00:07:13,000 Currently Neptune, which is the outermost planet, is at 30 110 00:07:13,000 --> 00:07:16,200 astronomical units. So we're hypothesizing that the 111 00:07:16,200 --> 00:07:20,320 solar system of roughly half as big and then some dynamical 112 00:07:20,320 --> 00:07:25,320 event threw them out. And what we think happened is 113 00:07:25,320 --> 00:07:31,000 that this compact planetary system was surrounded by a disk 114 00:07:31,280 --> 00:07:35,080 of small bodies that are extended just outside the orbit 115 00:07:35,120 --> 00:07:42,440 of the ice giants to around 30 AU and contain basically 2530 116 00:07:42,440 --> 00:07:46,440 Earth masses versus asteroids. And if you put that all on the 117 00:07:46,440 --> 00:07:49,840 computer and you integrate the orbits, you find that the system 118 00:07:49,840 --> 00:07:52,520 is not stable for long periods of time. 119 00:07:52,520 --> 00:07:56,640 And eventually what happens is that the orbits of the ice 120 00:07:56,640 --> 00:08:00,280 giants go unstable. They cross one another, they 121 00:08:00,280 --> 00:08:03,080 start gravitationally interacting with Jupiter and 122 00:08:03,080 --> 00:08:06,400 Saturn, which throws them out into this disk. 123 00:08:06,800 --> 00:08:10,640 The disk just goes Kapluy, not because of physical collisions, 124 00:08:10,640 --> 00:08:14,040 but because of the gravitational interaction with the planets. 125 00:08:14,400 --> 00:08:17,640 And that disk of material gets scattered all over the place. 126 00:08:18,120 --> 00:08:22,120 Most of it ends up in interstellar space, but some of 127 00:08:22,120 --> 00:08:25,000 it ends up trapped in the Trojans. 128 00:08:26,000 --> 00:08:28,200 And the gravitational interaction between the ice 129 00:08:28,200 --> 00:08:32,520 giants and that disk evolves them onto the orbits we see 130 00:08:32,520 --> 00:08:34,440 today. And actually, we can match the 131 00:08:34,440 --> 00:08:38,360 orbits of the giant planets in our numerical simulations. 132 00:08:39,000 --> 00:08:42,480 One of the premises I didn't quite follow is that you said we 133 00:08:42,480 --> 00:08:47,840 know that Uranus and Neptune could not have formed much later 134 00:08:48,200 --> 00:08:52,040 than Jupiter and Saturn and that was really one of the arguments 135 00:08:52,040 --> 00:08:55,240 that they must have formed closer in initially. 136 00:08:55,480 --> 00:08:57,680 How do we know that they wouldn't have formed much later 137 00:08:57,680 --> 00:08:59,960 on? Well, it's a theoretical 138 00:08:59,960 --> 00:09:02,880 understanding. Once you get a planet forming 139 00:09:02,880 --> 00:09:06,920 and it's slowly accruing these small bodies, Once we get to the 140 00:09:06,920 --> 00:09:11,880 point where the escape velocity from the planet is about the 141 00:09:11,880 --> 00:09:16,320 same as the planet's orbital velocity around the sun, the 142 00:09:16,640 --> 00:09:20,800 planets will scatter objects around rather than eating them. 143 00:09:21,520 --> 00:09:24,600 And so eventually what happens is they'll hand them off to 144 00:09:24,600 --> 00:09:27,680 Jupiter and Saturn if they're there, and Jupiter and Saturn 145 00:09:27,680 --> 00:09:29,560 will just knock them out of the solar system. 146 00:09:30,080 --> 00:09:33,680 So what we find is when you get to the point where the ice 147 00:09:33,680 --> 00:09:37,880 giants are a couple times the mass of the Earth, they will 148 00:09:37,880 --> 00:09:41,960 stop growing and all their food will be fed down to Jupiter and 149 00:09:41,960 --> 00:09:44,320 Saturn. So let's get back to the 150 00:09:44,320 --> 00:09:48,000 Trojans. Why do they in particular allow 151 00:09:48,000 --> 00:09:50,680 us to distinguish the various models of solar system 152 00:09:50,680 --> 00:09:53,160 evolution? And what does the NICE model 153 00:09:53,160 --> 00:09:55,200 predict as far as they're concerned? 154 00:09:55,720 --> 00:09:59,800 So one of the interesting aspects of Trojans when we 155 00:09:59,800 --> 00:10:04,000 started studying them from the ground is while we expected them 156 00:10:04,000 --> 00:10:08,480 to be a homogeneous population, basically all the same things 157 00:10:08,480 --> 00:10:13,120 that went into building the core of Jupiter, we see that they're 158 00:10:13,120 --> 00:10:15,720 very different from one another. In particular, the colors are 159 00:10:15,720 --> 00:10:20,000 very different from things that are basically Gray to things 160 00:10:20,000 --> 00:10:24,600 that are redder to Mars. And we think what that's saying 161 00:10:24,600 --> 00:10:27,880 is that this is represents material that comes from the 162 00:10:28,120 --> 00:10:31,960 many different regions of the solar system, this disk that 163 00:10:31,960 --> 00:10:37,080 existed outside the orbits of the giant planets initially, and 164 00:10:37,080 --> 00:10:39,560 that the different colors are due to different formation 165 00:10:39,560 --> 00:10:41,560 temperatures because you're further from the sun. 166 00:10:42,600 --> 00:10:46,880 And so the nice model mixes all that up and delivers them to the 167 00:10:47,280 --> 00:10:52,000 Trojan swarms. And so by understanding, first 168 00:10:52,000 --> 00:10:55,120 of all, what those different colors mean, we don't have a 169 00:10:55,120 --> 00:10:59,000 good understanding of that. So looking at these things close 170 00:10:59,040 --> 00:11:04,040 up will help us understand that we might be able to figure out 171 00:11:04,040 --> 00:11:07,960 where in the disk they formed. And then looking at the 172 00:11:07,960 --> 00:11:12,480 distribution of objects in the Trojans will allow us to 173 00:11:12,480 --> 00:11:15,080 understand how the planets moved around. 