BBC_The Comets Tale
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00:00On the edge of space, halfway to the nearest star, there is a vast cloud of debris, lumps
00:11of rock and ice that have drifted on the edge of our solar system for four billion years.
00:18They are among the most mysterious objects in the universe.
00:25Sometimes they are sent into the heart of our solar system, where they are transformed
00:29into the blazing stars we call comets.
00:35For centuries, our ancestors were in awe of comets.
00:47They were messengers from the gods, carrying the power of life and death.
01:05It's only now, as we have the power to uncover the comet's secrets, that we are discovering
01:11that those ancestors were right all along.
01:18The comet's tale is a story that really can tell us about life, the universe and everything.
01:41About once every ten years, a really bright comet lights up the skies.
02:04Some are bright enough to be seen in broad daylight.
02:10Others unfurl their tales across half the sky.
02:17They appear from nowhere and just as suddenly disappear.
02:24It's little wonder that throughout history, people have tried to explain their significance
02:29and the effects they have on the Earth.
02:37As far back as the second century BC, the Chinese had taken the trouble to classify
02:42comets into 29 separate types.
02:48The shapes they saw were so striking, they have penetrated human consciousness.
03:01Many other societies explain the sudden appearance of comets through their mythology.
03:08Comets mark the presence of gods in the skies.
03:13Comets are recorded in myths globally.
03:18Almost every population in the world talks about dragons, things that fall from the sky,
03:26stones and iron that fall from the heaven, giants with a single eye in their head,
03:34wizards and things like that that can be interpreted as comets.
03:42If you look hard enough, comets can even be found in the most famous folk stories.
03:54When you peel away all of the layers from the King Arthur story, what you're left with
03:59is a local lord who was left in Britain after the Romans had retreated.
04:04And that doesn't explain why or how he became such a huge figure in English folklore.
04:13There's another possibility, and this is that Arthur may have been a comet.
04:21When Arthur has a battle, he takes his sword out and that sword shines with the light of 30 torches.
04:31And generally, Arthur's battles occur at times of known meteor showers.
04:37And in those battles, there's widespread destruction.
04:42In one of the battles, 11 countries get destroyed.
04:46And that's characteristic of what you might expect from a comet.
04:49It's difficult to explain in human terms.
05:08Was King Arthur really a comet?
05:10It's an idea that requires quite a stretch of the imagination.
05:15But comets were often depicted as fiery swords.
05:19Perhaps the myths were created to help people interpret events in the sky that they couldn't
05:24otherwise explain.
05:29Whether you believe all the interpretations or not, there is no doubt that comets were
05:33the subject of deep superstition.
05:36The reason was simple.
05:43In times when the movement of the stars and planets was used to predict the future, the
05:47sudden appearance of something unusual was a dangerous omen.
05:52In the medieval world, philosophers and astronomers thought that everything further away from
05:57us than the moon is was perfect.
06:00Out there beyond the moon, nothing ever changed.
06:04And comets just don't look like that.
06:06Comets look like fiery signs moving across our sky.
06:13They were threats to the idea of a perfect, calm, orderly universe.
06:21Perhaps they were guided by God in order to send signs to humanity of God's purposes.
06:29When comets passed, astrologers almost automatically predicted great transformation, the death
06:35of kings, war, invasion, plague or famine.
06:43Not everyone viewed them with foreboding.
06:46Genghis Khan saw comets as a personal message, telling him to fulfill his destiny and wage
06:51war across Asia and Europe.
07:07To William the Conqueror, the appearance of a comet in 1066 was such an auspicious sign
07:13that it was immortalized on the Bayeux Tapestry.
07:17But for the Saxons, the comet was definitely a harbinger of doom.
07:34For centuries, all explanations of comets relied on superstition and astrology.
07:43It finally took the greatest mind of them all to make scientific sense of them.
07:52In 1680, a particularly bright comet caught the attention of Isaac Newton.
08:00It was a fascination that was to change the way we understand the universe.
08:07On November the 19th, at half past four in the morning, in Cambridge, the comet was seen
08:15by some young man.
08:18And on the same day, at five in the morning, at Boston, in New England, the comet was also
08:26observed.
08:28What was extraordinary about this comet was that it was visible in the sky.
08:32It was very bright.
08:34You could see it in October and November of 1680, and then it disappeared behind the sun.
08:45And then another comet appeared from behind the sun in December at the end of the year
08:51and was visible right through to March of 1681.
08:54So the big question is, is this one comet or two?
08:58If it's one comet, then it's bent a lot near the sun.
