0:00:05 > 0:00:07On the edge of space,
0:00:07 > 0:00:12halfway to the nearest star, there is a vast cloud of debris,
0:00:12 > 0:00:15lumps of rock and ice
0:00:15 > 0:00:19that have drifted on the edge of our solar system for four billion years.
0:00:19 > 0:00:23They are among the most mysterious objects in the universe.
0:00:26 > 0:00:30Sometimes they are sent into the heart of our solar system,
0:00:30 > 0:00:35where they are transformed into the blazing stars we call comets.
0:00:41 > 0:00:46For centuries, our ancestors were in awe of comets.
0:00:48 > 0:00:51They were messengers from the gods...
0:00:53 > 0:00:57..carrying the power of life and death.
0:01:06 > 0:01:12It's only now, as we have the power to uncover the comet's secrets,
0:01:12 > 0:01:16that we are discovering that those ancestors were right all along.
0:01:20 > 0:01:21The comet's tale is a story
0:01:21 > 0:01:26that really CAN tell us about life, the universe and everything.
0:01:57 > 0:02:03About once every ten years, a really bright comet lights up the skies.
0:02:05 > 0:02:08Some are bright enough to be seen in broad daylight.
0:02:11 > 0:02:15Others unfurl their tails across half the sky.
0:02:19 > 0:02:23They appear from nowhere, and just as suddenly disappear.
0:02:25 > 0:02:28It's little wonder that throughout history
0:02:28 > 0:02:31people have tried to explain their significance
0:02:31 > 0:02:33and the effects they have on the Earth.
0:02:38 > 0:02:41As far back as the second century BC,
0:02:41 > 0:02:44the Chinese had taken the trouble to classify comets
0:02:44 > 0:02:47into 29 separate types.
0:02:50 > 0:02:53The shapes they saw were so striking
0:02:53 > 0:02:55they have penetrated human consciousness.
0:03:02 > 0:03:06Many other societies explained the sudden appearance of comets
0:03:06 > 0:03:08through their mythology.
0:03:08 > 0:03:12Comets marked the presence of gods in the skies.
0:03:14 > 0:03:19Comets are recorded in myths globally.
0:03:19 > 0:03:24Almost every population in the world talks about dragons...
0:03:26 > 0:03:30..things that fall from the sky, stones and iron falling from the heavens...
0:03:31 > 0:03:34..giants with a single eye in their head...
0:03:36 > 0:03:39..wizards and things like that, that can be interpreted as comets.
0:03:43 > 0:03:45If you look hard enough,
0:03:45 > 0:03:49comets can even be found in the most famous folk stories.
0:03:55 > 0:04:00When you peel away all of the layers from the King Arthur story,
0:04:00 > 0:04:03what you're left with is a local lord
0:04:03 > 0:04:06who was left in Britain after the Romans had retreated.
0:04:06 > 0:04:09And that doesn't explain why or how
0:04:09 > 0:04:14he became such a huge figure in English folklore.
0:04:14 > 0:04:18There's another possibility, and this is that Arthur may have been a comet.
0:04:23 > 0:04:26When Arthur has a battle, he takes his sword out,
0:04:26 > 0:04:30and that sword shines with the light of 30 torches.
0:04:33 > 0:04:35And, generally, Arthur's battles occur
0:04:35 > 0:04:39at times of known meteor showers.
0:04:39 > 0:04:43And in those battles, there's widespread destruction.
0:04:43 > 0:04:48In one of the battles, 11 countries get destroyed,
0:04:48 > 0:04:51and that's characteristic of what you might expect from a comet,
0:04:51 > 0:04:54and it's difficult to explain in human terms.
0:05:08 > 0:05:11Was King Arthur really a comet?
0:05:12 > 0:05:15It's an idea that requires quite a stretch of the imagination.
0:05:17 > 0:05:21But comets were often depicted as fiery swords.
0:05:21 > 0:05:23Perhaps the myths were created to help people
0:05:23 > 0:05:28interpret events in the sky that they couldn't otherwise explain.
0:05:30 > 0:05:33Whether you believe all the interpretations or not,
0:05:33 > 0:05:38there is no doubt that comets were the subject of deep superstition.
0:05:38 > 0:05:40The reason was simple.
0:05:44 > 0:05:47In times when the movement of the stars and planets
0:05:47 > 0:05:49was used to predict the future,
0:05:49 > 0:05:53the sudden appearance of something unusual was a dangerous omen.
0:05:53 > 0:05:55In the medieval world,
0:05:55 > 0:05:59philosophers and astronomers thought that everything further away from us
0:05:59 > 0:06:01than the moon is was perfect.
0:06:01 > 0:06:05Out there, beyond the moon, nothing ever changed.
0:06:05 > 0:06:07And comets just don't look like that.
0:06:07 > 0:06:12Comets look like fiery signs moving across our sky.
0:06:14 > 0:06:18They were threats to the idea
0:06:18 > 0:06:22of a perfect, calm, orderly universe.
0:06:22 > 0:06:24Perhaps they were guided by God,
0:06:24 > 0:06:30in order to send signs to humanity of God's purposes.
0:06:30 > 0:06:33When comets passed,
0:06:33 > 0:06:37astrologers almost automatically predicted great transformation,
0:06:37 > 0:06:41the death of kings, war, invasion, plague or famine.
0:06:44 > 0:06:48Not everyone viewed them with foreboding.
0:06:48 > 0:06:51Genghis Khan saw comets as a personal message
0:06:51 > 0:06:56telling him to fulfil his destiny and wage war across Asia and Europe.
0:07:08 > 0:07:13To William the Conqueror, the appearance of a comet in 1066
0:07:13 > 0:07:19was such an auspicious sign, it was immortalised on the Bayeux Tapestry.
0:07:19 > 0:07:24But for the Saxons, the comet was definitely a harbinger of doom.
0:07:35 > 0:07:39For centuries, all explanations of comets
0:07:39 > 0:07:42relied on superstition and astrology.
0:07:45 > 0:07:48It finally took the greatest mind of them all
0:07:48 > 0:07:50to make scientific sense of them.
0:07:54 > 0:07:57In 1680, a particularly bright comet
0:07:57 > 0:08:01caught the attention of Isaac Newton.
0:08:01 > 0:08:06It was a fascination that was to change the way we understand the universe.
0:08:09 > 0:08:15"On November 19th, at half past four in the morning in Cambridge,
0:08:15 > 0:08:19"the comet was seen by some young man.
0:08:19 > 0:08:23"And on the same day, at five in the morning,
0:08:23 > 0:08:26"at Boston in New England,
0:08:26 > 0:08:30"the comet was also observed."
