1 00:00:43,091 --> 00:00:46,652 SAGAN: This is the age of planetary exploration... 2 00:00:46,860 --> 00:00:50,296 ...when our ships have begun to sail the heavens. 3 00:00:51,832 --> 00:00:55,427 In those heavens, there are some worlds much like hell. 4 00:00:55,636 --> 00:00:59,538 Our planet is, in comparison, much like a heaven. 5 00:00:59,740 --> 00:01:01,970 But the gates of heaven and hell... 6 00:01:02,175 --> 00:01:05,235 ...are adjacent and unmarked. 7 00:01:07,748 --> 00:01:10,216 The Earth is a lovely... 8 00:01:10,417 --> 00:01:12,681 ...and more or less placid place. 9 00:01:13,153 --> 00:01:15,951 Things change, but slowly. 10 00:01:16,156 --> 00:01:20,718 You can lead a full life and never encounter a natural catastrophe... 11 00:01:20,928 --> 00:01:22,759 ...more violent than a storm. 12 00:01:22,963 --> 00:01:25,431 And so we become complacent... 13 00:01:25,632 --> 00:01:26,826 ...relaxed... 14 00:01:27,034 --> 00:01:28,729 ...unconcerned. 15 00:01:28,969 --> 00:01:33,429 But in the history of the solar system and even in human history... 16 00:01:33,640 --> 00:01:36,302 ...there are clear records of extraordinary... 17 00:01:36,510 --> 00:01:38,671 ...and devastating catastrophes. 18 00:01:38,879 --> 00:01:41,473 We have now achieved the dubious distinction... 19 00:01:41,682 --> 00:01:43,843 ...of making our own major catastrophes... 20 00:01:44,051 --> 00:01:46,781 ...both intentional and inadvertent. 21 00:01:47,554 --> 00:01:49,886 On the landscapes of other planets... 22 00:01:50,090 --> 00:01:52,581 ...where past records are better preserved... 23 00:01:52,793 --> 00:01:55,261 ...there's abundant evidence of major catastrophes. 24 00:01:55,462 --> 00:01:57,259 It's all a matter of time scale. 25 00:01:57,464 --> 00:02:00,126 An event which is improbable in 100 years... 26 00:02:00,334 --> 00:02:03,098 ...may be inevitable in 100 million. 27 00:02:03,303 --> 00:02:06,466 But even on the Earth in this century... 28 00:02:06,673 --> 00:02:10,336 ...there have been bizarre natural events. 29 00:02:15,115 --> 00:02:17,811 In remote central Siberia... 30 00:02:18,018 --> 00:02:20,077 ...there was a time when the Tungus people... 31 00:02:20,287 --> 00:02:23,256 ...told strange tales of a giant fireball... 32 00:02:23,457 --> 00:02:26,483 ...that split the sky and shook the Earth. 33 00:02:28,095 --> 00:02:30,495 They told of a blast of searing wind... 34 00:02:30,697 --> 00:02:33,097 ...that knocked down people and forests. 35 00:02:34,835 --> 00:02:37,497 It happened, they said, on a summer's morning... 36 00:02:37,704 --> 00:02:39,535 ...in the year 1908. 37 00:02:39,840 --> 00:02:41,273 In the late 1920s... 38 00:02:41,475 --> 00:02:44,342 ...L.A. Kulik, a Soviet scientist... 39 00:02:44,544 --> 00:02:47,604 ...organized expeditions to try and solve the mystery. 40 00:02:49,983 --> 00:02:53,544 He built boats to penetrate this trackless land: 41 00:02:54,121 --> 00:02:55,713 Snowbound in winter... 42 00:02:55,922 --> 00:02:58,482 ...a swampy morass in summer. 43 00:03:01,294 --> 00:03:04,889 Eyewitnesses told of a ball of flame... 44 00:03:05,098 --> 00:03:06,622 ...larger than the sun... 45 00:03:06,833 --> 00:03:10,633 ...that had blazed across the sky 20 years before. 46 00:03:11,271 --> 00:03:15,105 Kulik assumed a giant meteorite had struck the Earth. 47 00:03:17,644 --> 00:03:21,205 He expected to find an enormous impact crater... 48 00:03:21,415 --> 00:03:23,542 ...and rare meteorite fragments... 49 00:03:23,750 --> 00:03:26,480 ...chipped off some distant asteroid. 50 00:03:29,489 --> 00:03:31,423 However, at ground zero... 51 00:03:31,625 --> 00:03:34,719 ...Kulik found upright trees stripped of their branches... 52 00:03:34,928 --> 00:03:37,726 ...but not a trace of the meteorite or its impact crater. 53 00:03:37,931 --> 00:03:39,592 He was deeply puzzled. 54 00:03:39,800 --> 00:03:43,793 He thought there were meteorite fragments buried in the swampy ground. 55 00:03:44,738 --> 00:03:48,299 So he set about digging trenches and pumping out the water. 56 00:03:48,508 --> 00:03:52,035 But the expected meteoritic rock and iron was not found. 57 00:03:54,181 --> 00:03:57,446 Undaunted, Kulik went on to make a thorough survey... 58 00:03:57,651 --> 00:04:00,415 ...despite the swarms of insects and other hardships. 59 00:04:00,620 --> 00:04:03,418 Because he discovered something that, in his own words... 60 00:04:03,623 --> 00:04:08,560 ..."exceeded all tales of eyewitnesses and my wildest expectations." 61 00:04:11,064 --> 00:04:14,795 For more than 20 kilometers in every direction from ground zero... 62 00:04:15,001 --> 00:04:19,938 ...the trees were flattened radially outward like broken matchsticks. 63 00:04:24,444 --> 00:04:26,469 There must've been a powerful explosion... 64 00:04:26,680 --> 00:04:28,705 ...several kilometers above the ground. 65 00:04:28,915 --> 00:04:31,713 The pressure wave, spreading out at the speed of sound... 66 00:04:31,918 --> 00:04:35,410 ...was reconstructed from barometric records at weather stations... 67 00:04:35,622 --> 00:04:40,252 ...across Siberia, through Russia and on into Western Europe. 68 00:04:40,460 --> 00:04:43,952 Dust from the explosion reflected so much sunlight back to Earth... 69 00:04:44,164 --> 00:04:46,223 ...that people could read by it at night... 70 00:04:46,433 --> 00:04:49,766 ...in London, 10,000 kilometers away. 71 00:04:55,041 --> 00:04:58,670 This really remarkable occurrence... 72 00:04:58,880 --> 00:05:01,781 ...is called the Tunguska Event. 73 00:05:02,316 --> 00:05:03,840 But what was it? 74 00:05:04,251 --> 00:05:08,381 Well, perhaps, some scientists have suggested... 75 00:05:08,588 --> 00:05:12,456 ...it was a chunk of antimatter from space... 76 00:05:12,659 --> 00:05:17,062 ...annihilated on contact with the ordinary matter of the Earth... 77 00:05:17,264 --> 00:05:20,597 ...disappearing in a flash of gamma rays. 78 00:05:21,001 --> 00:05:25,165 But the radioactivity you'd expect from matter-antimatter annihilation... 79 00:05:25,372 --> 00:05:28,864 ...is to be found nowhere at the impact site. 80 00:05:29,810 --> 00:05:34,076 Or, perhaps, other scientists have suggested... 81 00:05:34,281 --> 00:05:36,613 ...it was a mini black hole from space... 82 00:05:36,817 --> 00:05:38,808 ...which impacted the Earth in Siberia... 83 00:05:39,019 --> 00:05:41,385 ...tunneled through the solid body of Earth... 84 00:05:41,588 --> 00:05:43,852 ...and plunged out the other side. 85 00:05:44,057 --> 00:05:47,254 But the records of atmospheric shock waves give not a hint... 86 00:05:47,460 --> 00:05:51,954 ...of something booming out of the North Atlantic later that day. 87 00:05:52,199 --> 00:05:56,636 Or maybe, other people have speculated, it was a spaceship... 88 00:05:56,837 --> 00:06:01,171 ...of some unimaginably advanced extraterrestrial civilization... 89 00:06:01,374 --> 00:06:03,934 ...in desperate mechanical trouble... 90 00:06:04,144 --> 00:06:08,308 ...crashing in a remote region of an obscure planet. 91 00:06:08,615 --> 00:06:12,415 Well, if so, it's pretty startling that at the impact site... 92 00:06:12,619 --> 00:06:16,248 ...there is not a piece, not the tiniest transistor... 93 00:06:16,456 --> 00:06:18,822 ...of a crashed spacecraft. 94 00:06:19,025 --> 00:06:21,960 More prosaically, perhaps it was a large meteorite... 95 00:06:22,162 --> 00:06:24,562 ...or a small asteroid which hit the Earth. 96 00:06:24,764 --> 00:06:28,256 But even here, there are no observable telltale... 97 00:06:28,468 --> 00:06:31,460 ...rocky or metallic fragments of the sort... 98 00:06:31,671 --> 00:06:34,333 ...that you'd expect from such an impact. 99 00:06:34,641 --> 00:06:37,769 The key point of the Tunguska Event... 100 00:06:37,978 --> 00:06:42,278 ...is that there was a tremendous explosion, a great shock wave... 101 00:06:42,482 --> 00:06:45,713 ...many trees burned, an enormous forest fire... 102 00:06:45,952 --> 00:06:50,389 ...and yet, no crater in the ground. 103 00:06:50,590 --> 00:06:53,024 There seems to be only one explanation... 104 00:06:53,226 --> 00:06:55,786 ...which is consistent with these facts. 105 00:06:55,996 --> 00:06:59,022 And that explanation is this: 106 00:06:59,966 --> 00:07:03,868 In 1908, a piece of a comet... 107 00:07:04,070 --> 00:07:05,469 ...hit the Earth. 108 00:07:12,445 --> 00:07:14,470 No one saw it approach. 109 00:07:14,681 --> 00:07:19,175 A small point of light lost in the glare of the morning sun. 110 00:07:21,087 --> 00:07:24,488 It had been drifting for centuries through the inner solar system... 111 00:07:24,691 --> 00:07:28,923 ...like an iceberg in the ocean of interplanetary space. 112 00:07:35,969 --> 00:07:38,369 But this time, by accident... 113 00:07:38,571 --> 00:07:41,165 ...there was a planet in the way. 114 00:07:47,514 --> 00:07:51,245 From the time and direction of its approach, what hit the Earth... 115 00:07:51,451 --> 00:07:55,217 ...seems to have been a fragment of a comet named Encke. 