1 00:00:01,800 --> 00:00:05,300 The Milky Way galaxy is an expansive giant. 2 00:00:05,300 --> 00:00:07,400 Home to hundreds of billions of stars 3 00:00:07,400 --> 00:00:09,970 and at least 100 billion planets, 4 00:00:09,970 --> 00:00:12,870 many of which shine brightly in our own night sky. 5 00:00:14,100 --> 00:00:16,290 But there are others hiding in plain sight. 6 00:00:17,260 --> 00:00:19,440 If we gaze at the stars hard enough, 7 00:00:19,440 --> 00:00:21,630 we can even get lucky and catch a glimpse 8 00:00:21,630 --> 00:00:24,290 of the fluting remnants of solar system bodies. 9 00:00:25,300 --> 00:00:27,560 What are these near-Earth objects, 10 00:00:27,560 --> 00:00:30,110 and could they have played a role in our evolution? 11 00:00:33,060 --> 00:00:36,110 I look at these objects as tracers 12 00:00:36,110 --> 00:00:38,580 of the very earliest stages 13 00:00:38,580 --> 00:00:40,580 of planet formation in the solar system. 14 00:00:41,680 --> 00:00:43,740 We can study the planets and try to understand 15 00:00:43,740 --> 00:00:45,180 how they form and evolve. 16 00:00:45,180 --> 00:00:47,160 But the planets are complicated bodies. 17 00:00:47,160 --> 00:00:49,340 They have weather and wind and water. 18 00:00:49,340 --> 00:00:51,270 They have plate tectonics. 19 00:00:51,270 --> 00:00:52,240 They have volcanism. 20 00:00:52,240 --> 00:00:54,130 All of these things act to erase 21 00:00:54,130 --> 00:00:56,100 and reset the surfaces of the planet. 22 00:00:56,100 --> 00:00:57,710 And that makes it very difficult 23 00:00:57,710 --> 00:00:59,770 to learn about how the Earth formed 24 00:00:59,770 --> 00:01:01,890 four and a half billion years ago. 25 00:01:01,890 --> 00:01:04,910 In contrast, the asteroids are leftover fragments 26 00:01:04,910 --> 00:01:07,380 of the planet formation process, 27 00:01:07,380 --> 00:01:09,030 and they've been sitting out in the solar system 28 00:01:09,030 --> 00:01:10,700 for the past four and a half billion years 29 00:01:10,700 --> 00:01:12,140 relatively undisturbed. 30 00:01:13,540 --> 00:01:14,580 And so, by studying these objects, 31 00:01:14,580 --> 00:01:16,810 we're really able to peer back into the past, 32 00:01:16,810 --> 00:01:18,560 and understand what the conditions were like 33 00:01:18,560 --> 00:01:21,360 as planets were forming, both chemically and physically. 34 00:01:22,440 --> 00:01:25,020 So we have 600,000 of these things to study. 35 00:01:25,020 --> 00:01:26,400 We can't send a spacecraft to all of them, 36 00:01:26,400 --> 00:01:28,400 but we can use telescopes here on Earth. 37 00:01:30,920 --> 00:01:33,700 For a long time, the traditional definition 38 00:01:33,700 --> 00:01:34,730 of an asteroid and a comet, 39 00:01:34,730 --> 00:01:36,480 is that an asteroid's a rocky body, 40 00:01:37,320 --> 00:01:39,640 and that a comet's an icy body that shows activity. 41 00:01:39,640 --> 00:01:41,420 A comet injects a halo, 42 00:01:41,420 --> 00:01:44,610 those types of things that we associate with a comet. 43 00:01:44,610 --> 00:01:46,080 The past 10 or 15 years or so, 44 00:01:46,080 --> 00:01:48,240 we've started to discover asteroids 45 00:01:48,240 --> 00:01:50,790 that, for whatever reason, all of a sudden turn on. 46 00:01:50,790 --> 00:01:52,800 Just like a comet would, they start to show activity. 