0:08:30 > 0:08:32What was extraordinary about this comet was that
0:08:32 > 0:08:36it was visible in the sky, it was very bright,
0:08:36 > 0:08:39you could see it and October and November of 1680,
0:08:39 > 0:08:42and then it disappeared behind the sun.
0:08:47 > 0:08:50And then another comet appeared from behind the sun,
0:08:50 > 0:08:52in December, at the end of the year,
0:08:52 > 0:08:56and was visible right through to March of 1681.
0:08:56 > 0:09:00So the big question is, is this one comet or two?
0:09:00 > 0:09:06If it's one comet, then it's bent a lot near the sun.
0:09:06 > 0:09:10Here's a diagram that Newton himself made
0:09:10 > 0:09:14of the path of the comet, if it's one comet.
0:09:14 > 0:09:19You see here that it was approaching the orbit of the Earth and the sun
0:09:19 > 0:09:23in October and November of 1680,
0:09:23 > 0:09:28and then it disappears behind the sun, and reappeared in December
0:09:28 > 0:09:33and it stayed visible through February and March of 1681.
0:09:35 > 0:09:39For the sun to bend the path of a comet this much,
0:09:39 > 0:09:44Newton realised that there had to be an unseen force at work.
0:09:44 > 0:09:46He called it gravity...
0:09:50 > 0:09:54..an idea which didn't come from an apple falling from a tree,
0:09:54 > 0:09:58but from a comet passing behind the sun.
0:10:02 > 0:10:05It was by thinking about this puzzle,
0:10:05 > 0:10:09that Newton began to formulate the idea of gravitation.
0:10:09 > 0:10:16Before 1681, Newton had no notion of universal gravitation.
0:10:16 > 0:10:23Newton, between 1681 and 1684, began to suppose that comets come back,
0:10:23 > 0:10:27that they're therefore like planets, because they move in closed orbits,
0:10:27 > 0:10:30that you should be able to calculate the shape of the orbit,
0:10:30 > 0:10:34that you could, in fact, predict when comets come back.
0:10:35 > 0:10:41And that is exactly what he and his friend Edmund Halley set out to do.
0:10:41 > 0:10:44If they could use the theory of gravitation
0:10:44 > 0:10:48to predict the return of a comet,
0:10:48 > 0:10:51they would not only prove Newton's theory,
0:10:51 > 0:10:54but also show that comets were not omens or supernatural signs,
0:10:54 > 0:10:59but predictable, orbiting bodies like the planets.
0:11:05 > 0:11:09Halley collated tables of all the comet observations he could find,
0:11:09 > 0:11:11looking for similarities in their behaviour -
0:11:11 > 0:11:14what direction they came from,
0:11:14 > 0:11:18how close they got to the sun.
0:11:18 > 0:11:21At last he found three comets
0:11:21 > 0:11:24whose descriptions were almost identical,
0:11:24 > 0:11:28one recorded in 1531, one in 1607,
0:11:28 > 0:11:33and one he had observed himself in 1682.
0:11:33 > 0:11:37Each appearance was separated by 75 years.
0:11:40 > 0:11:43Convinced that these three sightings were of the same comet,
0:11:43 > 0:11:47Halley made a public prediction that it would return again
0:11:47 > 0:11:51in late 1758 or early 1759.
0:11:51 > 0:11:55It was an extraordinary piece of scientific bravado for the time.
0:12:07 > 0:12:11Neither Newton or Halley lived to see the prediction fulfilled.
0:12:11 > 0:12:14But when, on Boxing Day 1758,
0:12:14 > 0:12:18a comet was spotted in Germany, it was greeted with jubilation
0:12:18 > 0:12:22and as a triumph of science over the supernatural.
0:12:36 > 0:12:39Tracing back through the historical records,
0:12:39 > 0:12:42Comet Halley could be found reappearing like clockwork.
0:12:42 > 0:12:48This Babylonian tablet records its appearance in 164 BC.
0:12:49 > 0:12:52The comets that Genghis Khan and William the Conqueror saw
0:12:52 > 0:12:57weren't messengers from God, but the regular appearances of Comet Halley.
0:12:59 > 0:13:03'A once-in-a-lifetime meeting tonight, live on BBC1.
0:13:03 > 0:13:08'After its latest visit to Earth, Halley's Comet is already on course
0:13:08 > 0:13:12'back towards the icy wastes of outer space. In an attempt...'
0:13:12 > 0:13:17The last time it visited in 1986, it was greeted like an old friend.
0:13:17 > 0:13:21Around the world, people were transfixed, as the probe Giotto
0:13:21 > 0:13:25beamed back the first live pictures of a comet nucleus.
0:13:28 > 0:13:33It was the last we'll see of Comet Halley until 2061.
0:13:38 > 0:13:41Perhaps the most surprising thing about comets
0:13:41 > 0:13:44is that, 300 years after Newton and Halley,
0:13:44 > 0:13:47despite all that we now know about them,
0:13:47 > 0:13:49comets still have the power
0:13:49 > 0:13:53to capture and control people's imaginations.
0:13:56 > 0:14:00In 1997, a bright comet called Hale-Bopp
0:14:00 > 0:14:02suddenly appeared in the skies.
0:14:02 > 0:14:06It captivated sky-watchers around the world.
0:14:06 > 0:14:09It was particularly keenly watched
0:14:09 > 0:14:14by a cult in California known as Heaven's Gate.
0:14:14 > 0:14:17They believed that the comet was concealing a spaceship
0:14:17 > 0:14:20that had come to take them to another, better world.
0:14:35 > 0:14:39At this point, this is considered a mass suicide investigation.
0:14:44 > 0:14:47So deep was their conviction,
0:14:47 > 0:14:49that, as the comet came closest to the Earth,
0:14:49 > 0:14:54all 39 members poisoned themselves,
0:14:54 > 0:14:56believing that in taking their own lives,
0:14:56 > 0:15:00they would free their souls to board the spaceship.
0:15:01 > 0:15:04It seems no matter how far science progresses,
0:15:04 > 0:15:08comets can still hold the power of life and death.
0:15:16 > 0:15:20What is true, as we are now discovering scientifically,
0:15:20 > 0:15:24is that comets have had a profound effect on life on Earth.
0:15:27 > 0:15:30One of the reasons they retain so much power over us
0:15:30 > 0:15:34is that they seem to appear out of nowhere.
0:15:39 > 0:15:41On the island of La Palma in the Canaries,
0:15:41 > 0:15:46Alan Fitzsimmons is trying to unlock the comet's secrets.
0:15:52 > 0:15:57Here we have the William Herschel Telescope,
0:15:57 > 0:16:004.2 metres of astronomical loveliness,
0:16:00 > 0:16:04all ready and waiting to spot a few of these comets.