116 00:07:55,422 --> 00:07:58,755 Hurtling at more than 100,000 kilometers an hour... 117 00:07:58,959 --> 00:08:02,326 ...it was a mountain of ice about the size of a football field... 118 00:08:02,529 --> 00:08:05,965 ...and weighing almost a million tons. 119 00:08:07,167 --> 00:08:10,864 There was no warning, until it plunged into the atmosphere. 120 00:08:14,007 --> 00:08:16,305 (COMET RUMBLES) 121 00:08:51,945 --> 00:08:55,346 If such an explosion happened today... 122 00:08:55,548 --> 00:08:58,278 ...it might be thought, in the panic of the moment... 123 00:08:58,885 --> 00:09:01,217 ...to be produced by a nuclear weapon. 124 00:09:01,488 --> 00:09:04,218 Such a cometary impact and fireball... 125 00:09:04,424 --> 00:09:07,916 ...simulates all the effects of a 15-megaton nuclear burst... 126 00:09:08,128 --> 00:09:10,756 ...including the mushroom cloud, with one exception: 127 00:09:10,965 --> 00:09:12,956 There would be no radiation. 128 00:09:13,166 --> 00:09:16,158 So could a rare but natural event... 129 00:09:16,369 --> 00:09:18,394 ...the impact of a comet with Earth... 130 00:09:18,605 --> 00:09:21,199 ...trigger a nuclear war? 131 00:09:22,042 --> 00:09:25,478 It's a strange scenario: A small comet hits the Earth... 132 00:09:25,678 --> 00:09:28,112 ...as millions have during Earth's history... 133 00:09:28,314 --> 00:09:30,282 ...and the response of our civilization... 134 00:09:30,483 --> 00:09:33,714 ...is promptly to self-destruct. 135 00:09:35,288 --> 00:09:38,621 Maybe it's unlikely, but it might be a good idea... 136 00:09:38,825 --> 00:09:42,317 ...to understand comets and collisions and catastrophes... 137 00:09:42,530 --> 00:09:45,158 ...a little bit better than we do. 138 00:09:45,365 --> 00:09:49,426 Now, a comet, at least as far as we understand them today... 139 00:09:49,637 --> 00:09:50,968 ...is made mostly of ice: 140 00:09:51,171 --> 00:09:54,004 Water ice, maybe some ammonia ice... 141 00:09:54,207 --> 00:09:56,471 ...a little bit of methane ice. 142 00:09:56,877 --> 00:09:59,675 So in striking the Earth's atmosphere... 143 00:09:59,879 --> 00:10:01,972 ...a modest cometary fragment... 144 00:10:02,182 --> 00:10:07,051 ...will produce a great radiant fireball and a mighty blast wave. 145 00:10:07,254 --> 00:10:09,484 It'll burn trees and level forests... 146 00:10:09,689 --> 00:10:12,351 ...and make a sound heard around the world. 147 00:10:12,559 --> 00:10:15,426 But it need not make a crater in the ground. 148 00:10:15,628 --> 00:10:20,065 Why? Because the ices in the comet are all melted in the impact. 149 00:10:20,266 --> 00:10:23,724 And there's going to be very few recognizable pieces of comet... 150 00:10:23,937 --> 00:10:25,427 ...left on the ground. 151 00:10:32,078 --> 00:10:34,876 We humans like to think of the heavens as stable... 152 00:10:35,082 --> 00:10:37,573 ...serene, unchanging. 153 00:10:39,486 --> 00:10:41,386 But comets suddenly appear... 154 00:10:41,588 --> 00:10:45,718 ...and hang ominously in the sky, night after night, for weeks. 155 00:10:48,094 --> 00:10:52,497 So the idea developed that the comet had to be there for a reason. 156 00:10:52,700 --> 00:10:55,965 The reason was that comets were predictions of disaster... 157 00:10:56,170 --> 00:11:00,504 ...that they foretold the deaths of princes and the fall of kingdoms. 158 00:11:00,707 --> 00:11:03,198 In 1066, for example... 159 00:11:03,410 --> 00:11:07,506 ...the Normans witnessed an apparition or appearance of Halley's comet. 160 00:11:07,714 --> 00:11:11,514 Since a comet must, they thought, predict the fall of some kingdom... 161 00:11:11,718 --> 00:11:13,982 ...they promptly invaded England. 162 00:11:14,187 --> 00:11:16,781 Here's King Harold of England looking a little glum. 163 00:11:16,991 --> 00:11:19,357 The events were noted in the Bayeux tapestry... 164 00:11:19,559 --> 00:11:22,187 ...a kind of newspaper of the day. 165 00:11:22,395 --> 00:11:24,454 Or, in the early 13th century... 166 00:11:24,664 --> 00:11:28,191 ...Giotto, one of the founders of modern realistic painting... 167 00:11:28,401 --> 00:11:30,995 ...witnessed another apparition of comet Halley... 168 00:11:31,205 --> 00:11:34,470 ...and inserted it into a nativity he was painting. 169 00:11:34,675 --> 00:11:38,907 A harbinger of a different sort of change of kingdoms. 170 00:11:39,579 --> 00:11:41,877 Around 1517... 171 00:11:42,082 --> 00:11:45,848 ...another great comet appeared. This time it was seen in Mexico. 172 00:11:46,052 --> 00:11:47,917 And the Aztec emperor, Moctezuma... 173 00:11:48,254 --> 00:11:49,881 ...maybe this is he... 174 00:11:50,456 --> 00:11:52,447 ...promptly executed his astrologers. 175 00:11:52,659 --> 00:11:57,255 Why? They hadn't predicted the comet, and they sure hadn't explained it. 176 00:11:57,463 --> 00:12:02,366 Moctezuma was positive that the comet foretold some dreadful disaster. 177 00:12:02,570 --> 00:12:05,164 He became distant and gloomy... 178 00:12:05,371 --> 00:12:07,839 ...and in that way, helped to set the stage... 179 00:12:08,041 --> 00:12:11,943 ...for the successful Spanish conquest of Mexico under Cortés. 180 00:12:12,445 --> 00:12:16,074 In many cases, a superstitious belief in comets... 181 00:12:16,282 --> 00:12:19,217 ...becomes a self-fulfilling prophecy. 182 00:12:20,054 --> 00:12:22,614 Here are two quite different representations... 183 00:12:22,822 --> 00:12:25,450 ...of the great comet of 1577: 184 00:12:25,658 --> 00:12:28,354 This one pictured by the Turks... 185 00:12:31,231 --> 00:12:33,563 ...and this one by the Germans. 186 00:12:40,139 --> 00:12:43,438 In 1705, Edmund Halley finally... 187 00:12:43,643 --> 00:12:46,168 ...figured out that the same spectacular comet... 188 00:12:46,379 --> 00:12:50,179 ...was booming by the Earth every 76 years, like clockwork. 189 00:12:50,383 --> 00:12:54,114 That comet is now called, appropriately, comet Halley. 190 00:12:54,320 --> 00:12:58,120 And it's the same one that we talked about before, the comet of 1066. 191 00:12:58,324 --> 00:13:01,259 At that point, the subject began to lose a little... 192 00:13:01,461 --> 00:13:04,828 ...of its burden of superstition, but hardly all. 193 00:13:05,031 --> 00:13:08,797 Public fear of comets survived. Well, for example... 194 00:13:09,336 --> 00:13:12,499 ...look at this terribly nasty comet of 1857... 195 00:13:12,706 --> 00:13:16,142 ...that some people figured would splinter the Earth. 196 00:13:17,477 --> 00:13:21,538 By 1910, Halley's comet returned once more. 197 00:13:21,748 --> 00:13:25,206 But this time, astronomers using a new tool, the spectroscope... 198 00:13:25,418 --> 00:13:30,321 ...had discovered cyanogen gas in the tail of a comet. 199 00:13:30,524 --> 00:13:32,856 Now, cyanogen is a poison. 200 00:13:33,059 --> 00:13:36,688 The Earth was to pass through this poisonous tail. 201 00:13:36,896 --> 00:13:40,889 The fact that the gas was astonishingly, fabulously thin... 202 00:13:41,101 --> 00:13:43,262 ...reassured almost nobody. 203 00:13:43,469 --> 00:13:47,769 For example, look at the headlines in the Los Angeles Examiner... 204 00:13:47,975 --> 00:13:50,671 ...for May 9, 1910: 205 00:13:50,877 --> 00:13:54,870 "Say, Has That Comet 'Cyanogened' You Yet?" 206 00:13:55,082 --> 00:13:58,176 "Entire Human Race Due For Free Gaseous Bath. 207 00:13:58,385 --> 00:14:00,410 Expect High Jinks." 208 00:14:00,620 --> 00:14:05,489 Or take this from the San Francisco Chronicle, May 15, 1910: 209 00:14:05,693 --> 00:14:09,288 "Comet Comes And Husband Reforms." 210 00:14:09,495 --> 00:14:11,690 "Comet Parties Now Fad In New York." 211 00:14:11,898 --> 00:14:13,729 Amazing stuff! 212 00:14:13,933 --> 00:14:17,369 In 1910, people were holding comet parties, not so much to... 213 00:14:17,570 --> 00:14:21,165 ...celebrate the end of the world as to make merry before it happened. 214 00:14:21,407 --> 00:14:25,935 There were entrepreneurs who were hawking comet pills. 215 00:14:26,380 --> 00:14:28,439 I think I'm gonna take one for later. 216 00:14:28,648 --> 00:14:31,481 And there were those who were selling... 217 00:14:31,951 --> 00:14:36,888 ...gas masks to protect against the cyanogen. 218 00:14:37,291 --> 00:14:41,660 And comet nuttiness didn't stop in 1910. 219 00:14:46,899 --> 00:14:50,835 Long before 1066, humans marveled at comets. 220 00:14:51,038 --> 00:14:54,235 Our generation is beginning to understand them. 221 00:15:02,949 --> 00:15:05,281 Mercury, Venus, Earth and Mars... 222 00:15:05,485 --> 00:15:08,943 ...are small planets made mostly of rock and iron. 223 00:15:09,722 --> 00:15:12,885 Farther out where it's colder, are the giant planets... 224 00:15:13,092 --> 00:15:14,855 ...made mostly of gas. 225 00:15:15,061 --> 00:15:18,258 But comets originate from a great cloud beyond the planets... 226 00:15:18,464 --> 00:15:20,694 ...almost halfway to the nearest star. 227 00:15:20,900 --> 00:15:23,130 Occasionally, one falls in... 228 00:15:23,336 --> 00:15:25,429 ...accelerated by the sun's gravity. 229 00:15:25,639 --> 00:15:27,834 Because it's made mostly of ice, the comet... 