Halley's Comet: Once In A Lifetime
NOVA observes worldwide preparations as amateur comet hunters, astronomers and scientists armed with specialized cameras, high powered telescopes and spacecraft look to the heavens in search of the expected arrival in 1986 of Halley's Comet.
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00:00:00Hurtling into our neighborhood of the solar system, hundreds of glowing wanderers loop
00:00:23around the sun.
00:00:24They are comets, and the most famous is Halley's, named for the astronomer who discovered it
00:00:30would return again and again.
00:00:33In its visits over the centuries, Comet Halley has inspired both fascination and fear, confirmed
00:00:39Newton's law of gravity, and guarded the secrets of the solar system.
00:00:46This time those secrets may be revealed, because not only is Halley's visiting us, but we are
00:00:51planning to go out and meet it.
00:00:55If we fail, our next chance to intercept Halley's won't come until the year 2061.
00:01:00Halley's Comet, once in a lifetime, next on NOVA.
00:01:10Major funding for NOVA is provided by this station and other public television stations
00:01:15nationwide.
00:01:18Additional funding was provided by the Johnson & Johnson family of companies, supplying health
00:01:22care products worldwide.
00:01:27And Allied Signal, a technology leader in aerospace, electronics, automotive, advanced
00:02:03These astronomy buffs have an appointment with a visitor, who comes only once every
00:02:0776 years.
00:02:13They are here at the Harvard-Smithsonian Center for Astrophysics, to greet a cosmic traveler
00:02:18whose name is a household word, but whose mysteries remain locked within.
00:02:30These amateur astronomers have come to welcome a lonely sojourner returning from a trip more
00:02:34than three billion miles away from the sun.
00:02:38Okay, now, I've got my comet seeker all ready, or maybe I should say comet finder, because
00:02:52I intend to find it.
00:02:54When I dream, I go for broke.
00:02:57Well, we'll get it now.
00:03:01They have come to look for a comet called Halley.
00:03:13The winds are picking up and the temperature is dropping, but the sky is clear and Halley's
00:03:18Comet soon will move away from a constellation that is easy to find.
00:03:24So tonight, the amateur astronomers look toward the faint, fuzzy patch that is the comet.
00:03:29Okay, well, that's sort of where I thought I saw it, but I'll give it a try.
00:03:35Astronomy, amateur astronomy, has been an interest of mine since I was seven years old.
00:03:40And as long as I can remember, I've always heard the name Halley's Comet.
00:03:43My grandmother told me about it when she saw it when she was a kid.
00:03:48And I said, well, gee whiz, I'll be almost 60 years old when I see it.
00:03:54Once in a lifetime, you have this chance to see something which has been seen all through
00:03:59history and won't be seen again until we're all gone.
00:04:07And so suddenly, here it is.
00:04:09I saw it last Friday, and it was a thrill of a lifetime, really, just to see that fuzzy
00:04:13speck in the sky, that fuzzy glow.
00:04:17And so for the next few months, I'll be following it every single night.
00:04:23Astronomy is one of the few sciences in which amateurs can actively participate.
00:04:27Comet Halley will inspire many to join the ranks.
00:04:34Halley's Comet.
00:04:36This is how it looked in November 1985 through the eyes of a special image-intensifying video
00:04:41camera.
00:04:43In the course of its travels, its appearance radically changes.
00:04:52In 1948, near the outer edges of our solar system, Comet Halley reached the farthest
00:04:57point of its elliptical orbit and began again its 38-year fall toward the sun.
00:05:04Out beyond the orbit of Jupiter, where the sun's rays are weak, Halley's is just a
00:05:08ball of ice-bound dust some four miles in diameter.
00:05:12But as the comet moves closer to the sun, a magnificent transformation begins.
00:05:18Heat from the sun raises streams of gases from the surface.
00:05:22The head warms and glows with a celestial halo called a coma.
00:05:30Then pressure from sunlight and a stream of fast-moving particles called the solar wind
00:05:35blow the gases from the nucleus of the comet into a tail that can extend for 30 million
00:05:40miles.
00:05:48About 700 known comets come hurtling in to loop around the sun.
00:05:53Some can take hundreds of thousands of years to complete one orbit.
00:05:57Others like Halley's have been captured into an orbit of the inner solar system.
00:06:06Still others have been known to fall apart, leave the solar system, or plunge directly
00:06:10into the sun.
00:06:14Aside from their majestic beauty, comets have captured our attention because they may offer
00:06:19clues to unraveling the mysteries of the solar system.
00:06:23They may have even affected the course of life on Earth.
00:06:29But what explains our particular fascination with Halley's comet?
00:06:34First we know when this comet is coming.
00:06:38Halley's comet reappears once in about 76 years.
00:06:41In astronomical time, this is a short period.
00:06:45And Halley's is the brightest short-period comet.
00:06:50Its path lies at an angle to the plane of the solar system.
00:06:54As it orbits in a direction opposite that of the planet's, the tail always points away
00:06:58from the sun.
00:07:06This time, as Halley's continues its journey, for the first time ever, we have the technology
00:07:12to go to it.
00:07:15Spacecraft from many nations are now en route to fly by Halley's comet.
00:07:20Instruments also will be flown on the space shuttle to study.
00:07:24And the International Halley Watch, a network of hundreds of professional and amateur astronomers
00:07:29in 47 different countries, will keep detailed records on their observations of the comet.
00:07:34In the months to come, a clearer picture of both the nature of comets and matter in the
00:07:39universe will emerge from this most intense ever observation of a single comet.