0:16:09 > 0:16:14It may seem odd to use such a powerful telescope to observe comets,
0:16:14 > 0:16:17the brightest objects in the night sky,
0:16:17 > 0:16:20but the scientists want to study the comet's nucleus.
0:16:20 > 0:16:25To do that they must see them before they start producing their tails,
0:16:25 > 0:16:27when they are further away than Jupiter.
0:16:32 > 0:16:36The nucleus is tiny, just a few kilometres across,
0:16:36 > 0:16:41and in the vastness of space, they are incredibly difficult to spot.
0:16:43 > 0:16:49They only reflect, on average, 4% of the light that hits them,
0:16:49 > 0:16:54and a piece of coal reflects 8% of the light that hits it, so...
0:16:54 > 0:16:59a comet nucleus is twice as dark as a lump of coal.
0:16:59 > 0:17:03So we're trying to see these things from the feeble amount of sunlight
0:17:03 > 0:17:09they reflect out beyond Jupiter. It's not an easy thing to try and attempt to do.
0:17:24 > 0:17:26Oh, my God!
0:17:27 > 0:17:32That's it. That's it! HE LAUGHS
0:17:32 > 0:17:35- Oh, fantastic stuff!- Another one.
0:17:35 > 0:17:38Yep, yep, another one in the bag! Happy days.
0:17:44 > 0:17:46I do love this job, you know.
0:17:50 > 0:17:55This tiny moving blob of light is the comet's nucleus,
0:17:55 > 0:17:59a delicate lump of rock and ice floating slowly through space.
0:18:04 > 0:18:06The most powerful telescopes pick them up
0:18:06 > 0:18:08as they drift through the outer planets.
0:18:09 > 0:18:13But their orbits suggest they come from much further into space,
0:18:13 > 0:18:16far beyond the realm of the planets.
0:18:16 > 0:18:20Finding exactly where they come from hasn't been easy.
0:18:22 > 0:18:26Since the 1940s, astronomers have been looking
0:18:26 > 0:18:29for a huge reservoir of comets called the Kuiper Belt,
0:18:29 > 0:18:33which they believed lay beyond Neptune.
0:18:33 > 0:18:37The only problem was, no-one could find it.
0:18:37 > 0:18:39No matter how hard they looked,
0:18:39 > 0:18:42the only object anyone could see out there
0:18:42 > 0:18:44was the ex-planet Pluto.
0:18:50 > 0:18:53By the end of the 1980s,
0:18:53 > 0:18:56almost everyone had given up looking,
0:18:56 > 0:19:00apart from two maverick astronomers on Hawaii.
0:19:00 > 0:19:03I don't intentionally do things that are different,
0:19:03 > 0:19:06but I do...
0:19:06 > 0:19:10try to be sceptical of everything,
0:19:10 > 0:19:13because I think that's essential for science.
0:19:13 > 0:19:16So if you just buy into the prevailing wisdom,
0:19:16 > 0:19:19then all you can ever do is confirm what's already known.
0:19:24 > 0:19:28In 1987, Dave Jewitt and Jane Luu
0:19:28 > 0:19:31set up at the Mauna Kea observatory in Hawaii
0:19:31 > 0:19:34to stare into empty space...
0:19:34 > 0:19:39using up precious telescope time, finding nothing at all.
0:19:46 > 0:19:49At first, I couldn't get money to support any of this,
0:19:49 > 0:19:52because it seemed, I guess, so speculative or so crazy,
0:19:52 > 0:19:56and so instead I used money from other sources, probably illegally.
0:20:02 > 0:20:05Everyone, even their colleagues, thought they were mad.
0:20:05 > 0:20:08They started asking, "When are you going to stop?
0:20:08 > 0:20:11"You've been doing it for years and still haven't found anything.
0:20:11 > 0:20:14"Maybe there isn't anything out there."
0:20:20 > 0:20:25Eventually, their bloody-minded persistence paid off.
0:20:25 > 0:20:29After five years and over 400 hours observing,
0:20:29 > 0:20:32there was a small, moving anomaly on one of their pictures.
0:20:32 > 0:20:34He said, "Jane, come take a look at this,"
0:20:34 > 0:20:38and we saw something and it was moving at about the right speed
0:20:38 > 0:20:43we would expect a distant solar-system object would move.
0:20:43 > 0:20:45We were pretty excited about that,
0:20:45 > 0:20:48because we hadn't seen any other candidate object so good.
0:20:48 > 0:20:55It was just the first positive sign that we'd got after five years.
0:21:00 > 0:21:06This was it. At last they'd found the first Kuiper Belt object.
0:21:06 > 0:21:11They named it, catchily, "1992 QB1".
0:21:21 > 0:21:26Since then, over 1,000 Kuiper Belt objects have been found,
0:21:26 > 0:21:30quietly orbiting in the dark reaches beyond Neptune.
0:21:32 > 0:21:36But there are thought to be up to six billion potential comets here,
0:21:36 > 0:21:40orbiting in a belt three billion kilometres wide.
0:21:42 > 0:21:44Occasionally they collide...
0:21:50 > 0:21:54..and one is sent tumbling into the inner solar system,
0:21:54 > 0:21:58to become what we call a "short-period comet".
0:21:58 > 0:22:01Short-period comets orbit the sun frequently,
0:22:01 > 0:22:04and their orbital paths bring them back around the sun
0:22:04 > 0:22:07perhaps once every 10 or 20 years or so.
0:22:07 > 0:22:09And these are the objects that we believe
0:22:09 > 0:22:11have come from this trans-Neptunian region,
0:22:11 > 0:22:14just beyond the realm of the giant planets.
0:22:17 > 0:22:21Comet Halley is the most famous of the short-period comets.
0:22:21 > 0:22:24But there is another sort of comet,
0:22:24 > 0:22:29one whose orbit can be thousands, or even millions of years long.
0:22:29 > 0:22:32They come from far beyond the Kuiper Belt,
0:22:32 > 0:22:36from an almost mythical place called the Oort Cloud.
0:22:36 > 0:22:41The long-period comets, as the name suggests, take a really long time
0:22:41 > 0:22:44to go round the sun, perhaps anywhere between 200 years
0:22:44 > 0:22:46and a million years to go around once.
0:22:46 > 0:22:50And these are the objects that tell us that the Oort Cloud exists,
0:22:50 > 0:22:55stretching out tremendous distances towards the nearest stars.
0:22:57 > 0:23:00This is the very edge of the solar system,
0:23:00 > 0:23:02halfway to the nearest star,
0:23:02 > 0:23:0650,000 times further from the sun than the Earth.
0:23:06 > 0:23:12Here, trillions of comets cling tenuously to the sun's gravity.
0:23:13 > 0:23:16We now think that the Oort Cloud was formed
0:23:16 > 0:23:18during the violent formation of the solar system.
