1 00:00:50,091 --> 00:00:53,652 This is the age of planetary exploration... 2 00:00:53,860 --> 00:00:57,296 ...when our ships have begun to sail the heavens. 3 00:00:58,832 --> 00:01:02,427 In those heavens, there are some worlds much like hell. 4 00:01:02,636 --> 00:01:06,538 Our planet is, in comparison, much like a heaven. 5 00:01:06,740 --> 00:01:08,970 But the gates of heaven and hell... 6 00:01:09,175 --> 00:01:12,235 ...are adjacent and unmarked. 7 00:01:14,748 --> 00:01:17,216 The Earth is a lovely... 8 00:01:17,417 --> 00:01:19,681 ...and more or less placid place. 9 00:01:20,153 --> 00:01:22,951 Things change, but slowly. 10 00:01:23,156 --> 00:01:27,718 You can lead a full life and never encounter a natural catastrophe... 11 00:01:27,928 --> 00:01:29,759 ...more violent than a storm. 12 00:01:29,963 --> 00:01:32,431 And so we become complacent... 13 00:01:32,632 --> 00:01:33,826 ...relaxed... 14 00:01:34,034 --> 00:01:35,729 ...unconcerned. 15 00:01:35,969 --> 00:01:40,429 But in the history of the solar system and even in human history... 16 00:01:40,640 --> 00:01:43,302 ...there are clear records of extraordinary... 17 00:01:43,510 --> 00:01:45,671 ...and devastating catastrophes. 18 00:01:45,879 --> 00:01:48,473 We have now achieved the dubious distinction... 19 00:01:48,682 --> 00:01:50,843 ...of making our own major catastrophes... 20 00:01:51,051 --> 00:01:53,781 ...both intentional and inadvertent. 21 00:01:54,554 --> 00:01:56,886 On the landscapes of other planets... 22 00:01:57,090 --> 00:01:59,581 ...where past records are better preserved... 23 00:01:59,793 --> 00:02:02,261 ...there's abundant evidence of major catastrophes. 24 00:02:02,462 --> 00:02:04,259 It's all a matter of time scale. 25 00:02:04,464 --> 00:02:07,126 An event which is improbable in 100 years... 26 00:02:07,334 --> 00:02:10,098 ...may be inevitable in 100 million. 27 00:02:10,303 --> 00:02:13,466 But even on the Earth in this century... 28 00:02:13,673 --> 00:02:17,336 ...there have been bizarre natural events. 29 00:02:22,115 --> 00:02:24,811 In remote central Siberia... 30 00:02:25,018 --> 00:02:27,077 ...there was a time when the Tungus people... 31 00:02:27,287 --> 00:02:30,256 ...told strange tales of a giant fireball... 32 00:02:30,457 --> 00:02:33,483 ...that split the sky and shook the Earth. 33 00:02:35,095 --> 00:02:37,495 They told of a blast of searing wind... 34 00:02:37,697 --> 00:02:40,097 ...that knocked down people and forests. 35 00:02:41,835 --> 00:02:44,497 It happened, they said, on a summer's morning... 36 00:02:44,704 --> 00:02:46,535 ...in the year 1908. 37 00:02:46,840 --> 00:02:48,273 In the late 1920s... 38 00:02:48,475 --> 00:02:51,342 ...L.A. Kulik, a Soviet scientist... 39 00:02:51,544 --> 00:02:54,604 ...organized expeditions to try and solve the mystery. 40 00:02:56,983 --> 00:03:00,544 He built boats to penetrate this trackless land: 41 00:03:01,121 --> 00:03:02,713 Snowbound in winter... 42 00:03:02,922 --> 00:03:05,482 ...a swampy morass in summer. 43 00:03:08,294 --> 00:03:11,889 Eyewitnesses told of a ball of flame... 44 00:03:12,098 --> 00:03:13,622 ...larger than the sun... 45 00:03:13,833 --> 00:03:17,633 ...that had blazed across the sky 20 years before. 46 00:03:18,271 --> 00:03:22,105 Kulik assumed a giant meteorite had struck the Earth. 47 00:03:24,644 --> 00:03:28,205 He expected to find an enormous impact crater... 48 00:03:28,415 --> 00:03:30,542 ...and rare meteorite fragments... 49 00:03:30,750 --> 00:03:33,480 ...chipped off some distant asteroid. 50 00:03:36,489 --> 00:03:38,423 However, at ground zero... 51 00:03:38,625 --> 00:03:41,719 ...Kulik found upright trees stripped of their branches... 52 00:03:41,928 --> 00:03:44,726 ...but not a trace of the meteorite or its impact crater. 53 00:03:44,931 --> 00:03:46,592 He was deeply puzzled. 54 00:03:46,800 --> 00:03:50,793 He thought there were meteorite fragments buried in the swampy ground. 55 00:03:51,738 --> 00:03:55,299 So he set about digging trenches and pumping out the water. 56 00:03:55,508 --> 00:03:59,035 But the expected meteoritic rock and iron was not found. 57 00:04:01,181 --> 00:04:04,446 Undaunted, Kulik went on to make a thorough survey... 58 00:04:04,651 --> 00:04:07,415 ...despite the swarms of insects and other hardships. 59 00:04:07,620 --> 00:04:10,418 Because he discovered something that, in his own words... 60 00:04:10,623 --> 00:04:15,560 ..."exceeded all tales of eyewitnesses and my wildest expectations." 61 00:04:18,064 --> 00:04:21,795 For more than 20 kilometers in every direction from ground zero... 62 00:04:22,001 --> 00:04:26,938 ...the trees were flattened radially outward like broken matchsticks. 63 00:04:31,444 --> 00:04:33,469 There must've been a powerful explosion... 64 00:04:33,680 --> 00:04:35,705 ...several kilometers above the ground. 65 00:04:35,915 --> 00:04:38,713 The pressure wave, spreading out at the speed of sound... 66 00:04:38,918 --> 00:04:42,410 ...was reconstructed from barometric records at weather stations... 67 00:04:42,622 --> 00:04:47,252 ...across Siberia, through Russia and on into Western Europe. 68 00:04:47,460 --> 00:04:50,952 Dust from the explosion reflected so much sunlight back to Earth... 69 00:04:51,164 --> 00:04:53,223 ...that people could read by it at night... 70 00:04:53,433 --> 00:04:56,766 ...in London, 10,000 kilometers away. 71 00:05:02,041 --> 00:05:05,670 This really remarkable occurrence... 72 00:05:05,880 --> 00:05:08,781 ...is called the Tunguska Event. 73 00:05:09,316 --> 00:05:10,840 But what was it? 74 00:05:11,251 --> 00:05:15,381 Well, perhaps, some scientists have suggested... 75 00:05:15,588 --> 00:05:19,456 ...it was a chunk of antimatter from space... 76 00:05:19,659 --> 00:05:24,062 ...annihilated on contact with the ordinary matter of the Earth... 77 00:05:24,264 --> 00:05:27,597 ...disappearing in a flash of gamma rays. 78 00:05:28,001 --> 00:05:32,165 But the radioactivity you'd expect from matter-antimatter annihilation... 79 00:05:32,372 --> 00:05:35,864 ...is to be found nowhere at the impact site. 80 00:05:36,810 --> 00:05:41,076 Or, perhaps, other scientists have suggested... 81 00:05:41,281 --> 00:05:43,613 ...it was a mini black hole from space... 82 00:05:43,817 --> 00:05:45,808 ...which impacted the Earth in Siberia... 83 00:05:46,019 --> 00:05:48,385 ...tunneled through the solid body of Earth... 84 00:05:48,588 --> 00:05:50,852 ...and plunged out the other side. 85 00:05:51,057 --> 00:05:54,254 But the records of atmospheric shock waves give not a hint... 86 00:05:54,460 --> 00:05:58,954 ...of something booming out of the North Atlantic later that day. 87 00:05:59,199 --> 00:06:03,636 Or maybe, other people have speculated, it was a spaceship... 88 00:06:03,837 --> 00:06:08,171 ...of some unimaginably advanced extraterrestrial civilization... 89 00:06:08,374 --> 00:06:10,934 ...in desperate mechanical trouble... 90 00:06:11,144 --> 00:06:15,308 ...crashing in a remote region of an obscure planet. 91 00:06:15,615 --> 00:06:19,415 Well, if so, it's pretty startling that at the impact site... 92 00:06:19,619 --> 00:06:23,248 ...there is not a piece, not the tiniest transistor... 93 00:06:23,456 --> 00:06:25,822 ...of a crashed spacecraft. 94 00:06:26,025 --> 00:06:28,960 More prosaically, perhaps it was a large meteorite... 95 00:06:29,162 --> 00:06:31,562 ...or a small asteroid which hit the Earth. 96 00:06:31,764 --> 00:06:35,256 But even here, there are no observable telltale... 97 00:06:35,468 --> 00:06:38,460 ...rocky or metallic fragments of the sort... 98 00:06:38,671 --> 00:06:41,333 ...that you'd expect from such an impact. 99 00:06:41,641 --> 00:06:44,769 The key point of the Tunguska Event... 100 00:06:44,978 --> 00:06:49,278 ...is that there was a tremendous explosion, a great shock wave... 101 00:06:49,482 --> 00:06:52,713 ...many trees burned, an enormous forest fire... 102 00:06:52,952 --> 00:06:57,389 ...and yet, no crater in the ground. 103 00:06:57,590 --> 00:07:00,024 There seems to be only one explanation... 104 00:07:00,226 --> 00:07:02,786 ...which is consistent with these facts. 105 00:07:02,996 --> 00:07:06,022 And that explanation is this: 106 00:07:06,966 --> 00:07:10,868 In 1908, a piece of a comet... 107 00:07:11,070 --> 00:07:12,469 ...hit the Earth. 108 00:07:19,445 --> 00:07:21,470 No one saw it approach. 109 00:07:21,681 --> 00:07:26,175 A small point of light lost in the glare of the morning sun. 110 00:07:28,087 --> 00:07:31,488 It had been drifting for centuries through the inner solar system... 111 00:07:31,691 --> 00:07:35,923 ...like an iceberg in the ocean of interplanetary space. 112 00:07:42,969 --> 00:07:45,369 But this time, by accident... 113 00:07:45,571 --> 00:07:48,165 ...there was a planet in the way. 114 00:07:54,514 --> 00:07:58,245 From the time and direction of its approach, what hit the Earth... 115 00:07:58,451 --> 00:08:02,217 ...seems to have been a fragment of a comet named Encke. 