47 00:01:52,800 --> 00:01:55,150 They start to show evidence of outgassing, 48 00:01:55,150 --> 00:01:57,370 loss of volatiles, loss of dust. 49 00:01:57,370 --> 00:01:59,060 Just like a comet. 50 00:01:59,060 --> 00:02:01,590 And we now think that there are quite a few processes 51 00:02:01,590 --> 00:02:03,820 probably responsible for taking some of these objects, 52 00:02:03,820 --> 00:02:05,330 that we would have classified as asteroid, 53 00:02:05,330 --> 00:02:06,970 and turning them into comets. 54 00:02:06,970 --> 00:02:08,800 Collision is one possible way 55 00:02:08,800 --> 00:02:10,860 of making an asteroid look like a comet. 56 00:02:10,860 --> 00:02:13,420 If you have an asteroid and collide a body onto its surface, 57 00:02:13,420 --> 00:02:15,390 that's gonna kick up a big dust cloud 58 00:02:15,390 --> 00:02:16,610 and, as viewed from Earth, 59 00:02:16,610 --> 00:02:18,620 we will see an asteroid with a big dust cloud around it 60 00:02:18,620 --> 00:02:20,070 that looks just like a comet. 61 00:02:24,530 --> 00:02:25,750 They can be a hazard, 62 00:02:25,750 --> 00:02:28,270 and we certainly know that near-Earth asteroids 63 00:02:28,270 --> 00:02:29,680 can and do hit the Earth. 64 00:02:29,680 --> 00:02:33,320 We have numerous pieces of evidence all over the Earth, 65 00:02:33,320 --> 00:02:35,290 in the form of impact craters, 66 00:02:35,290 --> 00:02:37,950 that, in some cases, very large objects have hit the Earth 67 00:02:37,950 --> 00:02:40,210 and we think that there's a pretty good correlation 68 00:02:40,210 --> 00:02:41,990 between extinction events 69 00:02:41,990 --> 00:02:45,480 and impacts from asteroids and comets. 70 00:02:45,480 --> 00:02:47,830 While the most famous has been the impact 71 00:02:47,830 --> 00:02:50,460 that potentially killed off the dinosaurs, 72 00:02:50,460 --> 00:02:53,060 we have evidence of that impact 73 00:02:53,060 --> 00:02:56,010 off the coast of the Yucatan Peninsula in Mexico, 74 00:02:56,010 --> 00:02:58,220 but there are numerous others throughout history 75 00:02:58,220 --> 00:03:00,510 that we think correlate with extinction events 76 00:03:00,510 --> 00:03:02,590 that we see in the fossil record. 77 00:03:02,590 --> 00:03:05,430 With that said, though, those events are exceedingly rare. 78 00:03:05,430 --> 00:03:06,590 Exceedingly rare. 79 00:03:06,590 --> 00:03:10,060 And they're so rare that I don't stay up at night 80 00:03:10,060 --> 00:03:13,310 worrying about the next killer asteroid coming along. 81 00:03:13,310 --> 00:03:15,000 While impact events here on Earth 82 00:03:15,000 --> 00:03:17,030 are few and far between, 83 00:03:17,030 --> 00:03:18,580 parts of asteroids and comets 84 00:03:18,580 --> 00:03:21,080 have reached our planet in the form of meteorites. 85 00:03:22,220 --> 00:03:23,950 There's a whole host of terminology 86 00:03:23,950 --> 00:03:24,990 associated with meteors. 87 00:03:24,990 --> 00:03:28,280 There's meteors, there's meteoroids, there's meteorites. 88 00:03:28,280 --> 00:03:31,750 Meteors come from a variety of sources. 89 00:03:31,750 --> 00:03:33,380 Some of them are particles 90 00:03:33,380 --> 00:03:34,710 that have been released by comets. 91 00:03:34,710 --> 00:03:38,420 Some of them are dust particles released by asteroids, 92 00:03:38,420 --> 00:03:39,980 generally formed through collisions. 