0:23:22 > 0:23:26As the planets formed and their gravity increased,
0:23:26 > 0:23:30they catapulted chunks of debris into space.
0:23:30 > 0:23:32Some of it collected in the Oort Cloud,
0:23:32 > 0:23:35where it has been ever since.
0:23:38 > 0:23:42They've been out there in deep freeze, in deep space,
0:23:42 > 0:23:44and so, by studying comets,
0:23:44 > 0:23:50we're studying the formation and the evolution of our solar system.
0:23:51 > 0:23:55But studying the comets isn't easy.
0:23:55 > 0:23:57The Oort Cloud is so far away,
0:23:57 > 0:24:02and the objects in it so tiny, that we'll never be able to see it.
0:24:03 > 0:24:06But luckily for the astronomers,
0:24:06 > 0:24:11sometimes something happens that sends them tumbling towards us.
0:24:11 > 0:24:15All cloud objects become comets,
0:24:15 > 0:24:18not through the action of anything to do with our sun,
0:24:18 > 0:24:20but through the action of other stars, generally.
0:24:23 > 0:24:28Our sun is just one of 100,000 million stars
0:24:28 > 0:24:30that form our Milky Way galaxy,
0:24:30 > 0:24:34the Milky Way that you can see on a clear, dark, moonless night.
0:24:34 > 0:24:38And they're all moving in slightly different directions
0:24:38 > 0:24:41with slightly different velocities, and occasionally,
0:24:41 > 0:24:44a star will come close enough to our Oort Cloud
0:24:44 > 0:24:47that it will perturb some of those comets out there.
0:24:53 > 0:24:57Some of them will be thrown into interstellar space, never to return.
0:25:01 > 0:25:05But some fraction of them will be nudged inwards into our solar system.
0:25:07 > 0:25:11Until recently, it was assumed that our solar system
0:25:11 > 0:25:13was isolated from the rest of space
0:25:13 > 0:25:16by the vast distances surrounding it.
0:25:16 > 0:25:19It's because we see comets that we now know
0:25:19 > 0:25:21what happens around the Earth
0:25:21 > 0:25:25is driven by the movement of distant stars.
0:25:26 > 0:25:30This is one of the great discoveries of the last 20 years.
0:25:30 > 0:25:32Astronomers as a whole
0:25:32 > 0:25:37have increasingly begun to recognise that the solar system,
0:25:37 > 0:25:40not just the Earth, is open to its galactic environment.
0:25:40 > 0:25:44That's a very important change of mindset
0:25:44 > 0:25:48than our ancestors, even our parents probably had.
0:25:48 > 0:25:51Comets coming from the Oort Cloud are bringing with them
0:25:51 > 0:25:55a message from outer space, quite literally.
0:26:04 > 0:26:07Once pushed out from the Oort Cloud,
0:26:07 > 0:26:11these long-period comets start the two-light-year journey
0:26:11 > 0:26:14towards the sun.
0:26:14 > 0:26:19They drift through space, silently and unseen,
0:26:19 > 0:26:22for tens, even hundreds of thousands of years.
0:26:25 > 0:26:30But as they pass Jupiter, just 400 million miles away,
0:26:30 > 0:26:34they light up, and start to produce their tails.
0:26:34 > 0:26:37All of a sudden, we can see them.
0:26:41 > 0:26:44And we can also reach them.
0:26:51 > 0:26:53A number of missions have been sent
0:26:53 > 0:26:55to investigate comets at close quarters.
0:26:57 > 0:27:01The probe Deep Impact was sent
0:27:01 > 0:27:03to rendezvous with the comet Tempel 1,
0:27:03 > 0:27:08testing its strength and density by crashing into it.
0:27:14 > 0:27:18The Stardust mission flew past the Comet Wild 2...
0:27:20 > 0:27:23capturing grains of dust from its tail
0:27:23 > 0:27:25that it brought back to Earth.
0:27:27 > 0:27:29CONFIRMATION ON RADIO
0:27:29 > 0:27:31APPLAUSE
0:27:33 > 0:27:38'All stations, the main chute is open, we're coming down slowly.'
0:27:40 > 0:27:43These bits of dust have been unchanged
0:27:43 > 0:27:46since the formation of the solar system,
0:27:46 > 0:27:50and can tell us exactly what comets are made of.
0:27:50 > 0:27:54Perhaps surprisingly, the basic building blocks of comets
0:27:54 > 0:27:59aren't at all exotic. In fact, they're remarkably mundane.
0:27:59 > 0:28:02I'm going to show you a really neat demonstration
0:28:02 > 0:28:05of how to make a cometary nucleus
0:28:05 > 0:28:10using some relatively simple things you can find in the average house.
0:28:10 > 0:28:13To start with, I'll add some water.
0:28:16 > 0:28:21The second major constituent of a comet is the dark organic materials,
0:28:21 > 0:28:26the rubble and the crud and so on. To replicate that, I've got some soil.
0:28:26 > 0:28:30We also need to think about other ingredients that are in there.
0:28:30 > 0:28:33There are quite complex chemicals, organic chemicals.
0:28:33 > 0:28:36We can replicate that by putting in this soy sauce.
0:28:36 > 0:28:39No-one's suggesting there's actually soy sauce in a comet,
0:28:39 > 0:28:42but again, it's really nice analogue for it.
0:28:42 > 0:28:45So I'll give it a good glug, more than you'd use in an average dish.
0:28:47 > 0:28:50We can add a little ammonia, in this case, in these smelling salts.
0:28:51 > 0:28:54Just bash a few crystals in. What it really does, more than anything,
0:28:54 > 0:28:57is just make the comet a little smellier.
0:28:57 > 0:28:59There we go.
0:28:59 > 0:29:02Now, the other ingredient we're going add into this comet
0:29:02 > 0:29:06is carbon dioxide, or what you more commonly know as dry ice.
0:29:11 > 0:29:14You get a fairly violent reaction,
0:29:14 > 0:29:17because the dry ice is meeting water,
0:29:17 > 0:29:20which is about 80 or probably 100 degrees warmer.
0:29:20 > 0:29:23You can see the carbon dioxide turning to gas.
0:29:23 > 0:29:28Just give that a good squeeze and see how we're doing.
0:29:29 > 0:29:32That's really bubbling away violently now.
0:29:33 > 0:29:35And that looks good.
0:29:39 > 0:29:41This dirty snowball
0:29:41 > 0:29:44is a surprisingly accurate model of a comet nucleus.
0:29:44 > 0:29:51And as it warms up, it starts to behave exactly as real comet does.
0:29:51 > 0:29:53It produces a tail.
0:29:53 > 0:29:57You can see a beautiful - one of the best I've seen doing this -
0:29:57 > 0:30:00a beautiful jet coming out here, and that's exactly what happens
0:30:00 > 0:30:03when a cometary nucleus gets near the sun.