174 00:11:15,520 --> 00:11:19,120 That's the basic idea. Another thing that the NICE 175 00:11:19,120 --> 00:11:23,200 model predicts is I said that most of this disc goes away 176 00:11:23,680 --> 00:11:28,400 scattered into interstellar space, but some of it not only 177 00:11:28,400 --> 00:11:31,280 ends up in the Trojan swarms, but they end up in the Kuiper 178 00:11:31,280 --> 00:11:35,160 Belt and are the sources of comets that we see today. 179 00:11:35,840 --> 00:11:39,760 So the NICE model basically predicts that Trojan should be 180 00:11:39,760 --> 00:11:44,040 the same as comets. And we've sent spacecraft to 181 00:11:44,040 --> 00:11:47,400 visit Comets before, so we have decent understanding of what 182 00:11:47,400 --> 00:11:50,640 they are. So one of the things we're going 183 00:11:50,640 --> 00:11:54,440 to test is how different the Trojans are from the Comets. 184 00:11:54,800 --> 00:11:58,440 And does the model also predict that these small objects would 185 00:11:58,440 --> 00:12:01,480 be sampled from a whole lot of different distances and 186 00:12:01,480 --> 00:12:05,080 therefore you'd expect them to be very diverse rather than just 187 00:12:05,360 --> 00:12:08,480 homogeneous? That's right, and that diversity 188 00:12:08,480 --> 00:12:11,840 is the key. That's what Lucy is designed to 189 00:12:11,840 --> 00:12:14,560 do. As I said, we see this diversity 190 00:12:14,560 --> 00:12:17,680 from the ground, and understanding what that 191 00:12:17,680 --> 00:12:24,040 diversity means will help us constrain how the planets moved 192 00:12:24,040 --> 00:12:25,960 around. That's the basic story. 193 00:12:26,360 --> 00:12:30,960 Lucy is going to visit more objects in orbit around the Sun 194 00:12:31,160 --> 00:12:33,920 than any other spacecraft in history. 195 00:12:34,200 --> 00:12:37,120 Because we need to get to a lot of them and all. 196 00:12:37,120 --> 00:12:42,080 We're going to be studying 8 Trojans, 5 Trojan systems. 197 00:12:42,640 --> 00:12:45,600 Two of our objects are known to have satellites. 198 00:12:45,800 --> 00:12:48,480 We discovered the satellites as we were studying these things 199 00:12:48,480 --> 00:12:52,200 and preparing for the mission. One of these objects, which is, 200 00:12:52,520 --> 00:12:54,640 I must admit, my favorite. You know you're not supposed to 201 00:12:54,640 --> 00:12:58,120 love one of your children better than the others, but this one I 202 00:12:58,120 --> 00:13:03,920 actually do is an object called Petrocholis, which is a near 203 00:13:03,920 --> 00:13:07,720 equal mass binary. It's a system that contains 2 204 00:13:07,720 --> 00:13:12,320 objects that are almost exactly the same size, about 100 205 00:13:12,320 --> 00:13:15,640 kilometers in diameter, in a circular orbit around one 206 00:13:15,640 --> 00:13:21,040 another, and I think that that itself is an important clue to 207 00:13:21,040 --> 00:13:23,480 how the solar system formed and evolved. 208 00:13:24,280 --> 00:13:27,400 Well, that's fascinating. One of the things about the Lucy 209 00:13:27,400 --> 00:13:31,520 mission also is that it's the so-called exploration mission 210 00:13:31,520 --> 00:13:34,600 that has gone to places that we've never been before. 211 00:13:35,200 --> 00:13:39,840 Yeah, Lucy is going to this population. 212 00:13:40,200 --> 00:13:43,680 We've never seen them before. We've basically have sent 213 00:13:43,800 --> 00:13:47,760 spacecraft to all the other small body reservoirs in the 214 00:13:47,760 --> 00:13:51,720 solar system, the main asteroid belt, the Kuiper Belt, comets, 215 00:13:52,640 --> 00:13:55,960 but not Trojans. So I'm a theorist, right? 216 00:13:55,960 --> 00:13:59,800 So you start talking to me about Lucy and I'm going to be 217 00:14:00,280 --> 00:14:02,800 concentrating on what it's going to tell us about the history of 218 00:14:02,800 --> 00:14:06,880 the solar system. But in a way, more importantly, 219 00:14:07,120 --> 00:14:10,480 we're going to a population we've never seen and it's really 220 00:14:10,480 --> 00:14:14,440 the last small body population that we have to study. 221 00:14:14,920 --> 00:14:19,240 And what's more, while other small body reservoirs are 222 00:14:19,240 --> 00:14:23,560 contributing meteorites to the Earth, because of the Trojans 223 00:14:23,560 --> 00:14:29,240 proximity to Jupiter, meteorites can't get toss from there. 224 00:14:29,600 --> 00:14:32,800 So these objects are really, really a mystery. 