1 00:00:00,000 --> 00:00:04,608 We've talked about terrestrial planets. We've talked about jovian planets. 2 00:00:04,608 --> 00:00:09,533 we'll probably not have time to talk about the many wonderful moons in the 3 00:00:09,533 --> 00:00:14,395 solar system but I hope you have the tools to go out and read about them and 4 00:00:14,395 --> 00:00:19,446 understand what the criteria are that one would apply to try to understand what 5 00:00:19,446 --> 00:00:22,855 they're like. But there are two types of object that I 6 00:00:22,855 --> 00:00:25,570 still want to mention. The first is asteroids. 7 00:00:25,570 --> 00:00:29,926 So, asteroids are what we call planetesimals that, remember, never made 8 00:00:29,926 --> 00:00:34,198 it to planets. most, but not all also never melted and 9 00:00:34,198 --> 00:00:39,682 differientated and so they're still planetesimals, they never made it to 10 00:00:39,682 --> 00:00:43,071 protoplanet and these are of great interest to 11 00:00:43,071 --> 00:00:48,020 astronomers, because even the moon or a museum was chemically differentiated, 12 00:00:48,020 --> 00:00:53,156 so the chemistry on its surface reflects chemistry billions of years ago, but not 13 00:00:53,156 --> 00:00:57,854 the chemistry of the original nebula whereas, some asteroids might preserve 14 00:00:57,854 --> 00:01:02,739 the actual intact chemistry of the solar nebula because their surface has not 15 00:01:02,739 --> 00:01:06,623 undergone differentiation. You will find exactly the traces of 16 00:01:06,623 --> 00:01:11,070 whatever it was that was around. So, there are absolutely perfect museums, 17 00:01:11,070 --> 00:01:14,750 if you wish. some of them some of the rocks flying 18 00:01:14,750 --> 00:01:20,099 around in space maybe the debris of late collision, so do not reflect necessarily 19 00:01:20,099 --> 00:01:24,444 all of the early structure. There are many ways to generate rocks. 20 00:01:24,444 --> 00:01:29,792 Some never accreted, some might be the result of collisions later, and some did 21 00:01:29,792 --> 00:01:33,670 melt, and those are called dwarf planets. a dwarf planer, 22 00:01:33,670 --> 00:01:38,644 for the record, is something that is in solar orbit, was big enough to melt and 23 00:01:38,644 --> 00:01:42,980 be in hydro, ellipsoid in hydrostatic equilibrium, but is not big enough to 24 00:01:42,980 --> 00:01:45,659 have cleared its orbit of everything else. 25 00:01:45,659 --> 00:01:50,888 This is the definition that caused poor Pluto to be demoted from actual planet to 26 00:01:50,888 --> 00:01:56,166 a dwarf planet. the asteroid Ceres as we'll see, is a 27 00:01:56,166 --> 00:01:59,472 dwarf planet. what do asteroids look like? 28 00:01:59,472 --> 00:02:03,442 Well, this is an image of asteroid PA8, a near-Earth asteroid. 29 00:02:03,442 --> 00:02:07,478 we see as it rotates, we see various sides of the asteroid. 30 00:02:07,478 --> 00:02:10,588 There's a chunk of rock, these are radar images. 31 00:02:10,588 --> 00:02:14,293 This is a near-Earth asteroid, but not a very close call. 32 00:02:14,293 --> 00:02:19,586 This one at its closest was seventeen times the distance to the moon from 33 00:02:19,586 --> 00:02:22,299 Earth. But it was good practice at taking 34 00:02:22,299 --> 00:02:25,740 pictures of an object as it approaches the earth. 35 00:02:25,740 --> 00:02:31,778 these are Vesta, of which we saw beautiful images from the Dawn spacecraft 36 00:02:31,778 --> 00:02:37,440 previously, and Ceres, and we see that unlike Vesta, which is a planetesimal, 37 00:02:37,440 --> 00:02:43,101 Ceres is actually differentiated, it's a protoplanet, it's spherical. 38 00:02:43,101 --> 00:02:48,838 This is the largest of the asteroid, and it's the only object for now in the 39 00:02:48,838 --> 00:02:53,065 asteroid belt that qualifies for dwarf planet status. 