0:30:03 > 0:30:07Near the surface particularly, you see ice turning to gas,
0:30:07 > 0:30:11because there's no pressure in space to allow it to exist as a liquid.
0:30:11 > 0:30:14And so the frozen carbon dioxide, the frozen water,
0:30:14 > 0:30:18the other chemicals in a comet become gas-like and they jet out.
0:30:23 > 0:30:28In the vacuum of space, the escaping gas blows dust from the surface,
0:30:28 > 0:30:34and the debris builds up into a vast halo, called a coma.
0:30:36 > 0:30:39A single comet nucleus can produce a coma
0:30:39 > 0:30:44over a million kilometres across, bigger than the sun.
0:30:49 > 0:30:51It would expand for ever,
0:30:51 > 0:30:54if it wasn't for the pressure waves emitted from the sun
0:30:54 > 0:30:59that shape the coma into the comet's dramatic twin tails,
0:30:59 > 0:31:02one made of dust, and one made of gas.
0:31:04 > 0:31:08Which is why, no matter what direction the comet is travelling,
0:31:08 > 0:31:12the tails always point away from the sun.
0:31:17 > 0:31:21The longest tails can be half a billion kilometres long,
0:31:21 > 0:31:23and stretch halfway across the solar system.
0:31:28 > 0:31:32It's a grand display, one of the greatest shows in the universe,
0:31:32 > 0:31:36though it is a sign that, after four billion years,
0:31:36 > 0:31:39the comet is dying.
0:31:39 > 0:31:43The fact that we see them now as active comets
0:31:43 > 0:31:49means they have only a few thousand years left in their current orbits.
0:31:49 > 0:31:53So you can imagine, if you likened that
0:31:53 > 0:31:55to the lifetime of a human being,
0:31:55 > 0:31:59we're seeing the last few seconds of the lifetime of the comet.
0:31:59 > 0:32:06Every second, they are losing tonnes of dust and ice.
0:32:06 > 0:32:09Each time it goes round the sun,
0:32:09 > 0:32:12the comet nucleus shrinks.
0:32:12 > 0:32:17Most only have enough fuel for 1,000 revolutions around the sun.
0:32:18 > 0:32:24Even a large comet like Halley will only last another 150,000 years.
0:32:24 > 0:32:29And then, as their fuel supplies are exhausted, they die.
0:32:31 > 0:32:34Some perish as they crash into the sun.
0:32:40 > 0:32:43And some disintegrate,
0:32:43 > 0:32:46eventually turning into dust,
0:32:46 > 0:32:51dust we see as shooting stars as they enter the atmosphere.
0:32:56 > 0:32:58But not all of them disintegrate to dust.
0:33:06 > 0:33:09Some of them leave a more substantial skeleton
0:33:09 > 0:33:12drifting through space.
0:33:12 > 0:33:18We predict there should be hundreds, or maybe even thousands,
0:33:18 > 0:33:21of dead comets which still haven't yet been found.
0:33:21 > 0:33:24If the comet fades to dust and nothing else,
0:33:24 > 0:33:27then in a way they're not dangerous.
0:33:27 > 0:33:32But if they leave behind big chunks, hundreds of metres across possibly,
0:33:32 > 0:33:35then those objects could come through the atmosphere
0:33:35 > 0:33:38with some devastating consequences on the ground.
0:33:40 > 0:33:43Is the Earth really in danger from comets?
0:33:43 > 0:33:47Although big, they are incredibly fragile,
0:33:47 > 0:33:51no more substantial than cigarette ash.
0:33:51 > 0:33:54Still, some think a comparatively small chunk of comet
0:33:54 > 0:33:57could cause a global catastrophe.
0:34:15 > 0:34:19Mike Baillie is a paleoecologist,
0:34:19 > 0:34:22who's found the evidence of an ecological disaster
0:34:22 > 0:34:24in the fields of Northern Ireland.
0:34:24 > 0:34:2920-odd years ago, the farmer, when this field was being drained,
0:34:29 > 0:34:32found that these oaks were effectively floating to the surface,
0:34:32 > 0:34:34the land was shrinking round them.
0:34:34 > 0:34:38So he got a digger in, pulled them out and pushed them into a heap.
0:34:38 > 0:34:41And then in a flat area like this, it's a handy windbreak
0:34:41 > 0:34:44for sheltering animals, so that the farmers leave them,
0:34:44 > 0:34:49and it means that we have access to some ancient trees.
0:34:52 > 0:34:56Some of these oaks are thousands of years old,
0:34:56 > 0:35:01preserved perfectly in the oxygen-free conditions of the Irish peat bogs.
0:35:01 > 0:35:04By sampling tens of thousands of trees,
0:35:04 > 0:35:07Professor Baillie has built a year-by-year record
0:35:07 > 0:35:11of the Irish environment for the past 7,500 years.
0:35:14 > 0:35:17What he has found encoded in the annual rings
0:35:17 > 0:35:22is that the trees sometimes mysteriously stopped growing.
0:35:22 > 0:35:26This tree went from a period of extended, perfectly good,
0:35:26 > 0:35:29what we would regard as normal growth.
0:35:29 > 0:35:32When we get to this year, which is the year 540,
0:35:32 > 0:35:34there is a damage scar, and its growth
0:35:34 > 0:35:37was radically changed in character thereafter.
0:35:39 > 0:35:42When the same pattern emerged in many of the Irish trees,
0:35:42 > 0:35:46it was initially dismissed as the effects of savage storm
0:35:46 > 0:35:48that struck Ireland in 540.
0:35:55 > 0:35:58What caused a complete turnaround in this story
0:35:58 > 0:36:02was when we found that this same dated event
0:36:02 > 0:36:07occurs from Mongolia, Siberia, Northern Sweden,
0:36:07 > 0:36:10across Northern Europe, North America and South America.
0:36:10 > 0:36:14This was the realisation that this was a global event.
0:36:15 > 0:36:20Here was evidence of a worldwide catastrophe in 540 AD.
0:36:20 > 0:36:23What could have caused such a widespread disaster?
0:36:23 > 0:36:26There was one obvious suspect.
0:36:31 > 0:36:34The biggest volcanic eruptions
0:36:34 > 0:36:37blast so much ash into the upper atmosphere
0:36:37 > 0:36:40that it spreads around the world,
0:36:40 > 0:36:42blocking out the sun's rays.
0:36:48 > 0:36:51The eruption of Mount Pinatubo in 1991
0:36:51 > 0:36:53caused global temperatures to fall
0:36:53 > 0:36:55by over half a degree for the next two years.
0:36:56 > 0:37:01It's the same pattern that is seen in the 540 event.
