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SAGAN: This is the age of
planetary exploration...
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...when our ships have begun
to sail the heavens.
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00:00:53,832 --> 00:00:57,427
In those heavens, there are
some worlds much like hell.
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00:00:57,636 --> 00:01:01,538
Our planet is, in comparison,
much like a heaven.
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00:01:01,740 --> 00:01:03,970
But the gates of heaven and hell...
6
00:01:04,175 --> 00:01:07,235
...are adjacent and unmarked.
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00:01:09,748 --> 00:01:12,216
The Earth is a lovely...
8
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...and more or less placid place.
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Things change, but slowly.
10
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You can lead a full life and never
encounter a natural catastrophe...
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...more violent than a storm.
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And so we become complacent...
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00:01:27,632 --> 00:01:28,826
...relaxed...
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...unconcerned.
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But in the history of the solar
system and even in human history...
16
00:01:35,640 --> 00:01:38,302
...there are clear records
of extraordinary...
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...and devastating catastrophes.
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We have now achieved
the dubious distinction...
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...of making our own
major catastrophes...
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...both intentional and inadvertent.
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On the landscapes of other planets...
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...where past records
are better preserved...
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00:01:54,793 --> 00:01:57,261
...there's abundant evidence
of major catastrophes.
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It's all a matter of time scale.
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An event which is improbable
in 1 00 years...
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00:02:02,334 --> 00:02:05,098
...may be inevitable
in 1 00 million.
27
00:02:05,303 --> 00:02:08,466
But even on the Earth
in this century...
28
00:02:08,673 --> 00:02:12,336
...there have been
bizarre natural events.
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00:02:17,115 --> 00:02:19,811
In remote central Siberia...
30
00:02:20,018 --> 00:02:22,077
...there was a time
when the Tungus people...
31
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...told strange tales
of a giant fireball...
32
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...that split the sky
and shook the Earth.
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00:02:30,095 --> 00:02:32,495
They told of a blast
of searing wind...
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...that knocked down
people and forests.
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00:02:36,835 --> 00:02:39,497
It happened, they said,
on a summer's morning...
36
00:02:39,704 --> 00:02:41,535
...in the year 1908.
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In the late 1920s...
38
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...L.A. Kulik, a Soviet scientist...
39
00:02:46,544 --> 00:02:49,604
...organized expeditions
to try and solve the mystery.
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He built boats to penetrate
this trackless land:
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00:02:56,121 --> 00:02:57,713
Snowbound in winter...
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...a swampy morass in summer.
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00:03:03,294 --> 00:03:06,889
Eyewitnesses told of
a ball of flame...
44
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...larger than the sun...
45
00:03:08,833 --> 00:03:12,633
...that had blazed across the sky
20 years before.
46
00:03:13,271 --> 00:03:17,105
Kulik assumed a giant meteorite
had struck the Earth.
47
00:03:19,644 --> 00:03:23,205
He expected to find an enormous
impact crater...
48
00:03:23,415 --> 00:03:25,542
...and rare meteorite fragments...
49
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...chipped off some distant asteroid.
50
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However, at ground zero...
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00:03:33,625 --> 00:03:36,719
...Kulik found upright trees
stripped of their branches...
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...but not a trace of the meteorite
or its impact crater.
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He was deeply puzzled.
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00:03:41,800 --> 00:03:45,793
He thought there were meteorite
fragments buried in the swampy ground.
55
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So he set about digging trenches
and pumping out the water.
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But the expected meteoritic rock
and iron was not found.
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Undaunted, Kulik went on
to make a thorough survey...
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...despite the swarms of insects
and other hardships.
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00:04:02,620 --> 00:04:05,418
Because he discovered something that,
in his own words...
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00:04:05,623 --> 00:04:10,560
..."exceeded all tales of eyewitnesses
and my wildest expectations."
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00:04:13,064 --> 00:04:16,795
For more than 20 kilometers
in every direction from ground zero...
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...the trees were flattened radially
outward like broken matchsticks.
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There must've been
a powerful explosion...
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...several kilometers
above the ground.
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00:04:30,915 --> 00:04:33,713
The pressure wave, spreading out
at the speed of sound...
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00:04:33,918 --> 00:04:37,410
...was reconstructed from barometric
records at weather stations...
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00:04:37,622 --> 00:04:42,252
...across Siberia, through Russia
and on into Western Europe.
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00:04:42,460 --> 00:04:45,952
Dust from the explosion reflected
so much sunlight back to Earth...
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00:04:46,164 --> 00:04:48,223
...that people could
read by it at night...
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...in London, 10,000 kilometers away.
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This really remarkable occurrence...
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00:05:00,880 --> 00:05:03,781
...is called the Tunguska Event.
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00:05:04,316 --> 00:05:05,840
But what was it?
74
00:05:06,251 --> 00:05:10,381
Well, perhaps, some scientists
have suggested...
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00:05:10,588 --> 00:05:14,456
...it was a chunk of antimatter
from space...
76
00:05:14,659 --> 00:05:19,062
...annihilated on contact with
the ordinary matter of the Earth...
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...disappearing in a flash
of gamma rays.
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00:05:23,001 --> 00:05:27,165
But the radioactivity you'd expect
from matter-antimatter annihilation...
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00:05:27,372 --> 00:05:30,864
...is to be found nowhere
at the impact site.
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00:05:31,810 --> 00:05:36,076
Or, perhaps, other scientists
have suggested...
81
00:05:36,281 --> 00:05:38,613
...it was a mini black hole
from space...
82
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...which impacted the Earth
in Siberia...
83
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...tunneled through
the solid body of Earth...
84
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...and plunged out the other side.
85
00:05:46,057 --> 00:05:49,254
But the records of atmospheric
shock waves give not a hint...
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...of something booming out of
the North Atlantic later that day.
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00:05:54,199 --> 00:05:58,636
Or maybe, other people have speculated,
it was a spaceship...
88
00:05:58,837 --> 00:06:03,171
...of some unimaginably advanced
extraterrestrial civilization...
89
00:06:03,374 --> 00:06:05,934
...in desperate mechanical trouble...
90
00:06:06,144 --> 00:06:10,308
...crashing in a remote region
of an obscure planet.
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Well, if so, it's pretty startling
that at the impact site...
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...there is not a piece,
not the tiniest transistor...
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...of a crashed spacecraft.
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More prosaically, perhaps it was
a large meteorite...
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00:06:24,162 --> 00:06:26,562
...or a small asteroid
which hit the Earth.
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00:06:26,764 --> 00:06:30,256
But even here,
there are no observable telltale...
97
00:06:30,468 --> 00:06:33,460
...rocky or metallic fragments
of the sort...
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...that you'd expect
from such an impact.
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The key point of the Tunguska Event...
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...is that there was a tremendous
explosion, a great shock wave...
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...many trees burned,
an enormous forest fire...
102
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...and yet, no crater in the ground.
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There seems to be
only one explanation...
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...which is consistent
with these facts.
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And that explanation is this:
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In 1 908, a piece of a comet...
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...hit the Earth.
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No one saw it approach.
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A small point of light
lost in the glare of the morning sun.
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It had been drifting for centuries
through the inner solar system...
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...like an iceberg in the ocean
of interplanetary space.
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But this time, by accident...
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...there was a planet in the way.
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From the time and direction of
its approach, what hit the Earth...
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...seems to have been a fragment
of a comet named Encke.
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Hurtling at more than
100,000 kilometers an hour...
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...it was a mountain of ice
about the size of a football field...
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00:08:04,529 --> 00:08:07,965
...and weighing almost a million tons.
119
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There was no warning, until
it plunged into the atmosphere.
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00:08:16,007 --> 00:08:18,305
(COMET RUMBLES)
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If such an explosion happened today...
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...it might be thought,
in the panic of the moment...
123
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...to be produced by a nuclear weapon.
124
00:09:03,488 --> 00:09:06,218
Such a cometary impact and fireball...
125
00:09:06,424 --> 00:09:09,916
...simulates all the effects
of a 15-megaton nuclear burst...
126
00:09:10,128 --> 00:09:12,756
...including the mushroom cloud,
with one exception:
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00:09:12,965 --> 00:09:14,956
There would be no radiation.
128
00:09:15,166 --> 00:09:18,158
So could a rare but natural event...
129
00:09:18,369 --> 00:09:20,394
...the impact of a comet with Earth...
130
00:09:20,605 --> 00:09:23,199
...trigger a nuclear war?
131
00:09:24,042 --> 00:09:27,478
It's a strange scenario:
A small comet hits the Earth...
132
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...as millions have during
Earth's history...
133
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...and the response
of our civilization...
134
00:09:32,483 --> 00:09:35,714
...is promptly to self-destruct.
135
00:09:37,288 --> 00:09:40,621
Maybe it's unlikely,
but it might be a good idea...
136
00:09:40,825 --> 00:09:44,317
...to understand comets
and collisions and catastrophes...
137
00:09:44,530 --> 00:09:47,158
...a little bit better than we do.
138
00:09:47,365 --> 00:09:51,426
Now, a comet, at least as far as
we understand them today...
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...is made mostly of ice:
140
00:09:53,171 --> 00:09:56,004
Water ice, maybe some ammonia ice...
141
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...a little bit of methane ice.
142
00:09:58,877 --> 00:10:01,675
So in striking
the Earth's atmosphere...
143
00:10:01,879 --> 00:10:03,972
...a modest cometary fragment...
144
00:10:04,182 --> 00:10:09,051
...will produce a great radiant
fireball and a mighty blast wave.
145
00:10:09,254 --> 00:10:11,484
It'll burn trees and level forests...
146
00:10:11,689 --> 00:10:14,351
...and make a sound
heard around the world.
147
00:10:14,559 --> 00:10:17,426
But it need not make
a crater in the ground.
148
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Why? Because the ices in the comet
are all melted in the impact.
149
00:10:22,266 --> 00:10:25,724
And there's going to be very
few recognizable pieces of comet...
150
00:10:25,937 --> 00:10:27,427
...left on the ground.
151
00:10:34,078 --> 00:10:36,876
We humans like to think of
the heavens as stable...