00:07:52There's a very long history to observing comets, and it goes back farthest in China.
00:07:57The basic techniques of observational astronomy were developed here more than 2,500 years
00:08:02ago.
00:08:04And the passage of comets was always carefully recorded in the Jinshu Chronicles, Chinese
00:08:09historical records.
00:08:11Some of the ideas were a thousand years ahead of science in the West.
00:08:18For example, from 635 A.D., the Chinese astronomers realized that a comet doesn't produce its
00:08:27own light.
00:08:28Most of it is simply reflected from the sun.
00:08:34From its shape, a comet was called a broom star.
00:08:38Chinese astronomers were called upon to help formulate state affairs, and broom stars were
00:08:42powerful signs.
00:08:44The predictions were mostly catastrophic.
00:08:47These comets symbolized war, the spread of disease, the death of kings.
00:08:53For centuries, there seemed to be no other way to explain comets except as harbingers
00:08:57of doom or great events.
00:09:00For the Briton of 1066, it was an omen.
00:09:03Halley's comet was visible that year in the spring, before King Harold was killed in the
00:09:07Battle of Hastings.
00:09:10Artisans in Normandy embroidered this Bayeux tapestry.
00:09:13The Latin inscription tells us of the common people.
00:09:16They marvel at the star.
00:09:24Even distinguished philosophers and astronomers had confused ideas about comets.
00:09:29Aristotle believed comets were fiery objects in the Earth's own atmosphere.
00:09:35Kepler was first to suggest that comet tails are created by the pressure of sunlight, but
00:09:40he thought comets travel in straight lines.
00:09:45Avelius had the curve of the comet's path correct, but he believed comets spiral out
00:09:49of planets.
00:09:52What they all needed was more careful observation.
00:09:59It was Edmund Halley who gathered up the descriptions of 24 comets and set out to see whether comets
00:10:04orbit around the sun.
00:10:07In 1682, when a comet appeared, he carefully plotted its changing path across the night
00:10:12sky.
00:10:14Halley wondered if the same force that holds the planets in orbit around the sun also affects
00:10:19comets.
00:10:21But Halley did not have the mathematical ability to prove his theory.
00:10:25So he sought the help of Isaac Newton at Cambridge University.
00:10:29He was astonished to hear that Newton already had calculated a proof of the mysterious force,
00:10:33but had lost the work.
00:10:35Halley convinced Newton to redo the calculations.
00:10:38He even offered to pay for their publication.
00:10:41Twenty years later, Newton's famous Principia was published.
00:10:44In it, he stated the universal law of gravitation, which describes the force that holds planets
00:10:49in orbit.
00:10:51Was it possible that Newton's law governed the motion of comets, too?
00:10:56Halley looked at his data from 1682 and showed that the comet did in fact obey the law of
00:11:01gravity, traveling in an ellipse around the sun.
00:11:05Halley also suggested that the comets reported in 1531 and 1607 traveled the same path as
00:11:11the one he observed in 1682.
00:11:14In fact, they were all a single comet, which would return again.
00:11:19Halley went even further and explained why one return was separated by 76 years, another
00:11:24by 75 years.
00:11:27The gravitational pull of big planets, such as Jupiter and Saturn, could slow down or
00:11:31speed up the comet's journey.
00:11:35He would never live to see it, but he wrote, I would venture confidently to predict its
00:11:39return in 1758.
00:11:44If correct, this prediction would be the most dramatic confirmation of Newton's law of gravitation.
00:11:51Throughout the year, astronomers searched the skies, but the comet was late.
00:11:57Was Halley wrong?
00:11:58Then, at last, on Christmas night, the comet was spotted by Johann Pollege, a Dresden farmer.
00:12:06Forever after, the comet would carry the name of Edmund Halley for discovering its predictability.
00:12:15Scientists after Halley wanted to refine his predictions, even attempt to pinpoint the
00:12:19very day the comet would arrive.
00:12:23In 1835, the best predictions were three days off.
00:12:27Observers made detailed drawings of the comet's head, but the mystery of why their predictions
00:12:31fell short would not be resolved until well after the comet next returned in 1910.
00:12:39An astronomer in Heidelberg was first to spot and tell the world he'd found it.
00:12:43It was not much to look at yet, but the comet was on its way.
00:12:53Comet fever, they called it.
00:12:56It was 1910, and while it was hardly the advent of the space age, America had started to take to the skies.
00:13:12Comet fever was hard to escape.
00:13:37Among the many people fascinated by the 1910 return was the great American writer Mark Twain.
00:13:43I came in with Halley's comet in 1835, Twain once said.
00:13:47It is coming again next year, and I expect to go out with it.
00:13:51It will be the greatest disappointment of my life if I don't go out with Halley's comet.
00:13:58Mark Twain died on April 21st, 1910, the day after Halley's comet passed closest to the sun.
00:14:06In May that year, the Earth plunged through the tail of the comet.
00:14:09The tail was thought to contain the poisonous gas cyanogen.
00:14:14Despite assurances from scientists that the comet posed no threat, that the tail was too thin,
00:14:19there were many stories of public panic.
00:14:29The tail brushed the Earth, then anticlimax.
00:14:33No one felt a thing.
00:14:36But the close brush provided an opportunity to get good pictures.
00:14:40With the advent of photography and more sophisticated telescopes,
00:14:44scientists were able to study Halley's in 1910 as never before.
00:14:49To build up an exposure on the slow photographic film, the comet had to be followed manually.
00:14:54Target on the crosshairs, hand on the control paddle.