0:37:03 > 0:37:07But volcanic eruptions also leave another trace...
0:37:09 > 0:37:11a layer of sulphuric acid
0:37:11 > 0:37:15that is frozen into the Greenland ice sheets.
0:37:18 > 0:37:21But the samples taken from deep within the ice
0:37:21 > 0:37:24show no acid spike in 540.
0:37:24 > 0:37:29Whatever stopped the trees growing, it wasn't a volcano.
0:37:31 > 0:37:33If you've a global environmental event
0:37:33 > 0:37:35and it wasn't caused by a volcano,
0:37:35 > 0:37:37what's the next most likely explanation?
0:37:37 > 0:37:40And you're allowed to ask the question,
0:37:40 > 0:37:42"Could it have been some sort of loading from space?
0:37:42 > 0:37:45"Could it be an extra-terrestrial event?"
0:37:45 > 0:37:49And as soon as you start even asking that question,
0:37:49 > 0:37:52you realise that the most likely sort of event you'd be talking about
0:37:52 > 0:37:54would be some sort of brush with a comet.
0:37:58 > 0:38:04In theory, a lump of comet just 300 metres across
0:38:04 > 0:38:06could affect the global climate.
0:38:11 > 0:38:15As it hit the atmosphere at over 20,000 kilometres an hour,
0:38:15 > 0:38:17it would burst into flame....
0:38:19 > 0:38:23and blow itself to bits while still kilometres above the ground,
0:38:23 > 0:38:26spreading a shroud of dust around the world
0:38:26 > 0:38:28that could block out the sun.
0:38:31 > 0:38:35It's an event they call an airburst.
0:38:37 > 0:38:39And there is much more recent evidence
0:38:39 > 0:38:42that suggests these events do happen.
0:38:48 > 0:38:51On June 30th 1908, a huge explosion
0:38:51 > 0:38:56tore through the forests of Tunguska, Siberia.
0:38:56 > 0:38:59No-one witnessed the explosion itself...
0:39:06 > 0:39:10..but it threw up clouds of dust that reflected so much light
0:39:10 > 0:39:13they could be seen as far away as London.
0:39:21 > 0:39:25It was so bright, the story goes, people played cricket at midnight,
0:39:25 > 0:39:27and people were able to read newsprint.
0:39:29 > 0:39:34That gives you an idea of how much dust must have been liberated
0:39:34 > 0:39:38very, very high into the atmosphere, which was illuminating the ground,
0:39:38 > 0:39:42even as far away as London is from where the impact actually happened.
0:39:49 > 0:39:53It was 20 years before the Russians mounted an expedition to the site.
0:39:53 > 0:39:56What they found astounded them.
0:40:01 > 0:40:0660 million trees across an area the size of London
0:40:06 > 0:40:08had been levelled.
0:40:10 > 0:40:14Puzzled, the Russian scientists performed a set of experiments
0:40:14 > 0:40:19to try and figure out what could have caused such devastation.
0:40:19 > 0:40:23The only explanation was that the explosion was caused
0:40:23 > 0:40:26by an extraterrestrial object, just 60 metres across,
0:40:26 > 0:40:28running into the upper atmosphere.
0:40:28 > 0:40:31This was just a small airburst,
0:40:31 > 0:40:35but it showed the amount of damage that could be done.
0:40:35 > 0:40:39All that energy of motion is converted into heat,
0:40:39 > 0:40:42it's just like the explosion of a nuclear weapon
0:40:42 > 0:40:44in the Earth's atmosphere.
0:40:56 > 0:40:59One can only imagine the effects
0:40:59 > 0:41:03if an airburst exploded over a populated area.
0:41:03 > 0:41:07If this is the devastation comets can cause,
0:41:07 > 0:41:10perhaps our ancestors had good reason to be afraid of them.
0:41:16 > 0:41:19'The comet called Shoemaker-Levy, photographed...'
0:41:19 > 0:41:21'..About to smash into the planet Jupiter...'
0:41:21 > 0:41:26'..What could be the most dramatic astronomical event of the century...'
0:41:26 > 0:41:29'When the comet pieces strike, it will be a massive collision...'
0:41:29 > 0:41:33'..Crash into the planet with the force of millions of nuclear weapons.'
0:41:35 > 0:41:39If anyone was left in any doubt of the destructive power of comets,
0:41:39 > 0:41:42an event in 1994 would change their minds.
0:41:42 > 0:41:46It happened 400 million miles away,
0:41:46 > 0:41:50but, in solar system terms, it was in our backyard.
0:41:53 > 0:41:58Comet Shoemaker-Levy 9 was discovered in 1993.
0:41:58 > 0:42:01It had had been torn into 21 separate pieces
0:42:01 > 0:42:03by the gravity of the planet Jupiter,
0:42:03 > 0:42:07creating a line of debris, known as the String of Pearls,
0:42:07 > 0:42:10that was on a collision course with the giant planet.
0:42:16 > 0:42:19This was a unique astronomical event.
0:42:19 > 0:42:23No-one had witnessed two solar system objects collide before.
0:42:25 > 0:42:28As the scientists planned their observations,
0:42:28 > 0:42:32less rational comet-fever took hold once more.
0:42:32 > 0:42:34At the time of the comet impact,
0:42:34 > 0:42:39we may see the bio-field around people change.
0:42:39 > 0:42:41Some people will feel excited,
0:42:41 > 0:42:44some may feel more depressed, we don't know.
0:42:44 > 0:42:50We have a piece of software that tells us what the comet's impact
0:42:50 > 0:42:52will mean for you as an individual.
0:42:52 > 0:42:55So is it going to happen to me on that date, the 16th?
0:42:55 > 0:42:58Maybe discard some old ways that were holding you back.
0:42:58 > 0:43:02- July 16th, I should beware, right?- Yeah.
0:43:06 > 0:43:10No-one knew what effect, if any, the comet fragments,
0:43:10 > 0:43:14just a few kilometres across, would have on such a huge planet.
0:43:14 > 0:43:17As the comets approached,
0:43:17 > 0:43:20the world's astronomers watched the pictures coming in,
0:43:20 > 0:43:22and held their breath.
0:43:22 > 0:43:24Isn't that incredible?
0:43:25 > 0:43:27That's amazing!
0:43:27 > 0:43:29Look! Oh, my God!
0:43:29 > 0:43:31That's it.
0:43:31 > 0:43:32Look at that!
0:43:32 > 0:43:35THEY CHATTER EXCITEDLY
0:43:37 > 0:43:41The scale of the explosions surprised everyone.
0:43:41 > 0:43:44OK, we just blew up a section of the planet.
0:43:44 > 0:43:46This is the southern pole here.
0:43:46 > 0:43:50You can see there's a bright streak. See that bright streak?