230 00:15:28,040 --> 00:15:30,167 ...evaporates as it approaches the sun. 231 00:15:30,376 --> 00:15:33,243 The vapor is blown back by the solar wind... 232 00:15:33,447 --> 00:15:35,312 ...forming the cometary tail. 233 00:15:35,515 --> 00:15:37,881 Then it's flung back into outer darkness... 234 00:15:38,084 --> 00:15:39,415 ...its orbit so large... 235 00:15:39,620 --> 00:15:42,418 ...that it will not return for millions of years. 236 00:15:42,622 --> 00:15:45,182 These are the long-period comets. 237 00:15:45,391 --> 00:15:48,758 For every one plunging close enough to the sun to be discovered... 238 00:15:48,961 --> 00:15:50,690 ...there may be a billion others... 239 00:15:50,898 --> 00:15:53,958 ...slowly drifting beyond Pluto's orbit. 240 00:15:54,468 --> 00:15:58,962 Very rarely, a long-period comet is captured in the inner solar system... 241 00:15:59,172 --> 00:16:01,140 ...becoming a short-period comet. 242 00:16:01,341 --> 00:16:04,799 It passes near a major planet, like Saturn. 243 00:16:05,012 --> 00:16:07,640 The planet provides a small gravitational tug... 244 00:16:07,848 --> 00:16:10,681 ...enough to deflect it into a much smaller orbit. 245 00:16:10,883 --> 00:16:13,545 Though few are captured this way, those that are... 246 00:16:13,753 --> 00:16:17,587 ...become well-known because they all return in short intervals. 247 00:16:17,790 --> 00:16:21,453 Once trapped in the inner solar system, among the planets... 248 00:16:21,662 --> 00:16:24,859 ...the chances of another near-collision are increased. 249 00:16:26,566 --> 00:16:29,228 Here, a second encounter with Saturn... 250 00:16:29,436 --> 00:16:33,270 ...further reduces the comet's orbital period to decades. 251 00:16:33,473 --> 00:16:37,637 A comet may take 10,000 years between close planetary encounters. 252 00:16:37,844 --> 00:16:41,678 But in this computer study, we've sped things up. 253 00:16:42,315 --> 00:16:45,182 A third encounter, this time with Jupiter... 254 00:16:45,384 --> 00:16:48,353 ...further reduces the comet's orbital period. 255 00:16:48,554 --> 00:16:51,216 Now the comet must approach the sun... 256 00:16:51,424 --> 00:16:54,018 ...and grow a tail every few years. 257 00:16:54,227 --> 00:16:57,560 Since the dust and gas in the tail are lost forever to space... 258 00:16:57,763 --> 00:17:00,391 ...the comet must slowly be eroding. 259 00:17:00,601 --> 00:17:02,262 Pieces of it break off. 260 00:17:02,468 --> 00:17:05,562 Sometimes, as we've seen, they even strike the Earth. 261 00:17:05,771 --> 00:17:07,204 In a few thousand years... 262 00:17:07,407 --> 00:17:10,433 ...if a short-period comet hasn't hit a planet... 263 00:17:10,643 --> 00:17:13,271 ...it will have evaporated away almost entirely... 264 00:17:13,479 --> 00:17:17,575 ...leaving sand-sized fragments, which become meteors... 265 00:17:17,784 --> 00:17:21,880 ...and its core which, perhaps, becomes an asteroid. 266 00:17:24,156 --> 00:17:27,956 Suppose I were a pretty typical comet. 267 00:17:28,161 --> 00:17:30,686 And what you would see would be a kind of... 268 00:17:30,897 --> 00:17:33,491 ...tumbling snowball... 269 00:17:33,699 --> 00:17:38,033 ...spending most of my time out here in the outer solar system. 270 00:17:38,237 --> 00:17:40,262 I'd be a kilometer across. 271 00:17:40,473 --> 00:17:42,236 I'd be living most of my days... 272 00:17:42,442 --> 00:17:46,401 ...in the gloom beyond Saturn, orbiting the sun. 273 00:17:46,613 --> 00:17:48,979 But once every century, I would find myself... 274 00:17:49,182 --> 00:17:52,208 ...careening inward, faster and faster... 275 00:17:52,418 --> 00:17:54,784 ...towards the inner solar system. 276 00:17:56,323 --> 00:17:59,759 By the time I would cross the orbit of Jupiter... 277 00:17:59,960 --> 00:18:01,791 ...on my way to the orbit of Mars... 278 00:18:01,994 --> 00:18:05,395 ...I'd be heating up because I'd be getting closer to the sun. 279 00:18:05,598 --> 00:18:07,463 I'd be evaporating a little bit. 280 00:18:07,668 --> 00:18:09,795 Small pieces of dust and ice... 281 00:18:10,002 --> 00:18:13,233 ...would be blown behind me by the solar wind... 282 00:18:13,439 --> 00:18:16,772 ...forming an incipient cometary tail. 283 00:18:16,976 --> 00:18:19,410 On the scale of such a solar system model... 284 00:18:19,612 --> 00:18:22,513 ...l, me, a cometary nucleus... 285 00:18:22,715 --> 00:18:24,910 ...would be smaller than a snowflake. 286 00:18:25,118 --> 00:18:29,418 Although, when fully developed, my tail would be longer... 287 00:18:29,622 --> 00:18:32,921 ...than the spacing between the worlds. 288 00:18:34,293 --> 00:18:36,227 Now, sooner or later... 289 00:18:36,429 --> 00:18:40,195 ...comets on these long, elliptical trajectories around the sun... 290 00:18:40,399 --> 00:18:42,594 ...must collide with planets. 291 00:18:42,802 --> 00:18:45,270 The Earth and the moon... 292 00:18:45,472 --> 00:18:49,135 ...must have been bombarded by comets and asteroids... 293 00:18:49,343 --> 00:18:52,312 ...the debris from the early history of the solar system. 294 00:18:52,512 --> 00:18:56,312 In interplanetary space, there are more small objects than large ones. 295 00:18:56,516 --> 00:18:59,576 So there must be, on a given planetary surface... 296 00:18:59,786 --> 00:19:04,086 ...many more impacts of small objects than of large objects. 297 00:19:04,290 --> 00:19:08,124 So a thing like the Tunguska impact happens on the Earth... 298 00:19:08,327 --> 00:19:09,919 ...maybe every thousand years. 299 00:19:10,130 --> 00:19:13,327 But the impact of a giant cometary nucleus... 300 00:19:13,533 --> 00:19:15,467 ...like Halley's comet, let's say... 301 00:19:15,668 --> 00:19:18,330 ...happens only every billion years or so. 302 00:19:19,071 --> 00:19:22,370 Now, is there evidence of past collisions? 303 00:19:23,542 --> 00:19:24,941 When a large comet... 304 00:19:25,144 --> 00:19:27,806 ...or a large, rocky asteroid hits a planet... 305 00:19:28,015 --> 00:19:30,313 ...it makes a bowl-shaped crater. 306 00:19:30,516 --> 00:19:34,680 The well-preserved impact craters on Earth were all formed fairly recently. 307 00:19:34,888 --> 00:19:38,221 The older ones have been softened, filled in or rubbed out... 308 00:19:38,425 --> 00:19:40,985 ...by running water and mountain building. 309 00:19:41,193 --> 00:19:44,685 Impacts make craters on other worlds and about as often. 310 00:19:44,898 --> 00:19:46,365 But when the air is thin... 311 00:19:46,565 --> 00:19:50,262 ...when water rarely flows, when mountain building is feeble... 312 00:19:50,469 --> 00:19:52,767 ...the ancient craters are retained. 313 00:19:52,972 --> 00:19:55,770 This is the case on the moon and Mercury and Mars... 314 00:19:55,976 --> 00:19:58,672 ...our neighboring terrestrial planets. 315 00:20:00,179 --> 00:20:02,409 They huddle around the sun... 316 00:20:02,616 --> 00:20:04,948 ...their source of heat and light... 317 00:20:05,151 --> 00:20:08,450 ...a little bit like campers around a fire. 318 00:20:08,821 --> 00:20:11,415 They are about 4˝ billion years old. 319 00:20:11,624 --> 00:20:15,583 And all bear witness to an age long gone... 320 00:20:15,795 --> 00:20:18,491 ...of major collisions... 321 00:20:18,698 --> 00:20:23,101 ...which do not happen at that scale and frequency anymore. 322 00:20:23,737 --> 00:20:26,706 If we move out past... 323 00:20:26,907 --> 00:20:29,569 ...the terrestrial planets beyond Mars... 324 00:20:29,775 --> 00:20:33,836 ...we find ourselves in a different regime of the solar system... 325 00:20:34,047 --> 00:20:36,140 ...in the realm of Jupiter... 326 00:20:36,348 --> 00:20:39,579 ...and the other giant, or Jovian planets. 327 00:20:40,619 --> 00:20:45,113 These are great worlds composed largely of the gases... 328 00:20:45,325 --> 00:20:47,850 ...hydrogen and helium, some other stuff too. 329 00:20:48,061 --> 00:20:52,862 When we look at the surface, we do not see a solid surface... 330 00:20:53,065 --> 00:20:56,831 ...but only an occasional patch of atmosphere... 331 00:20:57,036 --> 00:21:00,563 ...and a complex array of multicolored clouds. 332 00:21:00,873 --> 00:21:02,534 These are serious planets... 333 00:21:02,743 --> 00:21:06,839 ...not fragmentary little world-lets like the Earth. 334 00:21:07,046 --> 00:21:10,743 In fact, 1000 Earths would fit... 335 00:21:10,950 --> 00:21:13,316 ...in the volume of Jupiter. 336 00:21:13,519 --> 00:21:17,387 If a comet or asteroid were to... 337 00:21:17,590 --> 00:21:22,527 ...accidentally impact Jupiter, it would be very unlikely to leave a crater. 338 00:21:22,728 --> 00:21:26,129 It might make a momentary hole in the clouds, but that's it. 339 00:21:26,333 --> 00:21:30,030 Nevertheless, we know that the outer solar system... 340 00:21:30,237 --> 00:21:33,229 ...has been subject to a many-billion-year history... 341 00:21:33,440 --> 00:21:35,533 ...of impact cratering. 342 00:21:35,975 --> 00:21:39,968 Jupiter's moon Callisto is studded with thousands of craters. 343 00:21:40,179 --> 00:21:43,671 Clear evidence of ancient collisions beyond Mars. 344 00:21:43,884 --> 00:21:47,411 And there are craters on other moons of Jupiter. 