225 00:14:33,240 --> 00:14:37,800 Let's talk about what the spacecraft looks like and what 226 00:14:37,800 --> 00:14:40,280 instruments it carries. Sure. 227 00:14:40,440 --> 00:14:43,440 Because Lucy's job is to study the diversity. 228 00:14:44,200 --> 00:14:47,560 We have to be moving really quickly to get from object to 229 00:14:47,560 --> 00:14:50,720 object to object. So we're travelling really fast. 230 00:14:51,040 --> 00:14:54,520 Our encounter velocities with our Trojans range between 231 00:14:54,520 --> 00:14:57,440 roughly 6 and 9 kilometers a second. 232 00:14:58,680 --> 00:15:02,840 As a result, we're not stopping, we're not orbiting, we're not 233 00:15:02,840 --> 00:15:05,920 scratching and sniffing. Everything that we're doing is 234 00:15:05,920 --> 00:15:09,320 remote sensing, but yet we're trying to do everything we 235 00:15:09,320 --> 00:15:13,080 possibly can with those remote sensing instruments. 236 00:15:13,600 --> 00:15:18,200 So in particular, we have panchromatic cameras which are 237 00:15:18,200 --> 00:15:22,120 going to be able to allow us the study at high resolution, the 238 00:15:22,120 --> 00:15:25,920 geology on the surface of these bodies, counting craters, 239 00:15:26,160 --> 00:15:29,120 looking for maybe vents because we think these things are 240 00:15:29,120 --> 00:15:32,000 related to comets. Maybe we'll see evidence that 241 00:15:32,000 --> 00:15:37,520 they were one time active. We have a near infrared 242 00:15:37,640 --> 00:15:40,880 spectrograph which is going to be studying the chemistry of 243 00:15:40,880 --> 00:15:45,120 these objects. We have all far thermal infrared 244 00:15:45,120 --> 00:15:48,600 spectrometer, which is going to tell us something about how 245 00:15:48,840 --> 00:15:54,080 fluffy the surfaces are. And as we fly by these objects, 246 00:15:54,080 --> 00:15:56,680 we're going to be able to measure the Doppler shift due to 247 00:15:56,680 --> 00:15:59,120 the gravity. And so we're going to be able to 248 00:15:59,120 --> 00:16:03,040 get their mass. And because we're going to know 249 00:16:03,040 --> 00:16:06,720 their shape after the encounter, we're going to get an idea of 250 00:16:06,720 --> 00:16:10,080 what their internal density is, which is going to tell us 251 00:16:10,080 --> 00:16:12,080 something about their internal structure. 252 00:16:12,560 --> 00:16:17,560 The interesting thing about the Trojans is that we have a few 253 00:16:17,560 --> 00:16:22,160 densities for these objects, and the densities are always less 254 00:16:22,160 --> 00:16:24,080 than one. So think about it. 255 00:16:24,080 --> 00:16:27,480 These objects are made out of rock and ice. 256 00:16:27,760 --> 00:16:30,840 We believe they're about half rock and half ice, and yet their 257 00:16:30,840 --> 00:16:34,600 density is less than ice, which means there are a lot of void 258 00:16:34,600 --> 00:16:38,080 spaces inside of it. And so we're going to be able to 259 00:16:38,080 --> 00:16:40,600 understand that as well. And of course, that tells us 260 00:16:40,600 --> 00:16:43,360 something about how they created, right, how fast they 261 00:16:43,360 --> 00:16:46,280 came together. Because if their density is low, 262 00:16:46,280 --> 00:16:50,240 whatever process formed them when these objects came together 263 00:16:50,240 --> 00:16:54,640 to form them, they had to be going at really low velocities 264 00:16:54,640 --> 00:16:56,880 or they would become more compact. 265 00:16:57,000 --> 00:17:00,680 So understanding their density is critical to understanding 266 00:17:00,680 --> 00:17:03,120 their history. You're going to be flying by at 267 00:17:03,120 --> 00:17:05,319 these very rapid speeds, as you said. 268 00:17:05,319 --> 00:17:09,480 So how long will you actually have upon approach and then as 269 00:17:09,480 --> 00:17:11,800 you leave to capture all your data? 270 00:17:12,640 --> 00:17:15,599 So one of the interesting things about Lucy and one of the 271 00:17:15,599 --> 00:17:20,599 challenging things about Lucy is that it's a 12 year mission, but 272 00:17:20,599 --> 00:17:24,359 all our important science is going to be collected within 273 00:17:24,359 --> 00:17:28,560 about 24 hours. Two hours on either side of 274 00:17:28,560 --> 00:17:31,320 close approach is where we're getting all our high quality 275 00:17:31,320 --> 00:17:35,560 scientific data. So first of all, we can't blink. 276 00:17:36,240 --> 00:17:40,920 Something can't go wrong because once an encounter is done, we 277 00:17:41,120 --> 00:17:44,800 can never go back. We can't do a do over. 278 00:17:45,160 --> 00:17:51,440 So we have to plan every second of what the spacecraft is going 279 00:17:51,440 --> 00:17:54,080 to be doing during these encounters. 280 00:17:54,080 --> 00:17:57,040 Also, we're not talking to the spacecraft during the encounter. 281 00:17:57,360 --> 00:18:00,920 After all, the light travel time is long enough that we couldn't 282 00:18:01,120 --> 00:18:05,880 say no, no, do this instead. So it's totally autonomous and 283 00:18:05,880 --> 00:18:07,960 everything needs to be planned beforehand. 