40 00:02:53,065 --> 00:02:59,180 And we expect to learn much more about it, when Dawn arrives at Ceres soon. 41 00:02:59,180 --> 00:03:02,407 So, various sizes of objects, where do we find them? 42 00:03:02,407 --> 00:03:06,990 Well, most of them are found in the belt between two and three one-half 43 00:03:06,990 --> 00:03:11,960 astronomical units away from the sun. But as this image to the right shows, 44 00:03:11,960 --> 00:03:15,665 this is actually an image as of December 1st, 2012, I believe. 45 00:03:15,665 --> 00:03:18,654 every green dot here is an individual asteroid. 46 00:03:18,654 --> 00:03:21,224 Every yellow dot is an individual asteroid. 47 00:03:21,224 --> 00:03:25,527 The red dots are also asteroids. These are asteroids that are near-Earth 48 00:03:25,527 --> 00:03:29,890 orbit, that come close to the orbit of Earth and, as you can see, of course, 49 00:03:29,890 --> 00:03:34,373 their sizes do not reflect the actual sizes of the asteroids on this scale, 50 00:03:34,373 --> 00:03:39,035 they'd be much too small for us to see. But as you can see, there are many, many 51 00:03:39,035 --> 00:03:43,816 hundreds of thousands of objects that we have studied and more and more are being 52 00:03:43,816 --> 00:03:46,531 discovered. I'll give you a link to the rate of 53 00:03:46,531 --> 00:03:49,651 discovery. There's a beautiful animation that makes 54 00:03:49,651 --> 00:03:52,597 this clear. And so, most of them move and then there 55 00:03:52,597 --> 00:03:57,161 are these gaps Kirk, Kirkwood gaps in the asteroid belt in which are very little 56 00:03:57,161 --> 00:03:59,819 orbits. Although, there is the possibility of a 57 00:03:59,819 --> 00:04:04,870 stable resonance with Jupiter and there are some asteroids that orbit in orbits 58 00:04:04,870 --> 00:04:09,689 that are resonant with Jupiter, these that we see in the first part of 59 00:04:09,689 --> 00:04:15,037 this animation orbit in a two to three resonance with Jupiter so that they go 60 00:04:15,037 --> 00:04:18,138 around three times each time Jupiter goes around once. 61 00:04:18,138 --> 00:04:21,884 We've here, frozen everything so that Jupiter appears stationary, so the 62 00:04:21,884 --> 00:04:24,997 picture is rotating. Together with Jupiter, we see the in, in, 63 00:04:24,997 --> 00:04:27,899 inner planets running around, and Jupiter is stationary. 64 00:04:27,899 --> 00:04:32,173 And we see these populations and note that in addition to streaming from sort 65 00:04:32,173 --> 00:04:36,130 of one accumulation point to the other, periodically one of them because of 66 00:04:36,130 --> 00:04:40,140 interactions with the rest of the swarm, gets shot down into the inner solar 67 00:04:40,140 --> 00:04:42,353 system. This collection are the Trojans. 68 00:04:42,353 --> 00:04:45,291 These orbit in a one to one residence with Jupiter. 69 00:04:45,291 --> 00:04:49,554 You know they're not going anywhere, they're just orbiting these points 60 70 00:04:49,554 --> 00:04:52,550 degrees west and east of the planet along its orbit. 71 00:04:52,550 --> 00:04:56,698 And again, once in a while because of interactions with the swarm, one of 72 00:04:56,698 --> 00:05:01,076 these, or several, will shoot down into the inner solar system or be shot way 73 00:05:01,076 --> 00:05:03,983 out. and this is where near-Earth asteroids 74 00:05:03,983 --> 00:05:06,050 can come from. Now, 75 00:05:06,050 --> 00:05:10,715 when they get deflected and they come moving near-Earth, they become meteors. 76 00:05:10,715 --> 00:05:15,144 When they hit our atmosphere, bits of rock in our atmosphere heat up due to 77 00:05:15,144 --> 00:05:16,916 friction. Most of them burn up. 78 00:05:16,916 --> 00:05:21,404 We see what we call shooting stars which are meteors hitting the atmosphere. 79 00:05:21,404 --> 00:05:26,246 some of them are large enough that even after burning, some survives to hit the 80 00:05:26,246 --> 00:05:28,313 ground, that makes them a meteorite. 81 00:05:28,313 --> 00:05:31,502 So, a meteorite is a meteor that made it to the ground. 