00:14:59Astronomers often worked with negatives because black on white makes for easier study of detail.
00:15:05Still, as the comet drew away from the Earth, mysteries remained.
00:15:09Once again, scientists had been several days off in predicting the comet's arrival.
00:15:14If gravity alone were acting upon the comet, their predictions would have been right on the mark.
00:15:19Some other mysterious force had to be at work.
00:15:22History would be solved when Halley's was at the outer edges of the solar system again,
00:15:26when scientists at last had better ideas about the nature of comets.
00:15:33Our modern ideas about what comets are and how they behave have been shaped by one man more than any other.
00:15:39Now 79, only recently has he seen so much interest in his subject.
00:15:45Let's be honest about it.
00:15:46Most astronomers were interested in the universe and the explosive universe and so forth
00:15:51and weren't worrying very much about comets at all.
00:15:54And back in the 1860s, it was established that comets distribute little solid bodies in orbits about the sun
00:16:05and that we run into them and get meteor showers.
00:16:08And that sort of dominated the thinking about what a comet was.
00:16:14A comet must just be a big floating mass going through space of small particles, gravel or sand-sized particles.
00:16:28So the gravel bank or the sand bank of this loosely distributed material
00:16:34became the general theory for comets from the 1860s for almost 100 years.
00:16:44But how could a loose ball of gravel stay together in orbit?
00:16:48And where would the gas that forms the tail come from?
00:16:51Whipple came up with a new theory.
00:16:53The nucleus of a comet is like a huge dirty snowball.
00:16:56Comets are just imposters because you see this great mass of gas and dust shining in sunlight.
00:17:04But the real comet is just a snowball down at the center.
00:17:08You never see it all.
00:17:11According to Whipple's theory, the nucleus of a comet must be a huge reservoir of ice, mostly water.
00:17:17The cosmic snowball could produce the gas observed flowing out of the comet when it nears the sun.
00:17:22A comet is a very dirty snowball.
00:17:25Much dirtier than this is about half dirt.
00:17:28Now, I just say, well, you can throw it without it spinning.
00:17:30How about it, Danny? Let's see you do it.
00:17:33No matter what you do, they'll always be spinning slow or fast.
00:17:37Everything spins in the universe.
00:17:39A spinning ball of ice mixed with tiny sand-like dust particles.
00:17:43Whipple's famous model for a comet.
00:17:49At Nagoya University in Japan, scientists are following Whipple's recipe,
00:17:53making homemade comets out of ice and sand.
00:17:57Whipple's famous dirty snowball theory may explain why predictions for the return of Halley's comet
00:18:02have been days off the mark.
00:18:04In the depths of space, a comet would be very cold.
00:18:07So plunge it into liquid nitrogen.
00:18:15This vacuum chamber will simulate space.
00:18:19And the laser, a ray of sunshine.
00:18:22Heat in the low pressures of the vacuum can turn the solid ice directly to gas,
00:18:27a process known as sublimation.
00:18:30Some places on the irregular surface are likely to absorb more heat than others.
00:18:35The snowball recoils as jets of expanding gas are thrown out.
00:18:40And here's the solution to the prediction mystery.
00:18:43The whole comet can be pushed slightly off course by these jets, affecting its arrival time.
00:18:53This time around, the United States planned to send a spacecraft to fly beside Halley's comet.
00:18:58So Don Yeomans of the Jet Propulsion Laboratory set out to forecast the comet's movements
00:19:03with space age precision.
00:19:05He knew that gravitational pull by big planets affects the period of the comet.
00:19:10It's even more variable than Halley knew, ranging from about 74 to 79 years.
00:19:17Yeomans also took into account the rocket-like jets that consistently throw the comet's schedule
00:19:21behind by several days.
00:19:23He factored in information from 1910, and in the end, he had a formula that could predict
00:19:28exactly where in the sky the comet would be at any point during its current return.
00:19:34Then he searched through the astronomical records and libraries,
00:19:37such as this at Mount Wilson, to check his model.
00:19:41If his formula is correct, he should be able to predict in reverse as well.
00:19:45He should be able to check his model against any observations he could find
00:19:49He should be able to check his model against any observations he could find from the past.
00:19:54This is the first edition.
00:19:58His starting point?
00:19:59Very slim volume for such an important work.
00:20:02Halley's own book of observations.
00:20:05Yeomans' model agrees fairly well with these detailed records.
00:20:11Here's the third edition.
00:20:13Continuing back in time, he is right on target for the path of the comet sighted in 1066.
00:20:19But for the year 837, when the comet came unusually close to the Earth,
00:20:23Yeomans' formula runs into trouble.
00:20:27The Earth's gravity must have changed the comet's orbit.
00:20:32Yeomans needs to determine the exact track of the comet.
00:20:35Perhaps the Jinshu chronicles hold the answer.
00:20:43The chronicle describes how Halley's comet moved two degrees across the background of
00:20:47the stars every hour on a particular day in 837 AD.
00:20:51From this precise observation, Yeomans can calculate when the comet passed closest to the sun.
00:20:58Now he can fine-tune his model.
00:21:02The refined model agrees with the Chinese records all the way back to around the time of Christ.
00:21:08The return of Halley's comet closest to the birth of Christ was 12 BC.
00:21:13So the comet was not the star of Bethlehem as the artist Giotto depicted.
00:21:17But Halley's did appear during the painter's lifetime.
00:21:22In the pre-Christian era, the Chinese records are not so well preserved.
00:21:25But here is one that looks good.