284 00:18:07,960 --> 00:18:12,640 So although we launched in 2021, you might think we've had a long 285 00:18:12,640 --> 00:18:17,520 time to wait and we're sitting and twiddling our thumbs, but we 286 00:18:17,520 --> 00:18:22,040 have been very, very busy planning each encounter in order 287 00:18:22,040 --> 00:18:25,080 to make sure that we're getting the best signs that we can. 288 00:18:25,560 --> 00:18:28,640 So how close will you get and what kind of surface resolution 289 00:18:28,640 --> 00:18:29,920 will you get with your instruments? 290 00:18:30,560 --> 00:18:35,160 So we're flying at a distance of a few 100 kilometers, five 600 291 00:18:35,160 --> 00:18:39,200 kilometers, and with the high resolution spectrograph that we 292 00:18:39,200 --> 00:18:44,120 have, we'll get about 15 meters per pixel on the camera. 293 00:18:44,720 --> 00:18:47,760 You mentioned that you're going to be flying by and going very 294 00:18:47,760 --> 00:18:50,880 fast and that you launched in 2021. 295 00:18:51,400 --> 00:18:55,560 So where has Lucy been travelling all that time and 296 00:18:55,560 --> 00:18:58,800 what trajectory is it taking to get to the Trojans? 297 00:18:59,600 --> 00:19:04,160 So Lucy's trajectory is really spectacular. 298 00:19:04,160 --> 00:19:08,080 It's one of my favorite aspects of the mission, to be able to 299 00:19:08,080 --> 00:19:11,480 get to these objects, and they're very special objects. 300 00:19:11,480 --> 00:19:16,960 I said we're going by 5 Trojan systems to study the diversity, 301 00:19:17,240 --> 00:19:22,000 but we've been lucky that the objects we've been able to get 302 00:19:22,200 --> 00:19:24,880 to are interesting as individuals. 303 00:19:25,160 --> 00:19:28,320 Several of them would be worth actually sending a spacecraft to 304 00:19:28,360 --> 00:19:32,160 if that's the only place the spacecraft could get to. 305 00:19:32,320 --> 00:19:34,960 I like to say is I'm a dynamicist. 306 00:19:34,960 --> 00:19:37,040 I study how things orbit the sun. 307 00:19:37,320 --> 00:19:42,000 So I've been worshipping at the feet of the celestial mechanics 308 00:19:42,160 --> 00:19:45,720 gods for 40 years, and they're paying us back. 309 00:19:45,720 --> 00:19:50,320 It's really, really a spectacular mission due to this 310 00:19:50,320 --> 00:19:54,120 really unique trajectory. But one of the things that we're 311 00:19:54,120 --> 00:19:59,680 doing in order to save money is loosely launched on a very small 312 00:19:59,680 --> 00:20:04,200 rocket, Atlas four O 1. And as a result, when it first 313 00:20:04,200 --> 00:20:07,760 left the vicinity to the Earth, its velocity with respect to the 314 00:20:07,840 --> 00:20:11,040 Earth was really small. So we could use a small rocket. 315 00:20:11,680 --> 00:20:17,040 And what we've been doing is using a series of Earth gravity 316 00:20:17,160 --> 00:20:19,720 assists where we fly by the Earth and use the Earth's 317 00:20:19,720 --> 00:20:25,400 gravity to slingshot Lucy out to slowly pump up the orbit of Lucy 318 00:20:25,400 --> 00:20:27,880 so it can cross the orbit of Jupiter. 319 00:20:28,560 --> 00:20:32,360 And so that's taking some time. And so that's what Lucy's been 320 00:20:32,360 --> 00:20:37,240 doing, basically. But on two of those orbits, we 321 00:20:37,240 --> 00:20:39,760 actually went through the main asteroid belt. 322 00:20:40,360 --> 00:20:43,800 Although we didn't get quite far enough to get to the Trojans, we 323 00:20:43,800 --> 00:20:48,360 did get to the point where we could get into the asteroid belt 324 00:20:48,760 --> 00:20:53,000 and we did 2 flybys as tests of our system, right? 325 00:20:53,000 --> 00:20:56,600 Again, there's no going back. So testing the system and making 326 00:20:56,600 --> 00:20:59,280 sure we understand how to fly it was key. 327 00:20:59,680 --> 00:21:03,920 And so we've studied 2 asteroids so far and they've turned out to 328 00:21:03,920 --> 00:21:06,960 be particularly interesting. One in particular is really 329 00:21:06,960 --> 00:21:09,280 interesting. Tell us about the afterwards you 330 00:21:09,320 --> 00:21:12,360 encountered them. Let me take a step back and 331 00:21:12,360 --> 00:21:15,680 point out that most asteroids that we know about are not 332 00:21:15,680 --> 00:21:18,080 named. All the objects that we're 333 00:21:18,080 --> 00:21:21,480 studying with Lucy, only two of them had names before we 334 00:21:21,480 --> 00:21:23,600 started. So we got to name a lot of these 335 00:21:23,600 --> 00:21:26,360 objects. For the main asteroid belt 336 00:21:26,360 --> 00:21:32,120 objects, we flew by an object that we named Dinkanesh, which 337 00:21:32,120 --> 00:21:36,640 is the Ethiopian name for the Lucy fossil. 338 00:21:36,960 --> 00:21:39,600 So let me take a second to talk about how we came up with the 339 00:21:39,600 --> 00:21:43,560 name Lucy. Lucy the mission is named after 340 00:21:43,560 --> 00:21:49,000 Lucy the fossil because we're arguing that these objects are 341 00:21:49,000 --> 00:21:54,040 the fossils of planet formation. And just like our ancestral 342 00:21:54,040 --> 00:22:00,080 fossils will tell us about our history, these objects are going 343 00:22:00,080 --> 00:22:02,200 to tell us about the history of the solar system. 