82 00:05:31,502 --> 00:05:36,918 farther out, there are more objects. there are objects between the orbit of 83 00:05:36,918 --> 00:05:41,794 Jupiter and Neptune, those are called Centaurs. And beyond Neptune, we have 84 00:05:41,794 --> 00:05:45,192 trans-Neptunian objects. They include the Kuiper belt. 85 00:05:45,192 --> 00:05:49,807 Here is a, a display of known Kuiper belt objects of which over a thousand are 86 00:05:49,807 --> 00:05:52,884 known. they stretch out from beyond Neptune's 87 00:05:52,884 --> 00:05:58,077 orbit at 30 astronomical units all the way out to 50 and are strongly influenced 88 00:05:58,077 --> 00:06:01,347 by Neptune's gravity. That's what qualifies them. 89 00:06:01,347 --> 00:06:07,052 As one would expect from objects way out in the distant outer regions of the solar 90 00:06:07,052 --> 00:06:11,347 nebula, they are very rich in ices so a lot of them is made of ice. 91 00:06:11,347 --> 00:06:14,287 these provide the source of short-term comets. 92 00:06:14,287 --> 00:06:18,528 Once in a while, gravitational interaction will shoot one of these into 93 00:06:18,528 --> 00:06:22,650 the inter solar nebulae and because they're so icy, rather than rocky, the 94 00:06:22,650 --> 00:06:27,309 big difference between a Kuiper belt object and an asteroid really is that, an 95 00:06:27,309 --> 00:06:31,431 asteroid is made of rock and a Kuiper belt object is rich in ices. 96 00:06:31,431 --> 00:06:37,654 And the icy these, these forms of source comets, as we will see, the prevalence of 97 00:06:37,654 --> 00:06:43,230 short period comets leads one to estimate that there are over a 100,000 Kuiper belt 98 00:06:43,230 --> 00:06:45,863 objects with radii above a 100 kilometers. 99 00:06:45,863 --> 00:06:51,063 So there are a lot of things out there, and then the long period comets as I 100 00:06:51,063 --> 00:06:56,066 said, predict this Oort cloud, which extends out to 50,000 astronomical units 101 00:06:56,066 --> 00:07:00,377 and is spherically symmetric so the solar system goes on for a while. 102 00:07:00,377 --> 00:07:04,717 And investigating what happens to these things, when they do get dipped into the 103 00:07:04,717 --> 00:07:08,135 solar system, is sufficiently interesting that it's worth doing. 104 00:07:08,135 --> 00:07:12,313 So, when something, collisions or the perturbation due to Neptune slows one of 105 00:07:12,313 --> 00:07:16,707 these trans-Neptunian objects down into an eccentric object or by taking them 106 00:07:16,707 --> 00:07:20,560 into the inner solar system, then they encounter, you have this ice ball. 107 00:07:20,560 --> 00:07:25,422 dusty snowball is what they're called with a radius of a few kilometers to a 108 00:07:25,422 --> 00:07:29,008 100 kilometers, say. And this now encounters the more intense 109 00:07:29,008 --> 00:07:33,749 solar radiation and the more intense solar wind in the interior of the solar 110 00:07:33,749 --> 00:07:36,180 system. And this is what creates a comet. 111 00:07:36,180 --> 00:07:41,285 This is what causes these objects to be both visible and sometimes, as the image 112 00:07:41,285 --> 00:07:46,013 nicely shows, very brilliant. they, the the interaction with the solar 113 00:07:46,013 --> 00:07:52,066 nebula, with the solar radiation, and the solar wind creates a coma, the head of a 114 00:07:52,066 --> 00:07:55,566 comet, which is the the brightest part we see. 115 00:07:55,566 --> 00:07:58,848 And then as we can see nicely in the picture, 116 00:07:58,848 --> 00:08:04,105 two tails, a whitish curved dust tail, and a straight blue-glowing ion tail. 117 00:08:04,105 --> 00:08:07,303 And let's discuss what it takes to make a comet. 118 00:08:07,303 --> 00:08:12,033 So, the nucleus of a comet, when it's visible, is hidden inside the coma. 119 00:08:12,033 --> 00:08:16,231 It's very hard to see. Only in the past few years have space 120 00:08:16,231 --> 00:08:21,094 missions been able to go and orbit the nuclei of comets far from the Sun. 121 00:08:21,094 --> 00:08:22,960 This is Comet Temple. And 122 00:08:22,960 --> 00:08:26,488 some of the things they found were somewhat surprising. 