09:04Now here's a diagram that Newton himself made of the path of the comet if it's one comet.
09:12You see here that it was approaching the orbit of the Earth and the sun in October and November
09:20of 1680.
09:22And then it disappears behind the sun and reappeared in December and it stayed visible
09:28through February and March of 1681.
09:33For the sun to bend the path of a comet this much, Newton realised that there had to be
09:39an unseen force at work.
09:42He called it gravity.
09:49An idea which didn't come from an apple falling from a tree, but from a comet passing behind the sun.
09:58It was by thinking about this puzzle that Newton began to formulate the idea of gravitation.
10:08Before 1681, Newton had no notion of universal gravitation.
10:15Newton between 1681 and 1684 began to suppose that comets come back, that they're therefore
10:22like planets because they move in closed orbits, that you should be able to calculate the shape
10:27of the orbit, that you could in fact predict when comets come back.
10:34And that is exactly what he and his friend Edmund Halley set out to do.
10:41If they could use the theory of gravitation to predict the return of a comet, they would
10:46not only prove Newton's theory, but would also show that comets were not omens or supernatural
10:52signs, but predictable orbiting bodies, like the planets.
11:03Halley collated tables of all the comet observations he could find, looking for similarities in
11:08their behaviour, what direction they came from, how close they got to the sun.
11:16At last, he found three comets whose descriptions were almost identical, one recorded in 1531,
11:24one in 1607, and one he had observed himself in 1682.
11:31Each appearance was separated by 75 years.
11:39Convinced that these three sightings were of the same comet, Halley made a public prediction
11:44that it would return again in late 1758 or early 1759.
11:49It was an extraordinary piece of scientific bravado for the time.
12:05Neither Newton nor Halley lived to see the prediction fulfilled.
12:10And when, on Boxing Day, 1758, a comet was spotted in Germany, it was greeted with jubilation
12:17and as a triumph of science over the supernatural.
12:24Tracing back through the historical records, comet Halley could be found reappearing like
12:39clockwork. This Babylonian tablet records its appearance
12:44in 164 BC. The comets that Genghis Khan and William the
12:49Conqueror saw weren't messages from God, but the regular appearances of comet Halley.
12:56A once-in-a-lifetime meeting, tonight, live on BBC One.
13:01After its latest visit to Earth, Halley's comet is already on course back towards the
13:06icy wastes of outer space. The last time it visited in 1986, it was greeted
13:13like an old friend. Around the world, people were transfixed as
13:18the probe Giotto beamed back the first live pictures of a comet nucleus.
13:27It was the last we'll see of comet Halley until 2061.
13:37Perhaps the most surprising thing about comets is that 300 years after Newton and Halley,
13:43despite all that we now know about them, comets still have the power to capture and
13:48control people's imaginations. In 1997, a bright comet called Hale-Bopp suddenly
13:58appeared in the skies. It captivated sky watchers around the world.
14:05It was particularly keenly watched by a cult in California known as Heaven's Gate.
14:12They believed that the comet was concealing a spaceship that had come to take them to
14:16another, better world.
14:33At this point, this is considered a mass suicide investigation.
14:42So deep was their conviction that as the comet came closest to the Earth, all 39 members
14:50poisoned themselves, believing that in taking their own lives, they would free their souls
14:56to board the spaceship. It seems no matter how far science progresses,
15:03comets can still hold the power of life and death.
15:12What is true, as we are now discovering scientifically, is that comets have had a profound effect on
15:21life on Earth. One of the reasons they retain so much power
15:27over us is that they seem to appear out of nowhere.
15:35On the island of La Palma in the Canaries, Alan Fitzsimmons is trying to unlock the comet's
15:44secrets. Here we have the William Herschel telescope,
15:554.2 metres of astronomical loveliness, already awaiting to spot a few of these comets.
16:05It may seem odd to use such a powerful telescope to observe comets, the brightest objects in
16:13the night sky, but the scientists want to study the comet's nucleus.
16:19And to do that, they must see them before they start producing their tails, when they
16:23are further away than Jupiter. The nucleus is tiny, just a few kilometres
16:31across, and in the vastness of space, they are incredibly difficult to spot.
16:39They only reflect on average 4% of the light that hits them, and a piece of coal reflects
16:478% of the light that hits it, so a comet nucleus is twice as dark as a lump of coal.
16:55And so we're trying to see these things from the feeble amount of sunlight they reflect
17:02out beyond Jupiter. It's not an easy thing to try and attempt to do.