152
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...serene, unchanging.
153
00:10:41,486 --> 00:10:43,386
But comets suddenly appear...
154
00:10:43,588 --> 00:10:47,718
...and hang ominously in the sky,
night after night, for weeks.
155
00:10:50,094 --> 00:10:54,497
So the idea developed that the comet
had to be there for a reason.
156
00:10:54,700 --> 00:10:57,965
The reason was that comets were
predictions of disaster...
157
00:10:58,170 --> 00:11:02,504
...that they foretold the deaths
of princes and the fall of kingdoms.
158
00:11:02,707 --> 00:11:05,198
In 1 066, for example...
159
00:11:05,410 --> 00:11:09,506
...the Normans witnessed an apparition
or appearance of Halley's comet.
160
00:11:09,714 --> 00:11:13,514
Since a comet must, they thought,
predict the fall of some kingdom...
161
00:11:13,718 --> 00:11:15,982
...they promptly invaded England.
162
00:11:16,187 --> 00:11:18,781
Here's King Harold of England
looking a little glum.
163
00:11:18,991 --> 00:11:21,357
The events were noted
in the Bayeux tapestry...
164
00:11:21,559 --> 00:11:24,187
...a kind of newspaper of the day.
165
00:11:24,395 --> 00:11:26,454
Or, in the early 13th century...
166
00:11:26,664 --> 00:11:30,191
...Giotto, one of the founders
of modern realistic painting...
167
00:11:30,401 --> 00:11:32,995
...witnessed another apparition
of comet Halley...
168
00:11:33,205 --> 00:11:36,470
...and inserted it into a nativity
he was painting.
169
00:11:36,675 --> 00:11:40,907
A harbinger of a different
sort of change of kingdoms.
170
00:11:41,579 --> 00:11:43,877
Around 1517...
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...another great comet appeared.
This time it was seen in Mexico.
172
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And the Aztec emperor, Moctezuma...
173
00:11:50,254 --> 00:11:51,881
...maybe this is he...
174
00:11:52,456 --> 00:11:54,447
...promptly executed his astrologers.
175
00:11:54,659 --> 00:11:59,255
Why? They hadn't predicted the comet,
and they sure hadn't explained it.
176
00:11:59,463 --> 00:12:04,366
Moctezuma was positive that the comet
foretold some dreadful disaster.
177
00:12:04,570 --> 00:12:07,164
He became distant and gloomy...
178
00:12:07,371 --> 00:12:09,839
...and in that way,
helped to set the stage...
179
00:12:10,041 --> 00:12:13,943
...for the successful Spanish conquest
of Mexico under Cort?s.
180
00:12:14,445 --> 00:12:18,074
In many cases, a superstitious
belief in comets...
181
00:12:18,282 --> 00:12:21,217
...becomes a self-fulfilling prophecy.
182
00:12:22,054 --> 00:12:24,614
Here are two quite different
representations...
183
00:12:24,822 --> 00:12:27,450
...of the great comet of 1 577:
184
00:12:27,658 --> 00:12:30,354
This one pictured by the Turks...
185
00:12:33,231 --> 00:12:35,563
...and this one by the Germans.
186
00:12:42,139 --> 00:12:45,438
In 1 705, Edmund Halley finally...
187
00:12:45,643 --> 00:12:48,168
...figured out that the same
spectacular comet...
188
00:12:48,379 --> 00:12:52,179
...was booming by the Earth
every 76 years, like clockwork.
189
00:12:52,383 --> 00:12:56,114
That comet is now called,
appropriately, comet Halley.
190
00:12:56,320 --> 00:13:00,120
And it's the same one that we talked
about before, the comet of 1 066.
191
00:13:00,324 --> 00:13:03,259
At that point, the subject began
to lose a little...
192
00:13:03,461 --> 00:13:06,828
...of its burden of superstition,
but hardly all.
193
00:13:07,031 --> 00:13:10,797
Public fear of comets survived.
Well, for example...
194
00:13:11,336 --> 00:13:14,499
...look at this terribly
nasty comet of 1 857...
195
00:13:14,706 --> 00:13:18,142
...that some people figured
would splinter the Earth.
196
00:13:19,477 --> 00:13:23,538
By 1 91 0, Halley's comet
returned once more.
197
00:13:23,748 --> 00:13:27,206
But this time, astronomers using
a new tool, the spectroscope...
198
00:13:27,418 --> 00:13:32,321
...had discovered cyanogen gas
in the tail of a comet.
199
00:13:32,524 --> 00:13:34,856
Now, cyanogen is a poison.
200
00:13:35,059 --> 00:13:38,688
The Earth was to pass through
this poisonous tail.
201
00:13:38,896 --> 00:13:42,889
The fact that the gas was
astonishingly, fabulously thin...
202
00:13:43,101 --> 00:13:45,262
...reassured almost nobody.
203
00:13:45,469 --> 00:13:49,769
For example, look at the headlines
in the Los Angeles Examiner...
204
00:13:49,975 --> 00:13:52,671
...for May 9, 1 91 0:
205
00:13:52,877 --> 00:13:56,870
"Say, Has That Comet
'Cyanogened' You Yet?"
206
00:13:57,082 --> 00:14:00,176
"Entire Human Race Due
For Free Gaseous Bath.
207
00:14:00,385 --> 00:14:02,410
Expect High Jinks."
208
00:14:02,620 --> 00:14:07,489
Or take this from the San Francisco
Chronicle, May 15, 1910:
209
00:14:07,693 --> 00:14:11,288
"Comet Comes And Husband Reforms."
210
00:14:11,495 --> 00:14:13,690
"Comet Parties Now Fad In New York."
211
00:14:13,898 --> 00:14:15,729
Amazing stuff!
212
00:14:15,933 --> 00:14:19,369
In 1 91 0, people were holding
comet parties, not so much to...
213
00:14:19,570 --> 00:14:23,165
...celebrate the end of the world
as to make merry before it happened.
214
00:14:23,407 --> 00:14:27,935
There were entrepreneurs
who were hawking comet pills.
215
00:14:28,380 --> 00:14:30,439
I think I'm gonna take one for later.
216
00:14:30,648 --> 00:14:33,481
And there were those
who were selling...
217
00:14:33,951 --> 00:14:38,888
...gas masks to protect
against the cyanogen.
218
00:14:39,291 --> 00:14:43,660
And comet nuttiness
didn't stop in 1910.
219
00:14:48,899 --> 00:14:52,835
Long before 1066,
humans marveled at comets.
220
00:14:53,038 --> 00:14:56,235
Our generation is beginning
to understand them.
221
00:15:04,949 --> 00:15:07,281
Mercury, Venus, Earth and Mars...
222
00:15:07,485 --> 00:15:10,943
...are small planets made mostly
of rock and iron.
223
00:15:11,722 --> 00:15:14,885
Farther out where it's colder,
are the giant planets...
224
00:15:15,092 --> 00:15:16,855
...made mostly of gas.
225
00:15:17,061 --> 00:15:20,258
But comets originate from
a great cloud beyond the planets...
226
00:15:20,464 --> 00:15:22,694
...almost halfway to the nearest star.
227
00:15:22,900 --> 00:15:25,130
Occasionally, one falls in...
228
00:15:25,336 --> 00:15:27,429
...accelerated by the sun's gravity.
229
00:15:27,639 --> 00:15:29,834
Because it's made mostly of ice,
the comet...
230
00:15:30,040 --> 00:15:32,167
...evaporates as it approaches
the sun.
231
00:15:32,376 --> 00:15:35,243
The vapor is blown back
by the solar wind...
232
00:15:35,447 --> 00:15:37,312
...forming the cometary tail.
233
00:15:37,515 --> 00:15:39,881
Then it's flung back
into outer darkness...
234
00:15:40,084 --> 00:15:41,415
...its orbit so large...
235
00:15:41,620 --> 00:15:44,418
...that it will not return
for millions of years.
236
00:15:44,622 --> 00:15:47,182
These are the long-period comets.
237
00:15:47,391 --> 00:15:50,758
For every one plunging close enough
to the sun to be discovered...
238
00:15:50,961 --> 00:15:52,690
...there may be a billion others...
239
00:15:52,898 --> 00:15:55,958
...slowly drifting
beyond Pluto's orbit.
240
00:15:56,468 --> 00:16:00,962
Very rarely, a long-period comet is
captured in the inner solar system...
241
00:16:01,172 --> 00:16:03,140
...becoming a short-period comet.
242
00:16:03,341 --> 00:16:06,799
It passes near a major planet,
like Saturn.
243
00:16:07,012 --> 00:16:09,640
The planet provides
a small gravitational tug...
244
00:16:09,848 --> 00:16:12,681
...enough to deflect it
into a much smaller orbit.
245
00:16:12,883 --> 00:16:15,545
Though few are captured this way,
those that are...
246
00:16:15,753 --> 00:16:19,587
...become well-known because
they all return in short intervals.
247
00:16:19,790 --> 00:16:23,453
Once trapped in the inner
solar system, among the planets...
248
00:16:23,662 --> 00:16:26,859
...the chances of another
near-collision are increased.
249
00:16:28,566 --> 00:16:31,228
Here, a second encounter
with Saturn...
250
00:16:31,436 --> 00:16:35,270
...further reduces the comet's
orbital period to decades.
251
00:16:35,473 --> 00:16:39,637
A comet may take 10,000 years
between close planetary encounters.
252
00:16:39,844 --> 00:16:43,678
But in this computer study,
we've sped things up.
253
00:16:44,315 --> 00:16:47,182
A third encounter,
this time with Jupiter...
254
00:16:47,384 --> 00:16:50,353
...further reduces
the comet's orbital period.
255
00:16:50,554 --> 00:16:53,216
Now the comet must approach the sun...
256
00:16:53,424 --> 00:16:56,018
...and grow a tail every few years.
257
00:16:56,227 --> 00:16:59,560
Since the dust and gas in the tail
are lost forever to space...
258
00:16:59,763 --> 00:17:02,391
...the comet must slowly be eroding.
259
00:17:02,601 --> 00:17:04,262
Pieces of it break off.