123 00:08:26,488 --> 00:08:31,044 This dirty snowball seems to have cratering in it, so it must be a more 124 00:08:31,044 --> 00:08:34,573 rigid thing than was imagined. In fact, somewhat brittle. 125 00:08:34,573 --> 00:08:39,513 So, there's this picture that what we have is sort of an interlaced structure, 126 00:08:39,513 --> 00:08:44,517 with ices and dust and ices and dust, ice serving as the glue, and what happens 127 00:08:44,517 --> 00:08:48,560 when this object approaches the sun is that the ices sublimate. 128 00:08:48,560 --> 00:08:50,815 They're heated. They turn to gas. 129 00:08:50,815 --> 00:08:55,116 At high pressure, they are ejected in jets carrying away dust. 130 00:08:55,116 --> 00:08:59,769 this is not a dust jet. This is the image of the Deep Impact. 131 00:08:59,769 --> 00:09:05,549 In fact what the Deep Impact spacecraft did is it shot a projectile into Comet 132 00:09:05,549 --> 00:09:11,612 Temple in order to generate a crater and study what the ejecta were to try to 133 00:09:11,612 --> 00:09:14,714 understand the inner structure of the comet. 134 00:09:14,714 --> 00:09:18,169 Studies of the out, of the results of this are still ongoing. 135 00:09:18,169 --> 00:09:24,228 Now, but this comet is closer to the sun. We see the sun light hitting it from the 136 00:09:24,228 --> 00:09:29,864 right and what we see is that on the side that the sun warms, we find jets emitted 137 00:09:29,864 --> 00:09:33,842 of these sublimated volatiles carrying away dust in jets. 138 00:09:33,842 --> 00:09:38,749 And this creates a tenuous dusty atmosphere around the comet that because 139 00:09:38,749 --> 00:09:43,986 the comet's gravity is very weak, this is an object a few tens of kilometers in 140 00:09:43,986 --> 00:09:47,364 radius this dust sort of drifts around in a huge 141 00:09:47,364 --> 00:09:52,146 cloud that can be the size of the Sun. And because it's so full of dust, 142 00:09:52,146 --> 00:09:56,024 reflects sunlight. And this is Comet Holmes, which in 2008 143 00:09:56,024 --> 00:10:00,612 underwent presumably some kind of collapse of an ice cave or something 144 00:10:00,612 --> 00:10:05,718 raising a big cloud of dust and generating a coma about the size of the 145 00:10:05,718 --> 00:10:09,596 Sun that was visible to the naked eye as a blurry object. 146 00:10:09,596 --> 00:10:15,288 but this is why you can not see the nucleus of a comet from Earth. Once a 147 00:10:15,288 --> 00:10:21,438 comet is visible, it's carrying around this huge tenuous atmosphere that is the 148 00:10:21,438 --> 00:10:24,195 coma. And then, besides that, there are two 149 00:10:24,195 --> 00:10:29,452 tails and we saw the two of them. the image of a comet zooming across the 150 00:10:29,452 --> 00:10:34,510 sky with its tail strung out behind it is very natural and intuitive but, of 151 00:10:34,510 --> 00:10:39,767 course, that intuition is based upon running around on Earth in the presence 152 00:10:39,767 --> 00:10:42,562 of an atmosphere. a comet does not zoom. 153 00:10:42,562 --> 00:10:46,488 A comet orbits. the orbital periods of comets range from 154 00:10:46,488 --> 00:10:51,678 a few decades to millions of years and so their motion across the sky is very slow. 155 00:10:51,678 --> 00:10:56,324 They rise and set like stars, of course, and then they move across the celestial 156 00:10:56,324 --> 00:11:01,676 sphere at a rather dignified slow pace, a, a comet and a shooting star are very 157 00:11:01,676 --> 00:11:04,617 different objects. shooting stars, as I said, are 158 00:11:04,617 --> 00:11:07,323 terrestrial. We'll talk about that in a second. 