93 00:03:39,980 --> 00:03:43,470 So two asteroids collide, and produce a shower of fragments. 94 00:03:43,470 --> 00:03:45,670 Some of those fragments can make their way to the Earth. 95 00:03:45,670 --> 00:03:47,480 Before that passage through the atmosphere, 96 00:03:47,480 --> 00:03:49,360 you could refer to that particle or that rock 97 00:03:49,360 --> 00:03:50,300 as a meteoroid. 98 00:03:51,160 --> 00:03:53,490 If that meteor makes its way to the ground, 99 00:03:53,490 --> 00:03:55,540 then that is when it becomes a meteorite. 100 00:03:57,620 --> 00:03:59,060 Generally what happens with meteorites, 101 00:03:59,060 --> 00:04:01,450 is that they will impact the atmosphere 102 00:04:01,450 --> 00:04:03,790 and as that body passes through the atmosphere 103 00:04:03,790 --> 00:04:06,360 it undergoes extreme heating and fragmentation effects. 104 00:04:06,360 --> 00:04:10,030 It fragments into many pieces and those pieces get scattered 105 00:04:10,030 --> 00:04:12,270 all over what we call strewn field. 106 00:04:12,270 --> 00:04:14,620 Those pieces can range in size from little dust particles 107 00:04:14,620 --> 00:04:16,740 all the way up to large blocks. 108 00:04:16,740 --> 00:04:18,540 The biggest meteorites in the world 109 00:04:18,540 --> 00:04:20,180 can be the size of rooms. 110 00:04:23,250 --> 00:04:26,350 In February of 2013, over Chelyabinsk, Russia, 111 00:04:26,350 --> 00:04:30,940 a modest sized meteoroid or asteroid hit the atmosphere, 112 00:04:30,940 --> 00:04:33,860 passed through the atmosphere as a meteoroid, 113 00:04:33,860 --> 00:04:36,760 actually blew up at a relatively high altitude 114 00:04:36,760 --> 00:04:40,540 and showered down fragments of that material 115 00:04:40,540 --> 00:04:42,700 onto the surface of the Earth. 116 00:04:42,700 --> 00:04:46,000 That was not predicted and part of the reason for that 117 00:04:46,000 --> 00:04:48,450 is that it was a relatively small object, 118 00:04:48,450 --> 00:04:51,040 about 15 meters in size or so. 119 00:04:51,040 --> 00:04:53,290 Something that small is hard to detect 120 00:04:53,290 --> 00:04:55,170 with ground-based telescopes. 121 00:04:55,170 --> 00:04:56,460 But perhaps more importantly, 122 00:04:56,460 --> 00:04:59,030 is that it approached from the direction of the Sun. 123 00:04:59,030 --> 00:05:00,200 If something of that size 124 00:05:00,200 --> 00:05:02,270 is coming from the direction of the Sun, 125 00:05:02,270 --> 00:05:03,900 you're not gonna know about it ahead of time, 126 00:05:03,900 --> 00:05:06,150 unless it's been discovered years previously. 127 00:05:07,120 --> 00:05:09,690 This amazing fireball, a streak of light 128 00:05:09,690 --> 00:05:10,950 that appeared in the sky, 129 00:05:10,950 --> 00:05:14,400 it left behind a vapor trail or a gas trail, 130 00:05:14,400 --> 00:05:17,690 and people ran to their windows to look at this. 131 00:05:17,690 --> 00:05:20,000 Several minutes after the fireball, 132 00:05:20,000 --> 00:05:21,540 the shockwave from that event 133 00:05:21,540 --> 00:05:23,040 made its way down to the surface, 134 00:05:23,040 --> 00:05:24,660 and that shockwave blew out windows 135 00:05:24,660 --> 00:05:26,700 all over this region in Russia 136 00:05:26,700 --> 00:05:28,550 and it was the shattering of windows 137 00:05:28,550 --> 00:05:30,300 onto people's faces that were looking out the window 138 00:05:30,300 --> 00:05:33,770 that caused the greatest amount of injuries in that event. 