260
00:17:04,468 --> 00:17:07,562
Sometimes, as we've seen,
they even strike the Earth.
261
00:17:07,771 --> 00:17:09,204
In a few thousand years...
262
00:17:09,407 --> 00:17:12,433
...if a short-period comet
hasn't hit a planet...
263
00:17:12,643 --> 00:17:15,271
...it will have evaporated away
almost entirely...
264
00:17:15,479 --> 00:17:19,575
...leaving sand-sized fragments,
which become meteors...
265
00:17:19,784 --> 00:17:23,880
...and its core which, perhaps,
becomes an asteroid.
266
00:17:26,156 --> 00:17:29,956
Suppose I were a pretty typical comet.
267
00:17:30,161 --> 00:17:32,686
And what you would see
would be a kind of...
268
00:17:32,897 --> 00:17:35,491
...tumbling snowball...
269
00:17:35,699 --> 00:17:40,033
...spending most of my time out here
in the outer solar system.
270
00:17:40,237 --> 00:17:42,262
I'd be a kilometer across.
271
00:17:42,473 --> 00:17:44,236
I'd be living most of my days...
272
00:17:44,442 --> 00:17:48,401
...in the gloom beyond Saturn,
orbiting the sun.
273
00:17:48,613 --> 00:17:50,979
But once every century,
I would find myself...
274
00:17:51,182 --> 00:17:54,208
...careening inward,
faster and faster...
275
00:17:54,418 --> 00:17:56,784
...towards the inner solar system.
276
00:17:58,323 --> 00:18:01,759
By the time I would cross
the orbit of Jupiter...
277
00:18:01,960 --> 00:18:03,791
...on my way to the orbit of Mars...
278
00:18:03,994 --> 00:18:07,395
...I'd be heating up because
I'd be getting closer to the sun.
279
00:18:07,598 --> 00:18:09,463
I'd be evaporating a little bit.
280
00:18:09,668 --> 00:18:11,795
Small pieces of dust and ice...
281
00:18:12,002 --> 00:18:15,233
...would be blown behind me
by the solar wind...
282
00:18:15,439 --> 00:18:18,772
...forming an incipient cometary tail.
283
00:18:18,976 --> 00:18:21,410
On the scale of such
a solar system model...
284
00:18:21,612 --> 00:18:24,513
...l, me, a cometary nucleus...
285
00:18:24,715 --> 00:18:26,910
...would be smaller than a snowflake.
286
00:18:27,118 --> 00:18:31,418
Although, when fully developed,
my tail would be longer...
287
00:18:31,622 --> 00:18:34,921
...than the spacing
between the worlds.
288
00:18:36,293 --> 00:18:38,227
Now, sooner or later...
289
00:18:38,429 --> 00:18:42,195
...comets on these long, elliptical
trajectories around the sun...
290
00:18:42,399 --> 00:18:44,594
...must collide with planets.
291
00:18:44,802 --> 00:18:47,270
The Earth and the moon...
292
00:18:47,472 --> 00:18:51,135
...must have been bombarded
by comets and asteroids...
293
00:18:51,343 --> 00:18:54,312
...the debris from the early history
of the solar system.
294
00:18:54,512 --> 00:18:58,312
In interplanetary space, there are
more small objects than large ones.
295
00:18:58,516 --> 00:19:01,576
So there must be,
on a given planetary surface...
296
00:19:01,786 --> 00:19:06,086
...many more impacts of small objects
than of large objects.
297
00:19:06,290 --> 00:19:10,124
So a thing like the Tunguska impact
happens on the Earth...
298
00:19:10,327 --> 00:19:11,919
...maybe every thousand years.
299
00:19:12,130 --> 00:19:15,327
But the impact of a giant
cometary nucleus...
300
00:19:15,533 --> 00:19:17,467
...like Halley's comet, let's say...
301
00:19:17,668 --> 00:19:20,330
...happens only every
billion years or so.
302
00:19:21,071 --> 00:19:24,370
Now, is there evidence
of past collisions?
303
00:19:25,542 --> 00:19:26,941
When a large comet...
304
00:19:27,144 --> 00:19:29,806
...or a large, rocky asteroid
hits a planet...
305
00:19:30,015 --> 00:19:32,313
...it makes a bowl-shaped crater.
306
00:19:32,516 --> 00:19:36,680
The well-preserved impact craters on
Earth were all formed fairly recently.
307
00:19:36,888 --> 00:19:40,221
The older ones have been softened,
filled in or rubbed out...
308
00:19:40,425 --> 00:19:42,985
...by running water
and mountain building.
309
00:19:43,193 --> 00:19:46,685
Impacts make craters on other worlds
and about as often.
310
00:19:46,898 --> 00:19:48,365
But when the air is thin...
311
00:19:48,565 --> 00:19:52,262
...when water rarely flows,
when mountain building is feeble...
312
00:19:52,469 --> 00:19:54,767
...the ancient craters are retained.
313
00:19:54,972 --> 00:19:57,770
This is the case on the moon
and Mercury and Mars...
314
00:19:57,976 --> 00:20:00,672
...our neighboring
terrestrial planets.
315
00:20:02,179 --> 00:20:04,409
They huddle around the sun...
316
00:20:04,616 --> 00:20:06,948
...their source of heat and light...
317
00:20:07,151 --> 00:20:10,450
...a little bit like campers
around a fire.
318
00:20:10,821 --> 00:20:13,415
They are about
4? billion years old.
319
00:20:13,624 --> 00:20:17,583
And all bear witness
to an age long gone...
320
00:20:17,795 --> 00:20:20,491
...of major collisions...
321
00:20:20,698 --> 00:20:25,101
...which do not happen at that scale
and frequency anymore.
322
00:20:25,737 --> 00:20:28,706
If we move out past...
323
00:20:28,907 --> 00:20:31,569
...the terrestrial planets
beyond Mars...
324
00:20:31,775 --> 00:20:35,836
...we find ourselves in a different
regime of the solar system...
325
00:20:36,047 --> 00:20:38,140
...in the realm of Jupiter...
326
00:20:38,348 --> 00:20:41,579
...and the other giant,
or Jovian planets.
327
00:20:42,619 --> 00:20:47,113
These are great worlds
composed largely of the gases...
328
00:20:47,325 --> 00:20:49,850
...hydrogen and helium,
some other stuff too.
329
00:20:50,061 --> 00:20:54,862
When we look at the surface,
we do not see a solid surface...
330
00:20:55,065 --> 00:20:58,831
...but only an occasional patch
of atmosphere...
331
00:20:59,036 --> 00:21:02,563
...and a complex array
of multicolored clouds.
332
00:21:02,873 --> 00:21:04,534
These are serious planets...
333
00:21:04,743 --> 00:21:08,839
...not fragmentary little world-lets
like the Earth.
334
00:21:09,046 --> 00:21:12,743
In fact, 1 000 Earths would fit...
335
00:21:12,950 --> 00:21:15,316
...in the volume of Jupiter.
336
00:21:15,519 --> 00:21:19,387
If a comet or asteroid were to...
337
00:21:19,590 --> 00:21:24,527
...accidentally impact Jupiter, it would
be very unlikely to leave a crater.
338
00:21:24,728 --> 00:21:28,129
It might make a momentary hole
in the clouds, but that's it.
339
00:21:28,333 --> 00:21:32,030
Nevertheless, we know
that the outer solar system...
340
00:21:32,237 --> 00:21:35,229
...has been subject to
a many-billion-year history...
341
00:21:35,440 --> 00:21:37,533
...of impact cratering.
342
00:21:37,975 --> 00:21:41,968
Jupiter's moon Callisto is studded
with thousands of craters.
343
00:21:42,179 --> 00:21:45,671
Clear evidence of ancient
collisions beyond Mars.
344
00:21:45,884 --> 00:21:49,411
And there are craters
on other moons of Jupiter.
345
00:21:49,654 --> 00:21:52,452
Most of the thousands of
large craters on our own moon...
346
00:21:52,656 --> 00:21:55,386
...were excavated
billions of years ago.
347
00:21:55,592 --> 00:21:58,117
But were any recorded
in historical times?
348
00:21:58,328 --> 00:22:02,196
The odds against it
are about 1000-to-one.
349
00:22:03,233 --> 00:22:05,724
(BELL RINGS)
350
00:22:10,174 --> 00:22:13,337
Nevertheless, there's a possible
eyewitness account...
351
00:22:13,544 --> 00:22:15,444
...of just such an event.
352
00:22:15,646 --> 00:22:19,639
It was the Sunday before the
feast of Saint John the Baptist...
353
00:22:20,050 --> 00:22:22,484
...in the summer of 1178.
354
00:22:23,353 --> 00:22:27,551
The monks of Canterbury Cathedral had
completed their evening prayers...
355
00:22:27,758 --> 00:22:29,953
...and were about to retire
for the night.
356
00:22:30,160 --> 00:22:31,923
The scholarly brother, Gervase...
357
00:22:32,130 --> 00:22:33,995
...returned to his cell to read...
358
00:22:34,198 --> 00:22:35,790
...while some of the others...
359
00:22:36,000 --> 00:22:39,595
...went outside to enjoy
the gentle June air.
360
00:22:39,803 --> 00:22:41,498
(PLAYS FLUTE)
361
00:22:45,376 --> 00:22:47,401
In the midst of their recreation...
362
00:22:47,611 --> 00:22:50,944
...they chanced to witness
an astonishing sight:
363
00:22:51,148 --> 00:22:54,515
A violent explosion on the moon.
364
00:23:03,494 --> 00:23:04,859
This was a time...
365
00:23:05,062 --> 00:23:07,587
...when the heavens were
thought to be changeless.
366
00:23:07,798 --> 00:23:11,632
The moon, the stars and the planets
were deemed pure...
367
00:23:11,835 --> 00:23:15,828
...because they followed
an unvarying celestial routine.
368
00:23:16,073 --> 00:23:19,941
They were expected to behave
without unseemly disruptions...
369
00:23:20,678 --> 00:23:22,509
...like monks in a monastery.