159 00:11:07,323 --> 00:11:12,060 but they do sprout tails. These tails are the ejecta gas and dust, 160 00:11:12,060 --> 00:11:17,739 ejected from the planet being pushed away from the sun by both the pressure of 161 00:11:17,739 --> 00:11:23,130 solar radiation, light carries energy and also momentum, and light applies a 162 00:11:23,130 --> 00:11:29,024 pressure, and so the solar radiation as well as the solar wind push some of the 163 00:11:29,024 --> 00:11:34,775 ejecta out, stringing it out in a tail that can be a few astronomical units long 164 00:11:34,775 --> 00:11:39,734 so astronomical unit, 150 million kilometers, the distance from Earth to 165 00:11:39,734 --> 00:11:44,951 the sun, these are solar system scale objects, these tails that always point 166 00:11:44,951 --> 00:11:50,004 away from the sun so that as a comet is approaching the sun, the tail stretches 167 00:11:50,004 --> 00:11:52,961 behind it. As it is moving away from the sun, the 168 00:11:52,961 --> 00:11:55,959 tail precedes it. this is not a problem. 169 00:11:55,959 --> 00:11:59,738 the two tails have to do with the two mechanisms. 170 00:11:59,738 --> 00:12:04,390 dust, which is indeed, propelled by the pressure of the sun 171 00:12:04,390 --> 00:12:11,407 of both radiation and of the solar wind is propelled away from the sun but as the 172 00:12:11,407 --> 00:12:17,910 comet moves along its trajectory, the dust it ejected weeks ago, remains away, 173 00:12:17,910 --> 00:12:23,006 pointed away from the sun at the position where the comet was so it produces this 174 00:12:23,006 --> 00:12:27,542 magnificent arching structure. So here, the sun is sort of has just set. 175 00:12:27,542 --> 00:12:30,959 So, it's below. the dust is being ejected in an upward 176 00:12:30,959 --> 00:12:35,930 direction but the comet itself is moving from right to left so that the dust it 177 00:12:35,930 --> 00:12:39,100 ejected a few days ago is now far to your right. 178 00:12:39,100 --> 00:12:44,587 on the other hand, the ion tail the gases that are pushed away and, and the dust 179 00:12:44,587 --> 00:12:49,140 tail is, of course, visible because dust reflects sunlight, so you see this 180 00:12:49,140 --> 00:12:53,193 beautiful white tail. the ion tail, not evident in this picture 181 00:12:53,193 --> 00:12:57,808 of comet Mcnaught, but certainly evident in this picture does not curve. 182 00:12:57,808 --> 00:13:01,362 It is always pointed directly straight away from the sun. 183 00:13:01,362 --> 00:13:06,475 the gases are ionized by solar wind, and then once the up charged particles, the 184 00:13:06,475 --> 00:13:10,404 interactions are governed by magnetic interactions essentially, 185 00:13:10,404 --> 00:13:14,855 the comet acquires a magnetosphere. The magnetosphere interacts in a 186 00:13:14,855 --> 00:13:19,693 complicated way with the solar wind. And the result is that the ion tail 187 00:13:19,693 --> 00:13:24,934 always points directly away from the sun and is redirected, if you will, as the 188 00:13:24,934 --> 00:13:29,855 comet moves so it does not curve. And ionized gas recombination, the same 189 00:13:29,855 --> 00:13:36,048 kind of ghostly blue glow that we saw in our electron tube is how you characterize 190 00:13:36,048 --> 00:13:39,736 the ion tail. So, these are these beautiful objects, 191 00:13:39,736 --> 00:13:43,494 comets, but a comet can't last forever, it's a this ice ball. 192 00:13:43,494 --> 00:13:48,574 As the ice evaporates, remember, ice was perhaps the glue that held things 193 00:13:48,574 --> 00:13:52,053 together. And a comet can't last forever, how do 194 00:13:52,053 --> 00:13:55,532 comets end? Well, one of four possible ends, I guess. 195 00:13:55,532 --> 00:13:58,230 One is, these things are at highly eccentric 196 00:13:58,230 --> 00:14:01,575 elliptic orbits. They can come close to the wrong object, 197 00:14:01,575 --> 00:14:05,637 Jupiter, Neptune Uranus. And collide gravitationally in such a way 198 00:14:05,637 --> 00:14:08,922 that they are completely ejected from the solar system. 