17:10Oh my god. That's it. That's it.
17:29Ha ha! Oh, fantastic stuff. Yep, yep, another one in the bag. Happy days.
17:40I do love this job, you know.
17:49This tiny, moving blob of light is the comet's nucleus, a delicate lump of rock and ice floating
17:56slowly through space. The most powerful telescopes pick them up
18:04as they drift through the outer planets, but their orbits suggest they come from much further
18:10into space, far beyond the realm of the planets. Finding exactly where they come from hasn't
18:17been easy. Since the 1940s, astronomers have been looking
18:23for a huge reservoir of comets, called the Kuiper belt, which they believed lay beyond
18:29Neptune. The only problem was, no one could find it.
18:35No matter how hard they looked, the only object anyone could see out there was the ex-planet
18:41Pluto.
18:48By the end of the 1980s, almost everyone had given up looking, apart from two maverick
18:56astronomers on Hawaii. I don't intentionally do things that are
19:01different, but I do try to be sceptical of everything, because I think that's essential
19:09for science. So if you just buy into the prevailing wisdom, then all you can ever do is confirm
19:16what's already known.
19:20In 1987, Dave Jewett and Jane Lu set up at the Mauna Kea Observatory in Hawaii, to stare
19:28into empty space, using up precious telescope time, finding nothing at all.
19:42At first I couldn't get money to support any of this, because it seemed, I guess, so
19:48speculative or so crazy, and so instead I used money from other sources, probably illegally.
19:58Everyone, even their colleagues, thought they were mad.
20:03They started asking, well, when are you going to stop? You've been doing it for years now,
20:07you still haven't found anything, or maybe there isn't anything out there.
20:18Eventually, their bloody-minded persistence paid off. After five years and over 400 hours
20:25observing, there was a small, moving anomaly on one of their pictures.
20:30He said, Jane, come take a look at this, and we saw something, and it was moving at about
20:35the right speed we would expect a distant solar system object would move.
20:40So we were instantly pretty excited about that, because we hadn't seen any other candidate
20:45object so good.
20:47It was just the first positive sign that we got after five years.
20:58This was it. At last, they'd found the first Kuiper Belt object.
21:04They named it, catchily, 1992 QB1.
21:15Since then, over a thousand Kuiper Belt objects have been found,
21:21quietly orbiting in the dark reaches beyond Neptune.
21:26But there are thought to be as many as six billion potential comets here,
21:31orbiting in a belt three billion kilometres wide.
21:36Occasionally, they collide.
21:42And one is sent tumbling into the inner solar system
21:46to become what we call a short-period comet.
21:50Short-period comets orbit the sun frequently, and their orbital paths bring them back around the sun
21:56perhaps once every 10 or 20 years or so.
21:59And these are the objects that we believe have come from this trans-Neptunian region,
22:03just beyond the realm of the giant planet.
22:07Comet Halley is the most famous of the short-period comets.
22:12But there is another sort of comet,
22:14one whose orbit can be thousands or even millions of years long.
22:19They come from far beyond the Kuiper Belt,
22:22from an almost mythical place called the Oort Cloud.
22:26The Longpierre comets, as the name suggests, have been around for thousands of years.
22:32The Longpierre comets, as the name suggests,
22:36take a really long time to go around the sun,
22:39perhaps anywhere between 200 years and a million years to go around once.
22:44And these are the objects that tell us that the Oort Cloud exists,
22:48stretching out tremendous distances towards the nearest stars.
22:55This is the very edge of the solar system.
22:58Halfway to the nearest star, 50,000 times further from the sun than the Earth.
23:05Here, trillions of comets cling tenuously to the sun's gravity.
23:11We now think that the Oort Cloud was formed
23:14during the violent formation of the solar system.
23:21As the planets formed, and their gravity increased,
23:24they catapulted chunks of debris into space,
23:27some of it collected in the Oort Cloud,
23:30where it has been ever since.
23:36They've been out there in deep freeze, in deep space,
23:40and so by studying comets,
23:42we're studying the formation and the evolution of our solar system.
23:49But studying the comets isn't easy.
23:52The Oort Cloud is so far away, and the object in it so tiny,
23:56that we'll never be able to see it.
24:01But luckily for the astronomers,
24:03sometimes something happens that sends them tumbling towards us.
24:09Oort Cloud objects become comets,
24:12not through the action of anything to do with our sun,
24:15but through the action of other stars, generally.
24:19Our sun is just one of 100,000 million stars
24:24that form our Milky Way galaxy,
24:26the Milky Way that you can see on a clear, dark, moonless night.