370
00:23:23,046 --> 00:23:26,038
Was it wise to discuss such a vision?
371
00:23:32,923 --> 00:23:34,652
In every time and culture...
372
00:23:34,859 --> 00:23:38,295
...there are pressures to conform
to the prevailing prejudices.
373
00:23:39,396 --> 00:23:42,263
But there are also,
in every place and epoch...
374
00:23:42,467 --> 00:23:46,870
...those who value the truth,
who record the evidence faithfully.
375
00:23:47,070 --> 00:23:50,164
Future generations are in their debt.
376
00:23:56,214 --> 00:23:58,546
A fire on the moon.
377
00:23:58,749 --> 00:24:02,412
Might it be some portent
of ill fortune?
378
00:24:02,820 --> 00:24:06,051
Should the chronicler
of the monastery be told?
379
00:24:06,925 --> 00:24:10,361
Was this event an apparition
of the evil one?
380
00:24:13,197 --> 00:24:16,189
Gervase of Canterbury was
a historian...
381
00:24:16,401 --> 00:24:19,165
...considered today a reliable
reporter of political...
382
00:24:19,369 --> 00:24:21,769
...and cultural events of his time.
383
00:24:21,972 --> 00:24:26,170
This is his account of the
eyewitness testimony he was given:
384
00:24:26,777 --> 00:24:28,836
"Now there was a bright new moon...
385
00:24:29,046 --> 00:24:30,980
...and as usual in that phase...
386
00:24:31,182 --> 00:24:33,548
...its horns were tilted
toward the east.
387
00:24:33,750 --> 00:24:37,413
And suddenly the upper horn
split in two.
388
00:24:37,622 --> 00:24:41,149
From the midpoint of this division,
a flaming torch sprang up...
389
00:24:41,359 --> 00:24:43,953
...spewing out
over a considerable distance...
390
00:24:44,161 --> 00:24:47,221
...fire, hot coals and sparks.
391
00:24:47,431 --> 00:24:50,594
After these transformations,"
Gervase continued...
392
00:24:50,801 --> 00:24:54,464
..."the moon from horn to horn
that is along its whole length...
393
00:24:54,671 --> 00:24:56,935
...took on a blackish appearance."
394
00:25:01,946 --> 00:25:06,212
Gervase took depositions
from all the eyewitnesses.
395
00:25:06,417 --> 00:25:07,714
He later wrote:
396
00:25:07,918 --> 00:25:12,514
"The writer was given this report by
men who saw it with their own eyes...
397
00:25:12,724 --> 00:25:15,215
...and are prepared to stake
their honor on an oath...
398
00:25:15,425 --> 00:25:18,826
...that they have made no addition
or falsification."
399
00:25:19,296 --> 00:25:21,321
Gervase committed the account
to paper...
400
00:25:21,531 --> 00:25:23,897
...enabling astronomers
eight centuries later...
401
00:25:24,102 --> 00:25:26,935
...to try and reconstruct
what really happened.
402
00:25:28,071 --> 00:25:31,006
It may be that 200 years
before Chaucer...
403
00:25:31,209 --> 00:25:34,201
...five monks saw an event
more wonderful...
404
00:25:34,412 --> 00:25:37,540
...than many another celebrated
Canterbury tale.
405
00:25:40,518 --> 00:25:43,715
If a small drifting mountain
were to hit the moon...
406
00:25:43,920 --> 00:25:46,684
...it would set our satellite
swinging like a bell.
407
00:25:46,890 --> 00:25:51,020
Eventually, the tremors would
die down, but not in a mere 800 years.
408
00:25:51,229 --> 00:25:54,426
So is the moon still quivering
from that impact?
409
00:25:54,632 --> 00:25:58,659
The Apollo astronauts emplaced arrays
of special mirrors on the moon.
410
00:25:58,870 --> 00:26:01,031
Reflectors made by
French scientists...
411
00:26:01,238 --> 00:26:04,366
...were also put on the moon
by Soviet Lunakhod vehicles.
412
00:26:04,574 --> 00:26:08,510
When a laser beam from Earth strikes
a mirror and bounces back...
413
00:26:08,713 --> 00:26:11,341
...the roundtrip travel time
can be measured.
414
00:26:11,548 --> 00:26:15,314
At the McDonald Observatory
of the University of Texas...
415
00:26:15,520 --> 00:26:19,354
...a laser beam is prepared for firing
at the reflectors on the moon...
416
00:26:19,557 --> 00:26:22,151
...380,000 kilometers away.
417
00:26:23,061 --> 00:26:26,121
By multiplying the travel time
by the speed of light...
418
00:26:26,330 --> 00:26:29,128
...the distance to that spot
can be determined...
419
00:26:29,332 --> 00:26:32,199
...to a precision of
7 to 10 centimeters:
420
00:26:32,402 --> 00:26:34,802
The width of a hand.
421
00:26:38,475 --> 00:26:41,410
When such measurements are
repeated over years...
422
00:26:41,611 --> 00:26:45,240
...even an extremely slight wobble
in the moon's motion...
423
00:26:45,449 --> 00:26:47,246
...can be determined.
424
00:26:47,452 --> 00:26:50,285
The accuracy is phenomenal.
425
00:26:50,488 --> 00:26:52,854
The error is much less...
426
00:26:53,056 --> 00:26:56,253
...than one-millionth of a percent.
427
00:26:57,962 --> 00:27:01,625
The moon, it turns out,
is gently swinging like a bell...
428
00:27:01,833 --> 00:27:04,393
...just as if it had been
hit by an asteroid...
429
00:27:04,602 --> 00:27:06,968
...less than 1000 years ago.
430
00:27:07,170 --> 00:27:09,138
(RINGING)
431
00:27:10,173 --> 00:27:14,337
So there may be physical evidence
in the age of space flight...
432
00:27:14,711 --> 00:27:19,148
...for the account of the Canterbury
monks in the 12th century.
433
00:27:22,053 --> 00:27:24,920
If 800 years ago
a big asteroid hit the moon...
434
00:27:25,122 --> 00:27:27,056
...the crater should be
prominent today...
435
00:27:27,257 --> 00:27:29,657
...still surrounded by bright rays...
436
00:27:29,861 --> 00:27:33,228
...thin streamers of dust
spewed out by the impact.
437
00:27:33,431 --> 00:27:35,865
In billions of years,
lunar rays are eroded...
438
00:27:36,067 --> 00:27:37,659
...but not in hundreds.
439
00:27:37,868 --> 00:27:41,531
And there is a recent ray crater
called Giordano Bruno...
440
00:27:41,738 --> 00:27:45,174
...in the region of the moon
where an explosion was reported...
441
00:27:45,375 --> 00:27:47,240
...in 1178.
442
00:27:52,916 --> 00:27:54,884
The entire evolution of the moon...
443
00:27:55,086 --> 00:27:57,554
...is a story of catastrophes.
444
00:27:57,755 --> 00:27:59,347
4 1/2 billion years ago...
445
00:27:59,556 --> 00:28:02,252
...the moon was accreting
from interplanetary boulders...
446
00:28:02,459 --> 00:28:05,053
...and craters were forming
all over its surface.
447
00:28:05,263 --> 00:28:08,096
The energy so released
helped melt the crust.
448
00:28:08,299 --> 00:28:12,759
After most of this debris was swept up
by the moon, the surface cooled.
449
00:28:13,203 --> 00:28:15,535
But about 3.9 billion years ago...
450
00:28:15,740 --> 00:28:18,675
...a great asteroid impacted.
451
00:28:23,446 --> 00:28:27,576
It generated an expanding shock wave
and re-melted some of the surface.
452
00:28:27,784 --> 00:28:29,809
The resulting basin
was then flooded...
453
00:28:30,021 --> 00:28:31,613
...probably by dark lava...
454
00:28:31,821 --> 00:28:35,188
...producing one of
the dry seas on the moon.
455
00:28:35,392 --> 00:28:38,589
More recent impacts excavated
craters with bright rays...
456
00:28:38,795 --> 00:28:42,492
...named after Eratosthenes
and Copernicus.
457
00:28:42,700 --> 00:28:44,725
The familiar features
of the man in the moon...
458
00:28:44,935 --> 00:28:47,961
...are a chronicle of ancient impacts.
459
00:28:49,540 --> 00:28:52,202
Most of the original asteroids
were swept up...
460
00:28:52,409 --> 00:28:54,400
...in the making
of the moon and planets.
461
00:28:54,612 --> 00:28:57,740
Many still orbit the sun
in the asteroid belt.
462
00:28:57,949 --> 00:29:01,578
Some, themselves almost fractured
by gravity tides...
463
00:29:01,785 --> 00:29:03,946
...and by impacts
with other asteroids...
464
00:29:04,155 --> 00:29:07,852
...have been captured by planets:
Phobos around Mars, for example...
465
00:29:08,059 --> 00:29:12,120
...or a close moon of Jupiter
called Amalthea.
466
00:29:13,730 --> 00:29:16,563
Similar to the asteroid belt
are the rings of Saturn...
467
00:29:16,766 --> 00:29:21,396
...composed of millions
of small, tumbling, icy moonlets.
468
00:29:21,606 --> 00:29:25,064
Maybe the rings of Saturn
are a moon...
469
00:29:25,276 --> 00:29:29,406
...which was prevented from
forming by the tides of Saturn.
470
00:29:29,614 --> 00:29:32,811
Or maybe it's the remains
of a moon that wandered too close...
471
00:29:33,017 --> 00:29:35,611
...and was torn apart by
the tides of Saturn.
472
00:29:35,820 --> 00:29:38,755
It's certainly a lovely place.
473
00:29:38,956 --> 00:29:43,086
Jupiter also has
a newly discovered ring system...
474
00:29:43,294 --> 00:29:46,024
...which is invisible from the Earth.
475
00:29:48,732 --> 00:29:53,669
Now, there is a curious argument...
476
00:29:53,938 --> 00:29:57,396
...alleging major recent collisions
in the solar system...
477
00:29:57,608 --> 00:30:00,099
...proposed by a psychiatrist...