199 00:14:08,922 --> 00:14:13,760 This certainly can happen especially to the Oort cloud comets, that are rather 200 00:14:13,760 --> 00:14:18,479 weakly bound so a small perturbation can send them completely out of the solar 201 00:14:18,479 --> 00:14:20,690 system. And then, they won't come back. 202 00:14:20,690 --> 00:14:25,070 Another possible end is extinction. Eventually, a comet might run out of 203 00:14:25,070 --> 00:14:30,067 volatiles, have them all sublimated away or maybe some of them might be sealed 204 00:14:30,067 --> 00:14:35,434 deep inside the comet but inaccessible and so you have basically a chunk of rock 205 00:14:35,434 --> 00:14:39,321 now or dust, orbiting the sun. It'll never make a comet any more. 206 00:14:39,321 --> 00:14:44,318 some suggestions are that as the ice leaves, the thing compacts and maybe some 207 00:14:44,318 --> 00:14:48,020 of what we call asteroids are essentially extinct comets 208 00:14:48,020 --> 00:14:53,103 having been deprived of their volatiles after passage through the sun. 209 00:14:53,103 --> 00:14:58,814 another possibility is disintegration. as you lose the icy glue that held 210 00:14:58,814 --> 00:15:04,455 together the dust, either the recoil from these jets submitted on the sun side or 211 00:15:04,455 --> 00:15:09,678 the tidal forces of the sun or some planet near which you come, can cause a, 212 00:15:09,678 --> 00:15:14,692 a comet to disintegrate as in these Hubble images brilliantly, Comet 73P. 213 00:15:14,692 --> 00:15:20,402 The P, by the way, stands for periodic. is shown disintegrating and then there, 214 00:15:20,402 --> 00:15:24,993 the, the bottom image shows from the ground, shows that there are many more 215 00:15:24,993 --> 00:15:29,760 fragments scattered around, so there's a, a hierarchical fragmentation going on. 216 00:15:29,760 --> 00:15:33,906 And then the other possible end for a comet is a collision. 217 00:15:33,906 --> 00:15:38,052 famously in 1994, Comet Shoemaker-Levy 9 this, did both. 218 00:15:38,052 --> 00:15:43,135 It first disintegrated into about nineteen pieces in the tidal, given the 219 00:15:43,135 --> 00:15:46,345 tidal force of Jupiter which it was approaching. 220 00:15:46,345 --> 00:15:51,494 And then, sequentially one after the other, all of these ploughed into Jupiter 221 00:15:51,494 --> 00:15:56,844 giving us a rare and at the time unique opportunity perhaps to see what ejecta 222 00:15:56,844 --> 00:16:02,461 they pushed out of the, pulled out of the atmosphere and to learn something about 223 00:16:02,461 --> 00:16:05,205 the constitution of Jupiter's atmosphere. 224 00:16:05,205 --> 00:16:10,250 Sadly, this collision occurred on the far side of Jupiter, so we couldn't see it. 225 00:16:10,250 --> 00:16:14,720 but every telescope on Earth was trained on Jupiter, because Jupiter rotates so 226 00:16:14,720 --> 00:16:19,416 rapidly, that within twenty minutes after the collision, the impacted areas faced 227 00:16:19,416 --> 00:16:22,302 the Earth. And in this Hubble telescope image, we 228 00:16:22,302 --> 00:16:26,603 see at least four of these dark spots in the lower right-hand corner of the 229 00:16:26,603 --> 00:16:28,810 planet, which are collision zones, where 230 00:16:28,810 --> 00:16:31,526 fragments of the comet collided with Jupiter. 231 00:16:31,526 --> 00:16:36,154 this was so successful that a decade or so later we dropped a probe 232 00:16:36,154 --> 00:16:41,499 of our own into Jupiter's atmosphere to try to understand the constitution and 233 00:16:41,499 --> 00:16:46,090 processes inside Jupiter's atmosphere with mixed results, I should say. 234 00:16:46,090 --> 00:16:50,469 so these are, one of these things is going to end a comet's career. 235 00:16:50,469 --> 00:16:55,235 A comet is not a stable object. It does not last billions upon billions 236 00:16:55,235 --> 00:16:59,351 of years like a planet. but it leaves a trace and these traces 237 00:16:59,351 --> 00:17:03,518 are interest, of interest to us. And if a comet does not collide with a 238 00:17:03,518 --> 00:17:07,273 planet like Jupiter, then, of course, there's always the option of 239 00:17:07,273 --> 00:17:10,852 collision with the sun. Here we see comet Lovejoy, ploughing on 240 00:17:10,852 --> 00:17:14,431 its way to the sun. We see the magnetic interactions between 241 00:17:14,431 --> 00:17:19,301 the solar wind and the tail, cause all this complicated dynamics in the tail the 242 00:17:19,301 --> 00:17:24,220 comet is on its way to a firey death. we see the comets diving into the sun 243 00:17:24,220 --> 00:17:28,610 with reasonable regularity. The exciting thing about comet Lovejoy, 244 00:17:28,610 --> 00:17:33,188 is that it actually survived the encounter, and came out the other end. 245 00:17:33,188 --> 00:17:37,955 And so, a year ago there was great joy among the sun grazing comet community, 246 00:17:37,955 --> 00:17:40,840 when Lovejoy actually survived the collision. 