00:21:27It's from 240 BC.
00:21:30This is the first certain record of Halley's comet.
00:21:39Yeomans' model is successfully matched to every recorded sighting back to 240 BC,
00:21:44with one exception, 164 BC.
00:21:47The observations from that year are sorely needed to resolve a competition
00:21:51that has arisen between Yeomans' model and a number of others.
00:21:55But the information is missing.
00:22:03Until in 1984, two historians of science paid a visit to the British Museum,
00:22:08where certain tablets were known to contain reports of astronomical events.
00:22:12From Durham University, Richard Stevenson and his student Kevin Yao
00:22:16homed in on ancient Babylon with the help of Christopher Walker.
00:22:23The story of the comet starts here after the record of market prices and political events.
00:22:30So what we have at the end of month eight here is a summary,
00:22:35after all the lunar and planetary observations,
00:22:39which includes a reference at the beginning of this line to the comet.
00:22:55And it tells us that the comet appeared for the first time
00:22:59in the northern part of the sky in the area of Taurus and the Pleiades.
00:23:04And just where it breaks off, it tells us that the comet started to move towards the west.
00:23:10And another tablet for 164 BC tells the same story,
00:23:14specifically that the comet was last seen in the constellation Sagittarius,
00:23:19seven and a half degrees above the planet Jupiter.
00:23:23This is precisely where Yeomans calculated it would be.
00:23:28His model emerged the most accurate and most likely to succeed
00:23:32in providing a precise path in the sky where the comet will be found this time around.
00:23:38From here on out, anyone who wants to look for the comet
00:23:40could refer to this track printed in an observer's guidebook.
00:23:53But the end of that competition brings another.
00:23:56Who would be the first to spot or recover, as it is called, Halley's comet for this return?
00:24:02Here on Kitt Peak in Arizona, the huge four-meter reflector and its electronic detectors
00:24:07had been searching since 1977, but still had not found it in 1982.
00:24:15Mike Belton was the odds-on favorite to recover the comet first.
00:24:20It looks pretty good. Let's go get the telescope.
00:24:24Belton wouldn't be able to actually see the comet, but the telescope's electronic detectors,
00:24:29more sensitive to light than the eye, might be able to capture its image.
00:24:35First, he worked out a track of where the comet would be using Yeoman's data.
00:24:39Our plan was to look at the sky very carefully,
00:24:43and we laid down a track on the sky, picked the best night,
00:24:46the night when there were fewer stars around, and one should be able to see the comet easiest.
00:24:51One particular night, on October the 16th, the comet was passing by a star that was about
00:24:57two million times brighter than the comet. So this bright star obviously was a problem,
00:25:03and we decided to skip that night, and we chose a night, two nights later,
00:25:09to do our recovery observations.
00:25:11Meanwhile, in California, a graduate student was about to enter the race to recover the comet.
00:25:16He didn't know about the star. Dave Jewett borrowed a CCD,
00:25:22a chip with a surface like a photographic plate, but that captures a video image.
00:25:30For three nights, the 200-inch Mount Palomar reflector was book solid,
00:25:34but Jewett was allowed time in the early morning hours.
00:25:37On the first night, it became very clear that we would have great difficulty in finding Halley's
00:25:41comet because of the presence of an extremely bright star near to the predicted location of
00:25:46the comet. By the third night, the last night of our observing run, we had decided that we would
00:25:55have a chance to see Comet Halley if we could find a way to remove the bright star from the field of
00:25:59view. So he had the best night of his life.
00:26:03So he had the biggest telescope in the West, the most sophisticated chip,
00:26:10and two razor blades to narrow down the image to a hundred lines in the middle.
00:26:17Even then, all he could see was a streaky mess. Following Yeoman's predicted path for an hour or
00:26:22so, he hurriedly collected electronic images at eight different times. He fed them into a computer
00:26:27and struggled to clean up the image. He programmed out light from bright stars and other information
00:26:33that might interfere, but he still couldn't see the comet. So then he added most of his
00:26:38pictures together, knowing that all the shifting stars would leave a blur. Then he subtracted the
00:26:43last picture, which would leave a dark shadow next to all the moving objects. But the comet,
00:26:49the object the telescope was following, should always be at the same spot in the electronic
00:26:53pictures. It should leave neither streak nor shadow. And there it is.
00:27:07Jewett was satisfied that he had captured the comet.
00:27:19He staked his claim by telegram to the Center for Astrophysics,
00:27:22the international clearinghouse for comet sightings.
00:27:27Brian Marsden serves as the Center's umpire. He prefers to receive comet pictures from two
00:27:32different nights, but Jewett sent data from only one night. The second night he tried,
00:27:37there was a bright star he couldn't work around. Should Marsden pass Jewett's message around the
00:27:41world? It's a tremendously competitive business. All the amateurs in the world seem to want to
00:27:47discover a comet, and many professionals too. They got eight images, I think, the first night,
00:27:54and then when they tried again, there was a star in the way. So this was a bare minimum of
00:27:59information. Now you see it, now you don't. And I was rather reluctant to accept it, but I made a
00:28:05few inquiries and convinced myself that they really did have the comet. So Marsden spread
00:28:10the word that Halley's comet had been recovered in October 1982. But I remember being exceedingly
00:28:16tired, having stayed awake for several days continuously. So once the identity of Halley's
00:28:23comet was confirmed, I went to bed. Jewett had captured an electronic image of the comet when
00:28:28it was still beyond the orbit of Saturn. It would have to become 19 million times brighter before it
00:28:37could be seen with the naked eye. So as Halley's hurtled in toward the sun, there would be other
00:28:43opportunities to be first, other chances for comet claim to fame. Some of these would be staked by
00:28:48amateurs. With equipment like this, it's hard to believe that Sumo Seki is a classical guitar
00:28:54teacher by profession, and only an amateur astronomer. If you're first to discover a
00:29:00comet, it will be named for you. Six comets, including this one, already carry Seki's name.