478
00:30:00,311 --> 00:30:04,213
...named Immanuel Velikovsky in 1 950.
479
00:30:04,615 --> 00:30:06,879
He suggested...
480
00:30:07,084 --> 00:30:10,611
...that an object of planetary mass,
which he called a comet...
481
00:30:10,821 --> 00:30:14,313
...was somehow produced
in the Jupiter system.
482
00:30:14,525 --> 00:30:17,790
He doesn't say exactly
how it's produced...
483
00:30:18,629 --> 00:30:19,994
...but maybe...
484
00:30:23,400 --> 00:30:25,630
...it's spat out...
485
00:30:28,773 --> 00:30:30,104
...of Jupiter.
486
00:30:30,940 --> 00:30:35,877
Anyway, however it was made
some 3500 years ago, he imagines...
487
00:30:36,446 --> 00:30:41,179
...it made repeated
close encounters with Mars...
488
00:30:42,019 --> 00:30:44,351
...with the Earth-moon system...
489
00:30:44,721 --> 00:30:49,658
...having as entertaining
biblical consequences...
490
00:30:50,527 --> 00:30:55,226
...the parting of the Red Sea so that
Moses and the Israelites could...
491
00:30:55,432 --> 00:30:58,060
...safely avoid the host of pharaoh...
492
00:30:58,268 --> 00:31:01,032
...and the stopping of
the Earth's rotation when...
493
00:31:01,237 --> 00:31:05,731
...Joshua commanded the sun
to stand still in Gibeon.
494
00:31:05,942 --> 00:31:08,570
He also imagined that
there was extensive flooding...
495
00:31:08,778 --> 00:31:12,009
...and the volcanoes all over
the Earth at that time.
496
00:31:12,515 --> 00:31:17,111
Well, then after
a very complicated game...
497
00:31:17,321 --> 00:31:21,052
...of interplanetary billiards
is completed...
498
00:31:21,258 --> 00:31:25,786
...Velikovsky proposed
that this comet...
499
00:31:25,995 --> 00:31:29,897
...entered into a stable,
almost perfectly circular orbit...
500
00:31:30,100 --> 00:31:31,397
...becoming...
501
00:31:34,205 --> 00:31:35,832
...the planet Venus...
502
00:31:36,039 --> 00:31:39,406
...which he claimed
never existed until then.
503
00:31:40,777 --> 00:31:45,612
Now, these ideas are
almost certainly wrong.
504
00:31:46,217 --> 00:31:48,879
There's no objection
in astronomy to collisions.
505
00:31:49,086 --> 00:31:51,281
We've seen collision fragments...
506
00:31:51,488 --> 00:31:54,787
...and evidence throughout
the solar system.
507
00:31:54,991 --> 00:31:58,825
The problem is with recent
and major collisions.
508
00:31:59,029 --> 00:32:01,259
In any scale model like this...
509
00:32:01,464 --> 00:32:04,729
...it's impossible to have both
the sizes of the planets...
510
00:32:04,934 --> 00:32:07,402
...and the sizes of their orbits
to the same scale...
511
00:32:07,604 --> 00:32:10,767
...because then the planets
would be too small to see.
512
00:32:10,975 --> 00:32:14,035
If the planets were really
to scale in such a model...
513
00:32:14,245 --> 00:32:16,713
...as grains of dust...
514
00:32:16,914 --> 00:32:19,178
...it would then be entirely clear...
515
00:32:19,382 --> 00:32:22,317
...that a comet entering
the inner solar system...
516
00:32:22,519 --> 00:32:25,420
...would have a negligible chance
of colliding with a planet...
517
00:32:25,622 --> 00:32:27,783
...in only a few thousand years.
518
00:32:27,992 --> 00:32:29,289
Moreover...
519
00:32:29,492 --> 00:32:32,950
...Venus is a rocky and metallic...
520
00:32:33,163 --> 00:32:35,529
...hydrogen-poor world...
521
00:32:35,732 --> 00:32:38,667
...whereas Jupiter, where Velikovsky
imagines it comes from...
522
00:32:38,869 --> 00:32:41,497
...is made of almost nothing
but hydrogen.
523
00:32:41,705 --> 00:32:46,506
There is no energy source in Jupiter
to eject planets or comets.
524
00:32:46,709 --> 00:32:50,440
If one did enter
the inner solar system...
525
00:32:50,647 --> 00:32:54,174
...there is no way it could stop
the Earth from rotating.
526
00:32:54,384 --> 00:32:57,785
And if it could, there's no way
Earth could start rotating again...
527
00:32:57,987 --> 00:33:00,387
...at anything like 24 hours a day.
528
00:33:00,590 --> 00:33:03,923
There's no geological evidence
for flooding and volcanism...
529
00:33:04,127 --> 00:33:06,186
...3500 years ago.
530
00:33:06,397 --> 00:33:09,560
Babylonian astronomers
observed Venus...
531
00:33:09,767 --> 00:33:12,395
...in its present stable orbit...
532
00:33:12,603 --> 00:33:15,595
...before Velikovsky said it existed.
533
00:33:15,806 --> 00:33:18,331
And so on.
534
00:33:22,847 --> 00:33:25,611
There are many hypotheses
in science which are wrong.
535
00:33:25,815 --> 00:33:29,342
That's all right. It's the aperture
to finding out what's right.
536
00:33:29,552 --> 00:33:32,385
Science is a self-correcting process.
537
00:33:32,589 --> 00:33:35,114
To be accepted,
new ideas must survive...
538
00:33:35,325 --> 00:33:39,625
...the most rigorous standards
of evidence and scrutiny.
539
00:33:39,963 --> 00:33:42,955
The worst aspect of
the Velikovsky affair is not...
540
00:33:43,166 --> 00:33:45,726
...that many of his ideas were
wrong or silly...
541
00:33:45,935 --> 00:33:48,597
...or in gross contradiction
to the facts.
542
00:33:48,806 --> 00:33:52,674
Rather, the worst aspect is
that some scientists...
543
00:33:52,876 --> 00:33:56,243
...attempted to suppress
Velikovsky's ideas.
544
00:33:56,479 --> 00:34:00,711
The suppression of uncomfortable ideas
may be common in religion...
545
00:34:00,917 --> 00:34:04,785
...or in politics,
but it is not the path to knowledge.
546
00:34:04,989 --> 00:34:08,288
And there's no place for it
in the endeavor of science.
547
00:34:08,492 --> 00:34:10,187
We do not know beforehand...
548
00:34:10,393 --> 00:34:14,420
...where fundamental insights
will arise from...
549
00:34:14,631 --> 00:34:18,761
...about our mysterious
and lovely solar system.
550
00:34:18,969 --> 00:34:22,598
And the history of our study
of the solar system shows clearly...
551
00:34:22,806 --> 00:34:26,833
...that accepted and conventional
ideas are often wrong...
552
00:34:27,044 --> 00:34:29,012
...and that fundamental insights...
553
00:34:29,212 --> 00:34:33,046
...can arise from
the most unexpected sources.
554
00:34:34,685 --> 00:34:36,448
We've evolved on the planet Earth...
555
00:34:36,654 --> 00:34:39,248
...and so we find it
a congenial place.
556
00:34:39,455 --> 00:34:41,787
But just next door is Venus...
557
00:34:41,991 --> 00:34:44,983
...until recently, enveloped
in mystery.
558
00:34:45,195 --> 00:34:48,096
It has almost the same size
and mass as the Earth.
559
00:34:48,299 --> 00:34:52,292
Might our sister world
be a balmy summer planet...
560
00:34:52,502 --> 00:34:55,835
...a little warmer than the Earth
because it's closer to the sun?
561
00:34:56,039 --> 00:35:00,373
Are there craters, volcanoes,
mountains, oceans, life?
562
00:35:01,412 --> 00:35:06,111
The first to look at Venus through
a telescope was Galileo in 1609.
563
00:35:06,316 --> 00:35:09,114
But all he could see
was a featureless disk.
564
00:35:09,687 --> 00:35:13,214
As optical telescopes got bigger,
that's all anybody could see:
565
00:35:13,423 --> 00:35:16,017
A disk with no details on it at all.
566
00:35:16,226 --> 00:35:19,992
Venus evidently was covered
with an opaque layer...
567
00:35:20,197 --> 00:35:23,325
...thick clouds concealing the surface.
568
00:35:23,534 --> 00:35:28,130
For centuries, even the composition
of the clouds of Venus was unknown.
569
00:35:28,338 --> 00:35:32,866
I mean, you could go outside, look up,
see Venus with the naked eye...
570
00:35:33,077 --> 00:35:36,171
...observe sunlight reflected
from the clouds of Venus.
571
00:35:36,380 --> 00:35:38,974
What were you looking at?
What were the clouds made of?
572
00:35:39,182 --> 00:35:40,740
Nobody knew.
573
00:35:40,951 --> 00:35:44,648
As a result, imagination ran riot.
574
00:35:44,855 --> 00:35:47,722
The absence of anything you
could see on Venus...
575
00:35:47,925 --> 00:35:51,361
...led some scientists and others
to deduce...
576
00:35:51,562 --> 00:35:53,723
...that the surface was a swamp.
577
00:35:55,032 --> 00:35:59,025
The argument, if we can dignify it
with such a phrase...
578
00:35:59,435 --> 00:36:00,367
...went like this:
579
00:36:00,571 --> 00:36:02,869
"I can't see a thing
on the surface of Venus."
580
00:36:03,072 --> 00:36:03,731
"Why not?"
581
00:36:03,941 --> 00:36:07,069
"Because it's covered with
a dense layer of clouds."
582
00:36:07,276 --> 00:36:08,743
"What are clouds made of?"
583
00:36:08,946 --> 00:36:12,575
"Water, of course. Therefore, Venus
must have a lot of water on it."
584
00:36:12,783 --> 00:36:14,273
"Then the surface must be wet."
585
00:36:14,485 --> 00:36:17,181
"If the surface is wet,
it's probably a swamp.
586
00:36:17,387 --> 00:36:20,220
If there's a swamp, there's ferns.
If there's ferns...
587
00:36:20,423 --> 00:36:22,789
...maybe there's even dinosaurs."