247 00:17:40,840 --> 00:17:44,101 You can look it up and find the briefless blogs. 248 00:17:44,101 --> 00:17:48,084 so, comets don't last forever, but they leave a trace. 249 00:17:48,084 --> 00:17:53,118 So, every time a comet comes near the sun, a bunch of it evaporates. 250 00:17:53,118 --> 00:17:57,715 It has this, it leaves behind this dust and gas that it ejected. 251 00:17:57,715 --> 00:18:03,115 all of this dust is scattered about the planet's orb, the comet's orbit. 252 00:18:03,115 --> 00:18:08,566 And as we discussed, what that does is the bits that are too close to the sun 253 00:18:08,566 --> 00:18:12,247 and inner orbits get stretched out ahead of the comet. 254 00:18:12,247 --> 00:18:17,497 These that are a little bit behind outside the orbit, become stretched out 255 00:18:17,497 --> 00:18:22,951 behind the comet, and essentially this dust forms a ring-like structure around 256 00:18:22,951 --> 00:18:27,587 the entire orbit of a comet. And after a few passes, the entire orbit 257 00:18:27,587 --> 00:18:33,109 of a comet is full of this orbiting space junk debris from prior passes near the 258 00:18:33,109 --> 00:18:36,109 sun. And what this means is that if a comets 259 00:18:36,109 --> 00:18:41,080 orbit happens to intersect or come close to the Earth's orbit, 260 00:18:41,080 --> 00:18:44,830 then, well, the timing is off. You might not collide with the comet. 261 00:18:44,830 --> 00:18:49,556 But once a year, when the Earth approaches that point in its orbit at 262 00:18:49,556 --> 00:18:54,968 which it is crossing the trajectory, the orbit of a comet then we will beats, we 263 00:18:54,968 --> 00:19:00,585 will experience an increased incidence of space junk, we call that a meteor shower 264 00:19:00,585 --> 00:19:05,928 and not only, and that's a predictable event because we know where the comet's 265 00:19:05,928 --> 00:19:10,860 orbit is, not only will the incident of space junk and shooting stars be 266 00:19:10,860 --> 00:19:16,135 increased, but because they are all orbiting in some particular orbit of a 267 00:19:16,135 --> 00:19:19,591 particular comet. The Earth is moving in a particular 268 00:19:19,591 --> 00:19:21,820 direction. There will be a 269 00:19:21,820 --> 00:19:25,992 predictable, repeatable direction from which all these shooting stars will 270 00:19:25,992 --> 00:19:30,390 appear to come because they all impinge the air, moving in roughly the same 271 00:19:30,390 --> 00:19:33,773 relevant direction. in this beautiful image from the 2001 272 00:19:33,773 --> 00:19:37,776 Leonid meteor shower in November 2001, which was particularly brilliant, 273 00:19:37,776 --> 00:19:41,385 what we see is indeed, there are shooting stars all over the sky. 274 00:19:41,385 --> 00:19:45,727 These meteors hit the atmosphere where they hit it but they all appeared to be 275 00:19:45,727 --> 00:19:49,617 coming from one point in the sky. This is called the quadrant, in this 276 00:19:49,617 --> 00:19:53,113 case, the quadrant for the Leonid is in the constellation Leo. 277 00:19:53,113 --> 00:19:55,820 They all appear to be coming from there, this is, 278 00:19:55,820 --> 00:19:58,991 if you look at this, you'll immediately see the perspective 279 00:19:58,991 --> 00:20:00,260 point of view. It's 280 00:20:00,260 --> 00:20:04,590 this is the direction from which all of these 281 00:20:04,590 --> 00:20:10,981 space debris is impinging upon the Earth and this is the source of our annually 282 00:20:10,981 --> 00:20:13,600 repeating periodic meteor showers.