24:30And they're all moving in slightly different directions
24:34with slightly different velocities,
24:36and occasionally a star will come close enough to our Oort Cloud
24:40that it will perturb some of those comets out there.
24:49Some of them will be thrown out into interstellar space, never to return.
24:57But some fraction of them will be nudged inwards into our solar system.
25:03Until recently, it was assumed that our solar system
25:06was isolated from the rest of space by the vast distances surrounding it.
25:12It's because we see comets that we now know what happens to them.
25:18The movement of comets around the Earth
25:20is driven by the movement of distant stars.
25:25This is one of the great discoveries of the last 20 years.
25:29Astronomers as a whole have increasingly begun to recognise
25:33that the solar system, not just the Earth, but the solar system,
25:36is open to its galactic environment.
25:39And that's a very important change of mindset
25:42than our ancestors, even our parents, probably had.
25:46Comets coming from the Oort Cloud
25:48are bringing with them a message from outer space, quite literally.
26:02Once pushed out from the Oort Cloud,
26:05these long-period comets start the two-light-year journey towards the Sun.
26:11They drift through space, silently and unseen,
26:15for tens, even hundreds of thousands of years.
26:22But as they pass Jupiter, just 400 million miles away,
26:26they light up and start to produce their tails.
26:31All of a sudden, we can see them.
26:34We can see them.
26:39And we can also reach them.
26:49A number of missions have been sent to investigate comets at close quarters.
26:56The probe Deep Impact was sent to rendezvous with the comet Tempel 1,
27:02testing its strength and density by crashing into it.
27:12The Stardust mission flew past the comet Wild 2,
27:18capturing grains of dust from its tail that it brought back to Earth.
27:24OK, we have confirmation of the main chute.
27:27Confirmation. Confirmation. OK.
27:30All stations, main chute is open. We're coming down slowly.
27:37These bits of dust have been unchanged since the formation of the Solar System
27:43and can tell us exactly what comets are made of.
27:47Perhaps surprisingly,
27:50the basic building blocks of comets aren't at all exotic.
27:54In fact, they're remarkably mundane.
27:57I'm going to show you a really neat demonstration
28:00of how to make a cometary nucleus using some relatively simple things.
28:05Many of these things you can actually find in the average house.
28:08To start with, I'll add some water.
28:14The second major constituent of a comet
28:17is the dark organic materials, the rubble and the crud and so on.
28:21And to replicate that, I've got some soil.
28:26We also need to think about other ingredients in there.
28:29There are quite complex chemicals, organic chemicals.
28:32We can replicate that by putting in the soy sauce.
28:35Now, of course, no-one's suggesting there's actually soy sauce in a comet,
28:38but again, it's a really nice analogue for it.
28:40So I'll give it a good glug there, rather more than you'd use in the average dish.
28:44We can add a little ammonia.
28:46And ammonia, in this case, is in these smelling salts.
28:49Just bash a few crystals of that in.
28:51What it really does, more than anything,
28:53is just make the comet a little smellier.
28:56There we go.
28:58Now, the other ingredient we're going to add into this comet
29:01is carbon dioxide, or what you more commonly know as dry ice.
29:05I think it's a fairly violent reaction
29:07because the dry ice is meeting water,
29:10which is about 80 or probably even 100 degrees warmer.
29:13You can see the carbon dioxide turning to gas.
29:16Let's give that a good squeeze and see how we're doing.
29:23It's really bubbling away violently now.
29:26And that looks good.
29:28This dirty snowball
29:30is a surprisingly accurate model of a comet nucleus.
29:34And as it warms up,
29:36it starts to behave exactly as a real comet does.
29:40It produces a tail.
29:42You can see a beautiful...
29:44One of the best I've seen, actually, in doing this,
29:46a beautiful jet coming out here.
29:48And that's exactly what happens when a comet does that.
29:51It produces a tail.
29:53One of the best I've seen, actually, in doing this,
29:55a beautiful jet coming out here.
29:57And that's exactly what happens
29:59when a cometary nucleus gets near the sun.
30:01Near the surface particularly, you see ice turning to gas.
30:04It doesn't turn to liquid
30:06because there's no pressure in space to allow it to exist as a liquid.
30:09And so the frozen carbon dioxide, the frozen water,
30:12the other chemicals in a comet
30:14become gas-like and they jet out.
30:18In the vacuum of space,
30:20the escaping gas blows dust from the surface
30:23and the debris builds up into a vast halo
30:26called the coma.