345 00:21:47,654 --> 00:21:50,452 Most of the thousands of large craters on our own moon... 346 00:21:50,656 --> 00:21:53,386 ...were excavated billions of years ago. 347 00:21:53,592 --> 00:21:56,117 But were any recorded in historical times? 348 00:21:56,328 --> 00:22:00,196 The odds against it are about 1000-to-one. 349 00:22:01,233 --> 00:22:03,724 (BELL RINGS) 350 00:22:08,174 --> 00:22:11,337 Nevertheless, there's a possible eyewitness account... 351 00:22:11,544 --> 00:22:13,444 ...of just such an event. 352 00:22:13,646 --> 00:22:17,639 It was the Sunday before the feast of Saint John the Baptist... 353 00:22:18,050 --> 00:22:20,484 ...in the summer of 1178. 354 00:22:21,353 --> 00:22:25,551 The monks of Canterbury Cathedral had completed their evening prayers... 355 00:22:25,758 --> 00:22:27,953 ...and were about to retire for the night. 356 00:22:28,160 --> 00:22:29,923 The scholarly brother, Gervase... 357 00:22:30,130 --> 00:22:31,995 ...returned to his cell to read... 358 00:22:32,198 --> 00:22:33,790 ...while some of the others... 359 00:22:34,000 --> 00:22:37,595 ...went outside to enjoy the gentle June air. 360 00:22:37,803 --> 00:22:39,498 (PLAYS FLUTE) 361 00:22:43,376 --> 00:22:45,401 In the midst of their recreation... 362 00:22:45,611 --> 00:22:48,944 ...they chanced to witness an astonishing sight: 363 00:22:49,148 --> 00:22:52,515 A violent explosion on the moon. 364 00:23:01,494 --> 00:23:02,859 This was a time... 365 00:23:03,062 --> 00:23:05,587 ...when the heavens were thought to be changeless. 366 00:23:05,798 --> 00:23:09,632 The moon, the stars and the planets were deemed pure... 367 00:23:09,835 --> 00:23:13,828 ...because they followed an unvarying celestial routine. 368 00:23:14,073 --> 00:23:17,941 They were expected to behave without unseemly disruptions... 369 00:23:18,678 --> 00:23:20,509 ...like monks in a monastery. 370 00:23:21,046 --> 00:23:24,038 Was it wise to discuss such a vision? 371 00:23:30,923 --> 00:23:32,652 In every time and culture... 372 00:23:32,859 --> 00:23:36,295 ...there are pressures to conform to the prevailing prejudices. 373 00:23:37,396 --> 00:23:40,263 But there are also, in every place and epoch... 374 00:23:40,467 --> 00:23:44,870 ...those who value the truth, who record the evidence faithfully. 375 00:23:45,070 --> 00:23:48,164 Future generations are in their debt. 376 00:23:54,214 --> 00:23:56,546 A fire on the moon. 377 00:23:56,749 --> 00:24:00,412 Might it be some portent of ill fortune? 378 00:24:00,820 --> 00:24:04,051 Should the chronicler of the monastery be told? 379 00:24:04,925 --> 00:24:08,361 Was this event an apparition of the evil one? 380 00:24:11,197 --> 00:24:14,189 Gervase of Canterbury was a historian... 381 00:24:14,401 --> 00:24:17,165 ...considered today a reliable reporter of political... 382 00:24:17,369 --> 00:24:19,769 ...and cultural events of his time. 383 00:24:19,972 --> 00:24:24,170 This is his account of the eyewitness testimony he was given: 384 00:24:24,777 --> 00:24:26,836 "Now there was a bright new moon... 385 00:24:27,046 --> 00:24:28,980 ...and as usual in that phase... 386 00:24:29,182 --> 00:24:31,548 ...its horns were tilted toward the east. 387 00:24:31,750 --> 00:24:35,413 And suddenly the upper horn split in two. 388 00:24:35,622 --> 00:24:39,149 From the midpoint of this division, a flaming torch sprang up... 389 00:24:39,359 --> 00:24:41,953 ...spewing out over a considerable distance... 390 00:24:42,161 --> 00:24:45,221 ...fire, hot coals and sparks. 391 00:24:45,431 --> 00:24:48,594 After these transformations," Gervase continued... 392 00:24:48,801 --> 00:24:52,464 ..."the moon from horn to horn that is along its whole length... 393 00:24:52,671 --> 00:24:54,935 ...took on a blackish appearance." 394 00:24:59,946 --> 00:25:04,212 Gervase took depositions from all the eyewitnesses. 395 00:25:04,417 --> 00:25:05,714 He later wrote: 396 00:25:05,918 --> 00:25:10,514 "The writer was given this report by men who saw it with their own eyes... 397 00:25:10,724 --> 00:25:13,215 ...and are prepared to stake their honor on an oath... 398 00:25:13,425 --> 00:25:16,826 ...that they have made no addition or falsification." 399 00:25:17,296 --> 00:25:19,321 Gervase committed the account to paper... 400 00:25:19,531 --> 00:25:21,897 ...enabling astronomers eight centuries later... 401 00:25:22,102 --> 00:25:24,935 ...to try and reconstruct what really happened. 402 00:25:26,071 --> 00:25:29,006 It may be that 200 years before Chaucer... 403 00:25:29,209 --> 00:25:32,201 ...five monks saw an event more wonderful... 404 00:25:32,412 --> 00:25:35,540 ...than many another celebrated Canterbury tale. 405 00:25:38,518 --> 00:25:41,715 If a small drifting mountain were to hit the moon... 406 00:25:41,920 --> 00:25:44,684 ...it would set our satellite swinging like a bell. 407 00:25:44,890 --> 00:25:49,020 Eventually, the tremors would die down, but not in a mere 800 years. 408 00:25:49,229 --> 00:25:52,426 So is the moon still quivering from that impact? 409 00:25:52,632 --> 00:25:56,659 The Apollo astronauts emplaced arrays of special mirrors on the moon. 410 00:25:56,870 --> 00:25:59,031 Reflectors made by French scientists... 411 00:25:59,238 --> 00:26:02,366 ...were also put on the moon by Soviet Lunakhod vehicles. 412 00:26:02,574 --> 00:26:06,510 When a laser beam from Earth strikes a mirror and bounces back... 413 00:26:06,713 --> 00:26:09,341 ...the roundtrip travel time can be measured. 414 00:26:09,548 --> 00:26:13,314 At the McDonald Observatory of the University of Texas... 415 00:26:13,520 --> 00:26:17,354 ...a laser beam is prepared for firing at the reflectors on the moon... 416 00:26:17,557 --> 00:26:20,151 ...380,000 kilometers away. 417 00:26:21,061 --> 00:26:24,121 By multiplying the travel time by the speed of light... 418 00:26:24,330 --> 00:26:27,128 ...the distance to that spot can be determined... 419 00:26:27,332 --> 00:26:30,199 ...to a precision of 7 to 10 centimeters: 420 00:26:30,402 --> 00:26:32,802 The width of a hand. 421 00:26:36,475 --> 00:26:39,410 When such measurements are repeated over years... 422 00:26:39,611 --> 00:26:43,240 ...even an extremely slight wobble in the moon's motion... 423 00:26:43,449 --> 00:26:45,246 ...can be determined. 424 00:26:45,452 --> 00:26:48,285 The accuracy is phenomenal. 425 00:26:48,488 --> 00:26:50,854 The error is much less... 426 00:26:51,056 --> 00:26:54,253 ...than one-millionth of a percent. 427 00:26:55,962 --> 00:26:59,625 The moon, it turns out, is gently swinging like a bell... 428 00:26:59,833 --> 00:27:02,393 ...just as if it had been hit by an asteroid... 429 00:27:02,602 --> 00:27:04,968 ...less than 1000 years ago. 430 00:27:05,170 --> 00:27:07,138 (RINGING) 431 00:27:08,173 --> 00:27:12,337 So there may be physical evidence in the age of space flight... 432 00:27:12,711 --> 00:27:17,148 ...for the account of the Canterbury monks in the 12th century. 433 00:27:20,053 --> 00:27:22,920 If 800 years ago a big asteroid hit the moon... 434 00:27:23,122 --> 00:27:25,056 ...the crater should be prominent today... 435 00:27:25,257 --> 00:27:27,657 ...still surrounded by bright rays... 436 00:27:27,861 --> 00:27:31,228 ...thin streamers of dust spewed out by the impact. 437 00:27:31,431 --> 00:27:33,865 In billions of years, lunar rays are eroded... 438 00:27:34,067 --> 00:27:35,659 ...but not in hundreds. 439 00:27:35,868 --> 00:27:39,531 And there is a recent ray crater called Giordano Bruno... 440 00:27:39,738 --> 00:27:43,174 ...in the region of the moon where an explosion was reported... 441 00:27:43,375 --> 00:27:45,240 ...in 1178. 442 00:27:50,916 --> 00:27:52,884 The entire evolution of the moon... 443 00:27:53,086 --> 00:27:55,554 ...is a story of catastrophes. 444 00:27:55,755 --> 00:27:57,347 4 1/2 billion years ago... 445 00:27:57,556 --> 00:28:00,252 ...the moon was accreting from interplanetary boulders... 446 00:28:00,459 --> 00:28:03,053 ...and craters were forming all over its surface. 447 00:28:03,263 --> 00:28:06,096 The energy so released helped melt the crust. 448 00:28:06,299 --> 00:28:10,759 After most of this debris was swept up by the moon, the surface cooled. 449 00:28:11,203 --> 00:28:13,535 But about 3.9 billion years ago... 450 00:28:13,740 --> 00:28:16,675 ...a great asteroid impacted. 451 00:28:21,446 --> 00:28:25,576 It generated an expanding shock wave and re-melted some of the surface. 452 00:28:25,784 --> 00:28:27,809 The resulting basin was then flooded... 453 00:28:28,021 --> 00:28:29,613 ...probably by dark lava... 454 00:28:29,821 --> 00:28:33,188 ...producing one of the dry seas on the moon. 455 00:28:33,392 --> 00:28:36,589 More recent impacts excavated craters with bright rays... 456 00:28:36,795 --> 00:28:40,492 ...named after Eratosthenes and Copernicus. 457 00:28:40,700 --> 00:28:42,725 The familiar features of the man in the moon... 458 00:28:42,935 --> 00:28:45,961 ...are a chronicle of ancient impacts. 459 00:28:47,540 --> 00:28:50,202 Most of the original asteroids were swept up... 460 00:28:50,409 --> 00:28:52,400 ...in the making of the moon and planets. 