116 00:08:02,422 --> 00:08:05,755 Hurtling at more than 100,000 kilometers an hour... 117 00:08:05,959 --> 00:08:09,326 ...it was a mountain of ice about the size of a football field... 118 00:08:09,529 --> 00:08:12,965 ...and weighing almost a million tons. 119 00:08:14,167 --> 00:08:17,864 There was no warning, until it plunged into the atmosphere. 120 00:08:58,945 --> 00:09:02,346 If such an explosion happened today... 121 00:09:02,548 --> 00:09:05,278 ...it might be thought, in the panic of the moment... 122 00:09:05,885 --> 00:09:08,217 ...to be produced by a nuclear weapon. 123 00:09:08,488 --> 00:09:11,218 Such a cometary impact and fireball... 124 00:09:11,424 --> 00:09:14,916 ...simulates all the effects of a 15-megaton nuclear burst... 125 00:09:15,128 --> 00:09:17,756 ...including the mushroom cloud, with one exception: 126 00:09:17,965 --> 00:09:19,956 There would be no radiation. 127 00:09:20,166 --> 00:09:23,158 So could a rare but natural event... 128 00:09:23,369 --> 00:09:25,394 ...the impact of a comet with Earth... 129 00:09:25,605 --> 00:09:28,199 ...trigger a nuclear war? 130 00:09:29,042 --> 00:09:32,478 It's a strange scenario: A small comet hits the Earth... 131 00:09:32,678 --> 00:09:35,112 ...as millions have during Earth's history... 132 00:09:35,314 --> 00:09:37,282 ...and the response of our civilization... 133 00:09:37,483 --> 00:09:40,714 ...is promptly to self-destruct. 134 00:09:42,288 --> 00:09:45,621 Maybe it's unlikely, but it might be a good idea... 135 00:09:45,825 --> 00:09:49,317 ...to understand comets and collisions and catastrophes... 136 00:09:49,530 --> 00:09:52,158 ...a little bit better than we do. 137 00:09:52,365 --> 00:09:56,426 Now, a comet, at least as far as we understand them today... 138 00:09:56,637 --> 00:09:57,968 ...is made mostly of ice: 139 00:09:58,171 --> 00:10:01,004 Water ice, maybe some ammonia ice... 140 00:10:01,207 --> 00:10:03,471 ...a little bit of methane ice. 141 00:10:03,877 --> 00:10:06,675 So in striking the Earth's atmosphere... 142 00:10:06,879 --> 00:10:08,972 ...a modest cometary fragment... 143 00:10:09,182 --> 00:10:14,051 ...will produce a great radiant fireball and a mighty blast wave. 144 00:10:14,254 --> 00:10:16,484 It'll burn trees and level forests... 145 00:10:16,689 --> 00:10:19,351 ...and make a sound heard around the world. 146 00:10:19,559 --> 00:10:22,426 But it need not make a crater in the ground. 147 00:10:22,628 --> 00:10:27,065 Why? Because the ices in the comet are all melted in the impact. 148 00:10:27,266 --> 00:10:30,724 And there's going to be very few recognizable pieces of comet... 149 00:10:30,937 --> 00:10:32,427 ...left on the ground. 150 00:10:39,078 --> 00:10:41,876 We humans like to think of the heavens as stable... 151 00:10:42,082 --> 00:10:44,573 ...serene, unchanging. 152 00:10:46,486 --> 00:10:48,386 But comets suddenly appear... 153 00:10:48,588 --> 00:10:52,718 ...and hang ominously in the sky, night after night, for weeks. 154 00:10:55,094 --> 00:10:59,497 So the idea developed that the comet had to be there for a reason. 155 00:10:59,700 --> 00:11:02,965 The reason was that comets were predictions of disaster... 156 00:11:03,170 --> 00:11:07,504 ...that they foretold the deaths of princes and the fall of kingdoms. 157 00:11:07,707 --> 00:11:10,198 In 1066, for example... 158 00:11:10,410 --> 00:11:14,506 ...the Normans witnessed an apparition or appearance of Halley's comet. 159 00:11:14,714 --> 00:11:18,514 Since a comet must, they thought, predict the fall of some kingdom... 160 00:11:18,718 --> 00:11:20,982 ...they promptly invaded England. 161 00:11:21,187 --> 00:11:23,781 Here's King Harold of England looking a little glum. 162 00:11:23,991 --> 00:11:26,357 The events were noted in the Bayeux tapestry... 163 00:11:26,559 --> 00:11:29,187 ...a kind of newspaper of the day. 164 00:11:29,395 --> 00:11:31,454 Or, in the early 13th century... 165 00:11:31,664 --> 00:11:35,191 ...Giotto, one of the founders of modern realistic painting... 166 00:11:35,401 --> 00:11:37,995 ...witnessed another apparition of comet Halley... 167 00:11:38,205 --> 00:11:41,470 ...and inserted it into a nativity he was painting. 168 00:11:41,675 --> 00:11:45,907 A harbinger of a different sort of change of kingdoms. 169 00:11:46,579 --> 00:11:48,877 Around 1517... 170 00:11:49,082 --> 00:11:52,848 ...another great comet appeared. This time it was seen in Mexico. 171 00:11:53,052 --> 00:11:54,917 And the Aztec emperor, Moctezuma... 172 00:11:55,254 --> 00:11:56,881 ...maybe this is he... 173 00:11:57,456 --> 00:11:59,447 ...promptly executed his astrologers. 174 00:11:59,659 --> 00:12:04,255 Why? They hadn't predicted the comet, and they sure hadn't explained it. 175 00:12:04,463 --> 00:12:09,366 Moctezuma was positive that the comet foretold some dreadful disaster. 176 00:12:09,570 --> 00:12:12,164 He became distant and gloomy... 177 00:12:12,371 --> 00:12:14,839 ...and in that way, helped to set the stage... 178 00:12:15,041 --> 00:12:18,943 ...for the successful Spanish conquest of Mexico under Cortés. 179 00:12:19,445 --> 00:12:23,074 In many cases, a superstitious belief in comets... 180 00:12:23,282 --> 00:12:26,217 ...becomes a self-fulfilling prophecy. 181 00:12:27,054 --> 00:12:29,614 Here are two quite different representations... 182 00:12:29,822 --> 00:12:32,450 ...of the great comet of 1577: 183 00:12:32,658 --> 00:12:35,354 This one pictured by the Turks... 184 00:12:38,231 --> 00:12:40,563 ...and this one by the Germans. 185 00:12:47,139 --> 00:12:50,438 In 1705, Edmund Halley finally... 186 00:12:50,643 --> 00:12:53,168 ...figured out that the same spectacular comet... 187 00:12:53,379 --> 00:12:57,179 ...was booming by the Earth every 76 years, like clockwork. 188 00:12:57,383 --> 00:13:01,114 That comet is now called, appropriately, comet Halley. 189 00:13:01,320 --> 00:13:05,120 And it's the same one that we talked about before, the comet of 1066. 190 00:13:05,324 --> 00:13:08,259 At that point, the subject began to lose a little... 191 00:13:08,461 --> 00:13:11,828 ...of its burden of superstition, but hardly all. 192 00:13:12,031 --> 00:13:15,797 Public fear of comets survived. Well, for example... 193 00:13:16,336 --> 00:13:19,499 ...look at this terribly nasty comet of 1857... 194 00:13:19,706 --> 00:13:23,142 ...that some people figured would splinter the Earth. 195 00:13:24,477 --> 00:13:28,538 By 1910, Halley's comet returned once more. 196 00:13:28,748 --> 00:13:32,206 But this time, astronomers using a new tool, the spectroscope... 197 00:13:32,418 --> 00:13:37,321 ...had discovered cyanogen gas in the tail of a comet. 198 00:13:37,524 --> 00:13:39,856 Now, cyanogen is a poison. 199 00:13:40,059 --> 00:13:43,688 The Earth was to pass through this poisonous tail. 200 00:13:43,896 --> 00:13:47,889 The fact that the gas was astonishingly, fabulously thin... 201 00:13:48,101 --> 00:13:50,262 ...reassured almost nobody. 202 00:13:50,469 --> 00:13:54,769 For example, look at the headlines in the Los Angeles Examiner... 203 00:13:54,975 --> 00:13:57,671 ...for May 9, 1910: 204 00:13:57,877 --> 00:14:01,870 "Say, Has That Comet 'Cyanogened' You Yet?" 205 00:14:02,082 --> 00:14:05,176 "Entire Human Race Due For Free Gaseous Bath. 206 00:14:05,385 --> 00:14:07,410 Expect High Jinks." 207 00:14:07,620 --> 00:14:12,489 Or take this from the San Francisco Chronicle, May 15, 1910: 208 00:14:12,693 --> 00:14:16,288 "Comet Comes And Husband Reforms." 209 00:14:16,495 --> 00:14:18,690 "Comet Parties Now Fad In New York." 210 00:14:18,898 --> 00:14:20,729 Amazing stuff! 211 00:14:20,933 --> 00:14:24,369 In 1910, people were holding comet parties, not so much to... 212 00:14:24,570 --> 00:14:28,165 ...celebrate the end of the world as to make merry before it happened. 213 00:14:28,407 --> 00:14:32,935 There were entrepreneurs who were hawking comet pills. 214 00:14:33,380 --> 00:14:35,439 I think I'm gonna take one for later. 215 00:14:35,648 --> 00:14:38,481 And there were those who were selling... 216 00:14:38,951 --> 00:14:43,888 ...gas masks to protect against the cyanogen. 217 00:14:44,291 --> 00:14:48,660 And comet nuttiness didn't stop in 1910. 218 00:14:53,899 --> 00:14:57,835 Long before 1066, humans marveled at comets. 219 00:14:58,038 --> 00:15:01,235 Our generation is beginning to understand them. 220 00:15:09,949 --> 00:15:12,281 Mercury, Venus, Earth and Mars... 221 00:15:12,485 --> 00:15:15,943 ...are small planets made mostly of rock and iron. 222 00:15:16,722 --> 00:15:19,885 Farther out where it's colder, are the giant planets... 223 00:15:20,092 --> 00:15:21,855 ...made mostly of gas. 224 00:15:22,061 --> 00:15:25,258 But comets originate from a great cloud beyond the planets... 225 00:15:25,464 --> 00:15:27,694 ...almost halfway to the nearest star. 226 00:15:27,900 --> 00:15:30,130 Occasionally, one falls in... 227 00:15:30,336 --> 00:15:32,429 ...accelerated by the sun's gravity. 228 00:15:32,639 --> 00:15:34,834 Because it's made mostly of ice, the comet... 