344 00:22:02,200 --> 00:22:06,400 So we named the Lucy mission after the Lucy fossil. 345 00:22:06,600 --> 00:22:10,120 I should also point out the Lucy fossil was named after Lucy in 346 00:22:10,120 --> 00:22:13,280 the sky with diamonds. So if you actually look at our 347 00:22:13,280 --> 00:22:17,080 logo, we made sort of a connection to that, that our 348 00:22:17,080 --> 00:22:20,680 logo was diamond shaped. So the first asteroid we 349 00:22:20,680 --> 00:22:25,240 encountered on route, we named Dinkanash, which is the 350 00:22:25,240 --> 00:22:27,360 Ethiopian name of the Lucy fossil. 351 00:22:27,360 --> 00:22:31,320 Lucy was discovered in Ethiopia and it currently is in their 352 00:22:31,320 --> 00:22:35,440 museums. Dinkanash turned out to be one 353 00:22:35,440 --> 00:22:38,400 of the most fascinating objects I think I've ever seen. 354 00:22:39,400 --> 00:22:43,280 Because remember I said at the beginning, these things are 355 00:22:43,280 --> 00:22:49,360 interesting because they basically remained unchanged 356 00:22:50,040 --> 00:22:53,560 over the history of the solar system, and that you need to 357 00:22:53,560 --> 00:22:56,920 have big objects in order to make geology happen. 358 00:22:57,960 --> 00:23:02,520 Turns out that's not right. If you have very small objects, 359 00:23:02,800 --> 00:23:08,200 objects that are about a kilometer in diameter, radiation 360 00:23:08,200 --> 00:23:13,960 forces can actually change the spin rate of these objects over 361 00:23:13,960 --> 00:23:17,280 time. And as a result they can spin 362 00:23:17,280 --> 00:23:19,760 up. And then centripetal forces 363 00:23:19,760 --> 00:23:23,760 start putting stresses on the bodies and they can crack and 364 00:23:24,040 --> 00:23:26,760 pieces can fall off and form satellites. 365 00:23:27,280 --> 00:23:32,200 And all that can happen on really fast time scales, 10 366 00:23:32,200 --> 00:23:35,200 million years or so. And that's what's happening on 367 00:23:35,200 --> 00:23:38,560 Dickenish. Dickenish is only 700 meters in 368 00:23:38,560 --> 00:23:42,080 diameter, and as a result it's been spinning up and slowing 369 00:23:42,080 --> 00:23:44,280 down. And we see evidence of a big 370 00:23:44,280 --> 00:23:48,520 crack that's opened up and in. And it has the satellite that we 371 00:23:48,520 --> 00:23:52,640 named Salaam, which is named after another fossil. 372 00:23:52,960 --> 00:23:56,720 And it is a contact binary of two objects that are roughly the 373 00:23:56,720 --> 00:23:59,080 same shape that are stuck together. 374 00:23:59,840 --> 00:24:04,120 And all these things are probably very young and form 375 00:24:04,120 --> 00:24:06,640 because of this radiation forces. 376 00:24:06,640 --> 00:24:10,640 So at a size scale of about a kilometer, really, really cool 377 00:24:10,640 --> 00:24:12,840 things can happen. And I didn't quite really 378 00:24:12,840 --> 00:24:16,920 appreciate that before we saw this object. 379 00:24:17,840 --> 00:24:21,440 The second object we went to, we named Donald Johansson after the 380 00:24:21,440 --> 00:24:25,480 discoverer of the Lucy fossil. Actually, one of the cool things 381 00:24:25,480 --> 00:24:29,360 is that he was able to come to the encounter. 382 00:24:29,640 --> 00:24:33,720 So I think this is the first time that a person for whom an 383 00:24:33,720 --> 00:24:36,800 asteroid was named after, I actually was there to see it 384 00:24:37,240 --> 00:24:40,520 when this thought of light in the sky became a world. 385 00:24:41,000 --> 00:24:45,920 So that was pretty cool. Donald Johansson is shaped like 386 00:24:45,920 --> 00:24:48,480 a bowling ball. Turns out that that's pretty 387 00:24:48,480 --> 00:24:52,000 common. So it didn't really hold any 388 00:24:52,000 --> 00:24:56,560 surprises when it came to its global structure, but it has 389 00:24:56,560 --> 00:25:00,760 some weird things going on on the neck between the two 390 00:25:00,960 --> 00:25:04,400 components. There seems to be some geology 391 00:25:04,400 --> 00:25:08,000 going on that we don't quite understand. 392 00:25:08,000 --> 00:25:09,400 So that's been interesting. Too. 393 00:25:10,800 --> 00:25:14,840 Let's look forward then to your encounter with the first set of 394 00:25:15,000 --> 00:25:20,080 Trojans that you're going to. And you mentioned that the gods 395 00:25:20,080 --> 00:25:22,800 of celestial mechanics are rewarding you. 396 00:25:22,800 --> 00:25:26,520 And is that because basically you have a limited supply of 397 00:25:26,520 --> 00:25:29,760 fuel on board, so you can't really change course once you're 398 00:25:29,760 --> 00:25:33,720 speeding along all that much? So they have to be kind of lined 399 00:25:33,720 --> 00:25:36,600 up pretty much in a straight line for you to go from one to 400 00:25:36,600 --> 00:25:39,720 the other. Did that actually leave you with 401 00:25:39,720 --> 00:25:42,280 a choice as to which asteroids to visit? 402 00:25:42,320 --> 00:25:45,200 And if so, how did you actually make that choice? 403 00:25:46,040 --> 00:25:51,360 Yeah, our first goal when we put together the mission was to try 404 00:25:51,360 --> 00:25:54,480 to run an experiment with as much control as possible. 