30:31A single comet nucleus can produce a coma
30:34over a million kilometres across.
30:37Bigger than the sun.
30:47It would expand forever
30:49if it wasn't for the pressure waves emitted from the sun
30:52that shape the coma into the comet's dramatic twin tails.
30:56One made of dust and one made of gas.
31:02Which is why, no matter what direction the comet is travelling,
31:06the tails always point away from the sun.
31:10The longest tails can be half a billion kilometres long
31:13and stretch halfway across the solar system.
31:21It's a grand display.
31:23One of the greatest shows in the universe.
31:26Though it is a sign that after four billion years,
31:29the comet is dying.
31:32The fact that we see them
31:34is a sign that they are dying.
31:37The fact that we see them now as active comets
31:41means that they have only a few thousand years left
31:45in their current orbits.
31:47So you can imagine,
31:49if you liken that to the lifetime of a human being,
31:53we're seeing the last few seconds of the lifetime of the comet.
31:58Every second, they are losing tonnes of dust and ice.
32:04Each time it goes round the sun,
32:06the comet nucleus shrinks.
32:10Most have only enough fuel
32:12for a thousand revolutions round the sun.
32:15Even a large comet like Halley
32:18will only last another 150,000 years.
32:22And then, as their fuel supplies are exhausted,
32:26they die.
32:29Some perish as they crash into the sun.
32:37And some disintegrate,
32:40eventually turning into dust.
32:43Dust we see as shooting stars
32:46as they enter the atmosphere.
32:51But not all of them disintegrate to dust.
32:56Some of them leave a more substantial skeleton
32:59drifting through space.
33:02We predict there should be hundreds
33:05or maybe even thousands of dead comets
33:08which still haven't yet been found.
33:11If the comet fades to dust and nothing else,
33:14then, in a way, they're not dangerous.
33:17But if the comet fades to dust,
33:20If the comet fades to dust and nothing else,
33:22then, in a way, they're not dangerous.
33:25But, on the other hand, if they leave behind big chunks
33:28hundreds of metres across, possibly,
33:30then those objects could come through the atmosphere
33:33with some devastating consequences on the ground.
33:38Is the Earth really in danger from comets?
33:42Although big, they are incredibly fragile.
33:45No more substantial than cigarette ash.
33:49Still, some think a comparatively small chunk of comet
33:53could cause a global catastrophe.
34:14Mike Bailey is a paleoecologist
34:17who's found the evidence of an ecological disaster
34:20in the fields of Northern Ireland.
34:22Twenty-odd years ago, the farmer,
34:25when this field was being drained,
34:28found that these oaks were effectively floating to the surface.
34:31The land was shrinking round them.
34:33So he got a digger in and pulled them out
34:35and pushed them into a heap.
34:37And in a flat area like this,
34:39it's a handy windbreak for sheltering animals.
34:41So the farmers leave them
34:43and it means that we have access to some ancient trees.
34:50Some of these oaks are thousands of years old,
34:53preserved perfectly in the oxygen-free conditions
34:56of the Irish peat bogs.
34:59By sampling tens of thousands of trees,
35:02Professor Bailey has built a year-by-year record
35:05of the Irish environment for the past 7,500 years.
35:10What he has found encoded in the annual rings
35:14is that the trees sometimes mysteriously stopped growing.
35:19This tree went from a period of extended, perfectly good,
35:23what we would regard as normal growth,
35:26when we get to this year, which is the year 540,
35:29there's a damaged scar
35:31and its growth was radically changed in character thereafter.
35:36When the same pattern emerged in many of the Irish trees,
35:40it was initially dismissed as the effects of a savage storm
35:44that struck Ireland in 540.
35:53What caused a complete turnaround in this story
35:56was when we found that this same dated event
36:00occurs from Mongolia, Siberia, northern Sweden,
36:04across northern Europe, North America and South America.
36:07This was the realisation that this was a global event.
36:12Here was evidence of a worldwide catastrophe in 540 AD.
36:17What could have caused such a widespread disaster?
36:20There was one obvious suspect.
36:30The biggest volcanic eruptions
36:32blast so much ash into the upper atmosphere
36:35that it spreads around the world, blocking out the sun's rays.
36:45The eruption of Mount Pinatubo in 1991
36:48caused global temperatures to fall by over half a degree
36:52for the next two years.
36:54It's the same pattern that is seen in the 540 event.
36:59But volcanic eruptions also leave another trace.
37:05A layer of sulfuric acid that is frozen into the Greenland ice sheets.