229 00:15:35,040 --> 00:15:37,167 ...evaporates as it approaches the sun. 230 00:15:37,376 --> 00:15:40,243 The vapor is blown back by the solar wind... 231 00:15:40,447 --> 00:15:42,312 ...forming the cometary tail. 232 00:15:42,515 --> 00:15:44,881 Then it's flung back into outer darkness... 233 00:15:45,084 --> 00:15:46,415 ...its orbit so large... 234 00:15:46,620 --> 00:15:49,418 ...that it will not return for millions of years. 235 00:15:49,622 --> 00:15:52,182 These are the long-period comets. 236 00:15:52,391 --> 00:15:55,758 For every one plunging close enough to the sun to be discovered... 237 00:15:55,961 --> 00:15:57,690 ...there may be a billion others... 238 00:15:57,898 --> 00:16:00,958 ...slowly drifting beyond Pluto's orbit. 239 00:16:01,468 --> 00:16:05,962 Very rarely, a long-period comet is captured in the inner solar system... 240 00:16:06,172 --> 00:16:08,140 ...becoming a short-period comet. 241 00:16:08,341 --> 00:16:11,799 It passes near a major planet, like Saturn. 242 00:16:12,012 --> 00:16:14,640 The planet provides a small gravitational tug... 243 00:16:14,848 --> 00:16:17,681 ...enough to deflect it into a much smaller orbit. 244 00:16:17,883 --> 00:16:20,545 Though few are captured this way, those that are... 245 00:16:20,753 --> 00:16:24,587 ...become well-known because they all return in short intervals. 246 00:16:24,790 --> 00:16:28,453 Once trapped in the inner solar system, among the planets... 247 00:16:28,662 --> 00:16:31,859 ...the chances of another near-collision are increased. 248 00:16:33,566 --> 00:16:36,228 Here, a second encounter with Saturn... 249 00:16:36,436 --> 00:16:40,270 ...further reduces the comet's orbital period to decades. 250 00:16:40,473 --> 00:16:44,637 A comet may take 10,000 years between close planetary encounters. 251 00:16:44,844 --> 00:16:48,678 But in this computer study, we've sped things up. 252 00:16:49,315 --> 00:16:52,182 A third encounter, this time with Jupiter... 253 00:16:52,384 --> 00:16:55,353 ...further reduces the comet's orbital period. 254 00:16:55,554 --> 00:16:58,216 Now the comet must approach the sun... 255 00:16:58,424 --> 00:17:01,018 ...and grow a tail every few years. 256 00:17:01,227 --> 00:17:04,560 Since the dust and gas in the tail are lost forever to space... 257 00:17:04,763 --> 00:17:07,391 ...the comet must slowly be eroding. 258 00:17:07,601 --> 00:17:09,262 Pieces of it break off. 259 00:17:09,468 --> 00:17:12,562 Sometimes, as we've seen, they even strike the Earth. 260 00:17:12,771 --> 00:17:14,204 In a few thousand years... 261 00:17:14,407 --> 00:17:17,433 ...if a short-period comet hasn't hit a planet... 262 00:17:17,643 --> 00:17:20,271 ...it will have evaporated away almost entirely... 263 00:17:20,479 --> 00:17:24,575 ...leaving sand-sized fragments, which become meteors... 264 00:17:24,784 --> 00:17:28,880 ...and its core which, perhaps, becomes an asteroid. 265 00:17:31,156 --> 00:17:34,956 Suppose I were a pretty typical comet. 266 00:17:35,161 --> 00:17:37,686 And what you would see would be a kind of... 267 00:17:37,897 --> 00:17:40,491 ...tumbling snowball... 268 00:17:40,699 --> 00:17:45,033 ...spending most of my time out here in the outer solar system. 269 00:17:45,237 --> 00:17:47,262 I'd be a kilometer across. 270 00:17:47,473 --> 00:17:49,236 I'd be living most of my days... 271 00:17:49,442 --> 00:17:53,401 ...in the gloom beyond Saturn, orbiting the sun. 272 00:17:53,613 --> 00:17:55,979 But once every century, I would find myself... 273 00:17:56,182 --> 00:17:59,208 ...careening inward, faster and faster... 274 00:17:59,418 --> 00:18:01,784 ...towards the inner solar system. 275 00:18:03,323 --> 00:18:06,759 By the time I would cross the orbit of Jupiter... 276 00:18:06,960 --> 00:18:08,791 ...on my way to the orbit of Mars... 277 00:18:08,994 --> 00:18:12,395 ...I'd be heating up because I'd be getting closer to the sun. 278 00:18:12,598 --> 00:18:14,463 I'd be evaporating a little bit. 279 00:18:14,668 --> 00:18:16,795 Small pieces of dust and ice... 280 00:18:17,002 --> 00:18:20,233 ...would be blown behind me by the solar wind... 281 00:18:20,439 --> 00:18:23,772 ...forming an incipient cometary tail. 282 00:18:23,976 --> 00:18:26,410 On the scale of such a solar system model... 283 00:18:26,612 --> 00:18:29,513 ...l, me, a cometary nucleus... 284 00:18:29,715 --> 00:18:31,910 ...would be smaller than a snowflake. 285 00:18:32,118 --> 00:18:36,418 Although, when fully developed, my tail would be longer... 286 00:18:36,622 --> 00:18:39,921 ...than the spacing between the worlds. 287 00:18:41,293 --> 00:18:43,227 Now, sooner or later... 288 00:18:43,429 --> 00:18:47,195 ...comets on these long, elliptical trajectories around the sun... 289 00:18:47,399 --> 00:18:49,594 ...must collide with planets. 290 00:18:49,802 --> 00:18:52,270 The Earth and the moon... 291 00:18:52,472 --> 00:18:56,135 ...must have been bombarded by comets and asteroids... 292 00:18:56,343 --> 00:18:59,312 ...the debris from the early history of the solar system. 293 00:18:59,512 --> 00:19:03,312 In interplanetary space, there are more small objects than large ones. 294 00:19:03,516 --> 00:19:06,576 So there must be, on a given planetary surface... 295 00:19:06,786 --> 00:19:11,086 ...many more impacts of small objects than of large objects. 296 00:19:11,290 --> 00:19:15,124 So a thing like the Tunguska impact happens on the Earth... 297 00:19:15,327 --> 00:19:16,919 ...maybe every thousand years. 298 00:19:17,130 --> 00:19:20,327 But the impact of a giant cometary nucleus... 299 00:19:20,533 --> 00:19:22,467 ...like Halley's comet, let's say... 300 00:19:22,668 --> 00:19:25,330 ...happens only every billion years or so. 301 00:19:26,071 --> 00:19:29,370 Now, is there evidence of past collisions? 302 00:19:30,542 --> 00:19:31,941 When a large comet... 303 00:19:32,144 --> 00:19:34,806 ...or a large, rocky asteroid hits a planet... 304 00:19:35,015 --> 00:19:37,313 ...it makes a bowl-shaped crater. 305 00:19:37,516 --> 00:19:41,680 The well-preserved impact craters on Earth were all formed fairly recently. 306 00:19:41,888 --> 00:19:45,221 The older ones have been softened, filled in or rubbed out... 307 00:19:45,425 --> 00:19:47,985 ...by running water and mountain building. 308 00:19:48,193 --> 00:19:51,685 Impacts make craters on other worlds and about as often. 309 00:19:51,898 --> 00:19:53,365 But when the air is thin... 310 00:19:53,565 --> 00:19:57,262 ...when water rarely flows, when mountain building is feeble... 311 00:19:57,469 --> 00:19:59,767 ...the ancient craters are retained. 312 00:19:59,972 --> 00:20:02,770 This is the case on the moon and Mercury and Mars... 313 00:20:02,976 --> 00:20:05,672 ...our neighboring terrestrial planets. 314 00:20:07,179 --> 00:20:09,409 They huddle around the sun... 315 00:20:09,616 --> 00:20:11,948 ...their source of heat and light... 316 00:20:12,151 --> 00:20:15,450 ...a little bit like campers around a fire. 317 00:20:15,821 --> 00:20:18,415 They are about 4½ billion years old. 318 00:20:18,624 --> 00:20:22,583 And all bear witness to an age long gone... 319 00:20:22,795 --> 00:20:25,491 ...of major collisions... 320 00:20:25,698 --> 00:20:30,101 ...which do not happen at that scale and frequency anymore. 321 00:20:30,737 --> 00:20:33,706 If we move out past... 322 00:20:33,907 --> 00:20:36,569 ...the terrestrial planets beyond Mars... 323 00:20:36,775 --> 00:20:40,836 ...we find ourselves in a different regime of the solar system... 324 00:20:41,047 --> 00:20:43,140 ...in the realm of Jupiter... 325 00:20:43,348 --> 00:20:46,579 ...and the other giant, or Jovian planets. 326 00:20:47,619 --> 00:20:52,113 These are great worlds composed largely of the gases... 327 00:20:52,325 --> 00:20:54,850 ...hydrogen and helium, some other stuff too. 328 00:20:55,061 --> 00:20:59,862 When we look at the surface, we do not see a solid surface... 329 00:21:00,065 --> 00:21:03,831 ...but only an occasional patch of atmosphere... 330 00:21:04,036 --> 00:21:07,563 ...and a complex array of multicolored clouds. 331 00:21:07,873 --> 00:21:09,534 These are serious planets... 332 00:21:09,743 --> 00:21:13,839 ...not fragmentary little world-lets like the Earth. 333 00:21:14,046 --> 00:21:17,743 In fact, 1000 Earths would fit... 334 00:21:17,950 --> 00:21:20,316 ...in the volume of Jupiter. 335 00:21:20,519 --> 00:21:24,387 If a comet or asteroid were to... 336 00:21:24,590 --> 00:21:29,527 ...accidentally impact Jupiter, it would be very unlikely to leave a crater. 337 00:21:29,728 --> 00:21:33,129 It might make a momentary hole in the clouds, but that's it. 338 00:21:33,333 --> 00:21:37,030 Nevertheless, we know that the outer solar system... 339 00:21:37,237 --> 00:21:40,229 ...has been subject to a many-billion-year history... 340 00:21:40,440 --> 00:21:42,533 ...of impact cratering. 341 00:21:42,975 --> 00:21:46,968 Jupiter's moon Callisto is studded with thousands of craters. 342 00:21:47,179 --> 00:21:50,671 Clear evidence of ancient collisions beyond Mars. 343 00:21:50,884 --> 00:21:54,411 And there are craters on other moons of Jupiter. 