139 00:05:33,770 --> 00:05:36,550 So it wasn't necessary there was a large impact crater 140 00:05:36,550 --> 00:05:37,940 and flash heating or anything like that. 141 00:05:37,940 --> 00:05:40,370 It was the shock of the fireball. 142 00:05:41,580 --> 00:05:43,760 This is actually a relatively small piece. 143 00:05:43,760 --> 00:05:45,880 The largest piece that was recovered 144 00:05:45,880 --> 00:05:48,370 was maybe half a meter in size. 145 00:05:48,370 --> 00:05:50,530 It was actually at the bottom of a lake. 146 00:05:50,530 --> 00:05:53,280 This is actually what we refer to as an ordinary chondrite, 147 00:05:53,280 --> 00:05:56,290 which is unsurprisingly a fairly ordinary meteorite. 148 00:05:56,290 --> 00:05:57,860 It's the most common type of meteorite 149 00:05:57,860 --> 00:05:59,300 that falls on the Earth. 150 00:05:59,300 --> 00:06:01,900 About 80% of all meteorites fall into this category. 151 00:06:03,210 --> 00:06:06,310 Well, this meteorite itself formed 152 00:06:06,310 --> 00:06:09,440 about four and a half billion years ago in the solar system, 153 00:06:09,440 --> 00:06:11,660 and has been floating around in space since then. 154 00:06:11,660 --> 00:06:13,570 It probably lived part of its life 155 00:06:13,570 --> 00:06:15,280 on the surface of a main-belt asteroid. 156 00:06:15,280 --> 00:06:19,110 At some point that main-belt asteroid was probably impacted 157 00:06:19,110 --> 00:06:22,190 and fragments were removed from that surface, 158 00:06:22,190 --> 00:06:23,940 and those fragments then made their way to Earth. 159 00:06:23,940 --> 00:06:27,630 It impacted and delivered the meteorites to the ground. 160 00:06:27,630 --> 00:06:29,950 So there's nothing terribly extraordinary about it, 161 00:06:29,950 --> 00:06:33,230 except for the fact that it has a very specific 162 00:06:33,230 --> 00:06:36,000 and very well documented impact event associated it with it, 163 00:06:36,000 --> 00:06:37,200 which is fairly unusual. 164 00:06:37,200 --> 00:06:39,510 Most of the time these things fall in remote regions, 165 00:06:39,510 --> 00:06:41,730 where people don't witness the fall. 166 00:06:41,730 --> 00:06:46,000 But this is the most documented impact in history. 167 00:06:46,000 --> 00:06:50,220 This is one of the more popular type of meteorites. 168 00:06:50,220 --> 00:06:51,500 It's called a pallasite. 169 00:06:51,500 --> 00:06:56,170 This is a mix of gem-quality peridot, or olivine, 170 00:06:56,170 --> 00:06:58,220 in a matrix of metal. 171 00:06:58,220 --> 00:07:00,500 And so it's a really striking, beautiful piece, 172 00:07:00,500 --> 00:07:02,170 and we think that something like this 173 00:07:02,170 --> 00:07:05,370 probably formed around the core-mantle boundary 174 00:07:05,370 --> 00:07:07,200 of a large protoplanetary body. 175 00:07:07,200 --> 00:07:09,750 So just like the Earth has a core, a mantle and a crust 176 00:07:09,750 --> 00:07:12,060 we think planetary bodies in the solar system 177 00:07:12,060 --> 00:07:13,320 some of them may have undergone 178 00:07:13,320 --> 00:07:14,990 a similar differentiation process. 