0:43:50 > 0:43:53And around the edge of the streak, there's some other stuff.
0:43:53 > 0:43:57It was not there the day before.
0:43:57 > 0:43:59It's a new feature on Jupiter.
0:44:03 > 0:44:07As the comet struck the Jovian atmosphere,
0:44:07 > 0:44:11fireballs 3,000km high erupted from the surface of the planet.
0:44:11 > 0:44:16The biggest impact released 600 times more energy
0:44:16 > 0:44:18than the entire world's nuclear arsenal.
0:44:20 > 0:44:24Scars bigger than the Earth were left on the surface of Jupiter.
0:44:24 > 0:44:28These were some of the biggest explosions
0:44:28 > 0:44:31that had ever been witnessed.
0:44:31 > 0:44:34It delivered, how about it? Yoo! HE LAUGHS
0:44:34 > 0:44:38Perhaps this was not the right time to celebrate.
0:44:38 > 0:44:41The Jupiter collision showed the enormous power
0:44:41 > 0:44:45that could be released by a full blown impact with a comet.
0:44:45 > 0:44:48Just imagine if it had hit the Earth.
0:45:40 > 0:45:43It's not just fantasy.
0:45:43 > 0:45:46Impacts this size have happened to the Earth.
0:45:46 > 0:45:49The last time was 65 million years ago.
0:45:54 > 0:45:56It spelled the end of the dinosaurs,
0:45:56 > 0:46:01and wiped out half of the rest of the species on the planet.
0:46:13 > 0:46:17Awakened to the danger, we now have projects to scan the skies
0:46:17 > 0:46:20looking for objects that are a threat to the Earth.
0:46:25 > 0:46:30But most of the sky surveys concentrate on finding asteroids.
0:46:30 > 0:46:33Unlike comets, which originate in deep space,
0:46:33 > 0:46:37asteroids mostly come from the belts between Mars and Jupiter,
0:46:37 > 0:46:40from where they are periodically deflected
0:46:40 > 0:46:43into an orbit that crosses the Earth's.
0:46:48 > 0:46:50But these rogue asteroids are easy to spot,
0:46:50 > 0:46:54and their trajectories can be plotted hundreds of years into the future...
0:46:59 > 0:47:02..giving scientists plenty of time
0:47:02 > 0:47:05to devise ways to divert them so they miss us.
0:47:07 > 0:47:09Unfortunately, avoiding an incoming comet
0:47:09 > 0:47:12would be a very different proposition.
0:47:16 > 0:47:20Comet impacts are likely to be more rare than asteroid impacts,
0:47:20 > 0:47:22but they're going to be much harder
0:47:22 > 0:47:27to mitigate against because a comet will be coming in from deep space,
0:47:27 > 0:47:30we'll have a few years at best, a few months at worst,
0:47:30 > 0:47:34knowledge before impact. And that may not be time enough
0:47:34 > 0:47:36to launch a spacecraft to, to deflect the orbit.
0:47:36 > 0:47:38People might remember a few years ago,
0:47:38 > 0:47:41we had a fantastic comet called Comet Hale-Bopp.
0:47:41 > 0:47:43Suddenly came out of nowhere.
0:47:43 > 0:47:49It was about 3,000 years since it had last passed by.
0:47:49 > 0:47:53And that was a very bright comet. And suddenly it was there in our midst.
0:47:53 > 0:47:56We've had, recently, a comet called Comet McNaught,
0:47:56 > 0:47:59which was a comet you could see with the naked eye.
0:47:59 > 0:48:04Nobody had predicted the appearance of these comets.
0:48:04 > 0:48:08You might get two, three, maybe six months' notice.
0:48:08 > 0:48:13So, it's possible that there could be a catastrophic collision
0:48:13 > 0:48:16with the Earth any time.
0:48:18 > 0:48:20It's a scary prospect,
0:48:20 > 0:48:25but perhaps not one to keep you awake at night.
0:48:25 > 0:48:28It's estimated that the Earth will endure a Tunguska-sized impact
0:48:28 > 0:48:31that could destroy a city, once every thousand years.
0:48:36 > 0:48:39And an extinction level event,
0:48:39 > 0:48:43like the one that wiped out the dinosaurs, every 100 million years.
0:48:53 > 0:48:55And rather than being afraid of comet impacts,
0:48:55 > 0:48:57perhaps we should be grateful for them.
0:48:57 > 0:49:01They might be responsible for us being here at all.
0:49:01 > 0:49:04MUSIC: "The Blue Danube Waltz" by Johann Strauss
0:49:08 > 0:49:11Way back at the beginning of the solar system,
0:49:11 > 0:49:13the planets were formed from the disc of debris
0:49:13 > 0:49:16that the early sun had thrown out around it.
0:49:20 > 0:49:23As the disc was stirred...
0:49:23 > 0:49:28bits of dust stuck together and became boulders...
0:49:28 > 0:49:32boulders stuck together and made asteroids...
0:49:34 > 0:49:38..which stuck into larger clumps...
0:49:38 > 0:49:40that became the planets.
0:49:48 > 0:49:51It was a hot, violent process,
0:49:51 > 0:49:53and when it was finished,
0:49:53 > 0:49:57it left the Earth a barren and sterile place.
0:50:03 > 0:50:05Yet within a few hundred million years,
0:50:05 > 0:50:08the Earth was covered in oceans,
0:50:08 > 0:50:10and life was thriving.
0:50:18 > 0:50:21Where can it all have come from?
0:50:21 > 0:50:24There is only one obvious source of water
0:50:24 > 0:50:26and organic molecules in the solar system -
0:50:26 > 0:50:29comets.
0:50:29 > 0:50:32But to explain how the Earth was transformed
0:50:32 > 0:50:34from a sterile boulder into the blue planet,
0:50:34 > 0:50:38astronomers have had to propose a massive hailstorm of comets,
0:50:38 > 0:50:413.8 billion years ago.
0:50:41 > 0:50:45They call it the "late heavy bombardment".
0:50:52 > 0:50:55The solar system wasn't as we see it today,
0:50:55 > 0:50:57there is still a lot of the stuff left over,
0:50:57 > 0:51:01the builders' rubble from the building of the planet.
0:51:01 > 0:51:04And Jupiter and all the other gas giants out there
0:51:04 > 0:51:07were doing a very good job at throwing stuff inwards to the sun.
0:51:08 > 0:51:12Some of which, unfortunately, we got in the way of,
0:51:12 > 0:51:14or, rather, I should say fortunately for us,
0:51:14 > 0:51:17if they really did bring the water that we see today.
0:51:17 > 0:51:22We're talking about a bombardment of the, if you like, the proto-Earth.