00:29:07In September 1984, Seki sent a telegram to the Center for Astrophysics, staking his claim as the
00:29:13first amateur to photograph Halley's comet. He had used a technique less powerful than CCDs,
00:29:20but more sensitive than the human eye. Photographic film and a long exposure time.
00:29:27Initially, his picture of the comet was suspicious. The comet seemed far too bright,
00:29:32until Seki explained it was retouched for the journalists.
00:29:37It would be several months before the comet was first seen by the human eye.
00:29:42An amateur astronomer with a reputation for keen visual acuity, Stephen O'Meara,
00:29:46set out to claim this distinction. In late 1984, I was presented with a visual challenge,
00:29:53and that was to actually be the first person to see Halley's comet. Until that time,
00:30:00and I went in January of 1985, the comet had been recorded electronically. It had been
00:30:05imaged with a CCD camera. Seki in Japan had actually photographed the comet,
00:30:13but until January, the comet had not been seen by the human eye.
00:30:19O'Meara looked for the comet through a telescope on the windy summit of Mauna Kea,
00:30:2314,000 feet above sea level in Hawaii.
00:30:26Professional astronomers were nearby to check his results.
00:30:30On the appointed night, O'Meara looked for Comet Halley in a section of the sky above
00:30:34the constellation Orion. For that section, he obtained a Palomar Sky Survey chart,
00:30:40a standard tool for astronomers worldwide. Only a handful of the stars on this map are
00:30:45visible to the naked eye. O'Meara then enlarged a small section of that map,
00:30:50on which a colleague plotted Yeoman's predicted path of the comet.
00:30:54He searched a very tiny area of this plot. Using a method of ancient astronomy,
00:30:58he looked for a faint object that would move relative to the stars.
00:31:02I was still looking at these very faint stars, waiting for one of them to give away its identity
00:31:08by movement. And the map that I had used went down to a certain limit, and I'm representing
00:31:17those stars as these dots. The comet would have been much fainter than these stars. Now,
00:31:25I had to draw in these fainter stars, hoping that perhaps one of them would turn out to be
00:31:31actually the comet. The way you would know that would be the comet would actually move.
00:31:36One object did move over the period of a half hour, and O'Meara plotted three positions for it.
00:31:43And O'Meara plotted three positions for it. He concluded that this was the comet.
00:31:49I felt very happy. I decided to go back and tell the astronomers of my accomplishment,
00:31:55what I was not aware of, but that the winds actually picked up to 75 miles an hour.
00:32:00I ran outside with my chart. As soon as I stepped away from the shelter of the dome,
00:32:05the wind almost knocked me over. It took my chart, which was the only evidence that I had,
00:32:11that I had seen the comet, and nearly ripped it out of my hand. I grabbed the charts, put them in
00:32:17my jacket, and zipped myself up and nearly crawled back to the location of the professional
00:32:23astronomers some 50 yards away. This is O'Meara's actual chart, torn and tattered. The astronomers
00:32:30checked this chart and agreed that his sighting of Halley's Comet was plausible. And he said,
00:32:34congratulations, you've just seen Halley's Comet, which I was ecstatic.
00:32:40O'Meara's observation was the first of perhaps millions to follow.
00:32:44By December 1985, the comet was visible to the naked eye in areas far from light-polluted cities.
00:32:52In February 1986, Comet Halley will disappear behind the sun. When it comes back into view
00:32:57late that month, it will be visible in the morning sky in the east. The view will be best from the
00:33:02southern hemisphere. Around mid-March, however, Halley should show some of its splendor to the
00:33:08northern hemisphere. To find it, all you need is an alarm clock to get you up before sunrise,
00:33:15and your hand at arm's length as a measuring device. Sky gazers should first note the point
00:33:21on the horizon where the sun rises around March 15th. The following day, an hour and a half before
00:33:28sunrise, measure five hand diameters to the right of that point and look one to two hands up from
00:33:35the horizon. That's five right, one to two up. Halley should be visible to the naked eye.
00:33:44While no one knows exactly how bright it will be, Halley should look a bit brighter than Comet
00:33:48Kahootek in 1974.
00:33:56When we gaze at a comet, we are looking not only at a spectacular show in the sky,
00:34:00but also 4.6 billion years into the past, when the solar system was just forming.
00:34:07The entire solar system formed from the gases and dust of the primordial solar nebula.
00:34:13When our sun coalesced, its great solar wind blew much of the lighter gases and materials
00:34:18of the primordial mix away from the inner earthy planets. Geological processes over
00:34:23time changed the character of Earth and the planets close to the sun.
00:34:31But comets, well preserved in the deep freeze of outer space, may be pristine ancient remnants of
00:34:37the primordial solar nebula. According to a popular theory, these dirty snowballs found
00:34:43a home in a huge shell or cloud far beyond the planets surrounding our solar system.
00:34:49There they move at a cosmic snail's pace, taking millions of years to orbit the sun.
00:34:55The theory of this cloud of comets was developed by Jan Oort of the University of Leiden.
00:35:00Oort's cloud explains why we can see five to ten new comets a year.