461 00:28:52,612 --> 00:28:55,740 Many still orbit the sun in the asteroid belt. 462 00:28:55,949 --> 00:28:59,578 Some, themselves almost fractured by gravity tides... 463 00:28:59,785 --> 00:29:01,946 ...and by impacts with other asteroids... 464 00:29:02,155 --> 00:29:05,852 ...have been captured by planets: Phobos around Mars, for example... 465 00:29:06,059 --> 00:29:10,120 ...or a close moon of Jupiter called Amalthea. 466 00:29:11,730 --> 00:29:14,563 Similar to the asteroid belt are the rings of Saturn... 467 00:29:14,766 --> 00:29:19,396 ...composed of millions of small, tumbling, icy moonlets. 468 00:29:19,606 --> 00:29:23,064 Maybe the rings of Saturn are a moon... 469 00:29:23,276 --> 00:29:27,406 ...which was prevented from forming by the tides of Saturn. 470 00:29:27,614 --> 00:29:30,811 Or maybe it's the remains of a moon that wandered too close... 471 00:29:31,017 --> 00:29:33,611 ...and was torn apart by the tides of Saturn. 472 00:29:33,820 --> 00:29:36,755 It's certainly a lovely place. 473 00:29:36,956 --> 00:29:41,086 Jupiter also has a newly discovered ring system... 474 00:29:41,294 --> 00:29:44,024 ...which is invisible from the Earth. 475 00:29:46,732 --> 00:29:51,669 Now, there is a curious argument... 476 00:29:51,938 --> 00:29:55,396 ...alleging major recent collisions in the solar system... 477 00:29:55,608 --> 00:29:58,099 ...proposed by a psychiatrist... 478 00:29:58,311 --> 00:30:02,213 ...named Immanuel Velikovsky in 1950. 479 00:30:02,615 --> 00:30:04,879 He suggested... 480 00:30:05,084 --> 00:30:08,611 ...that an object of planetary mass, which he called a comet... 481 00:30:08,821 --> 00:30:12,313 ...was somehow produced in the Jupiter system. 482 00:30:12,525 --> 00:30:15,790 He doesn't say exactly how it's produced... 483 00:30:16,629 --> 00:30:17,994 ...but maybe... 484 00:30:21,400 --> 00:30:23,630 ...it's spat out... 485 00:30:26,773 --> 00:30:28,104 ...of Jupiter. 486 00:30:28,940 --> 00:30:33,877 Anyway, however it was made some 3500 years ago, he imagines... 487 00:30:34,446 --> 00:30:39,179 ...it made repeated close encounters with Mars... 488 00:30:40,019 --> 00:30:42,351 ...with the Earth-moon system... 489 00:30:42,721 --> 00:30:47,658 ...having as entertaining biblical consequences... 490 00:30:48,527 --> 00:30:53,226 ...the parting of the Red Sea so that Moses and the Israelites could... 491 00:30:53,432 --> 00:30:56,060 ...safely avoid the host of pharaoh... 492 00:30:56,268 --> 00:30:59,032 ...and the stopping of the Earth's rotation when... 493 00:30:59,237 --> 00:31:03,731 ...Joshua commanded the sun to stand still in Gibeon. 494 00:31:03,942 --> 00:31:06,570 He also imagined that there was extensive flooding... 495 00:31:06,778 --> 00:31:10,009 ...and the volcanoes all over the Earth at that time. 496 00:31:10,515 --> 00:31:15,111 Well, then after a very complicated game... 497 00:31:15,321 --> 00:31:19,052 ...of interplanetary billiards is completed... 498 00:31:19,258 --> 00:31:23,786 ...Velikovsky proposed that this comet... 499 00:31:23,995 --> 00:31:27,897 ...entered into a stable, almost perfectly circular orbit... 500 00:31:28,100 --> 00:31:29,397 ...becoming... 501 00:31:32,205 --> 00:31:33,832 ...the planet Venus... 502 00:31:34,039 --> 00:31:37,406 ...which he claimed never existed until then. 503 00:31:38,777 --> 00:31:43,612 Now, these ideas are almost certainly wrong. 504 00:31:44,217 --> 00:31:46,879 There's no objection in astronomy to collisions. 505 00:31:47,086 --> 00:31:49,281 We've seen collision fragments... 506 00:31:49,488 --> 00:31:52,787 ...and evidence throughout the solar system. 507 00:31:52,991 --> 00:31:56,825 The problem is with recent and major collisions. 508 00:31:57,029 --> 00:31:59,259 In any scale model like this... 509 00:31:59,464 --> 00:32:02,729 ...it's impossible to have both the sizes of the planets... 510 00:32:02,934 --> 00:32:05,402 ...and the sizes of their orbits to the same scale... 511 00:32:05,604 --> 00:32:08,767 ...because then the planets would be too small to see. 512 00:32:08,975 --> 00:32:12,035 If the planets were really to scale in such a model... 513 00:32:12,245 --> 00:32:14,713 ...as grains of dust... 514 00:32:14,914 --> 00:32:17,178 ...it would then be entirely clear... 515 00:32:17,382 --> 00:32:20,317 ...that a comet entering the inner solar system... 516 00:32:20,519 --> 00:32:23,420 ...would have a negligible chance of colliding with a planet... 517 00:32:23,622 --> 00:32:25,783 ...in only a few thousand years. 518 00:32:25,992 --> 00:32:27,289 Moreover... 519 00:32:27,492 --> 00:32:30,950 ...Venus is a rocky and metallic... 520 00:32:31,163 --> 00:32:33,529 ...hydrogen-poor world... 521 00:32:33,732 --> 00:32:36,667 ...whereas Jupiter, where Velikovsky imagines it comes from... 522 00:32:36,869 --> 00:32:39,497 ...is made of almost nothing but hydrogen. 523 00:32:39,705 --> 00:32:44,506 There is no energy source in Jupiter to eject planets or comets. 524 00:32:44,709 --> 00:32:48,440 If one did enter the inner solar system... 525 00:32:48,647 --> 00:32:52,174 ...there is no way it could stop the Earth from rotating. 526 00:32:52,384 --> 00:32:55,785 And if it could, there's no way Earth could start rotating again... 527 00:32:55,987 --> 00:32:58,387 ...at anything like 24 hours a day. 528 00:32:58,590 --> 00:33:01,923 There's no geological evidence for flooding and volcanism... 529 00:33:02,127 --> 00:33:04,186 ...3500 years ago. 530 00:33:04,397 --> 00:33:07,560 Babylonian astronomers observed Venus... 531 00:33:07,767 --> 00:33:10,395 ...in its present stable orbit... 532 00:33:10,603 --> 00:33:13,595 ...before Velikovsky said it existed. 533 00:33:13,806 --> 00:33:16,331 And so on. 534 00:33:20,847 --> 00:33:23,611 There are many hypotheses in science which are wrong. 535 00:33:23,815 --> 00:33:27,342 That's all right. It's the aperture to finding out what's right. 536 00:33:27,552 --> 00:33:30,385 Science is a self-correcting process. 537 00:33:30,589 --> 00:33:33,114 To be accepted, new ideas must survive... 538 00:33:33,325 --> 00:33:37,625 ...the most rigorous standards of evidence and scrutiny. 539 00:33:37,963 --> 00:33:40,955 The worst aspect of the Velikovsky affair is not... 540 00:33:41,166 --> 00:33:43,726 ...that many of his ideas were wrong or silly... 541 00:33:43,935 --> 00:33:46,597 ...or in gross contradiction to the facts. 542 00:33:46,806 --> 00:33:50,674 Rather, the worst aspect is that some scientists... 543 00:33:50,876 --> 00:33:54,243 ...attempted to suppress Velikovsky's ideas. 544 00:33:54,479 --> 00:33:58,711 The suppression of uncomfortable ideas may be common in religion... 545 00:33:58,917 --> 00:34:02,785 ...or in politics, but it is not the path to knowledge. 546 00:34:02,989 --> 00:34:06,288 And there's no place for it in the endeavor of science. 547 00:34:06,492 --> 00:34:08,187 We do not know beforehand... 548 00:34:08,393 --> 00:34:12,420 ...where fundamental insights will arise from... 549 00:34:12,631 --> 00:34:16,761 ...about our mysterious and lovely solar system. 550 00:34:16,969 --> 00:34:20,598 And the history of our study of the solar system shows clearly... 551 00:34:20,806 --> 00:34:24,833 ...that accepted and conventional ideas are often wrong... 552 00:34:25,044 --> 00:34:27,012 ...and that fundamental insights... 553 00:34:27,212 --> 00:34:31,046 ...can arise from the most unexpected sources. 554 00:34:32,685 --> 00:34:34,448 We've evolved on the planet Earth... 555 00:34:34,654 --> 00:34:37,248 ...and so we find it a congenial place. 556 00:34:37,455 --> 00:34:39,787 But just next door is Venus... 557 00:34:39,991 --> 00:34:42,983 ...until recently, enveloped in mystery. 558 00:34:43,195 --> 00:34:46,096 It has almost the same size and mass as the Earth. 559 00:34:46,299 --> 00:34:50,292 Might our sister world be a balmy summer planet... 560 00:34:50,502 --> 00:34:53,835 ...a little warmer than the Earth because it's closer to the sun? 561 00:34:54,039 --> 00:34:58,373 Are there craters, volcanoes, mountains, oceans, life? 562 00:34:59,412 --> 00:35:04,111 The first to look at Venus through a telescope was Galileo in 1609. 563 00:35:04,316 --> 00:35:07,114 But all he could see was a featureless disk. 564 00:35:07,687 --> 00:35:11,214 As optical telescopes got bigger, that's all anybody could see: 565 00:35:11,423 --> 00:35:14,017 A disk with no details on it at all. 566 00:35:14,226 --> 00:35:17,992 Venus evidently was covered with an opaque layer... 567 00:35:18,197 --> 00:35:21,325 ...thick clouds concealing the surface. 568 00:35:21,534 --> 00:35:26,130 For centuries, even the composition of the clouds of Venus was unknown. 569 00:35:26,338 --> 00:35:30,866 I mean, you could go outside, look up, see Venus with the naked eye... 570 00:35:31,077 --> 00:35:34,171 ...observe sunlight reflected from the clouds of Venus. 571 00:35:34,380 --> 00:35:36,974 What were you looking at? What were the clouds made of? 572 00:35:37,182 --> 00:35:38,740 Nobody knew. 573 00:35:38,951 --> 00:35:42,648 As a result, imagination ran riot. 574 00:35:42,855 --> 00:35:45,722 The absence of anything you could see on Venus... 