405 00:25:55,000 --> 00:25:57,560 I told you that Trojans have very different colors from one 406 00:25:57,560 --> 00:26:03,200 another, so we tried to find 2 objects that are the same size 407 00:26:03,200 --> 00:26:07,120 and on the same orbit. So they've had the same basic 408 00:26:07,120 --> 00:26:10,800 history but yet have very different colors. 409 00:26:11,240 --> 00:26:14,360 We did that so that we could say anything that we saw. 410 00:26:14,400 --> 00:26:17,400 The in differences between these two objects are due to their 411 00:26:17,400 --> 00:26:22,440 different compositions and not to say the different histories, 412 00:26:23,040 --> 00:26:26,600 and we put together a list of about 1/2 a dozen. 413 00:26:26,600 --> 00:26:30,200 That's all we could find candidate objects and then ask 414 00:26:30,200 --> 00:26:34,720 the people that designed the mission to see which ones we can 415 00:26:34,720 --> 00:26:38,200 actually get to within a reasonable amount of time and a 416 00:26:38,200 --> 00:26:41,920 reasonable amount of fuel. And it turns out there was one 417 00:26:41,920 --> 00:26:47,320 pair, OK. And that is the center of our 418 00:26:47,360 --> 00:26:50,480 mission, which is going to an optic called Euribides, which is 419 00:26:50,480 --> 00:26:54,280 a Gray type asteroid, which is the first one we encounter in 420 00:26:54,280 --> 00:26:58,560 2027. And then Oris, which is a very 421 00:26:58,560 --> 00:27:03,200 red object that we encountered in 2029. 422 00:27:03,200 --> 00:27:06,160 And that's when we're going through the 1st loop, going 423 00:27:06,160 --> 00:27:11,040 through the leading swarms of Trojans, the ones that lead 424 00:27:11,040 --> 00:27:12,760 Jupiter. So that's how we started. 425 00:27:13,240 --> 00:27:20,160 Everything else was just luck. And you might think it didn't 426 00:27:20,160 --> 00:27:24,480 require much luck, but one of the things NASA cares about when 427 00:27:24,480 --> 00:27:29,520 you put in a proposal to fly a mission through them is you need 428 00:27:29,520 --> 00:27:33,120 to make an argument of why you want to do it now. 429 00:27:33,720 --> 00:27:37,600 Why not wait to do the mission? Because after all, technology 430 00:27:37,600 --> 00:27:40,840 gets better as time goes on. And if you waited a decade, 431 00:27:40,840 --> 00:27:42,680 maybe you would get more signs out of it. 432 00:27:43,320 --> 00:27:47,200 So we ran the experiment asking ourselves what would happen if 433 00:27:47,200 --> 00:27:52,440 we were launching in 2030 instead of 2021, and tried to 434 00:27:52,440 --> 00:27:58,800 put together a list of targets and we didn't find anything 435 00:27:58,840 --> 00:28:03,400 anywhere near as interesting. And again, we were really lucky. 436 00:28:03,400 --> 00:28:09,640 We maybe got three objects or 4 objects rather than the eight 437 00:28:09,640 --> 00:28:14,040 that we're going to, and they were just uninteresting objects 438 00:28:14,040 --> 00:28:16,600 compared to the ones we're actually visiting. 439 00:28:17,080 --> 00:28:20,120 Well, that's really very fortuitous indeed then, now that 440 00:28:20,120 --> 00:28:23,240 we know how unlikely this alignment is. 441 00:28:23,600 --> 00:28:28,320 So how long will it take to fly by all these systems in the 442 00:28:28,320 --> 00:28:31,160 first set of Trojans? And then what does the Path of 443 00:28:31,160 --> 00:28:33,200 Lucy look like after you're done with those? 444 00:28:33,680 --> 00:28:36,680 OK, so as I said, we rattled around the inner part of the 445 00:28:36,680 --> 00:28:40,560 solar system for a while, using Earth gravity assist to pump up 446 00:28:40,560 --> 00:28:45,080 the orbit of Lucy. Right now it's in an orbit with 447 00:28:45,160 --> 00:28:50,280 a perihelion, a close approach distance to the Sun just inside 448 00:28:50,280 --> 00:28:54,000 the orbit of the Earth, and an aphelion distance, the furthest 449 00:28:54,000 --> 00:28:57,120 distance it gets from the Sun, just outside the orbit of 450 00:28:57,160 --> 00:28:59,960 Jupiter. And that's where it'll be for 451 00:28:59,960 --> 00:29:02,800 most of its life. We're doing 2 orbits. 452 00:29:03,040 --> 00:29:06,520 The first orbit will take us into the leading swarm, and the 453 00:29:06,520 --> 00:29:09,000 second orbit will take us into the trailing swarm. 454 00:29:09,720 --> 00:29:13,320 And in the leading swarm, we're having 4 encounters in 15 455 00:29:13,320 --> 00:29:15,640 months. So it's going to be very fast, 456 00:29:15,640 --> 00:29:18,040 but we're going to be very busy. As a matter of fact, I'm worried 457 00:29:18,040 --> 00:29:20,200 about it. Then we come back into the inner 458 00:29:20,200 --> 00:29:23,480 part of the solar system, have another Earth gravity assist, 459 00:29:23,480 --> 00:29:29,520 which is going to allow us to target our final Trojan, which 460 00:29:29,520 --> 00:29:32,720 like I said, this is my favorite system of Patrocalis. 