00:35:04The cloud provides this seemingly endless reservoir. So how many comets in the cloud?
00:35:10Well, I wish I knew, but there must be something of the order of 100,000 million comets,
00:35:18or 100 billion as the Americans would say. The comets are held in their huge orbits by the sun's
00:35:23gravity, but close encounters with other stars can boot comets into our solar system.
00:35:32Some of these comets fall towards the sun, where the gravity of huge planets traps them into a
00:35:37lower orbit. Brand new comets offer clues about the distance of the Oort cloud.
00:35:44If one computes the orbits with great accuracy, one sees that these comets come from very large
00:35:51distances of the order of 50,000 times the distance from the earth to the sun,
00:35:5750,000 astronomical units. Comets from the distant Oort cloud recently have been blamed
00:36:03for disasters on earth. For years, scientists have believed that long ago, some object from
00:36:09space hit the earth, kicking up dust that blocked out the sun's rays. This, they believe,
00:36:14wiped out the dinosaurs. Now, University of Chicago scientists say there is evidence that
00:36:20the demise of the dinosaurs was just one in a series of periodic extinctions that seem to occur
00:36:26about every 30 million years. Periodic extinctions imply mechanisms that operate like clockwork.
00:36:35Perhaps a star on a 30 million year orbit periodically dips into the Oort cloud,
00:36:40sending in a whole storm of comets.
00:36:45This twist on the extinction theory was proposed by Richard Muller.
00:36:50The earth is a rather small target. The probability of hitting the earth,
00:36:55even if you come within the earth's orbit, is only one in a billion.
00:36:58So it's very unlikely to happen for any single comet, such as Halley's comet.
00:37:03But if you have a storm of comets, if you have this unusual occurrence, when you have a billion
00:37:08comets coming into the inner solar system, perhaps deflected by Jupiter so that they get trapped
00:37:13into recurrent orbits, then the probability can get pretty high. If you have a billion comets
00:37:19coming in, then it's likely that one or perhaps a few will hit the earth. If that happens,
00:37:26then we expect a catastrophe. We know there have been catastrophes on earth caused by objects
00:37:33from space, but there is little evidence that scars like this were caused by periodic showers
00:37:38of comets. Still, the search for the Oort cloud star, dubbed Nemesis, has begun. The Nemesis theory
00:37:45is controversial, but it raises the possibility that comets have profoundly affected life on earth.
00:37:54At the same time, there are theories holding comets responsible for mass extinctions.
00:37:58Other research suggests that comet impacts, when our planet was very young,
00:38:02may have brought the building blocks of life.
00:38:05This experiment begins with a mixture of the gases thought to be in comets.
00:38:10At the University of Leiden in Holland, Mayo Greenberg has designed a machine that simulates
00:38:15the conditions of interstellar space. He looks into a vacuum chamber containing an intensely
00:38:20cold metal block, which simulates interstellar dust grains that could end up in comets.
00:38:26Onto its surface, Greenberg directs a thin stream of the gas mixture.
00:38:31The temperature is only 15 degrees above absolute zero,
00:38:34but it's also bathed in ultraviolet light, which simulates light from the sun.
00:38:38In previous experiments, Greenberg has seen tiny flashes,
00:38:42miniature explosions in the frozen gases on the surface of the block.
00:38:49They're difficult to see, so he turns off the light and watches the dark, icy surface.
00:38:56These explosions leave a crater of water ice mixed with yellow stuff,
00:39:01complex organic chemicals that could have been the building blocks of life.
00:39:05Could comets carrying the stuff of life have added chemicals to the primordial soup of Earth?
00:39:11Could they shower down upon us periodically to cause mass extinctions?
00:39:17Are they deep frozen remnants of the early solar system?
00:39:26Scientists have attempted to answer some of these questions by determining exactly what
00:39:30Capturing light from comets and dissecting it through prism-like devices
00:39:35gives clues about the molecules emitting this light.
00:39:38Every molecule emits light at characteristic wavelengths.
00:39:43By interpreting the spectrum of emitted light,
00:39:45scientists estimate that comets contain 10 to 50 percent dust,
00:39:49and the rest frozen water and other ices, including methane, ammonia, and carbon dioxide.
00:39:55This is just a guess, because the spectral fingerprints of comets do not directly identify these materials.
00:40:01Instead, they identify molecules after they have been broken up by sunlight.
00:40:05See, one of the problems is that a comet is a chemical laboratory factory,
00:40:12and the material that's coming out into sunlight is changed chemically by the sunlight and by
00:40:18molecules and atoms striking each other, and so unless we can get in very close,
00:40:23we can't be sure what the so-called parent, original molecules are that produce
00:40:29all of these broken up, busted molecules that we actually observe.
00:40:34So far, there have been no real opportunities to get close to a comet.
00:40:38But when the Earth crosses the horizon, the comet will be the first object to cross the horizon.
00:40:43There have been no real opportunities to get close to a comet.
00:40:46But when the Earth crosses the path of a comet's dust trail,
00:40:49scientists have attempted to catch bits of these particles.
00:40:53Some of the particles burn up in the Earth's atmosphere,
00:40:55causing meteor showers, commonly called shooting stars.
00:41:04It is the other particles, drifting invisibly through the atmosphere without burning up,
00:41:08that scientists try to capture with high-flying U-2 aircraft.
00:41:14It takes one hour of flight to catch a single grain, using probes with sticky surfaces.
00:41:22This tiny particle of dust, magnified 15,000 times, is composed of silicon compounds.