37:14But the samples taken from deep within the ice
37:17show no acid spike in 540.
37:20Whatever stopped the trees growing, it wasn't a volcano.
37:29If you have a global environmental event
37:31and it wasn't caused by a volcano,
37:33what's the next most likely explanation?
37:35And you're allowed to ask the question,
37:38could it have been some sort of loading from space?
37:41Could it be an extraterrestrial event?
37:43And as soon as you start even asking that question,
37:46you realise that the most likely sort of event you would be talking about
37:50would be some sort of brush with a comet.
37:54In theory, a lump of comet just 300 metres across
37:59could affect the global climate.
38:07As it hit the atmosphere at over 20,000 kilometres an hour,
38:11it would burst into flame
38:14and blow itself to bits while still kilometres above the ground,
38:19spreading a shroud of dust around the world
38:22that could block out the sun.
38:28It's an event they call an airburst.
38:33And there is much more recent evidence
38:36that suggests these events do happen.
38:44On June 30, 1908,
38:48a huge explosion tore through the forests of Tungushka, Siberia.
38:55No-one witnessed the explosion itself.
39:04But it threw up clouds of dust that reflected so much light
39:08they could be seen as far away as London.
39:18It was so bright, so the story goes,
39:21people were able to play cricket at midnight
39:24and people were able to read newsprint.
39:27That gives you an idea of how much dust must have been liberated
39:31very, very high into the atmosphere,
39:34which was illuminating the ground even as far away as London is
39:38from where the impact actually happened.
39:47It was 20 years before the Russians mounted an expedition to the site.
39:52What they found astounded them.
40:0060 million trees across an area the size of London had been levelled.
40:08Puzzled, the Russian scientists performed a set of experiments
40:12to try and figure out what could have caused such devastation.
40:17The only explanation was that the explosion was caused
40:20by an extraterrestrial object just 60 metres across,
40:24running into the upper atmosphere.
40:27This was just a small airburst,
40:29but it showed the amount of damage that could be done.
40:33All that energy of motion is converted into heat
40:37and it's just like the explosion of a nuclear weapon
40:40in the Earth's atmosphere.
40:48EXPLOSION
40:56One can only imagine the effects
40:58if an airburst exploded over a populated area.
41:02If this is the devastation comets can cause,
41:05perhaps our ancestors had good reason to be afraid of them.
41:10MUSIC
41:14The comet called Shoemaker-Levy...
41:16..about to smash into the planet Jupiter...
41:19..what could be the most dramatic astronomical event of the century.
41:23When the comet pieces strike Jupiter,
41:25it will be the most massive collision of celestial objects...
41:28..with the force of millions of nuclear weapons.
41:33If anyone was left in any doubt of the true destructive power of comets,
41:37an event in 1994 would change their minds.
41:41It happened 400 million miles away,
41:44but in Solar System terms, it was in our backyard.
41:52Comet Shoemaker-Levy 9 was discovered in 1993.
41:56It had been torn into 21 separate pieces
41:59by the gravity of the planet Jupiter,
42:01creating a line of debris known as the String of Pearls
42:05that was on a collision course with the giant planet.
42:14This was a unique astronomical event.
42:17No-one had witnessed two Solar System objects collide before.
42:23As the scientists planned their observations,
42:26less rational comet fever took hold once more.
42:30At the time of the comet impact,
42:32we may see the biofield around people change.
42:36Some people will feel excited,
42:38some may feel more depressed.
42:41We don't know.
42:43We have a piece of software that tells us
42:46what the comet's impact will mean for you as an individual.
42:50So is it going to happen to me on that date, the 16th?
42:53Maybe discard some old ways of beholding you back.
42:56July 16th, I should beware, right?
42:59Yeah.
43:03No-one knew what effect, if any, the comet fragments,
43:07just a few kilometres across, would have on such a huge planet.
43:12As the comets approached,
43:14the world's astronomers watched the pictures coming in
43:18and held their breath.
43:20Isn't that incredible?
43:22It's right in the middle. That's amazing.
43:24Look!
43:26Yeah, that's it.
43:28Look at that.
43:31Look!
43:35The scale of the explosions surprised everyone.
43:39OK, we just blew up a section of the planet.
43:42This is the southern pole here.
43:44You see there's a little, there's a bright streak.
43:47See that bright streak?
43:49And around the edge of the streak, there's some other stuff.
43:52It was not there the day before.
43:55It's a new feature on Jupiter.
44:01As the comet struck the Jovian atmosphere,
44:04fireballs 3,000 kilometres high erupted from the surface of the planet.