575 00:35:45,925 --> 00:35:49,361 ...led some scientists and others to deduce... 576 00:35:49,562 --> 00:35:51,723 ...that the surface was a swamp. 577 00:35:53,032 --> 00:35:57,025 The argument, if we can dignify it with such a phrase... 578 00:35:57,435 --> 00:35:58,367 ...went like this: 579 00:35:58,571 --> 00:36:00,869 "I can't see a thing on the surface of Venus." 580 00:36:01,072 --> 00:36:01,731 "Why not?" 581 00:36:01,941 --> 00:36:05,069 "Because it's covered with a dense layer of clouds." 582 00:36:05,276 --> 00:36:06,743 "What are clouds made of?" 583 00:36:06,946 --> 00:36:10,575 "Water, of course. Therefore, Venus must have a lot of water on it." 584 00:36:10,783 --> 00:36:12,273 "Then the surface must be wet." 585 00:36:12,485 --> 00:36:15,181 "If the surface is wet, it's probably a swamp. 586 00:36:15,387 --> 00:36:18,220 If there's a swamp, there's ferns. If there's ferns... 587 00:36:18,423 --> 00:36:20,789 ...maybe there's even dinosaurs." 588 00:36:20,993 --> 00:36:22,927 Observation: You couldn't see a thing. 589 00:36:23,129 --> 00:36:25,723 Conclusion: dinosaurs. 590 00:36:26,298 --> 00:36:29,096 If just looking at Venus was so unproductive... 591 00:36:29,300 --> 00:36:30,733 ...what else could you do? 592 00:36:30,936 --> 00:36:34,303 The next clue came from early work with that: 593 00:36:34,507 --> 00:36:36,031 A glass prism. 594 00:36:36,241 --> 00:36:40,371 An intense beam of ordinary white light is passed through a narrow slit... 595 00:36:40,579 --> 00:36:42,171 ...and then through the prism. 596 00:36:42,381 --> 00:36:44,975 The result is to spread the white light out... 597 00:36:45,184 --> 00:36:48,551 ...into its constituent rainbow of colors. 598 00:36:49,188 --> 00:36:52,521 This rainbow pattern is called a spectrum. 599 00:36:52,725 --> 00:36:55,523 Think about it. White light enters the prism... 600 00:36:55,728 --> 00:36:58,492 ...what comes out of the prism is colored light. 601 00:36:58,698 --> 00:37:00,723 Lots of colors. Where did they come from? 602 00:37:00,932 --> 00:37:02,957 They must've been hiding in the white light. 603 00:37:03,169 --> 00:37:06,297 White light must be a mixture of many colors. 604 00:37:06,505 --> 00:37:09,269 Here we see the spectrum running from... 605 00:37:09,475 --> 00:37:12,876 ...violet, blue, green, yellow, orange, to red. 606 00:37:13,079 --> 00:37:17,516 Since we see these colors, we call this the spectrum of visible light. 607 00:37:18,550 --> 00:37:22,577 The sun emits lots of visible light. The air is transparent to it. 608 00:37:22,788 --> 00:37:25,450 So our eyes evolved to work in visible light. 609 00:37:25,657 --> 00:37:29,149 But there are many other frequencies of light which our eyes can't detect. 610 00:37:29,361 --> 00:37:31,420 Beyond the violet is the ultraviolet. 611 00:37:31,629 --> 00:37:34,757 It's just as real, but you need instruments to detect it. 612 00:37:34,967 --> 00:37:38,300 Beyond the ultraviolet are the x-rays and then the gamma rays. 613 00:37:38,837 --> 00:37:41,203 On the other side of visible light, beyond the red... 614 00:37:41,407 --> 00:37:44,137 ...is the infrared, again real, again invisible. 615 00:37:44,343 --> 00:37:47,540 Beyond the infrared are the radio waves. 616 00:37:47,747 --> 00:37:51,513 Now, this entire range from the gamma rays way over there... 617 00:37:51,717 --> 00:37:54,117 ...to the radio waves all the way over here... 618 00:37:54,320 --> 00:37:56,686 ...are simply different kinds of light. 619 00:37:56,888 --> 00:37:59,083 They differ only in the frequency. 620 00:37:59,291 --> 00:38:02,260 They're all useful, by the way, in astronomy. 621 00:38:02,461 --> 00:38:05,089 But because of the limitations of our eyes... 622 00:38:05,297 --> 00:38:09,631 ...we have a prejudice, a bias, a chauvinism... 623 00:38:09,835 --> 00:38:13,430 ...to this tiny rainbow band of visible light. 624 00:38:13,638 --> 00:38:17,802 Now, a spectrum can be used in a simple and elegant way... 625 00:38:18,042 --> 00:38:21,307 ...to determine the chemical composition of the atmosphere... 626 00:38:21,513 --> 00:38:22,878 ...of a planet or star. 627 00:38:23,082 --> 00:38:25,448 Different atoms and molecules absorb... 628 00:38:25,651 --> 00:38:28,415 ...different frequencies or colors of light. 629 00:38:28,620 --> 00:38:33,114 And those absorbed or missing frequencies appear as black lines... 630 00:38:33,325 --> 00:38:37,159 ...in the spectrum of the light we receive from the planet or star. 631 00:38:37,363 --> 00:38:41,697 Each and every substance has a characteristic fingerprint... 632 00:38:41,900 --> 00:38:44,198 ...a spectral signature... 633 00:38:44,403 --> 00:38:47,338 ...which permits it to be detected over a great distance. 634 00:38:47,540 --> 00:38:51,101 As a result, the gases in the atmosphere of Venus... 635 00:38:51,310 --> 00:38:54,211 ...at a distance of 60 million kilometers... 636 00:38:54,413 --> 00:38:58,144 ...their composition's been determined from the Earth. 637 00:38:58,350 --> 00:39:02,844 It's amazing to me still, we can tell what a thing is made out of... 638 00:39:03,054 --> 00:39:06,683 ...at an enormous distance away, without ever touching it. 639 00:39:08,027 --> 00:39:12,157 Our eyes can't see in the near infrared part of the spectrum. 640 00:39:12,364 --> 00:39:13,763 But our instruments can. 641 00:39:13,965 --> 00:39:17,799 Here's the absorption pattern of lots and lots of carbon dioxide: 642 00:39:18,003 --> 00:39:22,463 Dark lines in characteristic patterns at specific frequencies. 643 00:39:22,708 --> 00:39:25,506 You'd detect a different set of infrared lines... 644 00:39:25,711 --> 00:39:28,373 ...if, say, water vapor were present. 645 00:39:29,148 --> 00:39:34,051 If Venus were really soaking wet, then you could determine that... 646 00:39:34,419 --> 00:39:38,219 ...by finding the pattern of water vapor in its atmosphere. 647 00:39:38,423 --> 00:39:41,517 But around 1920, when this experiment was first performed... 648 00:39:41,726 --> 00:39:44,388 ...the Venus atmosphere seemed to have not a hint... 649 00:39:44,597 --> 00:39:48,795 ...not a smidgen, not a trace of water vapor above the clouds. 650 00:39:49,000 --> 00:39:53,096 And so instead of a swampy, soaking wet surface... 651 00:39:53,305 --> 00:39:56,968 ...it was suggested that Venus was bone-dry, a desert planet... 652 00:39:57,176 --> 00:40:00,668 ...with clouds composed of fine silicate dust. 653 00:40:01,247 --> 00:40:04,114 But later, spectroscopic observations revealed... 654 00:40:04,316 --> 00:40:06,216 ...the characteristic absorption lines... 655 00:40:06,417 --> 00:40:09,284 ...of an enormous amount of carbon dioxide. 656 00:40:09,487 --> 00:40:13,856 Scientists thought there must be lots of carbon compounds on the surface... 657 00:40:14,059 --> 00:40:17,654 ...making this a planet covered with petroleum. 658 00:40:17,897 --> 00:40:21,856 Others agreed that the atmosphere was dry but thought the surface was wet. 659 00:40:22,067 --> 00:40:25,559 With all that CO 2, it had to be carbonated water. 660 00:40:25,771 --> 00:40:29,673 Venus, they thought, was covered with a vast ocean of seltzer. 661 00:40:29,875 --> 00:40:33,311 The first hint of the true situation on Venus came... 662 00:40:33,512 --> 00:40:37,004 ...not from the visible, ultraviolet or infrared part of the spectrum... 663 00:40:37,216 --> 00:40:40,185 ...but from over here in the radio region. 664 00:40:40,386 --> 00:40:43,787 We're used to the idea of radio signals from intelligent life... 665 00:40:43,989 --> 00:40:47,948 ...or at least semi-intelligent life, radio and television stations. 666 00:40:48,159 --> 00:40:52,152 But there are all kinds of reasons why natural objects emit radio waves. 667 00:40:52,363 --> 00:40:54,957 One reason is that they're hot. 668 00:40:55,266 --> 00:40:57,234 And when, in 1956... 669 00:40:57,436 --> 00:41:00,599 ...Venus was, for the first time, observed by a radio telescope... 670 00:41:00,806 --> 00:41:03,866 ...the planet was discovered to be emitting radio waves... 671 00:41:04,076 --> 00:41:07,341 ...as if it were at an extremely high temperature. 672 00:41:07,546 --> 00:41:12,279 But the real demonstration that Venus' surface was astonishingly hot... 673 00:41:12,483 --> 00:41:17,420 ...came when the first spacecraft penetrated the clouds of Venus... 674 00:41:17,756 --> 00:41:21,988 ...and slowly settled on the surface of the nearest planet. 675 00:41:25,064 --> 00:41:30,001 These were the unmanned spacecraft of the Soviet Venera series. 676 00:41:32,604 --> 00:41:37,268 In our spaceship of the imagination, we retrace their course. 677 00:41:38,677 --> 00:41:43,114 From a distance, our sister planet seems serene and peaceful... 678 00:41:43,315 --> 00:41:45,510 ...its clouds motionless. 679 00:41:48,520 --> 00:41:51,978 These clouds are near the top of a great ocean of air... 680 00:41:52,191 --> 00:41:56,958 ...about 100 kilometers thick, composed mainly of carbon dioxide. 681 00:42:00,566 --> 00:42:03,660 There's some nitrogen, a little water vapor and other gases... 