00:41:28While it is believed to have fallen from a passing comet,
00:41:31scientists cannot be entirely sure of its origin.
00:41:34So definitive answers await a space mission to a comet,
00:41:37and Halley's, the largest comet with a precisely known orbit, is the obvious target.
00:41:43NASA PROPOSED TO JOIN FORCES WITH EUROPEAN SCIENTISTS
00:41:48Initially, NASA proposed sending a spacecraft that would fly alongside Halley's for months,
00:41:53and then attempt to land on its nucleus.
00:41:56But developing the exotic ion-drive propulsion necessary would be too expensive, it was decided.
00:42:02Next, NASA proposed joining forces with European scientists.
00:42:06In the end, this mission, too, was cancelled.
00:42:08It was downhill all of the way.
00:42:10The background in planetary exploration in general was very poor,
00:42:15and for those of us who had worked on comets for,
00:42:18some people had been working on a comet mission for over a decade,
00:42:21and seeing Halley coming and knowing that this was absolutely the last chance
00:42:25to have a United States mission, or even participation in a mission to Halley,
00:42:30was very dismal, thinking that here was the chance of a lifetime, and we blew it.
00:42:35But the 11 nations of the European Space Agency forged ahead with their own Halley project.
00:42:41They call their spacecraft, Giotto, after the 14th century painter.
00:42:46Giotto will fly dangerously close to the comet to study the coma.
00:42:51Every grain of dust will strike it like a bullet.
00:42:55A bumper, an aluminum sheet followed by a second layer,
00:42:58may protect it from all but the largest grains.
00:43:01Survival will be touch-and-go as the density of dust increases a hundred-fold
00:43:05in the final few minutes before encounter.
00:43:10The Japanese, too, decided to set their sights on Halley's.
00:43:14They are sending two spacecraft in the direction of the comet,
00:43:17their first-ever venture into interplanetary space.
00:43:22Finally, the Soviet Union embarked on a Halley's flyby mission
00:43:25with two identical spacecraft called the Giotto and the Giotto.
00:43:29The Giotto was the first to be launched in December 1984.
00:43:32The Soviet spacecraft will carry U.S. instruments on board.
00:43:37The Soviet spaceships must swing by Venus first,
00:43:40so they were the first to be launched in December 1984.
00:43:50Followed by the launch of Giotto in July 1985.
00:43:59And the launch of the Japanese craft called Suisei in August 1985.
00:44:10And they're on their way.
00:44:15It's a unique international collaboration.
00:44:19While NASA's initial plans to be part of the flyby were scuttled,
00:44:23Bob Farquhar saw an opportunity to rejoin the comet hunt.
00:44:26Farquhar was flight director of an American spacecraft launched in 1978,
00:44:31and he proposed rerouting it to Halley's.
00:44:33He knew the spacecraft had more fuel in reserve than most people thought.
00:44:37Well, we certainly tried to get extra fuel on there.
00:44:41I guess I didn't exactly lie to him or anything,
00:44:44but I did stretch the point a little bit.
00:44:47I wanted to make sure I had plenty of margin for any type of launch vehicle errors.
00:44:52The spacecraft, called ICE, was designed to study the spacecraft's
00:44:56solar wind in the Earth's magnetic field,
00:44:59and had spent the last two years in odd looping orbits around the Earth.
00:45:03Farquhar saw that he could use the moon's gravity
00:45:05to slingshot the spacecraft toward the comet.
00:45:12On December 22nd, 1983,
00:45:15the ICE spacecraft passed within 75 miles of the surface of the moon.
00:45:21And with that, we got a kick in the pants from the moon's gravity.
00:45:26As it turned out, Halley's would be too far away
00:45:29for the craft's weak radio signals to be picked up on Earth.
00:45:32But another comet, Giacobini-Zinner, would be well within radio range.
00:45:39September 11th, 1985, NASA Goddard near Washington, D.C.
00:45:44Fred Whipple is followed in by Soviet, European,
00:45:46and Japanese scientists associated with the Halley's comet flybys.
00:45:50Today is like a rehearsal for that main event.
00:45:54No one really knows what's going to happen.
00:45:56Will dust from the comet knock the American spacecraft into a spin or destroy it?
00:46:01Farquhar half expects it will.
00:46:04It is shortly after 5 a.m., about two hours before closest approach,
00:46:08and the instruments on the spacecraft
00:46:10already have been signaling electronic and magnetic activity.
00:46:14The spacecraft has no camera,
00:46:16but pictures are being taken by a telescope at Kitt Peak.
00:46:19The comet barely shows a hint of tail,
00:46:21but it's there, about 14,000 miles wide, in fact.
00:46:26And at about 7 a.m., the International Cometary Explorer,
00:46:29or ICE spacecraft, plunges smack into it, traveling 46,000 miles an hour.
00:46:36Ten minutes after the midpoint of the encounter, Farquhar talks to the press.
00:46:41Well, I'm very pleased that we're still alive.
00:46:44It looks like we can start thinking about Halley after all,
00:46:48the upstream measurements of Halley.
00:46:51Also looks like we could be taking cruise data for quite some time.
00:46:57The ICE spacecraft escaped undamaged.
00:46:59Its instruments may even be able to radio back information
00:47:02about Halley's when that comet gets closer.
00:47:05Farquhar, who feared the ICE spacecraft might miss the tail
00:47:08or be wiped out by dust, was relieved.
00:47:10Well, it's a funny thing here.
00:47:14There wasn't as much dust as we thought.