44:10The biggest impact released 600 times more energy
44:14than the entire world's nuclear arsenal.
44:18Scars bigger than the Earth were left on the surface of Jupiter.
44:24These were some of the biggest explosions that had ever been witnessed.
44:30How about it?
44:33Perhaps this was not the right time to celebrate.
44:36The Jupiter collision showed the enormous power
44:39that could be released by a full-blown impact with a comet.
44:43Just imagine if it had hit the Earth.
45:30It's not just fantasy.
45:33Impacts this size have happened to the Earth.
45:36The last time was 65 million years ago.
45:45It spelled the end of the dinosaurs
45:48and wiped out half of the rest of the species on the planet.
45:54Awakened to the danger,
45:57we now have projects to scan the skies,
46:00looking for objects that are a threat to the Earth.
46:07But most of the time,
46:09it's not just a matter of finding objects that are a threat to the Earth.
46:14It's also a matter of finding objects that are a threat to the Earth.
46:20But most of the sky surveys concentrate on finding asteroids.
46:25Unlike comets, which originate in deep space,
46:28asteroids mostly come from the belt between Mars and Jupiter,
46:32from where they are periodically deflected
46:35into an orbit that crosses the Earth's.
46:43But these rogue asteroids are easy to spot
46:46and their trajectories can be plotted hundreds of years into the future.
46:55Giving scientists plenty of time to devise ways to divert them
46:59so they miss us.
47:02Unfortunately, avoiding an incoming comet
47:05would be a very different proposition.
47:11Comet impacts are likely to be a threat to the Earth
47:14Comet impacts are likely to be more rare than asteroid impacts,
47:18but they're going to be much harder to mitigate against
47:22because a comet will be coming in from deep space,
47:25will have a few years at best, a few months at worst,
47:28knowledge before impact,
47:30and that may not be time enough to launch a spacecraft to deflect the orbit.
47:34People might remember a few years ago we had a fantastic comet
47:38called Comet Hale-Bopp.
47:40Suddenly it came out of nowhere.
47:42It's about 3,000 years since it had last passed by
47:46and that was a very bright comet
47:48and suddenly it was there in our midst.
47:51We've had recently a comet called Comet McNaught,
47:54which was a comet you could see with the naked eye.
47:57Nobody had predicted the appearance of these comets.
48:01You might get two, three, maybe six months' notice.
48:06So it's possible that there could be a catastrophic collision
48:11with the Earth any time.
48:16It's a scary prospect,
48:18but perhaps not one to keep you awake at night.
48:22It's estimated that the Earth will endure a Tunguska-sized impact
48:26that could destroy a city once every 1,000 years.
48:35And an extinction-level event,
48:37like the one that wiped out the dinosaurs every 100 million years.
48:50And rather than being afraid of comet impacts,
48:53perhaps we should be grateful for them.
48:56They might be responsible for us being here at all.
49:00MUSIC PLAYS
49:06Way back at the beginning of the solar system,
49:09the planets were formed from the disk of debris
49:12that the early sun had thrown out around it.
49:19As the disk was stirred,
49:21bits of dust stuck together and became boulders.
49:26Boulders stuck together.
49:29And made asteroids.
49:33Which stuck into larger clumps that became the planets.
49:46It was a hot, violent process.
49:49And when it was finished,
49:51it left the Earth a barren and sterile place.
50:00Yet within a few hundred million years,
50:03the Earth was covered in oceans and life was thriving.
50:16Where can it all have come from?
50:19There is only one obvious source of water
50:22and organic molecules in the solar system.
50:25Comets.
50:27But to explain how the Earth was transformed
50:30from a sterile boulder into the blue planet,
50:33astronomers have had to propose a massive hailstorm of comets,
50:373.8 billion years ago.
50:39They call it the Late Heavy Bombardment.
50:50The solar system wasn't as we see it today.
50:53There was still a lot of the stuff left over,
50:56the gas bubble from the building of the planets.
50:59And Jupiter and all the other gas giants out there
51:02were doing a very good job at throwing stuff inwards towards the sun.
51:07Some of which, unfortunately, we got in the way of,
51:10or rather I should say fortunately for us
51:12if they really did bring the water that we see today.
51:16We're talking about a bombardment of the, if you like, the proto-Earth.
51:21That takes millions or tens of millions of years
51:25with an impact rate that may have been
51:281,000 or 10,000 times what it is today.
51:38The bombardment hasn't finished. It's still going on today.