461 00:29:33,080 --> 00:29:37,560 We're only going to one object in the trailing swarm, and 462 00:29:37,560 --> 00:29:41,800 that's because we made the decision to go to a very low 463 00:29:41,800 --> 00:29:46,480 density region of the Trojan swarms in order to get to 464 00:29:46,480 --> 00:29:49,560 Petrocholis. We could have flown through the 465 00:29:49,560 --> 00:29:53,360 center of the swarm and probably gotten a few more objects, but 466 00:29:53,360 --> 00:29:58,680 Petrocholis is so cool that I decided that we needed to go 467 00:29:58,680 --> 00:30:00,920 after it rather than more targets. 468 00:30:00,920 --> 00:30:02,280 And so that's what we're going to do. 469 00:30:02,560 --> 00:30:05,440 And the fact that it's an equal mass binary, does that make you 470 00:30:05,440 --> 00:30:09,320 think that it's more likely to be like a really pristine 471 00:30:09,320 --> 00:30:11,920 fossil, as it were? Yes. 472 00:30:11,920 --> 00:30:16,400 So let's talk about this object and objects like it. 473 00:30:17,000 --> 00:30:20,160 When we look around the solar system, the inner part, let's 474 00:30:20,160 --> 00:30:22,000 call it the inner part of the solar system. 475 00:30:22,440 --> 00:30:26,080 And by inner part, I'm talking inside the orbit of Neptune. 476 00:30:26,840 --> 00:30:32,640 So in the region where planets actually form, seeing 2 objects 477 00:30:33,000 --> 00:30:37,560 like this, these near equal mass binaries are incredibly rare. 478 00:30:37,960 --> 00:30:40,440 I think we know of three or four of them. 479 00:30:40,720 --> 00:30:43,680 But if you look out in the region of the Kuiper Belt, the 480 00:30:43,680 --> 00:30:46,920 region beyond the orbit of Neptune, there's a place that we 481 00:30:46,920 --> 00:30:49,160 call the cold classical Kuiper Belt. 482 00:30:49,840 --> 00:30:55,520 And it is a place where we see asteroid type objects that we 483 00:30:55,520 --> 00:30:59,160 believe to be pristine because they were far enough from 484 00:30:59,160 --> 00:31:03,400 Neptune that they weren't affected by the planet formation 485 00:31:03,400 --> 00:31:06,480 process themselves. When you're looking at that 486 00:31:06,480 --> 00:31:11,240 area, it turns out they're all near equal mass binaries, almost 487 00:31:11,240 --> 00:31:13,960 at 100%. So I think what that's telling 488 00:31:13,960 --> 00:31:18,880 us is that the first macroscopic objects to form in the solar 489 00:31:18,880 --> 00:31:27,240 system were formed as binaries. And indeed, modern theories of 490 00:31:27,720 --> 00:31:31,960 planet formation and planetesimal formation actually 491 00:31:31,960 --> 00:31:36,280 can explain that. And the idea is that the first 492 00:31:36,720 --> 00:31:43,000 macroscopic objects form because small things, things the size of 493 00:31:43,000 --> 00:31:48,040 a meter or the size of your fist, were concentrated due to 494 00:31:48,040 --> 00:31:52,560 their interaction with gas. And when they got concentrated 495 00:31:52,760 --> 00:31:55,560 enough, sort of like, think of them as like little star 496 00:31:55,560 --> 00:31:58,640 clusters, they became gravitationally bound to one 497 00:31:58,640 --> 00:32:03,640 another and then collapsed to form the big objects. 498 00:32:03,760 --> 00:32:07,280 By the way, they collapse very slowly, which is probably why 499 00:32:07,280 --> 00:32:09,040 these objects have very low density. 500 00:32:10,160 --> 00:32:14,960 But when you model this, you find out is what's happening is 501 00:32:14,960 --> 00:32:19,720 that these clumps actually have a little bit of rotation, and as 502 00:32:19,720 --> 00:32:24,040 they collapse, angular momentum has to be conserved. 503 00:32:24,040 --> 00:32:27,680 So they spin up faster and faster and faster, like a dancer 504 00:32:27,680 --> 00:32:31,320 bringing in their arms. Angular momentum is conserved, 505 00:32:31,320 --> 00:32:33,920 so they need to spin faster as their arms move in. 506 00:32:35,200 --> 00:32:39,320 There's so much angular momentum in these systems that you can't 507 00:32:39,320 --> 00:32:42,680 fit all of it into one object, it would fly apart. 508 00:32:43,400 --> 00:32:47,760 So what nature naturally does, it turns out, is it makes 2 509 00:32:47,760 --> 00:32:52,480 objects the same mass, but in orbit around one another. 510 00:32:53,040 --> 00:32:56,360 And that's exactly what we see in the cold classic Kuiper belt. 511 00:32:57,080 --> 00:33:02,280 So we really think that these are the first real big objects. 512 00:33:02,280 --> 00:33:06,480 And if that's true, then Petrocholis is one of those rare 513 00:33:06,480 --> 00:33:11,200 survivors that survived the planet formation process, which 514 00:33:11,200 --> 00:33:13,000 is very violent. After all, things are hitting 515 00:33:13,040 --> 00:33:14,920 each other. Things are gravitationally 516 00:33:14,920 --> 00:33:18,400 flying by one another, which could RIP these binaries apart 517 00:33:18,400 --> 00:33:21,080 gravitationally. That's why they didn't survive, 518 00:33:22,160 --> 00:33:25,320 but Patroclus did. So it's going to be really cool 519 00:33:25,320 --> 00:33:28,880 to look at that. So after you finished with the 520 00:33:29,080 --> 00:33:34,760 second Trojan encounter with Patroclus, what's going to 521 00:33:34,760 --> 00:33:36,880 happen to Lucy? Where will its orbit take it? 522 00:33:37,760 --> 00:33:40,840 It's going to be in the orbit that it's on basically. 