682 00:42:03,869 --> 00:42:07,134 ...but only the merest trace of hydrocarbons. 683 00:42:07,539 --> 00:42:09,973 The clouds turn out to be, not water... 684 00:42:10,175 --> 00:42:13,736 ...but a concentrated solution of sulfuric acid. 685 00:42:22,854 --> 00:42:24,617 Even in the high clouds... 686 00:42:24,823 --> 00:42:28,520 ...Venus is a thoroughly nasty place. 687 00:42:36,701 --> 00:42:39,465 The clouds are stained yellow by sulfur. 688 00:42:39,671 --> 00:42:41,468 There are great lightning storms. 689 00:42:41,673 --> 00:42:43,971 As we descend, there are increasing amounts... 690 00:42:44,175 --> 00:42:46,143 ...of the noxious gas sulfur dioxide. 691 00:42:46,812 --> 00:42:49,975 The pressures become so high that early Venera spacecraft... 692 00:42:50,182 --> 00:42:52,776 ...were crushed like old tin cans... 693 00:42:52,985 --> 00:42:55,613 ...by the weight of the surrounding atmosphere. 694 00:42:58,423 --> 00:43:01,449 Beneath the clouds in the dense, clear air... 695 00:43:01,659 --> 00:43:04,219 ...it's as bright as on an overcast day on Earth. 696 00:43:04,430 --> 00:43:08,059 But the atmosphere is so thick that the ground seems to ripple... 697 00:43:08,266 --> 00:43:09,756 ...and distort. 698 00:43:10,002 --> 00:43:13,904 The atmospheric pressure down here is 90 times that on Earth. 699 00:43:14,173 --> 00:43:19,110 The temperature is 380 degrees centigrade, 900 degrees Fahrenheit. 700 00:43:19,510 --> 00:43:22,138 Hotter than the hottest household oven. 701 00:43:22,346 --> 00:43:24,906 This is a world marked by searing heat... 702 00:43:25,117 --> 00:43:27,677 ...crushing pressures, sulfurous gases... 703 00:43:27,886 --> 00:43:30,548 ...and a desolate, reddish landscape. 704 00:43:30,756 --> 00:43:34,749 Far from the balmy paradise imagined by some early scientists... 705 00:43:34,960 --> 00:43:39,226 ...Venus is the one place in the solar system most like hell. 706 00:43:45,137 --> 00:43:47,628 But today, as in ancient tradition... 707 00:43:47,840 --> 00:43:52,436 ...there are travelers who will dare a visit to the underworld. 708 00:43:52,678 --> 00:43:55,476 Venera 9 was the first spacecraft in human history... 709 00:43:55,681 --> 00:43:58,411 ...to return a photograph from the surface of Venus. 710 00:43:58,617 --> 00:44:01,347 It found the rocks curiously eroded... 711 00:44:01,920 --> 00:44:03,547 ...perhaps by the corrosive gases... 712 00:44:03,755 --> 00:44:06,053 ...perhaps because the temperature is so high... 713 00:44:06,258 --> 00:44:09,955 ...that the rocks are partly molten and sluggishly flow. 714 00:44:10,162 --> 00:44:14,929 The Soviet Venera spacecraft, their electronics long ago fried... 715 00:44:15,133 --> 00:44:18,569 ...are slowly corroding on the surface of Venus. 716 00:44:18,769 --> 00:44:20,828 They are the first spaceships from Earth... 717 00:44:21,039 --> 00:44:23,803 ...ever to land on another planet. 718 00:44:28,580 --> 00:44:30,480 The reason Venus is like hell... 719 00:44:30,682 --> 00:44:33,810 ...seems to be what's called the greenhouse effect. 720 00:44:34,019 --> 00:44:37,716 Ordinary visible sunlight penetrates the clouds and heats the surface. 721 00:44:37,923 --> 00:44:40,653 But the dense atmosphere blankets the surface... 722 00:44:40,859 --> 00:44:43,521 ...and prevents it from cooling off to space. 723 00:44:43,729 --> 00:44:46,459 An atmosphere 90 times as dense as ours... 724 00:44:46,664 --> 00:44:49,394 ...made of carbon dioxide, water vapor and other gases... 725 00:44:49,601 --> 00:44:51,398 ...lets in visible light from the sun... 726 00:44:51,603 --> 00:44:55,664 ...but will not let out the infrared light radiated by the surface. 727 00:44:55,874 --> 00:44:57,364 The temperature rises... 728 00:44:57,575 --> 00:45:00,544 ...until the infrared radiation trickling out to space... 729 00:45:00,745 --> 00:45:03,839 ...just balances the sunlight reaching the surface. 730 00:45:07,318 --> 00:45:10,082 The greenhouse effect can make an Earth-like world... 731 00:45:10,289 --> 00:45:12,723 ...into a planetary inferno. 732 00:45:14,959 --> 00:45:18,019 In this caldron, there's not likely to be anything alive... 733 00:45:18,229 --> 00:45:20,356 ...even creatures very different from us. 734 00:45:20,566 --> 00:45:23,660 Organic and other conceivable biological molecules... 735 00:45:23,869 --> 00:45:26,667 ...would simply fall to pieces. 736 00:45:39,217 --> 00:45:41,913 The hell of Venus is in stark contrast... 737 00:45:42,120 --> 00:45:45,612 ...with the comparative heaven of its neighboring world... 738 00:45:45,823 --> 00:45:49,224 ...our little planetary home, the Earth. 739 00:45:50,896 --> 00:45:54,889 Here, the atmosphere is 90 times thinner. 740 00:45:55,100 --> 00:45:58,069 Here, the carbon dioxide and water vapor... 741 00:45:58,270 --> 00:46:00,295 ...make a modest greenhouse effect... 742 00:46:00,504 --> 00:46:03,496 ...which heats the ground above the freezing point of water. 743 00:46:03,708 --> 00:46:08,202 Without it, our oceans would be frozen solid. 744 00:46:08,413 --> 00:46:11,780 A little greenhouse effect is a good thing. 745 00:46:23,161 --> 00:46:25,823 But Venus is an ominous reminder... 746 00:46:26,031 --> 00:46:28,192 ...that on a world rather like the Earth... 747 00:46:28,399 --> 00:46:30,731 ...things can go wrong. 748 00:46:31,270 --> 00:46:35,798 There is no guarantee that our planet will always be so hospitable. 749 00:46:36,008 --> 00:46:38,169 To maintain this clement world... 750 00:46:38,377 --> 00:46:41,938 ...we must understand it and appreciate it. 751 00:46:45,183 --> 00:46:47,879 The Earth is a place to our eyes... 752 00:46:48,086 --> 00:46:50,987 ...more beautiful than any other that we know. 753 00:46:51,189 --> 00:46:54,647 But this beauty has been sculpted by change: 754 00:46:54,860 --> 00:46:57,488 Gentle, almost undetectable change... 755 00:46:57,696 --> 00:46:59,994 ...and sudden, violent change. 756 00:47:00,198 --> 00:47:04,328 In the cosmos, there is no refuge from change. 757 00:47:05,070 --> 00:47:08,267 The Sphinx: human head, lion's body... 758 00:47:08,473 --> 00:47:11,909 ...constructed more than 5500 years ago. 759 00:47:12,344 --> 00:47:15,279 That face was once crisp and cleanly rendered... 760 00:47:15,480 --> 00:47:17,175 ...like this paw I am standing on. 761 00:47:17,381 --> 00:47:20,407 The paw has been buried in the sand until recently... 762 00:47:20,618 --> 00:47:22,779 ...and protected from erosion. 763 00:47:23,288 --> 00:47:26,951 The face is now muddled and softened... 764 00:47:27,159 --> 00:47:30,822 ...because of thousands of years of sandblasting in the desert... 765 00:47:31,028 --> 00:47:32,928 ...and a little rainfall. 766 00:47:33,432 --> 00:47:37,232 In New York City, there is an obelisk called Cleopatra's Needle... 767 00:47:37,436 --> 00:47:38,960 ...which comes from Egypt. 768 00:47:39,171 --> 00:47:43,301 In only a little more than a century in New York's Central Park... 769 00:47:43,508 --> 00:47:47,945 ...the inscriptions on that obelisk have been almost totally obliterated. 770 00:47:48,146 --> 00:47:50,637 Not by sand and water... 771 00:47:50,849 --> 00:47:53,317 ...but by smog and industrial pollution. 772 00:47:53,518 --> 00:47:55,509 A bit like the atmosphere of Venus. 773 00:47:56,053 --> 00:48:00,012 Slow erosion wipes out information. 774 00:48:00,225 --> 00:48:01,249 On the Earth... 775 00:48:01,460 --> 00:48:03,792 ...mountain ranges are destroyed by erosion... 776 00:48:03,995 --> 00:48:06,190 ...in maybe tens of millions of years... 777 00:48:06,397 --> 00:48:10,424 ...small impact craters in maybe hundreds of thousands of years. 778 00:48:10,635 --> 00:48:13,399 And the greatest artifacts of human beings... 779 00:48:13,604 --> 00:48:16,971 ...in thousands or tens of thousands of years. 780 00:48:19,211 --> 00:48:22,669 In addition to such slow and uniform processes... 781 00:48:22,881 --> 00:48:26,248 ...there are rare but sudden catastrophes. 782 00:48:26,451 --> 00:48:29,215 The Sphinx is missing a nose. 783 00:48:29,421 --> 00:48:33,653 In an act of idle desecration, some soldiers once shot it off. 784 00:48:33,859 --> 00:48:38,193 If you wait long enough, everything changes. 785 00:48:56,481 --> 00:49:00,645 Slow, uniform processes, unheralded events: 786 00:49:00,852 --> 00:49:02,479 The sting of a sand grain... 787 00:49:02,686 --> 00:49:04,711 ...the fall of a drop of water... 788 00:49:04,923 --> 00:49:08,586 ...can, over the ages, totally rework the landscape. 789 00:49:29,281 --> 00:49:31,841 And rare, violent processes... 790 00:49:32,050 --> 00:49:35,019 ...exceptional events that will not recur in a lifetime... 791 00:49:35,287 --> 00:49:37,915 ...also make major changes. 792 00:49:58,609 --> 00:50:02,739 Both the insignificant and the extraordinary... 793 00:50:02,948 --> 00:50:06,111 ...are the architects of the natural world. 794 00:50:50,861 --> 00:50:53,887 The destruction of trees and grasslands... 795 00:50:54,099 --> 00:50:56,533 ...makes the surface of the Earth brighter. 