344 00:21:54,654 --> 00:21:57,452 Most of the thousands of large craters on our own moon... 345 00:21:57,656 --> 00:22:00,386 ...were excavated billions of years ago. 346 00:22:00,592 --> 00:22:03,117 But were any recorded in historical times? 347 00:22:03,328 --> 00:22:07,196 The odds against it are about 1000-to-one. 348 00:22:15,174 --> 00:22:18,337 Nevertheless, there's a possible eyewitness account... 349 00:22:18,544 --> 00:22:20,444 ...of just such an event. 350 00:22:20,646 --> 00:22:24,639 It was the Sunday before the feast of Saint John the Baptist... 351 00:22:25,050 --> 00:22:27,484 ...in the summer of 1178. 352 00:22:28,353 --> 00:22:32,551 The monks of Canterbury Cathedral had completed their evening prayers... 353 00:22:32,758 --> 00:22:34,953 ...and were about to retire for the night. 354 00:22:35,160 --> 00:22:36,923 The scholarly brother, Gervase... 355 00:22:37,130 --> 00:22:38,995 ...returned to his cell to read... 356 00:22:39,198 --> 00:22:40,790 ...while some of the others... 357 00:22:41,000 --> 00:22:44,595 ...went outside to enjoy the gentle June air. 358 00:22:50,376 --> 00:22:52,401 In the midst of their recreation... 359 00:22:52,611 --> 00:22:55,944 ...they chanced to witness an astonishing sight: 360 00:22:56,148 --> 00:22:59,515 A violent explosion on the moon. 361 00:23:08,494 --> 00:23:09,859 This was a time... 362 00:23:10,062 --> 00:23:12,587 ...when the heavens were thought to be changeless. 363 00:23:12,798 --> 00:23:16,632 The moon, the stars and the planets were deemed pure... 364 00:23:16,835 --> 00:23:20,828 ...because they followed an unvarying celestial routine. 365 00:23:21,073 --> 00:23:24,941 They were expected to behave without unseemly disruptions... 366 00:23:25,678 --> 00:23:27,509 ...like monks in a monastery. 367 00:23:28,046 --> 00:23:31,038 Was it wise to discuss such a vision? 368 00:23:37,923 --> 00:23:39,652 In every time and culture... 369 00:23:39,859 --> 00:23:43,295 ...there are pressures to conform to the prevailing prejudices. 370 00:23:44,396 --> 00:23:47,263 But there are also, in every place and epoch... 371 00:23:47,467 --> 00:23:51,870 ...those who value the truth, who record the evidence faithfully. 372 00:23:52,070 --> 00:23:55,164 Future generations are in their debt. 373 00:24:01,214 --> 00:24:03,546 A fire on the moon. 374 00:24:03,749 --> 00:24:07,412 Might it be some portent of ill fortune? 375 00:24:07,820 --> 00:24:11,051 Should the chronicler of the monastery be told? 376 00:24:11,925 --> 00:24:15,361 Was this event an apparition of the evil one? 377 00:24:18,197 --> 00:24:21,189 Gervase of Canterbury was a historian... 378 00:24:21,401 --> 00:24:24,165 ...considered today a reliable reporter of political... 379 00:24:24,369 --> 00:24:26,769 ...and cultural events of his time. 380 00:24:26,972 --> 00:24:31,170 This is his account of the eyewitness testimony he was given: 381 00:24:31,777 --> 00:24:33,836 "Now there was a bright new moon... 382 00:24:34,046 --> 00:24:35,980 ...and as usual in that phase... 383 00:24:36,182 --> 00:24:38,548 ...its horns were tilted toward the east. 384 00:24:38,750 --> 00:24:42,413 And suddenly the upper horn split in two. 385 00:24:42,622 --> 00:24:46,149 From the midpoint of this division, a flaming torch sprang up... 386 00:24:46,359 --> 00:24:48,953 ...spewing out over a considerable distance... 387 00:24:49,161 --> 00:24:52,221 ...fire, hot coals and sparks. 388 00:24:52,431 --> 00:24:55,594 After these transformations," Gervase continued... 389 00:24:55,801 --> 00:24:59,464 ..."the moon from horn to horn that is along its whole length... 390 00:24:59,671 --> 00:25:01,935 ...took on a blackish appearance." 391 00:25:06,946 --> 00:25:11,212 Gervase took depositions from all the eyewitnesses. 392 00:25:11,417 --> 00:25:12,714 He later wrote: 393 00:25:12,918 --> 00:25:17,514 "The writer was given this report by men who saw it with their own eyes... 394 00:25:17,724 --> 00:25:20,215 ...and are prepared to stake their honor on an oath... 395 00:25:20,425 --> 00:25:23,826 ...that they have made no addition or falsification." 396 00:25:24,296 --> 00:25:26,321 Gervase committed the account to paper... 397 00:25:26,531 --> 00:25:28,897 ...enabling astronomers eight centuries later... 398 00:25:29,102 --> 00:25:31,935 ...to try and reconstruct what really happened. 399 00:25:33,071 --> 00:25:36,006 It may be that 200 years before Chaucer... 400 00:25:36,209 --> 00:25:39,201 ...five monks saw an event more wonderful... 401 00:25:39,412 --> 00:25:42,540 ...than many another celebrated Canterbury tale. 402 00:25:45,518 --> 00:25:48,715 If a small drifting mountain were to hit the moon... 403 00:25:48,920 --> 00:25:51,684 ...it would set our satellite swinging like a bell. 404 00:25:51,890 --> 00:25:56,020 Eventually, the tremors would die down, but not in a mere 800 years. 405 00:25:56,229 --> 00:25:59,426 So is the moon still quivering from that impact? 406 00:25:59,632 --> 00:26:03,659 The Apollo astronauts emplaced arrays of special mirrors on the moon. 407 00:26:03,870 --> 00:26:06,031 Reflectors made by French scientists... 408 00:26:06,238 --> 00:26:09,366 ...were also put on the moon by Soviet Lunakhod vehicles. 409 00:26:09,574 --> 00:26:13,510 When a laser beam from Earth strikes a mirror and bounces back... 410 00:26:13,713 --> 00:26:16,341 ...the roundtrip travel time can be measured. 411 00:26:16,548 --> 00:26:20,314 At the McDonald Observatory of the University of Texas... 412 00:26:20,520 --> 00:26:24,354 ...a laser beam is prepared for firing at the reflectors on the moon... 413 00:26:24,557 --> 00:26:27,151 ...380,000 kilometers away. 414 00:26:28,061 --> 00:26:31,121 By multiplying the travel time by the speed of light... 415 00:26:31,330 --> 00:26:34,128 ...the distance to that spot can be determined... 416 00:26:34,332 --> 00:26:37,199 ...to a precision of 7 to 10 centimeters: 417 00:26:37,402 --> 00:26:39,802 The width of a hand. 418 00:26:43,475 --> 00:26:46,410 When such measurements are repeated over years... 419 00:26:46,611 --> 00:26:50,240 ...even an extremely slight wobble in the moon's motion... 420 00:26:50,449 --> 00:26:52,246 ...can be determined. 421 00:26:52,452 --> 00:26:55,285 The accuracy is phenomenal. 422 00:26:55,488 --> 00:26:57,854 The error is much less... 423 00:26:58,056 --> 00:27:01,253 ...than one-millionth of a percent. 424 00:27:02,962 --> 00:27:06,625 The moon, it turns out, is gently swinging like a bell... 425 00:27:06,833 --> 00:27:09,393 ...just as if it had been hit by an asteroid... 426 00:27:09,602 --> 00:27:11,968 ...less than 1000 years ago. 427 00:27:15,173 --> 00:27:19,337 So there may be physical evidence in the age of space flight... 428 00:27:19,711 --> 00:27:24,148 ...for the account of the Canterbury monks in the 12th century. 429 00:27:27,053 --> 00:27:29,920 If 800 years ago a big asteroid hit the moon... 430 00:27:30,122 --> 00:27:32,056 ...the crater should be prominent today... 431 00:27:32,257 --> 00:27:34,657 ...still surrounded by bright rays... 432 00:27:34,861 --> 00:27:38,228 ...thin streamers of dust spewed out by the impact. 433 00:27:38,431 --> 00:27:40,865 In billions of years, lunar rays are eroded... 434 00:27:41,067 --> 00:27:42,659 ...but not in hundreds. 435 00:27:42,868 --> 00:27:46,531 And there is a recent ray crater called Giordano Bruno... 436 00:27:46,738 --> 00:27:50,174 ...in the region of the moon where an explosion was reported... 437 00:27:50,375 --> 00:27:52,240 ...in 1178. 438 00:27:57,916 --> 00:27:59,884 The entire evolution of the moon... 439 00:28:00,086 --> 00:28:02,554 ...is a story of catastrophes. 440 00:28:02,755 --> 00:28:04,347 4 1/2 billion years ago... 441 00:28:04,556 --> 00:28:07,252 ...the moon was accreting from interplanetary boulders... 442 00:28:07,459 --> 00:28:10,053 ...and craters were forming all over its surface. 443 00:28:10,263 --> 00:28:13,096 The energy so released helped melt the crust. 444 00:28:13,299 --> 00:28:17,759 After most of this debris was swept up by the moon, the surface cooled. 445 00:28:18,203 --> 00:28:20,535 But about 3.9 billion years ago... 446 00:28:20,740 --> 00:28:23,675 ...a great asteroid impacted. 447 00:28:28,446 --> 00:28:32,576 It generated an expanding shock wave and re-melted some of the surface. 448 00:28:32,784 --> 00:28:34,809 The resulting basin was then flooded... 449 00:28:35,021 --> 00:28:36,613 ...probably by dark lava... 450 00:28:36,821 --> 00:28:40,188 ...producing one of the dry seas on the moon. 451 00:28:40,392 --> 00:28:43,589 More recent impacts excavated craters with bright rays... 452 00:28:43,795 --> 00:28:47,492 ...named after Eratosthenes and Copernicus. 453 00:28:47,700 --> 00:28:49,725 The familiar features of the man in the moon... 454 00:28:49,935 --> 00:28:52,961 ...are a chronicle of ancient impacts. 455 00:28:54,540 --> 00:28:57,202 Most of the original asteroids were swept up... 456 00:28:57,409 --> 00:28:59,400 ...in the making of the moon and planets. 