588
00:36:22,993 --> 00:36:24,927
Observation:
You couldn't see a thing.
589
00:36:25,129 --> 00:36:27,723
Conclusion: dinosaurs.
590
00:36:28,298 --> 00:36:31,096
If just looking at Venus
was so unproductive...
591
00:36:31,300 --> 00:36:32,733
...what else could you do?
592
00:36:32,936 --> 00:36:36,303
The next clue came from
early work with that:
593
00:36:36,507 --> 00:36:38,031
A glass prism.
594
00:36:38,241 --> 00:36:42,371
An intense beam of ordinary white
light is passed through a narrow slit...
595
00:36:42,579 --> 00:36:44,171
...and then through the prism.
596
00:36:44,381 --> 00:36:46,975
The result is to spread
the white light out...
597
00:36:47,184 --> 00:36:50,551
...into its constituent
rainbow of colors.
598
00:36:51,188 --> 00:36:54,521
This rainbow pattern
is called a spectrum.
599
00:36:54,725 --> 00:36:57,523
Think about it.
White light enters the prism...
600
00:36:57,728 --> 00:37:00,492
...what comes out of the prism
is colored light.
601
00:37:00,698 --> 00:37:02,723
Lots of colors.
Where did they come from?
602
00:37:02,932 --> 00:37:04,957
They must've been hiding
in the white light.
603
00:37:05,169 --> 00:37:08,297
White light must be
a mixture of many colors.
604
00:37:08,505 --> 00:37:11,269
Here we see the spectrum
running from...
605
00:37:11,475 --> 00:37:14,876
...violet, blue, green,
yellow, orange, to red.
606
00:37:15,079 --> 00:37:19,516
Since we see these colors, we call
this the spectrum of visible light.
607
00:37:20,550 --> 00:37:24,577
The sun emits lots of visible light.
The air is transparent to it.
608
00:37:24,788 --> 00:37:27,450
So our eyes evolved
to work in visible light.
609
00:37:27,657 --> 00:37:31,149
But there are many other frequencies
of light which our eyes can't detect.
610
00:37:31,361 --> 00:37:33,420
Beyond the violet
is the ultraviolet.
611
00:37:33,629 --> 00:37:36,757
It's just as real, but you need
instruments to detect it.
612
00:37:36,967 --> 00:37:40,300
Beyond the ultraviolet are
the x-rays and then the gamma rays.
613
00:37:40,837 --> 00:37:43,203
On the other side of visible light,
beyond the red...
614
00:37:43,407 --> 00:37:46,137
...is the infrared,
again real, again invisible.
615
00:37:46,343 --> 00:37:49,540
Beyond the infrared
are the radio waves.
616
00:37:49,747 --> 00:37:53,513
Now, this entire range from
the gamma rays way over there...
617
00:37:53,717 --> 00:37:56,117
...to the radio waves
all the way over here...
618
00:37:56,320 --> 00:37:58,686
...are simply different
kinds of light.
619
00:37:58,888 --> 00:38:01,083
They differ only in the frequency.
620
00:38:01,291 --> 00:38:04,260
They're all useful, by the way,
in astronomy.
621
00:38:04,461 --> 00:38:07,089
But because of the limitations
of our eyes...
622
00:38:07,297 --> 00:38:11,631
...we have a prejudice,
a bias, a chauvinism...
623
00:38:11,835 --> 00:38:15,430
...to this tiny rainbow band
of visible light.
624
00:38:15,638 --> 00:38:19,802
Now, a spectrum can be used
in a simple and elegant way...
625
00:38:20,042 --> 00:38:23,307
...to determine the chemical
composition of the atmosphere...
626
00:38:23,513 --> 00:38:24,878
...of a planet or star.
627
00:38:25,082 --> 00:38:27,448
Different atoms and molecules
absorb...
628
00:38:27,651 --> 00:38:30,415
...different frequencies
or colors of light.
629
00:38:30,620 --> 00:38:35,114
And those absorbed or missing
frequencies appear as black lines...
630
00:38:35,325 --> 00:38:39,159
...in the spectrum of the light
we receive from the planet or star.
631
00:38:39,363 --> 00:38:43,697
Each and every substance
has a characteristic fingerprint...
632
00:38:43,900 --> 00:38:46,198
...a spectral signature...
633
00:38:46,403 --> 00:38:49,338
...which permits it to be detected
over a great distance.
634
00:38:49,540 --> 00:38:53,101
As a result, the gases
in the atmosphere of Venus...
635
00:38:53,310 --> 00:38:56,211
...at a distance of
60 million kilometers...
636
00:38:56,413 --> 00:39:00,144
...their composition's been determined
from the Earth.
637
00:39:00,350 --> 00:39:04,844
It's amazing to me still, we can tell
what a thing is made out of...
638
00:39:05,054 --> 00:39:08,683
...at an enormous distance away,
without ever touching it.
639
00:39:10,027 --> 00:39:14,157
Our eyes can't see in the near
infrared part of the spectrum.
640
00:39:14,364 --> 00:39:15,763
But our instruments can.
641
00:39:15,965 --> 00:39:19,799
Here's the absorption pattern
of lots and lots of carbon dioxide:
642
00:39:20,003 --> 00:39:24,463
Dark lines in characteristic patterns
at specific frequencies.
643
00:39:24,708 --> 00:39:27,506
You'd detect a different set
of infrared lines...
644
00:39:27,711 --> 00:39:30,373
...if, say, water vapor were present.
645
00:39:31,148 --> 00:39:36,051
If Venus were really soaking wet,
then you could determine that...
646
00:39:36,419 --> 00:39:40,219
...by finding the pattern
of water vapor in its atmosphere.
647
00:39:40,423 --> 00:39:43,517
But around 1 920, when this experiment
was first performed...
648
00:39:43,726 --> 00:39:46,388
...the Venus atmosphere seemed to
have not a hint...
649
00:39:46,597 --> 00:39:50,795
...not a smidgen, not a trace
of water vapor above the clouds.
650
00:39:51,000 --> 00:39:55,096
And so instead of a swampy,
soaking wet surface...
651
00:39:55,305 --> 00:39:58,968
...it was suggested that Venus
was bone-dry, a desert planet...
652
00:39:59,176 --> 00:40:02,668
...with clouds composed
of fine silicate dust.
653
00:40:03,247 --> 00:40:06,114
But later, spectroscopic
observations revealed...
654
00:40:06,316 --> 00:40:08,216
...the characteristic
absorption lines...
655
00:40:08,417 --> 00:40:11,284
...of an enormous amount
of carbon dioxide.
656
00:40:11,487 --> 00:40:15,856
Scientists thought there must be lots
of carbon compounds on the surface...
657
00:40:16,059 --> 00:40:19,654
...making this a planet
covered with petroleum.
658
00:40:19,897 --> 00:40:23,856
Others agreed that the atmosphere was
dry but thought the surface was wet.
659
00:40:24,067 --> 00:40:27,559
With all that CO 2,
it had to be carbonated water.
660
00:40:27,771 --> 00:40:31,673
Venus, they thought, was covered
with a vast ocean of seltzer.
661
00:40:31,875 --> 00:40:35,311
The first hint of the true situation
on Venus came...
662
00:40:35,512 --> 00:40:39,004
...not from the visible, ultraviolet
or infrared part of the spectrum...
663
00:40:39,216 --> 00:40:42,185
...but from over here
in the radio region.
664
00:40:42,386 --> 00:40:45,787
We're used to the idea of
radio signals from intelligent life...
665
00:40:45,989 --> 00:40:49,948
...or at least semi-intelligent life,
radio and television stations.
666
00:40:50,159 --> 00:40:54,152
But there are all kinds of reasons
why natural objects emit radio waves.
667
00:40:54,363 --> 00:40:56,957
One reason is that they're hot.
668
00:40:57,266 --> 00:40:59,234
And when, in 1 956...
669
00:40:59,436 --> 00:41:02,599
...Venus was, for the first time,
observed by a radio telescope...
670
00:41:02,806 --> 00:41:05,866
...the planet was discovered
to be emitting radio waves...
671
00:41:06,076 --> 00:41:09,341
...as if it were at
an extremely high temperature.
672
00:41:09,546 --> 00:41:14,279
But the real demonstration that Venus'
surface was astonishingly hot...
673
00:41:14,483 --> 00:41:19,420
...came when the first spacecraft
penetrated the clouds of Venus...
674
00:41:19,756 --> 00:41:23,988
...and slowly settled on the surface
of the nearest planet.
675
00:41:27,064 --> 00:41:32,001
These were the unmanned spacecraft
of the Soviet Venera series.
676
00:41:34,604 --> 00:41:39,268
In our spaceship of the imagination,
we retrace their course.
677
00:41:40,677 --> 00:41:45,114
From a distance, our sister planet
seems serene and peaceful...
678
00:41:45,315 --> 00:41:47,510
...its clouds motionless.
679
00:41:50,520 --> 00:41:53,978
These clouds are near the top
of a great ocean of air...
680
00:41:54,191 --> 00:41:58,958
...about 100 kilometers thick,
composed mainly of carbon dioxide.
681
00:42:02,566 --> 00:42:05,660
There's some nitrogen, a little
water vapor and other gases...
682
00:42:05,869 --> 00:42:09,134
...but only the merest trace
of hydrocarbons.
683
00:42:09,539 --> 00:42:11,973
The clouds turn out to be,
not water...
684
00:42:12,175 --> 00:42:15,736
...but a concentrated solution
of sulfuric acid.
685
00:42:24,854 --> 00:42:26,617
Even in the high clouds...
686
00:42:26,823 --> 00:42:30,520
...Venus is a thoroughly nasty place.
687
00:42:38,701 --> 00:42:41,465
The clouds are stained yellow
by sulfur.
688
00:42:41,671 --> 00:42:43,468
There are great lightning storms.
689
00:42:43,673 --> 00:42:45,971
As we descend, there are
increasing amounts...
690
00:42:46,175 --> 00:42:48,143
...of the noxious gas sulfur dioxide.