51:41We're not quite so aware of it
51:44because that bombardment is by tiny, tiny dust particles
51:48that float down through the atmosphere every day.
51:51About 60,000 tonnes over the whole of the Earth every year
51:55comes down as dust particles.
51:57Water is bound into those mineral grains.
52:00So we're still being bombarded
52:03by grains that carry water and carbon, even today.
52:15It's an amazing thought that all the water
52:18and all the organic molecules that make up every living thing on the planet
52:22may have started out on a comet beyond the edge of the solar system.
52:37But there are people who have taken the idea even further.
52:41They believe that comets brought actual living creatures to the Earth.
52:48Earth life is essentially alien life.
52:51It is not a life that was indigenous to the Earth by any means.
52:55And if we evolve from that life,
52:58then I think we are the products of evolution from alien life.
53:10In the 1960s, astronomers Fred Hoyle and Chandra Vikramasinghe
53:15proposed that the Earth was colonised by bacteria from outer space.
53:20It was an idea called panspermia, and it's never quite gone away.
53:25I think it's true to say, isn't it, that if your theory is correct,
53:29life not only may be spread throughout the universe, it must be.
53:33It would really be very surprising if life is not...
53:36if the galaxy is not just teeming with life.
53:40Since then, there have been a number of strange discoveries
53:44that may, or may not, back up the theory.
53:48In 2001, the southern Indian state of Kerala
53:52was shocked by a loud explosion
53:55followed by storms of blood-red rain over the next two months.
54:00Local scientist Dr Godfrey Lewis
54:03collected samples from all over the state and analysed them.
54:12What he found was that the rain was being turned red
54:16by cells the like of which he had never seen before.
54:21It's quite exciting to see this.
54:25They look like red blood cells, but they are not,
54:28because this is having a very thick cell wall,
54:31and that cell wall is not there in the blood cells.
54:36What made the cells even more mysterious
54:39was that he could find no trace of DNA in them.
54:43If that was the case, the cells were like nothing else on Earth.
54:51The cells were in the blood.
54:53The blood was in the blood.
54:55The blood was in the blood.
54:57The blood was in the blood.
54:59The blood was in the blood.
55:01The blood was in the blood.
55:04The staggering claim is that this is possibly extraterrestrial.
55:09That is a big claim, I know.
55:11But all the experiments are supporting this claim.
55:20This is some of the samples we have collected.
55:24A very good sample we have.
55:27This is the real stuff?
55:30It's the real stuff, man. It's the real stuff.
55:32It's red for sure.
55:34For Chandra Bikramasinghe,
55:36this was vital evidence to back up his theory.
55:39A fragment of comet must have blown up in the upper atmosphere
55:43and rained extraterrestrial bacteria down on the Earth.
55:49If it is true that these are alien bugs from space,
55:52then it is an absolute clear-cut proof of panspermia,
55:56ongoing panspermia,
55:58and that would be absolutely fascinating,
56:00and that would be over the moon.
56:05But unfortunately for Bikramasinghe,
56:08there was bad news.
56:10On closer inspection,
56:12the cells in the red rain didn't seem so alien after all.
56:17This is DNA from the red rain.
56:19Wow, there's no doubt about it.
56:21There's no doubt about it.
56:23It's absolutely conclusive as far as I can see.
56:25The material when examined by astrophysicists
56:30and people who are looking for evidence to support a view
56:35doesn't appear to be anything that they've ever seen before.
56:38But people who actually have seen things before say,
56:41looks like red algae to me.
56:43I would tend to go with the people who have seen more things
56:47in the biological kingdom
56:48rather than those who are looking to support their own ideas
56:52about how the world should work without the data to back it up.
56:55The journey towards the truth is always a rewarding one, I think,
56:59and I saw this as a journey towards discovering a truth
57:03that was quite plain to me
57:05and is becoming plainer and plainer to other people now
57:09after 30-odd years.
57:14Whatever their actual role,
57:16it seems comets were vital in the evolution of life on Earth.
57:23And they maintain the ability to destroy it.
57:34It seems our ancestors were right all along.
57:37Comets do carry the power of the gods.
57:41Maybe that is why they still cause so much excitement
57:45when they appear in the sky.
57:52What's more, when astronomers turn their telescopes to other stars,
57:56they see clouds of dust,
57:59the telltale signs of comet activity.
58:07Just as comets helped bring life to our solar system,
58:10they may be doing the same elsewhere.
58:17Perhaps that is the next message that comets will bring us.
58:22That we are not alone.
58:52NASA Jet Propulsion Laboratory, California Institute of Technology