0:51:22 > 0:51:26That takes millions, or tens of millions of years,
0:51:26 > 0:51:31with an impact rate that may have been
0:51:31 > 0:51:341,000 or 10,000 times what it is today.
0:51:40 > 0:51:43The bombardment hasn't finished, it's still going on today.
0:51:43 > 0:51:46We're not quite so aware of it,
0:51:46 > 0:51:50because that bombardment is by tiny, tiny dust particles,
0:51:50 > 0:51:54that float down through the atmosphere every day,
0:51:54 > 0:51:57about 60,000 tonnes over the whole of the Earth, every year,
0:51:57 > 0:51:59comes down as dust particles.
0:51:59 > 0:52:02Water is bound into those mineral grains.
0:52:02 > 0:52:05So, we're still being bombarded
0:52:05 > 0:52:10by grains that carry water and carbon, even today.
0:52:18 > 0:52:22It's an amazing thought that all the water and organic molecules
0:52:22 > 0:52:24that make up every living thing on the planet
0:52:24 > 0:52:29may have started out on a comet beyond the edge of the solar system.
0:52:40 > 0:52:44But there are people who have taken the idea even further.
0:52:44 > 0:52:49They believe comets brought actual living creatures to the Earth.
0:52:51 > 0:52:53Earth life is essentially alien life,
0:52:53 > 0:52:57it is not a life that is indigenous to the Earth, by any means.
0:52:57 > 0:53:03And if we evolved from that life, then I think we are the products
0:53:03 > 0:53:05of evolution from alien life.
0:53:13 > 0:53:18In the 1960s, astronomers Fred Hoyle and Chandra Wickramasinghe
0:53:18 > 0:53:21proposed that the Earth was colonised by bacteria from space.
0:53:23 > 0:53:28It was an idea called panspermia, and it's never quite gone away.
0:53:28 > 0:53:31I think it's true to say, isn't it, that if your theory is correct,
0:53:31 > 0:53:35life not only MAY be spread throughout the universe, it MUST be?
0:53:35 > 0:53:38It would really be very surprising if life is not...
0:53:38 > 0:53:41if the galaxy is not just teeming with life.
0:53:45 > 0:53:48Since then there have been a number of strange discoveries
0:53:48 > 0:53:52that may, or may not, back up the theory.
0:53:52 > 0:53:59In 2001, the Indian state of Kerala was shocked by loud explosion,
0:53:59 > 0:54:03followed by storms of blood-red rain over the next two months.
0:54:10 > 0:54:12Local scientist Doctor Godfrey Louis
0:54:12 > 0:54:15collected samples from all over the state and analysed them.
0:54:21 > 0:54:26What he found was that the rain was being turned red by cells
0:54:26 > 0:54:28the like of which he had never seen before.
0:54:32 > 0:54:34It's quite exciting to see this.
0:54:36 > 0:54:38They look like red blood cells but they're not.
0:54:38 > 0:54:41Because these are having a very thick cell wall.
0:54:41 > 0:54:45And that cell wall is not there in blood cells.
0:54:55 > 0:54:58What made the cells even more mysterious
0:54:58 > 0:55:01was that he could find no trace of DNA in them.
0:55:01 > 0:55:08If that was the case, the cells were like nothing else on Earth.
0:55:08 > 0:55:10The staggering claim is that
0:55:10 > 0:55:15this is possibly extra-terrestrial. That's a big claim, I know,
0:55:15 > 0:55:18but all the experiments are supporting this claim.
0:55:21 > 0:55:23Come in, please.
0:55:24 > 0:55:27This is some of the samples we have collected.
0:55:27 > 0:55:32- Pretty good sample, we have... - So, this is the real stuff?
0:55:32 > 0:55:35- Yeah, this is the real stuff. - It's red, for sure.
0:55:35 > 0:55:39For Chandra Wickramasinghe, this was vital evidence
0:55:39 > 0:55:41to back up his theory.
0:55:41 > 0:55:45A fragment of comet must have blown up in the upper atmosphere
0:55:45 > 0:55:49and rained extra-terrestrial bacteria down on the Earth.
0:55:51 > 0:55:55If it is true that these are alien bugs from space,
0:55:55 > 0:55:59then it is an absolute clear cut proof of ongoing panspermia,
0:55:59 > 0:56:03and that would be absolutely fascinating, and I'd be over the moon!
0:56:07 > 0:56:11But unfortunately for Wickramasinghe, there was bad news.
0:56:11 > 0:56:14On closer inspection, the cells in the red rain
0:56:14 > 0:56:19didn't seem so alien after all.
0:56:19 > 0:56:21This is DNA from the red rain.
0:56:21 > 0:56:23Wow, there is no doubt about it.
0:56:23 > 0:56:27- No doubt about it.- Absolutely conclusive, as far as I can see.
0:56:27 > 0:56:30- Yeah.- The material, when examined by astrophysicists
0:56:30 > 0:56:37and people who are looking for evidence to support a view...
0:56:37 > 0:56:40doesn't appear to be anything that they've ever seen before.
0:56:40 > 0:56:43But people who actually have seen things before say,
0:56:43 > 0:56:45"It looks like red algae to me."
0:56:45 > 0:56:48I would tend to go with the people who have seen more things
0:56:48 > 0:56:51in the biological kingdom, rather than those
0:56:51 > 0:56:54who are looking to support their own ideas
0:56:54 > 0:56:57about how the world should work without the data to back it up.
0:56:57 > 0:57:01The journey towards the truth is always a rewarding one, I think,
0:57:01 > 0:57:05and I saw this as a journey toward discovering a truth
0:57:05 > 0:57:09that was quite plain to me and is becoming plainer and plainer
0:57:09 > 0:57:13to other people now, after 30-odd years.
0:57:15 > 0:57:18Whatever their actual role,
0:57:18 > 0:57:22it seems comets were vital in the evolution of life on Earth.
0:57:26 > 0:57:30And they maintain the ability to destroy it.
0:57:37 > 0:57:40It seems our ancestors were right all along,
0:57:40 > 0:57:44comets do carry the power of the gods.
0:57:44 > 0:57:48Maybe that is why they still cause so much excitement
0:57:48 > 0:57:50when they appear in the sky.
0:57:54 > 0:57:59What's more, when astronomers turn their telescopes to other stars,
0:57:59 > 0:58:04they see clouds of dust, the tell-tell signs of comet activity.
0:58:09 > 0:58:12Just as comets helped bring life to our solar system,
0:58:12 > 0:58:15they may be doing the same elsewhere.
0:58:19 > 0:58:23Perhaps that is the next message that comets will bring us,
0:58:23 > 0:58:26that we are not alone.
0:58:43 > 0:58:46Subtitles by Richard J Boyle Red Bee Media Ltd
0:58:46 > 0:58:49Email subtitling@bbc.co.uk