457 00:28:59,612 --> 00:29:02,740 Many still orbit the sun in the asteroid belt. 458 00:29:02,949 --> 00:29:06,578 Some, themselves almost fractured by gravity tides... 459 00:29:06,785 --> 00:29:08,946 ...and by impacts with other asteroids... 460 00:29:09,155 --> 00:29:12,852 ...have been captured by planets: Phobos around Mars, for example... 461 00:29:13,059 --> 00:29:17,120 ...or a close moon of Jupiter called Amalthea. 462 00:29:18,730 --> 00:29:21,563 Similar to the asteroid belt are the rings of Saturn... 463 00:29:21,766 --> 00:29:26,396 ...composed of millions of small, tumbling, icy moonlets. 464 00:29:26,606 --> 00:29:30,064 Maybe the rings of Saturn are a moon... 465 00:29:30,276 --> 00:29:34,406 ...which was prevented from forming by the tides of Saturn. 466 00:29:34,614 --> 00:29:37,811 Or maybe it's the remains of a moon that wandered too close... 467 00:29:38,017 --> 00:29:40,611 ...and was torn apart by the tides of Saturn. 468 00:29:40,820 --> 00:29:43,755 It's certainly a lovely place. 469 00:29:43,956 --> 00:29:48,086 Jupiter also has a newly discovered ring system... 470 00:29:48,294 --> 00:29:51,024 ...which is invisible from the Earth. 471 00:29:53,732 --> 00:29:58,669 Now, there is a curious argument... 472 00:29:58,938 --> 00:30:02,396 ...alleging major recent collisions in the solar system... 473 00:30:02,608 --> 00:30:05,099 ...proposed by a psychiatrist... 474 00:30:05,311 --> 00:30:09,213 ...named Immanuel Velikovsky in 1950. 475 00:30:09,615 --> 00:30:11,879 He suggested... 476 00:30:12,084 --> 00:30:15,611 ...that an object of planetary mass, which he called a comet... 477 00:30:15,821 --> 00:30:19,313 ...was somehow produced in the Jupiter system. 478 00:30:19,525 --> 00:30:22,790 He doesn't say exactly how it's produced... 479 00:30:23,629 --> 00:30:24,994 ...but maybe... 480 00:30:28,400 --> 00:30:30,630 ...it's spat out... 481 00:30:33,773 --> 00:30:35,104 ...of Jupiter. 482 00:30:35,940 --> 00:30:40,877 Anyway, however it was made some 3500 years ago, he imagines... 483 00:30:41,446 --> 00:30:46,179 ...it made repeated close encounters with Mars... 484 00:30:47,019 --> 00:30:49,351 ...with the Earth-moon system... 485 00:30:49,721 --> 00:30:54,658 ...having as entertaining biblical consequences... 486 00:30:55,527 --> 00:31:00,226 ...the parting of the Red Sea so that Moses and the Israelites could... 487 00:31:00,432 --> 00:31:03,060 ...safely avoid the host of pharaoh... 488 00:31:03,268 --> 00:31:06,032 ...and the stopping of the Earth's rotation when... 489 00:31:06,237 --> 00:31:10,731 ...Joshua commanded the sun to stand still in Gibeon. 490 00:31:10,942 --> 00:31:13,570 He also imagined that there was extensive flooding... 491 00:31:13,778 --> 00:31:17,009 ...and the volcanoes all over the Earth at that time. 492 00:31:17,515 --> 00:31:22,111 Well, then after a very complicated game... 493 00:31:22,321 --> 00:31:26,052 ...of interplanetary billiards is completed... 494 00:31:26,258 --> 00:31:30,786 ...Velikovsky proposed that this comet... 495 00:31:30,995 --> 00:31:34,897 ...entered into a stable, almost perfectly circular orbit... 496 00:31:35,100 --> 00:31:36,397 ...becoming... 497 00:31:39,205 --> 00:31:40,832 ...the planet Venus... 498 00:31:41,039 --> 00:31:44,406 ...which he claimed never existed until then. 499 00:31:45,777 --> 00:31:50,612 Now, these ideas are almost certainly wrong. 500 00:31:51,217 --> 00:31:53,879 There's no objection in astronomy to collisions. 501 00:31:54,086 --> 00:31:56,281 We've seen collision fragments... 502 00:31:56,488 --> 00:31:59,787 ...and evidence throughout the solar system. 503 00:31:59,991 --> 00:32:03,825 The problem is with recent and major collisions. 504 00:32:04,029 --> 00:32:06,259 In any scale model like this... 505 00:32:06,464 --> 00:32:09,729 ...it's impossible to have both the sizes of the planets... 506 00:32:09,934 --> 00:32:12,402 ...and the sizes of their orbits to the same scale... 507 00:32:12,604 --> 00:32:15,767 ...because then the planets would be too small to see. 508 00:32:15,975 --> 00:32:19,035 If the planets were really to scale in such a model... 509 00:32:19,245 --> 00:32:21,713 ...as grains of dust... 510 00:32:21,914 --> 00:32:24,178 ...it would then be entirely clear... 511 00:32:24,382 --> 00:32:27,317 ...that a comet entering the inner solar system... 512 00:32:27,519 --> 00:32:30,420 ...would have a negligible chance of colliding with a planet... 513 00:32:30,622 --> 00:32:32,783 ...in only a few thousand years. 514 00:32:32,992 --> 00:32:34,289 Moreover... 515 00:32:34,492 --> 00:32:37,950 ...Venus is a rocky and metallic... 516 00:32:38,163 --> 00:32:40,529 ...hydrogen-poor world... 517 00:32:40,732 --> 00:32:43,667 ...whereas Jupiter, where Velikovsky imagines it comes from... 518 00:32:43,869 --> 00:32:46,497 ...is made of almost nothing but hydrogen. 519 00:32:46,705 --> 00:32:51,506 There is no energy source in Jupiter to eject planets or comets. 520 00:32:51,709 --> 00:32:55,440 If one did enter the inner solar system... 521 00:32:55,647 --> 00:32:59,174 ...there is no way it could stop the Earth from rotating. 522 00:32:59,384 --> 00:33:02,785 And if it could, there's no way Earth could start rotating again... 523 00:33:02,987 --> 00:33:05,387 ...at anything like 24 hours a day. 524 00:33:05,590 --> 00:33:08,923 There's no geological evidence for flooding and volcanism... 525 00:33:09,127 --> 00:33:11,186 ...3500 years ago. 526 00:33:11,397 --> 00:33:14,560 Babylonian astronomers observed Venus... 527 00:33:14,767 --> 00:33:17,395 ...in its present stable orbit... 528 00:33:17,603 --> 00:33:20,595 ...before Velikovsky said it existed. 529 00:33:20,806 --> 00:33:23,331 And so on. 530 00:33:27,847 --> 00:33:30,611 There are many hypotheses in science which are wrong. 531 00:33:30,815 --> 00:33:34,342 That's all right. It's the aperture to finding out what's right. 532 00:33:34,552 --> 00:33:37,385 Science is a self-correcting process. 533 00:33:37,589 --> 00:33:40,114 To be accepted, new ideas must survive... 534 00:33:40,325 --> 00:33:44,625 ...the most rigorous standards of evidence and scrutiny. 535 00:33:44,963 --> 00:33:47,955 The worst aspect of the Velikovsky affair is not... 536 00:33:48,166 --> 00:33:50,726 ...that many of his ideas were wrong or silly... 537 00:33:50,935 --> 00:33:53,597 ...or in gross contradiction to the facts. 538 00:33:53,806 --> 00:33:57,674 Rather, the worst aspect is that some scientists... 539 00:33:57,876 --> 00:34:01,243 ...attempted to suppress Velikovsky's ideas. 540 00:34:01,479 --> 00:34:05,711 The suppression of uncomfortable ideas may be common in religion... 541 00:34:05,917 --> 00:34:09,785 ...or in politics, but it is not the path to knowledge. 542 00:34:09,989 --> 00:34:13,288 And there's no place for it in the endeavor of science. 543 00:34:13,492 --> 00:34:15,187 We do not know beforehand... 544 00:34:15,393 --> 00:34:19,420 ...where fundamental insights will arise from... 545 00:34:19,631 --> 00:34:23,761 ...about our mysterious and lovely solar system. 546 00:34:23,969 --> 00:34:27,598 And the history of our study of the solar system shows clearly... 547 00:34:27,806 --> 00:34:31,833 ...that accepted and conventional ideas are often wrong... 548 00:34:32,044 --> 00:34:34,012 ...and that fundamental insights... 549 00:34:34,212 --> 00:34:38,046 ...can arise from the most unexpected sources. 550 00:34:39,685 --> 00:34:41,448 We've evolved on the planet Earth... 551 00:34:41,654 --> 00:34:44,248 ...and so we find it a congenial place. 552 00:34:44,455 --> 00:34:46,787 But just next door is Venus... 553 00:34:46,991 --> 00:34:49,983 ...until recently, enveloped in mystery. 554 00:34:50,195 --> 00:34:53,096 It has almost the same size and mass as the Earth. 555 00:34:53,299 --> 00:34:57,292 Might our sister world be a balmy summer planet... 556 00:34:57,502 --> 00:35:00,835 ...a little warmer than the Earth because it's closer to the sun? 557 00:35:01,039 --> 00:35:05,373 Are there craters, volcanoes, mountains, oceans, life? 558 00:35:06,412 --> 00:35:11,111 The first to look at Venus through a telescope was Galileo in 1609. 559 00:35:11,316 --> 00:35:14,114 But all he could see was a featureless disk. 560 00:35:14,687 --> 00:35:18,214 As optical telescopes got bigger, that's all anybody could see: 561 00:35:18,423 --> 00:35:21,017 A disk with no details on it at all. 562 00:35:21,226 --> 00:35:24,992 Venus evidently was covered with an opaque layer... 563 00:35:25,197 --> 00:35:28,325 ...thick clouds concealing the surface. 564 00:35:28,534 --> 00:35:33,130 For centuries, even the composition of the clouds of Venus was unknown. 565 00:35:33,338 --> 00:35:37,866 I mean, you could go outside, look up, see Venus with the naked eye... 566 00:35:38,077 --> 00:35:41,171 ...observe sunlight reflected from the clouds of Venus. 567 00:35:41,380 --> 00:35:43,974 What were you looking at? What were the clouds made of? 568 00:35:44,182 --> 00:35:45,740 Nobody knew. 569 00:35:45,951 --> 00:35:49,648 As a result, imagination ran riot. 570 00:35:49,855 --> 00:35:52,722 The absence of anything you could see on Venus... 