179 00:07:14,990 --> 00:07:18,800 And so this is a chunk of a core-mantle boundary 180 00:07:18,800 --> 00:07:20,790 of one of these differentiated protoplanets, 181 00:07:20,790 --> 00:07:22,960 and obviously that protoplanet has been disrupted 182 00:07:22,960 --> 00:07:27,690 so that it has been collisionally fragmented into a piece 183 00:07:27,690 --> 00:07:30,520 that we have now collected as this meteorite. 184 00:07:30,520 --> 00:07:32,060 Many of them are cut into these slices 185 00:07:32,060 --> 00:07:34,510 so that you can see the sort of appearance 186 00:07:34,510 --> 00:07:37,740 and internal texture of the meteorite. 187 00:07:37,740 --> 00:07:38,830 The last piece here that I'll show 188 00:07:38,830 --> 00:07:40,980 is a fascinating meteorite. 189 00:07:40,980 --> 00:07:42,000 This is an iron meteorite. 190 00:07:42,000 --> 00:07:43,550 It a slice of the iron meteorite. 191 00:07:43,550 --> 00:07:45,650 Its surface has been etched 192 00:07:45,650 --> 00:07:47,720 so that you can see these crystallized, 193 00:07:47,720 --> 00:07:50,770 interleaved pattern of crystals on the surface of it. 194 00:07:50,770 --> 00:07:52,800 This particular iron meteorite 195 00:07:52,800 --> 00:07:56,700 has the oldest date of any rock in the solar system, 196 00:07:56,700 --> 00:08:00,690 and that 4.567 billion years old. 197 00:08:00,690 --> 00:08:02,690 This rock was probably the first thing 198 00:08:02,690 --> 00:08:03,700 that formed in the solar system, 199 00:08:03,700 --> 00:08:05,950 the first planetary body that formed in the solar system 200 00:08:05,950 --> 00:08:07,750 four and a half billion years ago. 201 00:08:07,750 --> 00:08:09,260 And there's a really interesting history 202 00:08:09,260 --> 00:08:11,250 about how we think something like this 203 00:08:11,250 --> 00:08:13,150 could have formed so quickly. 204 00:08:13,150 --> 00:08:15,530 It requires very rapid accretion, 205 00:08:15,530 --> 00:08:17,180 so the body accreted very quickly, 206 00:08:17,180 --> 00:08:18,800 and because we're looking at iron, 207 00:08:18,800 --> 00:08:20,690 this is a chunk of iron-nickel metal, 208 00:08:20,690 --> 00:08:22,070 this is almost certainly the core 209 00:08:22,070 --> 00:08:23,230 of the differentiated body. 210 00:08:23,230 --> 00:08:26,170 So you're looking at rapid accretion and differentiation 211 00:08:26,170 --> 00:08:28,170 in the first few million years of solar system history, 212 00:08:28,170 --> 00:08:29,110 which is staggering to think 213 00:08:29,110 --> 00:08:31,370 that such complicated geological processes 214 00:08:31,370 --> 00:08:33,870 were happening so early in the solar system. 215 00:08:33,870 --> 00:08:36,830 And it's those bodies that this meteorite represents 216 00:08:36,830 --> 00:08:38,480 that were the precursors to the Earth 217 00:08:38,480 --> 00:08:40,030 and the planets in the solar system. 218 00:08:40,030 --> 00:08:41,540 So it was objects like this 219 00:08:41,540 --> 00:08:43,090 that were colliding and sticking together 220 00:08:43,090 --> 00:08:46,180 that ultimately formed and built up the planets. 221 00:08:46,180 --> 00:08:47,980 So by studying these objects 222 00:08:47,980 --> 00:08:49,920 we can understand the building blocks and the ingredients 223 00:08:49,920 --> 00:08:52,670 that went in to form the Earth and set up the conditions 224 00:08:52,670 --> 00:08:55,400 that make the Earth habitable and sustainable for life.