523 00:33:40,840 --> 00:33:44,200 Now that takes it out to Jupiter's orbit and brings it in 524 00:33:44,200 --> 00:33:46,960 the Earth's orbit. We think there should be fuel 525 00:33:46,960 --> 00:33:50,680 enough and if the spacecraft is healthy, I'm sure NASA will find 526 00:33:50,680 --> 00:33:52,880 A use for the spacecraft and do something cool with it. 527 00:33:53,440 --> 00:33:57,920 One of the interesting aspects of the orbit that it's on is it 528 00:33:57,920 --> 00:34:02,400 turns out it's very long lived. It'll last in this orbit if no 529 00:34:02,400 --> 00:34:06,560 one does anything with it, for about on average 2,000,000 530 00:34:06,560 --> 00:34:09,440 years. Though we did something I think 531 00:34:09,440 --> 00:34:13,199 that was interesting for those of you who know about the 532 00:34:13,199 --> 00:34:17,080 history of NASA. NASA, particularly early on, had 533 00:34:17,080 --> 00:34:22,760 this idea of sending messages to aliens, right? 534 00:34:22,760 --> 00:34:26,320 So the first spacecraft to leave the solar system, which were The 535 00:34:26,679 --> 00:34:30,280 Pioneers and the Voyagers, all have messages to the aliens. 536 00:34:30,280 --> 00:34:32,480 The Pioneers have these famous plaques. 537 00:34:32,840 --> 00:34:35,440 The Voyagers have record player, the sounds of the Earth. 538 00:34:35,840 --> 00:34:39,000 All that is focused on aliens because they're leaving the 539 00:34:39,000 --> 00:34:41,719 solar system. Lucy, on the other hand, is 540 00:34:41,719 --> 00:34:45,920 going to be in orbit in the solar system and it'll last for 541 00:34:45,920 --> 00:34:49,000 a very long time. So we had an idea to put a 542 00:34:49,000 --> 00:34:53,520 plaque on Lucy, not with messages to aliens, both to 543 00:34:53,520 --> 00:34:58,160 messages to our descendants. You could easily imagine 100 544 00:34:58,160 --> 00:35:00,520 years from now, maybe it won't be so interesting then. 545 00:35:00,520 --> 00:35:04,320 But 1000 years from now, 10,000 years from now, hundreds of 546 00:35:04,320 --> 00:35:07,720 thousands of years from now, our descendants want to learn about 547 00:35:07,720 --> 00:35:12,000 us by going around the solar system and picking up our space 548 00:35:12,000 --> 00:35:14,880 junk. So we left them a little 549 00:35:14,960 --> 00:35:17,240 message. So take a look, it's online. 550 00:35:17,240 --> 00:35:18,600 You can see a picture of our plaque. 551 00:35:19,080 --> 00:35:22,920 I know this is probably a bad time to ask you this, but once 552 00:35:22,920 --> 00:35:26,160 you have the data from a successful Lucy mission in hand, 553 00:35:26,680 --> 00:35:29,960 what would be your next preferred destination for a new 554 00:35:29,960 --> 00:35:33,760 space mission to help resolve further questions about solar 555 00:35:33,760 --> 00:35:36,880 system evolution? Well, there are two places that 556 00:35:36,880 --> 00:35:41,200 I think I would like to go. The first is send another 557 00:35:41,200 --> 00:35:44,680 spacecraft out to the carper belt and study the cold 558 00:35:44,680 --> 00:35:48,240 classical carper belt. As I said earlier, there are 559 00:35:48,240 --> 00:35:52,560 regions of the carper belt that we think should contain the same 560 00:35:52,560 --> 00:35:59,600 stuff as the Trojans. And so being able to compare 100 561 00:35:59,600 --> 00:36:03,480 kilometer sized thing in the Trojan swarms to something in 562 00:36:03,480 --> 00:36:05,800 the carper belt would be, I think, really interesting. 563 00:36:06,560 --> 00:36:10,480 But there is another place that we could go that's much closer. 564 00:36:10,840 --> 00:36:14,840 There's a population of asteroids just outside the main 565 00:36:14,840 --> 00:36:18,760 asteroid belt called Hilda's. If the nice bottle is correct, 566 00:36:18,840 --> 00:36:22,920 They were also captured at the same time the Trojans were. 567 00:36:24,240 --> 00:36:29,200 So one of the really interesting places to go is to a Hilda. 568 00:36:29,200 --> 00:36:35,760 And indeed, we've recently discovered that we can get Lucy 569 00:36:36,520 --> 00:36:40,000 to a Hilda at the end of its main mission. 570 00:36:41,080 --> 00:36:45,600 So maybe if NASA decides to extend the mission, we'll be 571 00:36:45,600 --> 00:36:47,480 able to go to one of those objects, too. 572 00:36:48,520 --> 00:36:52,600 Because comparing populations to one another, looking at 573 00:36:52,600 --> 00:36:56,080 diversity, like I said, it's the key to figuring all this out. 574 00:36:57,160 --> 00:36:59,000 Al. Levison, thank you very much. 575 00:36:59,360 --> 00:37:03,120 My pleasure, it's fun. To see pictures and 576 00:37:03,120 --> 00:37:08,520 illustrations that support this podcast, go to geologybytes.com, 577 00:37:08,640 --> 00:37:11,400 where you'll also find a subject matter index of all the 578 00:37:11,400 --> 00:37:13,760 episodes. There you can also give me 579 00:37:13,760 --> 00:37:17,680 feedback which I welcome, as well as sign up to get my emails 580 00:37:17,680 --> 00:37:18,920 about new episodes.