571 00:35:52,925 --> 00:35:56,361 ...led some scientists and others to deduce... 572 00:35:56,562 --> 00:35:58,723 ...that the surface was a swamp. 573 00:36:00,032 --> 00:36:04,025 The argument, if we can dignify it with such a phrase... 574 00:36:04,435 --> 00:36:05,367 ...went like this: 575 00:36:05,571 --> 00:36:07,869 "I can't see a thing on the surface of Venus." 576 00:36:08,072 --> 00:36:08,731 "Why not?" 577 00:36:08,941 --> 00:36:12,069 "Because it's covered with a dense layer of clouds." 578 00:36:12,276 --> 00:36:13,743 "What are clouds made of?" 579 00:36:13,946 --> 00:36:17,575 "Water, of course. Therefore, Venus must have a lot of water on it." 580 00:36:17,783 --> 00:36:19,273 "Then the surface must be wet." 581 00:36:19,485 --> 00:36:22,181 "If the surface is wet, it's probably a swamp. 582 00:36:22,387 --> 00:36:25,220 If there's a swamp, there's ferns. If there's ferns... 583 00:36:25,423 --> 00:36:27,789 ...maybe there's even dinosaurs." 584 00:36:27,993 --> 00:36:29,927 Observation: You couldn't see a thing. 585 00:36:30,129 --> 00:36:32,723 Conclusion: dinosaurs. 586 00:36:33,298 --> 00:36:36,096 If just looking at Venus was so unproductive... 587 00:36:36,300 --> 00:36:37,733 ...what else could you do? 588 00:36:37,936 --> 00:36:41,303 The next clue came from early work with that: 589 00:36:41,507 --> 00:36:43,031 A glass prism. 590 00:36:43,241 --> 00:36:47,371 An intense beam of ordinary white light is passed through a narrow slit... 591 00:36:47,579 --> 00:36:49,171 ...and then through the prism. 592 00:36:49,381 --> 00:36:51,975 The result is to spread the white light out... 593 00:36:52,184 --> 00:36:55,551 ...into its constituent rainbow of colors. 594 00:36:56,188 --> 00:36:59,521 This rainbow pattern is called a spectrum. 595 00:36:59,725 --> 00:37:02,523 Think about it. White light enters the prism... 596 00:37:02,728 --> 00:37:05,492 ...what comes out of the prism is colored light. 597 00:37:05,698 --> 00:37:07,723 Lots of colors. Where did they come from? 598 00:37:07,932 --> 00:37:09,957 They must've been hiding in the white light. 599 00:37:10,169 --> 00:37:13,297 White light must be a mixture of many colors. 600 00:37:13,505 --> 00:37:16,269 Here we see the spectrum running from... 601 00:37:16,475 --> 00:37:19,876 ...violet, blue, green, yellow, orange, to red. 602 00:37:20,079 --> 00:37:24,516 Since we see these colors, we call this the spectrum of visible light. 603 00:37:25,550 --> 00:37:29,577 The sun emits lots of visible light. The air is transparent to it. 604 00:37:29,788 --> 00:37:32,450 So our eyes evolved to work in visible light. 605 00:37:32,657 --> 00:37:36,149 But there are many other frequencies of light which our eyes can't detect. 606 00:37:36,361 --> 00:37:38,420 Beyond the violet is the ultraviolet. 607 00:37:38,629 --> 00:37:41,757 It's just as real, but you need instruments to detect it. 608 00:37:41,967 --> 00:37:45,300 Beyond the ultraviolet are the x-rays and then the gamma rays. 609 00:37:45,837 --> 00:37:48,203 On the other side of visible light, beyond the red... 610 00:37:48,407 --> 00:37:51,137 ...is the infrared, again real, again invisible. 611 00:37:51,343 --> 00:37:54,540 Beyond the infrared are the radio waves. 612 00:37:54,747 --> 00:37:58,513 Now, this entire range from the gamma rays way over there... 613 00:37:58,717 --> 00:38:01,117 ...to the radio waves all the way over here... 614 00:38:01,320 --> 00:38:03,686 ...are simply different kinds of light. 615 00:38:03,888 --> 00:38:06,083 They differ only in the frequency. 616 00:38:06,291 --> 00:38:09,260 They're all useful, by the way, in astronomy. 617 00:38:09,461 --> 00:38:12,089 But because of the limitations of our eyes... 618 00:38:12,297 --> 00:38:16,631 ...we have a prejudice, a bias, a chauvinism... 619 00:38:16,835 --> 00:38:20,430 ...to this tiny rainbow band of visible light. 620 00:38:20,638 --> 00:38:24,802 Now, a spectrum can be used in a simple and elegant way... 621 00:38:25,042 --> 00:38:28,307 ...to determine the chemical composition of the atmosphere... 622 00:38:28,513 --> 00:38:29,878 ...of a planet or star. 623 00:38:30,082 --> 00:38:32,448 Different atoms and molecules absorb... 624 00:38:32,651 --> 00:38:35,415 ...different frequencies or colors of light. 625 00:38:35,620 --> 00:38:40,114 And those absorbed or missing frequencies appear as black lines... 626 00:38:40,325 --> 00:38:44,159 ...in the spectrum of the light we receive from the planet or star. 627 00:38:44,363 --> 00:38:48,697 Each and every substance has a characteristic fingerprint... 628 00:38:48,900 --> 00:38:51,198 ...a spectral signature... 629 00:38:51,403 --> 00:38:54,338 ...which permits it to be detected over a great distance. 630 00:38:54,540 --> 00:38:58,101 As a result, the gases in the atmosphere of Venus... 631 00:38:58,310 --> 00:39:01,211 ...at a distance of 60 million kilometers... 632 00:39:01,413 --> 00:39:05,144 ...their composition's been determined from the Earth. 633 00:39:05,350 --> 00:39:09,844 It's amazing to me still, we can tell what a thing is made out of... 634 00:39:10,054 --> 00:39:13,683 ...at an enormous distance away, without ever touching it. 635 00:39:15,027 --> 00:39:19,157 Our eyes can't see in the near infrared part of the spectrum. 636 00:39:19,364 --> 00:39:20,763 But our instruments can. 637 00:39:20,965 --> 00:39:24,799 Here's the absorption pattern of lots and lots of carbon dioxide: 638 00:39:25,003 --> 00:39:29,463 Dark lines in characteristic patterns at specific frequencies. 639 00:39:29,708 --> 00:39:32,506 You'd detect a different set of infrared lines... 640 00:39:32,711 --> 00:39:35,373 ...if, say, water vapor were present. 641 00:39:36,148 --> 00:39:41,051 If Venus were really soaking wet, then you could determine that... 642 00:39:41,419 --> 00:39:45,219 ...by finding the pattern of water vapor in its atmosphere. 643 00:39:45,423 --> 00:39:48,517 But around 1920, when this experiment was first performed... 644 00:39:48,726 --> 00:39:51,388 ...the Venus atmosphere seemed to have not a hint... 645 00:39:51,597 --> 00:39:55,795 ...not a smidgen, not a trace of water vapor above the clouds. 646 00:39:56,000 --> 00:40:00,096 And so instead of a swampy, soaking wet surface... 647 00:40:00,305 --> 00:40:03,968 ...it was suggested that Venus was bone-dry, a desert planet... 648 00:40:04,176 --> 00:40:07,668 ...with clouds composed of fine silicate dust. 649 00:40:08,247 --> 00:40:11,114 But later, spectroscopic observations revealed... 650 00:40:11,316 --> 00:40:13,216 ...the characteristic absorption lines... 651 00:40:13,417 --> 00:40:16,284 ...of an enormous amount of carbon dioxide. 652 00:40:16,487 --> 00:40:20,856 Scientists thought there must be lots of carbon compounds on the surface... 653 00:40:21,059 --> 00:40:24,654 ...making this a planet covered with petroleum. 654 00:40:24,897 --> 00:40:28,856 Others agreed that the atmosphere was dry but thought the surface was wet. 655 00:40:29,067 --> 00:40:32,559 With all that CO 2, it had to be carbonated water. 656 00:40:32,771 --> 00:40:36,673 Venus, they thought, was covered with a vast ocean of seltzer. 657 00:40:36,875 --> 00:40:40,311 The first hint of the true situation on Venus came... 658 00:40:40,512 --> 00:40:44,004 ...not from the visible, ultraviolet or infrared part of the spectrum... 659 00:40:44,216 --> 00:40:47,185 ...but from over here in the radio region. 660 00:40:47,386 --> 00:40:50,787 We're used to the idea of radio signals from intelligent life... 661 00:40:50,989 --> 00:40:54,948 ...or at least semi-intelligent life, radio and television stations. 662 00:40:55,159 --> 00:40:59,152 But there are all kinds of reasons why natural objects emit radio waves. 663 00:40:59,363 --> 00:41:01,957 One reason is that they're hot. 664 00:41:02,266 --> 00:41:04,234 And when, in 1956... 665 00:41:04,436 --> 00:41:07,599 ...Venus was, for the first time, observed by a radio telescope... 666 00:41:07,806 --> 00:41:10,866 ...the planet was discovered to be emitting radio waves... 667 00:41:11,076 --> 00:41:14,341 ...as if it were at an extremely high temperature. 668 00:41:14,546 --> 00:41:19,279 But the real demonstration that Venus' surface was astonishingly hot... 669 00:41:19,483 --> 00:41:24,420 ...came when the first spacecraft penetrated the clouds of Venus... 670 00:41:24,756 --> 00:41:28,988 ...and slowly settled on the surface of the nearest planet. 671 00:41:32,064 --> 00:41:37,001 These were the unmanned spacecraft of the Soviet Venera series. 672 00:41:39,604 --> 00:41:44,268 In our spaceship of the imagination, we retrace their course. 673 00:41:45,677 --> 00:41:50,114 From a distance, our sister planet seems serene and peaceful... 674 00:41:50,315 --> 00:41:52,510 ...its clouds motionless. 675 00:41:55,520 --> 00:41:58,978 These clouds are near the top of a great ocean of air... 676 00:41:59,191 --> 00:42:03,958 ...about 100 kilometers thick, composed mainly of carbon dioxide. 