796 00:50:56,735 --> 00:51:00,796 It reflects more sunlight back to space and cools our planet. 797 00:51:01,705 --> 00:51:03,400 After we discovered fire... 798 00:51:03,608 --> 00:51:06,805 ...we began to incinerate forests intentionally... 799 00:51:07,012 --> 00:51:09,139 ...to clear the land by a process called... 800 00:51:09,548 --> 00:51:12,608 ..."slash and burn" agriculture. 801 00:51:12,817 --> 00:51:17,379 And today, forests and grasslands are being destroyed... 802 00:51:17,589 --> 00:51:21,889 ...frivolously, carelessly by humans who are... 803 00:51:22,093 --> 00:51:26,120 ...heedless of the beauty of our cousins the trees... 804 00:51:26,331 --> 00:51:29,698 ...and ignorant of the possible climatic catastrophes... 805 00:51:29,901 --> 00:51:33,962 ...which large-scale burning of forests may bring. 806 00:51:34,439 --> 00:51:36,805 (TREES BREAKING) 807 00:51:39,544 --> 00:51:42,012 The indiscriminate destruction of vegetation... 808 00:51:42,213 --> 00:51:43,805 ...may alter the global climate... 809 00:51:44,015 --> 00:51:46,950 ...in ways that no scientist can yet predict. 810 00:51:48,453 --> 00:51:50,683 It has already deadened large patches... 811 00:51:50,889 --> 00:51:53,323 ...of the Earth's life-supporting skin. 812 00:52:05,403 --> 00:52:09,601 And yet, we ravage the Earth at an accelerated pace... 813 00:52:09,808 --> 00:52:12,140 ...as if it belonged to this one generation... 814 00:52:12,344 --> 00:52:15,939 ...as if it were ours to do with as we please. 815 00:52:24,789 --> 00:52:27,189 The Earth has mechanisms to cleanse itself... 816 00:52:27,392 --> 00:52:30,725 ...to neutralize the toxic substances in its system. 817 00:52:30,929 --> 00:52:33,523 But these mechanisms work only up to a point. 818 00:52:33,732 --> 00:52:37,099 Beyond some critical threshold, they break down. 819 00:52:37,302 --> 00:52:40,533 The damage becomes irreversible. 820 00:53:08,600 --> 00:53:10,534 Our generation must choose. 821 00:53:10,735 --> 00:53:13,829 Which do we value more: short-term profits... 822 00:53:14,039 --> 00:53:18,032 ...or the long-term habitability of our planetary home? 823 00:53:21,079 --> 00:53:23,047 The world is divided politically. 824 00:53:23,248 --> 00:53:25,682 But ecologically it is tightly interwoven. 825 00:53:25,884 --> 00:53:29,581 There are no useless threads in the fabric of the ecosystem. 826 00:53:29,854 --> 00:53:33,688 If you cut any one of them, you will unravel many others. 827 00:53:35,060 --> 00:53:36,652 We have uncovered other worlds... 828 00:53:36,861 --> 00:53:40,126 ...with choking atmospheres and deadly surfaces. 829 00:53:40,398 --> 00:53:43,731 Shall we then re-create these hells on Earth? 830 00:53:46,504 --> 00:53:50,406 We have encountered desolate moons and barren asteroids. 831 00:53:50,608 --> 00:53:55,545 Shall we then scar and crater this blue-green world in their likeness? 832 00:54:14,099 --> 00:54:16,897 Natural catastrophes are rare. 833 00:54:17,102 --> 00:54:18,535 But they come often enough. 834 00:54:18,737 --> 00:54:22,366 We need not force the hand of nature. 835 00:54:32,016 --> 00:54:36,316 If we ruin the Earth, there is no place else to go. 836 00:54:36,521 --> 00:54:39,217 This is not a disposable world. 837 00:54:39,424 --> 00:54:43,190 And we are not yet able to re-engineer other planets. 838 00:54:51,703 --> 00:54:54,228 The cruelest desert on Earth... 839 00:54:54,439 --> 00:54:58,705 ...is far more hospitable than any place on Mars. 840 00:55:00,011 --> 00:55:03,469 The bright, sandy surface and dusty atmosphere of Mars... 841 00:55:03,681 --> 00:55:07,276 ...reflect enough sunlight back to space to cool the planet... 842 00:55:07,485 --> 00:55:12,422 ...freezing out all its water, locking it in a perpetual ice age. 843 00:55:13,391 --> 00:55:17,953 Human activities brighten our landscape and our atmosphere. 844 00:55:18,163 --> 00:55:21,394 Might this ultimately make an ice age here? 845 00:55:22,500 --> 00:55:26,527 At the same time, we are releasing vast quantities of carbon dioxide... 846 00:55:26,738 --> 00:55:29,468 ...increasing the greenhouse effect. 847 00:55:29,674 --> 00:55:32,302 The Earth need not resemble Venus very closely... 848 00:55:32,510 --> 00:55:35,377 ...for it to become barren and lifeless. 849 00:55:39,017 --> 00:55:42,453 It may not take much to destabilize the Earth's climate... 850 00:55:42,654 --> 00:55:46,112 ...to convert this heaven, our only home in the cosmos... 851 00:55:46,324 --> 00:55:48,292 ...into a kind of hell. 852 00:55:50,829 --> 00:55:54,390 The study of the global climate, the sun's influence... 853 00:55:54,599 --> 00:55:57,227 ...the comparison of the Earth with other worlds... 854 00:55:57,435 --> 00:56:00,871 These are subjects in their earliest stages of development. 855 00:56:01,072 --> 00:56:04,235 They are funded poorly and grudgingly. 856 00:56:04,442 --> 00:56:08,572 Meanwhile, we continue to load the Earth's atmosphere with materials... 857 00:56:08,780 --> 00:56:13,217 ...about whose long-term influence we are almost entirely ignorant. 858 00:56:14,586 --> 00:56:19,080 There are worlds that began with as much apparent promise as Earth. 859 00:56:19,290 --> 00:56:22,157 But something went wrong. 860 00:56:22,360 --> 00:56:26,888 Knowing that worlds can die alerts us to our danger. 861 00:56:28,032 --> 00:56:31,934 If a visitor arrived from another world, what account would we give... 862 00:56:32,136 --> 00:56:35,401 ...of our stewardship of the planet Earth? 863 00:56:42,614 --> 00:56:47,483 In the history of the solar system, have worlds ever been destroyed? 864 00:56:48,586 --> 00:56:51,783 Most of the moons in the outer solar system have craters on them... 865 00:56:51,990 --> 00:56:54,424 ...made by cometary impacts. 866 00:56:55,126 --> 00:56:57,060 Some have such large craters though... 867 00:56:57,262 --> 00:57:00,925 ...that if the impacting comets had been just a little bit bigger... 868 00:57:01,132 --> 00:57:03,225 ...the moons would have been shattered. 869 00:57:05,803 --> 00:57:08,363 What would the results of such a collision look like? 870 00:57:08,573 --> 00:57:10,507 (EXPLOSION) 871 00:57:11,409 --> 00:57:13,104 Maybe a planetary ring. 872 00:57:15,580 --> 00:57:18,447 The idea has been growing that little worlds are... 873 00:57:18,650 --> 00:57:21,517 ...every now and then, demolished by a cometary impact. 874 00:57:21,719 --> 00:57:26,281 The fragments then slowly coalesce, and a moon arises again... 875 00:57:26,491 --> 00:57:27,822 ...from its own ashes. 876 00:57:28,026 --> 00:57:32,554 Some moons may have been destroyed and reconstituted many times. 877 00:57:33,798 --> 00:57:37,564 For our own world, the peril is more subtle. 878 00:57:38,202 --> 00:57:39,897 Since this series was first broadcast... 879 00:57:40,104 --> 00:57:42,902 ...the dangers of the increasing greenhouse effect... 880 00:57:43,107 --> 00:57:44,665 ...have become much more clear. 881 00:57:44,876 --> 00:57:49,108 We burn fossil fuels, like coal and gas and petroleum... 882 00:57:49,314 --> 00:57:51,839 ...putting more carbon dioxide into the atmosphere... 883 00:57:52,050 --> 00:57:54,382 ...and thereby heating the Earth. 884 00:57:54,585 --> 00:57:57,679 The hellish conditions on Venus are a reminder that... 885 00:57:57,889 --> 00:57:59,322 ...this is serious business. 886 00:57:59,724 --> 00:58:02,022 Computer models that successfully explain... 887 00:58:02,226 --> 00:58:04,160 ...the climates of other planets... 888 00:58:04,362 --> 00:58:07,331 ...predict the deaths of forests... 889 00:58:07,532 --> 00:58:10,797 ...parched croplands, the flooding of coastal cities... 890 00:58:11,002 --> 00:58:13,095 ...environmental refugees... 891 00:58:13,304 --> 00:58:16,569 ...widespread disasters in the next century... 892 00:58:16,874 --> 00:58:18,466 ...unless we change our ways. 893 00:58:18,676 --> 00:58:20,166 What do we have to do? 894 00:58:20,611 --> 00:58:22,238 Four things. 895 00:58:22,447 --> 00:58:25,848 One: much more efficient use of fossil fuels. 896 00:58:26,050 --> 00:58:30,180 Why not cars that get 70 miles a gallon instead of 25? 897 00:58:30,388 --> 00:58:34,688 Two: research and development on safe alternative energy sources... 898 00:58:34,892 --> 00:58:36,587 ...especially solar power. 899 00:58:37,195 --> 00:58:40,164 Three: reforestation on a grand scale. 900 00:58:40,365 --> 00:58:43,857 And four: helping to bring the billion poorest people... 901 00:58:44,068 --> 00:58:46,195 ...on the planet to self-sufficiency... 902 00:58:46,404 --> 00:58:49,737 ...which is the key step in curbing world population growth. 903 00:58:49,941 --> 00:58:53,843 Every one of these steps makes sense apart from greenhouse warming. 904 00:58:54,679 --> 00:58:57,147 No one has proposed that the trouble with Venus is... 905 00:58:57,348 --> 00:59:01,375 ...that there once was Venusians who drove fuel-inefficient cars. 906 00:59:01,586 --> 00:59:04,851 But our nearest neighbor, nevertheless, is a stark warning... 907 00:59:05,056 --> 00:59:08,457 ...on the possible fate of an Earth-like world.