691
00:42:48,812 --> 00:42:51,975
The pressures become so high
that early Venera spacecraft...
692
00:42:52,182 --> 00:42:54,776
...were crushed like old tin cans...
693
00:42:54,985 --> 00:42:57,613
...by the weight
of the surrounding atmosphere.
694
00:43:00,423 --> 00:43:03,449
Beneath the clouds
in the dense, clear air...
695
00:43:03,659 --> 00:43:06,219
...it's as bright as
on an overcast day on Earth.
696
00:43:06,430 --> 00:43:10,059
But the atmosphere is so thick
that the ground seems to ripple...
697
00:43:10,266 --> 00:43:11,756
...and distort.
698
00:43:12,002 --> 00:43:15,904
The atmospheric pressure down here is
90 times that on Earth.
699
00:43:16,173 --> 00:43:21,110
The temperature is 380 degrees
centigrade, 900 degrees Fahrenheit.
700
00:43:21,510 --> 00:43:24,138
Hotter than the hottest
household oven.
701
00:43:24,346 --> 00:43:26,906
This is a world
marked by searing heat...
702
00:43:27,117 --> 00:43:29,677
...crushing pressures,
sulfurous gases...
703
00:43:29,886 --> 00:43:32,548
...and a desolate, reddish landscape.
704
00:43:32,756 --> 00:43:36,749
Far from the balmy paradise
imagined by some early scientists...
705
00:43:36,960 --> 00:43:41,226
...Venus is the one place
in the solar system most like hell.
706
00:43:47,137 --> 00:43:49,628
But today, as in ancient tradition...
707
00:43:49,840 --> 00:43:54,436
...there are travelers who will dare
a visit to the underworld.
708
00:43:54,678 --> 00:43:57,476
Venera 9 was the first spacecraft
in human history...
709
00:43:57,681 --> 00:44:00,411
...to return a photograph
from the surface of Venus.
710
00:44:00,617 --> 00:44:03,347
It found the rocks curiously eroded...
711
00:44:03,920 --> 00:44:05,547
...perhaps by the corrosive gases...
712
00:44:05,755 --> 00:44:08,053
...perhaps because
the temperature is so high...
713
00:44:08,258 --> 00:44:11,955
...that the rocks are partly molten
and sluggishly flow.
714
00:44:12,162 --> 00:44:16,929
The Soviet Venera spacecraft,
their electronics long ago fried...
715
00:44:17,133 --> 00:44:20,569
...are slowly corroding
on the surface of Venus.
716
00:44:20,769 --> 00:44:22,828
They are the first spaceships
from Earth...
717
00:44:23,039 --> 00:44:25,803
...ever to land on another planet.
718
00:44:30,580 --> 00:44:32,480
The reason Venus is like hell...
719
00:44:32,682 --> 00:44:35,810
...seems to be what's called
the greenhouse effect.
720
00:44:36,019 --> 00:44:39,716
Ordinary visible sunlight penetrates
the clouds and heats the surface.
721
00:44:39,923 --> 00:44:42,653
But the dense atmosphere
blankets the surface...
722
00:44:42,859 --> 00:44:45,521
...and prevents it from
cooling off to space.
723
00:44:45,729 --> 00:44:48,459
An atmosphere 90 times
as dense as ours...
724
00:44:48,664 --> 00:44:51,394
...made of carbon dioxide,
water vapor and other gases...
725
00:44:51,601 --> 00:44:53,398
...lets in visible light
from the sun...
726
00:44:53,603 --> 00:44:57,664
...but will not let out the infrared
light radiated by the surface.
727
00:44:57,874 --> 00:44:59,364
The temperature rises...
728
00:44:59,575 --> 00:45:02,544
...until the infrared radiation
trickling out to space...
729
00:45:02,745 --> 00:45:05,839
...just balances the sunlight
reaching the surface.
730
00:45:09,318 --> 00:45:12,082
The greenhouse effect can make
an Earth-like world...
731
00:45:12,289 --> 00:45:14,723
...into a planetary inferno.
732
00:45:16,959 --> 00:45:20,019
In this caldron, there's not likely
to be anything alive...
733
00:45:20,229 --> 00:45:22,356
...even creatures
very different from us.
734
00:45:22,566 --> 00:45:25,660
Organic and other conceivable
biological molecules...
735
00:45:25,869 --> 00:45:28,667
...would simply fall to pieces.
736
00:45:41,217 --> 00:45:43,913
The hell of Venus is
in stark contrast...
737
00:45:44,120 --> 00:45:47,612
...with the comparative heaven
of its neighboring world...
738
00:45:47,823 --> 00:45:51,224
...our little planetary home,
the Earth.
739
00:45:52,896 --> 00:45:56,889
Here, the atmosphere is
90 times thinner.
740
00:45:57,100 --> 00:46:00,069
Here, the carbon dioxide
and water vapor...
741
00:46:00,270 --> 00:46:02,295
...make a modest greenhouse effect...
742
00:46:02,504 --> 00:46:05,496
...which heats the ground
above the freezing point of water.
743
00:46:05,708 --> 00:46:10,202
Without it, our oceans
would be frozen solid.
744
00:46:10,413 --> 00:46:13,780
A little greenhouse effect
is a good thing.
745
00:46:25,161 --> 00:46:27,823
But Venus is an ominous reminder...
746
00:46:28,031 --> 00:46:30,192
...that on a world
rather like the Earth...
747
00:46:30,399 --> 00:46:32,731
...things can go wrong.
748
00:46:33,270 --> 00:46:37,798
There is no guarantee that our planet
will always be so hospitable.
749
00:46:38,008 --> 00:46:40,169
To maintain this clement world...
750
00:46:40,377 --> 00:46:43,938
...we must understand it
and appreciate it.
751
00:46:47,183 --> 00:46:49,879
The Earth is a place to our eyes...
752
00:46:50,086 --> 00:46:52,987
...more beautiful than
any other that we know.
753
00:46:53,189 --> 00:46:56,647
But this beauty has been
sculpted by change:
754
00:46:56,860 --> 00:46:59,488
Gentle, almost undetectable change...
755
00:46:59,696 --> 00:47:01,994
...and sudden, violent change.
756
00:47:02,198 --> 00:47:06,328
In the cosmos,
there is no refuge from change.
757
00:47:07,070 --> 00:47:10,267
The Sphinx:
human head, lion's body...
758
00:47:10,473 --> 00:47:13,909
...constructed more than
5500 years ago.
759
00:47:14,344 --> 00:47:17,279
That face was once crisp
and cleanly rendered...
760
00:47:17,480 --> 00:47:19,175
...like this paw I am standing on.
761
00:47:19,381 --> 00:47:22,407
The paw has been buried
in the sand until recently...
762
00:47:22,618 --> 00:47:24,779
...and protected from erosion.
763
00:47:25,288 --> 00:47:28,951
The face is now muddled
and softened...
764
00:47:29,159 --> 00:47:32,822
...because of thousands of years
of sandblasting in the desert...
765
00:47:33,028 --> 00:47:34,928
...and a little rainfall.
766
00:47:35,432 --> 00:47:39,232
In New York City, there is an obelisk
called Cleopatra's Needle...
767
00:47:39,436 --> 00:47:40,960
...which comes from Egypt.
768
00:47:41,171 --> 00:47:45,301
In only a little more than a century
in New York's Central Park...
769
00:47:45,508 --> 00:47:49,945
...the inscriptions on that obelisk
have been almost totally obliterated.
770
00:47:50,146 --> 00:47:52,637
Not by sand and water...
771
00:47:52,849 --> 00:47:55,317
...but by smog
and industrial pollution.
772
00:47:55,518 --> 00:47:57,509
A bit like the atmosphere of Venus.
773
00:47:58,053 --> 00:48:02,012
Slow erosion wipes out information.
774
00:48:02,225 --> 00:48:03,249
On the Earth...
775
00:48:03,460 --> 00:48:05,792
...mountain ranges are destroyed
by erosion...
776
00:48:05,995 --> 00:48:08,190
...in maybe tens of millions
of years...
777
00:48:08,397 --> 00:48:12,424
...small impact craters in maybe
hundreds of thousands of years.
778
00:48:12,635 --> 00:48:15,399
And the greatest artifacts
of human beings...
779
00:48:15,604 --> 00:48:18,971
...in thousands
or tens of thousands of years.
780
00:48:21,211 --> 00:48:24,669
In addition to such slow
and uniform processes...
781
00:48:24,881 --> 00:48:28,248
...there are rare but sudden
catastrophes.
782
00:48:28,451 --> 00:48:31,215
The Sphinx is missing a nose.
783
00:48:31,421 --> 00:48:35,653
In an act of idle desecration,
some soldiers once shot it off.
784
00:48:35,859 --> 00:48:40,193
If you wait long enough,
everything changes.
785
00:48:58,481 --> 00:49:02,645
Slow, uniform processes,
unheralded events:
786
00:49:02,852 --> 00:49:04,479
The sting of a sand grain...
787
00:49:04,686 --> 00:49:06,711
...the fall of a drop of water...
788
00:49:06,923 --> 00:49:10,586
...can, over the ages,
totally rework the landscape.
789
00:49:31,281 --> 00:49:33,841
And rare, violent processes...
790
00:49:34,050 --> 00:49:37,019
...exceptional events
that will not recur in a lifetime...
791
00:49:37,287 --> 00:49:39,915
...also make major changes.
792
00:50:00,609 --> 00:50:04,739
Both the insignificant
and the extraordinary...
793
00:50:04,948 --> 00:50:08,111
...are the architects
of the natural world.
794
00:50:52,861 --> 00:50:55,887
The destruction of trees
and grasslands...
795
00:50:56,099 --> 00:50:58,533
...makes the surface
of the Earth brighter.
796
00:50:58,735 --> 00:51:02,796
It reflects more sunlight
back to space and cools our planet.
797
00:51:03,705 --> 00:51:05,400
After we discovered fire...
798
00:51:05,608 --> 00:51:08,805
...we began to incinerate forests
intentionally...
799
00:51:09,012 --> 00:51:11,139
...to clear the land
by a process called...