677 00:42:07,566 --> 00:42:10,660 There's some nitrogen, a little water vapor and other gases... 678 00:42:10,869 --> 00:42:14,134 ...but only the merest trace of hydrocarbons. 679 00:42:14,539 --> 00:42:16,973 The clouds turn out to be, not water... 680 00:42:17,175 --> 00:42:20,736 ...but a concentrated solution of sulfuric acid. 681 00:42:29,854 --> 00:42:31,617 Even in the high clouds... 682 00:42:31,823 --> 00:42:35,520 ...Venus is a thoroughly nasty place. 683 00:42:43,701 --> 00:42:46,465 The clouds are stained yellow by sulfur. 684 00:42:46,671 --> 00:42:48,468 There are great lightning storms. 685 00:42:48,673 --> 00:42:50,971 As we descend, there are increasing amounts... 686 00:42:51,175 --> 00:42:53,143 ...of the noxious gas sulfur dioxide. 687 00:42:53,812 --> 00:42:56,975 The pressures become so high that early Venera spacecraft... 688 00:42:57,182 --> 00:42:59,776 ...were crushed like old tin cans... 689 00:42:59,985 --> 00:43:02,613 ...by the weight of the surrounding atmosphere. 690 00:43:05,423 --> 00:43:08,449 Beneath the clouds in the dense, clear air... 691 00:43:08,659 --> 00:43:11,219 ...it's as bright as on an overcast day on Earth. 692 00:43:11,430 --> 00:43:15,059 But the atmosphere is so thick that the ground seems to ripple... 693 00:43:15,266 --> 00:43:16,756 ...and distort. 694 00:43:17,002 --> 00:43:20,904 The atmospheric pressure down here is 90 times that on Earth. 695 00:43:21,173 --> 00:43:26,110 The temperature is 380 degrees centigrade, 900 degrees Fahrenheit. 696 00:43:26,510 --> 00:43:29,138 Hotter than the hottest household oven. 697 00:43:29,346 --> 00:43:31,906 This is a world marked by searing heat... 698 00:43:32,117 --> 00:43:34,677 ...crushing pressures, sulfurous gases... 699 00:43:34,886 --> 00:43:37,548 ...and a desolate, reddish landscape. 700 00:43:37,756 --> 00:43:41,749 Far from the balmy paradise imagined by some early scientists... 701 00:43:41,960 --> 00:43:46,226 ...Venus is the one place in the solar system most like hell. 702 00:43:52,137 --> 00:43:54,628 But today, as in ancient tradition... 703 00:43:54,840 --> 00:43:59,436 ...there are travelers who will dare a visit to the underworld. 704 00:43:59,678 --> 00:44:02,476 Venera 9 was the first spacecraft in human history... 705 00:44:02,681 --> 00:44:05,411 ...to return a photograph from the surface of Venus. 706 00:44:05,617 --> 00:44:08,347 It found the rocks curiously eroded... 707 00:44:08,920 --> 00:44:10,547 ...perhaps by the corrosive gases... 708 00:44:10,755 --> 00:44:13,053 ...perhaps because the temperature is so high... 709 00:44:13,258 --> 00:44:16,955 ...that the rocks are partly molten and sluggishly flow. 710 00:44:17,162 --> 00:44:21,929 The Soviet Venera spacecraft, their electronics long ago fried... 711 00:44:22,133 --> 00:44:25,569 ...are slowly corroding on the surface of Venus. 712 00:44:25,769 --> 00:44:27,828 They are the first spaceships from Earth... 713 00:44:28,039 --> 00:44:30,803 ...ever to land on another planet. 714 00:44:35,580 --> 00:44:37,480 The reason Venus is like hell... 715 00:44:37,682 --> 00:44:40,810 ...seems to be what's called the greenhouse effect. 716 00:44:41,019 --> 00:44:44,716 Ordinary visible sunlight penetrates the clouds and heats the surface. 717 00:44:44,923 --> 00:44:47,653 But the dense atmosphere blankets the surface... 718 00:44:47,859 --> 00:44:50,521 ...and prevents it from cooling off to space. 719 00:44:50,729 --> 00:44:53,459 An atmosphere 90 times as dense as ours... 720 00:44:53,664 --> 00:44:56,394 ...made of carbon dioxide, water vapor and other gases... 721 00:44:56,601 --> 00:44:58,398 ...lets in visible light from the sun... 722 00:44:58,603 --> 00:45:02,664 ...but will not let out the infrared light radiated by the surface. 723 00:45:02,874 --> 00:45:04,364 The temperature rises... 724 00:45:04,575 --> 00:45:07,544 ...until the infrared radiation trickling out to space... 725 00:45:07,745 --> 00:45:10,839 ...just balances the sunlight reaching the surface. 726 00:45:14,318 --> 00:45:17,082 The greenhouse effect can make an Earth-like world... 727 00:45:17,289 --> 00:45:19,723 ...into a planetary inferno. 728 00:45:21,959 --> 00:45:25,019 In this caldron, there's not likely to be anything alive... 729 00:45:25,229 --> 00:45:27,356 ...even creatures very different from us. 730 00:45:27,566 --> 00:45:30,660 Organic and other conceivable biological molecules... 731 00:45:30,869 --> 00:45:33,667 ...would simply fall to pieces. 732 00:45:46,217 --> 00:45:48,913 The hell of Venus is in stark contrast... 733 00:45:49,120 --> 00:45:52,612 ...with the comparative heaven of its neighboring world... 734 00:45:52,823 --> 00:45:56,224 ...our little planetary home, the Earth. 735 00:45:57,896 --> 00:46:01,889 Here, the atmosphere is 90 times thinner. 736 00:46:02,100 --> 00:46:05,069 Here, the carbon dioxide and water vapor... 737 00:46:05,270 --> 00:46:07,295 ...make a modest greenhouse effect... 738 00:46:07,504 --> 00:46:10,496 ...which heats the ground above the freezing point of water. 739 00:46:10,708 --> 00:46:15,202 Without it, our oceans would be frozen solid. 740 00:46:15,413 --> 00:46:18,780 A little greenhouse effect is a good thing. 741 00:46:30,161 --> 00:46:32,823 But Venus is an ominous reminder... 742 00:46:33,031 --> 00:46:35,192 ...that on a world rather like the Earth... 743 00:46:35,399 --> 00:46:37,731 ...things can go wrong. 744 00:46:38,270 --> 00:46:42,798 There is no guarantee that our planet will always be so hospitable. 745 00:46:43,008 --> 00:46:45,169 To maintain this clement world... 746 00:46:45,377 --> 00:46:48,938 ...we must understand it and appreciate it. 747 00:46:52,183 --> 00:46:54,879 The Earth is a place to our eyes... 748 00:46:55,086 --> 00:46:57,987 ...more beautiful than any other that we know. 749 00:46:58,189 --> 00:47:01,647 But this beauty has been sculpted by change: 750 00:47:01,860 --> 00:47:04,488 Gentle, almost undetectable change... 751 00:47:04,696 --> 00:47:06,994 ...and sudden, violent change. 752 00:47:07,198 --> 00:47:11,328 In the cosmos, there is no refuge from change. 753 00:47:12,070 --> 00:47:15,267 The Sphinx: human head, lion's body... 754 00:47:15,473 --> 00:47:18,909 ...constructed more than 5500 years ago. 755 00:47:19,344 --> 00:47:22,279 That face was once crisp and cleanly rendered... 756 00:47:22,480 --> 00:47:24,175 ...like this paw I am standing on. 757 00:47:24,381 --> 00:47:27,407 The paw has been buried in the sand until recently... 758 00:47:27,618 --> 00:47:29,779 ...and protected from erosion. 759 00:47:30,288 --> 00:47:33,951 The face is now muddled and softened... 760 00:47:34,159 --> 00:47:37,822 ...because of thousands of years of sandblasting in the desert... 761 00:47:38,028 --> 00:47:39,928 ...and a little rainfall. 762 00:47:40,432 --> 00:47:44,232 In New York City, there is an obelisk called Cleopatra's Needle... 763 00:47:44,436 --> 00:47:45,960 ...which comes from Egypt. 764 00:47:46,171 --> 00:47:50,301 In only a little more than a century in New York's Central Park... 765 00:47:50,508 --> 00:47:54,945 ...the inscriptions on that obelisk have been almost totally obliterated. 766 00:47:55,146 --> 00:47:57,637 Not by sand and water... 767 00:47:57,849 --> 00:48:00,317 ...but by smog and industrial pollution. 768 00:48:00,518 --> 00:48:02,509 A bit like the atmosphere of Venus. 769 00:48:03,053 --> 00:48:07,012 Slow erosion wipes out information. 770 00:48:07,225 --> 00:48:08,249 On the Earth... 771 00:48:08,460 --> 00:48:10,792 ...mountain ranges are destroyed by erosion... 772 00:48:10,995 --> 00:48:13,190 ...in maybe tens of millions of years... 773 00:48:13,397 --> 00:48:17,424 ...small impact craters in maybe hundreds of thousands of years. 774 00:48:17,635 --> 00:48:20,399 And the greatest artifacts of human beings... 775 00:48:20,604 --> 00:48:23,971 ...in thousands or tens of thousands of years. 776 00:48:26,211 --> 00:48:29,669 In addition to such slow and uniform processes... 777 00:48:29,881 --> 00:48:33,248 ...there are rare but sudden catastrophes. 778 00:48:33,451 --> 00:48:36,215 The Sphinx is missing a nose. 779 00:48:36,421 --> 00:48:40,653 In an act of idle desecration, some soldiers once shot it off. 780 00:48:40,859 --> 00:48:45,193 If you wait long enough, everything changes. 781 00:49:03,481 --> 00:49:07,645 Slow, uniform processes, unheralded events: 782 00:49:07,852 --> 00:49:09,479 The sting of a sand grain... 783 00:49:09,686 --> 00:49:11,711 ...the fall of a drop of water... 784 00:49:11,923 --> 00:49:15,586 ...can, over the ages, totally rework the landscape. 785 00:49:36,281 --> 00:49:38,841 And rare, violent processes... 786 00:49:39,050 --> 00:49:42,019 ...exceptional events that will not recur in a lifetime... 787 00:49:42,287 --> 00:49:44,915 ...also make major changes. 788 00:50:05,609 --> 00:50:09,739 Both the insignificant and the extraordinary... 789 00:50:09,948 --> 00:50:13,111 ...are the architects of the natural world. 790 00:50:57,861 --> 00:51:00,887 The destruction of trees and grasslands... 