800
00:51:11,548 --> 00:51:14,608
..."slash and burn" agriculture.
801
00:51:14,817 --> 00:51:19,379
And today, forests and grasslands
are being destroyed...
802
00:51:19,589 --> 00:51:23,889
...frivolously, carelessly
by humans who are...
803
00:51:24,093 --> 00:51:28,120
...heedless of the beauty
of our cousins the trees...
804
00:51:28,331 --> 00:51:31,698
...and ignorant of the possible
climatic catastrophes...
805
00:51:31,901 --> 00:51:35,962
...which large-scale burning
of forests may bring.
806
00:51:36,439 --> 00:51:38,805
(TREES BREAKING)
807
00:51:41,544 --> 00:51:44,012
The indiscriminate destruction
of vegetation...
808
00:51:44,213 --> 00:51:45,805
...may alter the global climate...
809
00:51:46,015 --> 00:51:48,950
...in ways that no scientist
can yet predict.
810
00:51:50,453 --> 00:51:52,683
It has already deadened
large patches...
811
00:51:52,889 --> 00:51:55,323
...of the Earth's
life-supporting skin.
812
00:52:07,403 --> 00:52:11,601
And yet, we ravage the Earth
at an accelerated pace...
813
00:52:11,808 --> 00:52:14,140
...as if it belonged
to this one generation...
814
00:52:14,344 --> 00:52:17,939
...as if it were ours
to do with as we please.
815
00:52:26,789 --> 00:52:29,189
The Earth has mechanisms
to cleanse itself...
816
00:52:29,392 --> 00:52:32,725
...to neutralize the toxic substances
in its system.
817
00:52:32,929 --> 00:52:35,523
But these mechanisms work
only up to a point.
818
00:52:35,732 --> 00:52:39,099
Beyond some critical threshold,
they break down.
819
00:52:39,302 --> 00:52:42,533
The damage becomes irreversible.
820
00:53:10,600 --> 00:53:12,534
Our generation must choose.
821
00:53:12,735 --> 00:53:15,829
Which do we value more:
short-term profits...
822
00:53:16,039 --> 00:53:20,032
...or the long-term habitability
of our planetary home?
823
00:53:23,079 --> 00:53:25,047
The world is divided politically.
824
00:53:25,248 --> 00:53:27,682
But ecologically
it is tightly interwoven.
825
00:53:27,884 --> 00:53:31,581
There are no useless threads
in the fabric of the ecosystem.
826
00:53:31,854 --> 00:53:35,688
If you cut any one of them,
you will unravel many others.
827
00:53:37,060 --> 00:53:38,652
We have uncovered other worlds...
828
00:53:38,861 --> 00:53:42,126
...with choking atmospheres
and deadly surfaces.
829
00:53:42,398 --> 00:53:45,731
Shall we then re-create
these hells on Earth?
830
00:53:48,504 --> 00:53:52,406
We have encountered desolate moons
and barren asteroids.
831
00:53:52,608 --> 00:53:57,545
Shall we then scar and crater this
blue-green world in their likeness?
832
00:54:16,099 --> 00:54:18,897
Natural catastrophes are rare.
833
00:54:19,102 --> 00:54:20,535
But they come often enough.
834
00:54:20,737 --> 00:54:24,366
We need not force the hand of nature.
835
00:54:34,016 --> 00:54:38,316
If we ruin the Earth,
there is no place else to go.
836
00:54:38,521 --> 00:54:41,217
This is not a disposable world.
837
00:54:41,424 --> 00:54:45,190
And we are not yet able
to re-engineer other planets.
838
00:54:53,703 --> 00:54:56,228
The cruelest desert on Earth...
839
00:54:56,439 --> 00:55:00,705
...is far more hospitable
than any place on Mars.
840
00:55:02,011 --> 00:55:05,469
The bright, sandy surface
and dusty atmosphere of Mars...
841
00:55:05,681 --> 00:55:09,276
...reflect enough sunlight
back to space to cool the planet...
842
00:55:09,485 --> 00:55:14,422
...freezing out all its water,
locking it in a perpetual ice age.
843
00:55:15,391 --> 00:55:19,953
Human activities brighten
our landscape and our atmosphere.
844
00:55:20,163 --> 00:55:23,394
Might this ultimately
make an ice age here?
845
00:55:24,500 --> 00:55:28,527
At the same time, we are releasing
vast quantities of carbon dioxide...
846
00:55:28,738 --> 00:55:31,468
...increasing the greenhouse effect.
847
00:55:31,674 --> 00:55:34,302
The Earth need not
resemble Venus very closely...
848
00:55:34,510 --> 00:55:37,377
...for it to become
barren and lifeless.
849
00:55:41,017 --> 00:55:44,453
It may not take much
to destabilize the Earth's climate...
850
00:55:44,654 --> 00:55:48,112
...to convert this heaven,
our only home in the cosmos...
851
00:55:48,324 --> 00:55:50,292
...into a kind of hell.
852
00:55:52,829 --> 00:55:56,390
The study of the global climate,
the sun's influence...
853
00:55:56,599 --> 00:55:59,227
...the comparison of the Earth
with other worlds...
854
00:55:59,435 --> 00:56:02,871
These are subjects in their
earliest stages of development.
855
00:56:03,072 --> 00:56:06,235
They are funded poorly
and grudgingly.
856
00:56:06,442 --> 00:56:10,572
Meanwhile, we continue to load the
Earth's atmosphere with materials...
857
00:56:10,780 --> 00:56:15,217
...about whose long-term influence
we are almost entirely ignorant.
858
00:56:16,586 --> 00:56:21,080
There are worlds that began with
as much apparent promise as Earth.
859
00:56:21,290 --> 00:56:24,157
But something went wrong.
860
00:56:24,360 --> 00:56:28,888
Knowing that worlds can die
alerts us to our danger.
861
00:56:30,032 --> 00:56:33,934
If a visitor arrived from another
world, what account would we give...
862
00:56:34,136 --> 00:56:37,401
...of our stewardship
of the planet Earth?
863
00:56:44,614 --> 00:56:49,483
In the history of the solar system,
have worlds ever been destroyed?
864
00:56:50,586 --> 00:56:53,783
Most of the moons in the outer
solar system have craters on them...
865
00:56:53,990 --> 00:56:56,424
...made by cometary impacts.
866
00:56:57,126 --> 00:56:59,060
Some have such
large craters though...
867
00:56:59,262 --> 00:57:02,925
...that if the impacting comets
had been just a little bit bigger...
868
00:57:03,132 --> 00:57:05,225
...the moons would have
been shattered.
869
00:57:07,803 --> 00:57:10,363
What would the results of
such a collision look like?
870
00:57:10,573 --> 00:57:12,507
(EXPLOSION)
871
00:57:13,409 --> 00:57:15,104
Maybe a planetary ring.
872
00:57:17,580 --> 00:57:20,447
The idea has been growing
that little worlds are...
873
00:57:20,650 --> 00:57:23,517
...every now and then,
demolished by a cometary impact.
874
00:57:23,719 --> 00:57:28,281
The fragments then slowly coalesce,
and a moon arises again...
875
00:57:28,491 --> 00:57:29,822
...from its own ashes.
876
00:57:30,026 --> 00:57:34,554
Some moons may have been destroyed
and reconstituted many times.
877
00:57:35,798 --> 00:57:39,564
For our own world,
the peril is more subtle.
878
00:57:40,202 --> 00:57:41,897
Since this series was
first broadcast...
879
00:57:42,104 --> 00:57:44,902
...the dangers of the increasing
greenhouse effect...
880
00:57:45,107 --> 00:57:46,665
...have become much more clear.
881
00:57:46,876 --> 00:57:51,108
We burn fossil fuels, like coal
and gas and petroleum...
882
00:57:51,314 --> 00:57:53,839
...putting more carbon dioxide
into the atmosphere...
883
00:57:54,050 --> 00:57:56,382
...and thereby heating the Earth.
884
00:57:56,585 --> 00:57:59,679
The hellish conditions on Venus
are a reminder that...
885
00:57:59,889 --> 00:58:01,322
...this is serious business.
886
00:58:01,724 --> 00:58:04,022
Computer models that
successfully explain...
887
00:58:04,226 --> 00:58:06,160
...the climates of other planets...
888
00:58:06,362 --> 00:58:09,331
...predict the deaths of forests...
889
00:58:09,532 --> 00:58:12,797
...parched croplands,
the flooding of coastal cities...
890
00:58:13,002 --> 00:58:15,095
...environmental refugees...
891
00:58:15,304 --> 00:58:18,569
...widespread disasters
in the next century...
892
00:58:18,874 --> 00:58:20,466
...unless we change our ways.
893
00:58:20,676 --> 00:58:22,166
What do we have to do?
894
00:58:22,611 --> 00:58:24,238
Four things.
895
00:58:24,447 --> 00:58:27,848
One: much more efficient use
of fossil fuels.
896
00:58:28,050 --> 00:58:32,180
Why not cars that get 70 miles
a gallon instead of 25?
897
00:58:32,388 --> 00:58:36,688
Two: research and development
on safe alternative energy sources...
898
00:58:36,892 --> 00:58:38,587
...especially solar power.
899
00:58:39,195 --> 00:58:42,164
Three: reforestation
on a grand scale.
900
00:58:42,365 --> 00:58:45,857
And four: helping to bring
the billion poorest people...
901
00:58:46,068 --> 00:58:48,195
...on the planet
to self-sufficiency...
902
00:58:48,404 --> 00:58:51,737
...which is the key step
in curbing world population growth.
903
00:58:51,941 --> 00:58:55,843
Every one of these steps makes sense
apart from greenhouse warming.
904
00:58:56,679 --> 00:58:59,147
No one has proposed
that the trouble with Venus is...
905
00:58:59,348 --> 00:59:03,375
...that there once was Venusians
who drove fuel-inefficient cars.
906
00:59:03,586 --> 00:59:06,851
But our nearest neighbor,
nevertheless, is a stark warning...
907
00:59:07,056 --> 00:59:10,457
...on the possible fate
of an Earth-like world.
77968
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