791 00:51:01,099 --> 00:51:03,533 ...makes the surface of the Earth brighter. 792 00:51:03,735 --> 00:51:07,796 It reflects more sunlight back to space and cools our planet. 793 00:51:08,705 --> 00:51:10,400 After we discovered fire... 794 00:51:10,608 --> 00:51:13,805 ...we began to incinerate forests intentionally... 795 00:51:14,012 --> 00:51:16,139 ...to clear the land by a process called... 796 00:51:16,548 --> 00:51:19,608 ..."slash and burn" agriculture. 797 00:51:19,817 --> 00:51:24,379 And today, forests and grasslands are being destroyed... 798 00:51:24,589 --> 00:51:28,889 ...frivolously, carelessly by humans who are... 799 00:51:29,093 --> 00:51:33,120 ...heedless of the beauty of our cousins the trees... 800 00:51:33,331 --> 00:51:36,698 ...and ignorant of the possible climatic catastrophes... 801 00:51:36,901 --> 00:51:40,962 ...which large-scale burning of forests may bring. 802 00:51:46,544 --> 00:51:49,012 The indiscriminate destruction of vegetation... 803 00:51:49,213 --> 00:51:50,805 ...may alter the global climate... 804 00:51:51,015 --> 00:51:53,950 ...in ways that no scientist can yet predict. 805 00:51:55,453 --> 00:51:57,683 It has already deadened large patches... 806 00:51:57,889 --> 00:52:00,323 ...of the Earth's life-supporting skin. 807 00:52:12,403 --> 00:52:16,601 And yet, we ravage the Earth at an accelerated pace... 808 00:52:16,808 --> 00:52:19,140 ...as if it belonged to this one generation... 809 00:52:19,344 --> 00:52:22,939 ...as if it were ours to do with as we please. 810 00:52:31,789 --> 00:52:34,189 The Earth has mechanisms to cleanse itself... 811 00:52:34,392 --> 00:52:37,725 ...to neutralize the toxic substances in its system. 812 00:52:37,929 --> 00:52:40,523 But these mechanisms work only up to a point. 813 00:52:40,732 --> 00:52:44,099 Beyond some critical threshold, they break down. 814 00:52:44,302 --> 00:52:47,533 The damage becomes irreversible. 815 00:53:15,600 --> 00:53:17,534 Our generation must choose. 816 00:53:17,735 --> 00:53:20,829 Which do we value more: short-term profits... 817 00:53:21,039 --> 00:53:25,032 ...or the long-term habitability of our planetary home? 818 00:53:28,079 --> 00:53:30,047 The world is divided politically. 819 00:53:30,248 --> 00:53:32,682 But ecologically it is tightly interwoven. 820 00:53:32,884 --> 00:53:36,581 There are no useless threads in the fabric of the ecosystem. 821 00:53:36,854 --> 00:53:40,688 If you cut any one of them, you will unravel many others. 822 00:53:42,060 --> 00:53:43,652 We have uncovered other worlds... 823 00:53:43,861 --> 00:53:47,126 ...with choking atmospheres and deadly surfaces. 824 00:53:47,398 --> 00:53:50,731 Shall we then re-create these hells on Earth? 825 00:53:53,504 --> 00:53:57,406 We have encountered desolate moons and barren asteroids. 826 00:53:57,608 --> 00:54:02,545 Shall we then scar and crater this blue-green world in their likeness? 827 00:54:21,099 --> 00:54:23,897 Natural catastrophes are rare. 828 00:54:24,102 --> 00:54:25,535 But they come often enough. 829 00:54:25,737 --> 00:54:29,366 We need not force the hand of nature. 830 00:54:39,016 --> 00:54:43,316 If we ruin the Earth, there is no place else to go. 831 00:54:43,521 --> 00:54:46,217 This is not a disposable world. 832 00:54:46,424 --> 00:54:50,190 And we are not yet able to re-engineer other planets. 833 00:54:58,703 --> 00:55:01,228 The cruelest desert on Earth... 834 00:55:01,439 --> 00:55:05,705 ...is far more hospitable than any place on Mars. 835 00:55:07,011 --> 00:55:10,469 The bright, sandy surface and dusty atmosphere of Mars... 836 00:55:10,681 --> 00:55:14,276 ...reflect enough sunlight back to space to cool the planet... 837 00:55:14,485 --> 00:55:19,422 ...freezing out all its water, locking it in a perpetual ice age. 838 00:55:20,391 --> 00:55:24,953 Human activities brighten our landscape and our atmosphere. 839 00:55:25,163 --> 00:55:28,394 Might this ultimately make an ice age here? 840 00:55:29,500 --> 00:55:33,527 At the same time, we are releasing vast quantities of carbon dioxide... 841 00:55:33,738 --> 00:55:36,468 ...increasing the greenhouse effect. 842 00:55:36,674 --> 00:55:39,302 The Earth need not resemble Venus very closely... 843 00:55:39,510 --> 00:55:42,377 ...for it to become barren and lifeless. 844 00:55:46,017 --> 00:55:49,453 It may not take much to destabilize the Earth's climate... 845 00:55:49,654 --> 00:55:53,112 ...to convert this heaven, our only home in the cosmos... 846 00:55:53,324 --> 00:55:55,292 ...into a kind of hell. 847 00:55:57,829 --> 00:56:01,390 The study of the global climate, the sun's influence... 848 00:56:01,599 --> 00:56:04,227 ...the comparison of the Earth with other worlds... 849 00:56:04,435 --> 00:56:07,871 These are subjects in their earliest stages of development. 850 00:56:08,072 --> 00:56:11,235 They are funded poorly and grudgingly. 851 00:56:11,442 --> 00:56:15,572 Meanwhile, we continue to load the Earth's atmosphere with materials... 852 00:56:15,780 --> 00:56:20,217 ...about whose long-term influence we are almost entirely ignorant. 853 00:56:21,586 --> 00:56:26,080 There are worlds that began with as much apparent promise as Earth. 854 00:56:26,290 --> 00:56:29,157 But something went wrong. 855 00:56:29,360 --> 00:56:33,888 Knowing that worlds can die alerts us to our danger. 856 00:56:35,032 --> 00:56:38,934 If a visitor arrived from another world, what account would we give... 857 00:56:39,136 --> 00:56:42,401 ...of our stewardship of the planet Earth? 858 00:56:49,614 --> 00:56:54,483 In the history of the solar system, have worlds ever been destroyed? 859 00:56:55,586 --> 00:56:58,783 Most of the moons in the outer solar system have craters on them... 860 00:56:58,990 --> 00:57:01,424 ...made by cometary impacts. 861 00:57:02,126 --> 00:57:04,060 Some have such large craters though... 862 00:57:04,262 --> 00:57:07,925 ...that if the impacting comets had been just a little bit bigger... 863 00:57:08,132 --> 00:57:10,225 ...the moons would have been shattered. 864 00:57:12,803 --> 00:57:15,363 What would the results of such a collision look like? 865 00:57:18,409 --> 00:57:20,104 Maybe a planetary ring. 866 00:57:22,580 --> 00:57:25,447 The idea has been growing that little worlds are... 867 00:57:25,650 --> 00:57:28,517 ...every now and then, demolished by a cometary impact. 868 00:57:28,719 --> 00:57:33,281 The fragments then slowly coalesce, and a moon arises again... 869 00:57:33,491 --> 00:57:34,822 ...from its own ashes. 870 00:57:35,026 --> 00:57:39,554 Some moons may have been destroyed and reconstituted many times. 871 00:57:40,798 --> 00:57:44,564 For our own world, the peril is more subtle. 872 00:57:45,202 --> 00:57:46,897 Since this series was first broadcast... 873 00:57:47,104 --> 00:57:49,902 ...the dangers of the increasing greenhouse effect... 874 00:57:50,107 --> 00:57:51,665 ...have become much more clear. 875 00:57:51,876 --> 00:57:56,108 We burn fossil fuels, like coal and gas and petroleum... 876 00:57:56,314 --> 00:57:58,839 ...putting more carbon dioxide into the atmosphere... 877 00:57:59,050 --> 00:58:01,382 ...and thereby heating the Earth. 878 00:58:01,585 --> 00:58:04,679 The hellish conditions on Venus are a reminder that... 879 00:58:04,889 --> 00:58:06,322 ...this is serious business. 880 00:58:06,724 --> 00:58:09,022 Computer models that successfully explain... 881 00:58:09,226 --> 00:58:11,160 ...the climates of other planets... 882 00:58:11,362 --> 00:58:14,331 ...predict the deaths of forests... 883 00:58:14,532 --> 00:58:17,797 ...parched croplands, the flooding of coastal cities... 884 00:58:18,002 --> 00:58:20,095 ...environmental refugees... 885 00:58:20,304 --> 00:58:23,569 ...widespread disasters in the next century... 886 00:58:23,874 --> 00:58:25,466 ...unless we change our ways. 887 00:58:25,676 --> 00:58:27,166 What do we have to do? 888 00:58:27,611 --> 00:58:29,238 Four things. 889 00:58:29,447 --> 00:58:32,848 One: much more efficient use of fossil fuels. 890 00:58:33,050 --> 00:58:37,180 Why not cars that get 70 miles a gallon instead of 25? 891 00:58:37,388 --> 00:58:41,688 Two: research and development on safe alternative energy sources... 892 00:58:41,892 --> 00:58:43,587 ...especially solar power. 893 00:58:44,195 --> 00:58:47,164 Three: reforestation on a grand scale. 894 00:58:47,365 --> 00:58:50,857 And four: helping to bring the billion poorest people... 895 00:58:51,068 --> 00:58:53,195 ...on the planet to self-sufficiency... 896 00:58:53,404 --> 00:58:56,737 ...which is the key step in curbing world population growth. 897 00:58:56,941 --> 00:59:00,843 Every one of these steps makes sense apart from greenhouse warming. 898 00:59:01,679 --> 00:59:04,147 No one has proposed that the trouble with Venus is... 899 00:59:04,348 --> 00:59:08,375 ...that there once was Venusians who drove fuel-inefficient cars. 900 00:59:08,586 --> 00:59:11,851 But our nearest neighbor, nevertheless, is a stark warning... 901 00:59:12,056 --> 00:59:15,457 ...on the possible fate of an Earth-like world.