00:47:15I guess I was Mr. Doom and Gloom here, but things worked out pretty good.
00:47:21I'm beginning to wonder if there's any dust in comets, period.
00:47:26The scientists were clearly proud of the mission,
00:47:29but in the excitement of the moment,
00:47:30it proved difficult to explain their accomplishments in simple terms.
00:47:34Hit the cold plasma in the middle.
00:47:36It actually penetrated the neutral sheet
00:47:39where the polarity of the magnitude of the magnetic field flipped.
00:47:43And that we wanted to hit, but we really weren't sure we were going to get it.
00:47:47But we've got even that.
00:47:48Jack Brandt was speaking in terms of a relatively new branch of physics.
00:47:52At a press conference a week later, he tried again.
00:47:57On Monday, I said if I said the word plasma physics,
00:48:00that that tended to cause people to exit the room.
00:48:03It still does.
00:48:05But we have a little problem here.
00:48:07Most of the universe is in the plasma form.
00:48:11Depending on who you ask, you get estimates of 90% of the matter,
00:48:1595% of the matter, 99% of the matter.
00:48:19Bottom line here is if you're doing modern astrophysics,
00:48:25you cannot avoid plasma physics.
00:48:29Here on Earth, we're most familiar with matter in one of three forms,
00:48:33a solid, a liquid, or a gas.
00:48:43But when gas molecules are ionized, that is stripped of their electrons,
00:48:47a fourth state of matter is formed, plasma.
00:48:52You don't have to go far to see examples, the sun itself,
00:48:59solar flares,
00:49:03the northern lights.
00:49:10Everyone in the room is probably familiar with the currents,
00:49:13if they are flowing in a wire.
00:49:17They're probably familiar with magnetic fields,
00:49:20if you have a bar magnet in your hand.
00:49:23You have to now think of as a cosmic environment,
00:49:27where there are ionized gases and forces,
00:49:31and where the magnetic fields and the currents are flowing around in this gas,
00:49:37unconfined by a magnet and unconfined by a copper wire.
00:49:42And that is the domain of plasma physics.
00:49:46In a comet, gas molecules vaporize off the head.
00:49:50Ultraviolet light kicks electrons out of these molecules, thereby ionizing the gas.
00:49:56A plasma is born.
00:49:58One of the best examples of plasma physics in a comet
00:50:01happens when the ionized gas interacts with a solar wind,
00:50:04a plasma flowing outward from the sun at 250 miles a second.
00:50:10The solar wind carries a magnetic field.
00:50:12It's tough for this field to cross the plasma ions from a comet and vice versa.
00:50:17So the interaction of the magnetic field with the comet's plasma
00:50:21lends the unique structure to the comet,
00:50:23a plasma tail that extends straight back.
00:50:26While the dust tail, less affected by the magnetic field,
00:50:29curves like water from a moving hose.
00:50:34Here, a comet releases a vast amount of gas and dust into the two characteristic tails.
00:50:40Red shows the dust tail strewn along the orbit of the comet,
00:50:43while the plasma tail shoots directly away from the sun.
00:50:49When the ice spacecraft passed right through such a plasma tail,
00:50:52it detected an abundance of water ions,
00:50:54supporting the picture of comets as large, dirty snowballs.
00:50:59But there were also surprises.
00:51:01Interactions between comet plasma and the solar wind
00:51:04were detected over a region much larger than expected.
00:51:08In fact, the comet, a mile and a half across,
00:51:10was affecting a region in space five times the diameter of the sun.
00:51:16This was of particular interest to scientists on the Halley missions,
00:51:20who may have to switch on their experiments earlier than planned.
00:51:24What we have done with the ICE mission
00:51:29is to set a very high standard for the contributions yet to come.
00:51:35There are going to be other spacecraft.
00:51:37There's going to be important observations from the ground,
00:51:40all organized and held together by the International Halley Watch.
00:51:44Amateurs are going to contribute.
00:51:47Spacecraft and Earth orbit are going to contribute.
00:51:50But when you think of this knowledge
00:51:52that we're going to gain in 1985 and 1986 as a crown,
00:51:58we have put the first jewel in, and it is a jewel of very high quality.
00:52:05Come March 1986,
00:52:07scientists hope to place more jewels in the crown of comet science
00:52:11when the International Armada of Five spacecraft
00:52:13makes its closest approach to Halley's.
00:52:19For the first time, the icy wanderer will have visitors.
00:52:25The Soviet flybys occur on March 6th and March 9th.
00:52:29With television cameras mounted on a scanning platform,
00:52:33the Vega spacecraft will observe the distant coma, tails, and nucleus.
00:52:39The Japanese spacecraft, Suisei, will fly by the comet on March 8th.
00:52:44It will take measurements of the outermost region of the comet,
00:52:47where it interacts with the solar wind.
00:52:50Suisei's sister ship, Sagikake, will not actually approach the comet,
00:52:57but will study distant interplanetary space for comparison purposes.
00:53:02And finally, the most treasured jewel will be placed in the crown of comet science
00:53:06on March 13th, when Giotto flies closest of all spacecraft to Halley's.
00:53:13Unlike other instruments that analyze chemicals remotely from emitted light,
00:53:17Giotto's mass spectrometer will scoop up and identify comet chemicals firsthand.
00:53:25Giotto is aimed extremely close to the nucleus, within a scant 300 miles.
00:53:31Closing in at 150,000 miles per hour, there is a danger at this range.
00:53:36Collision with comet dust could knock the spacecraft over,
00:53:39so its antenna no longer points to Earth, effectively ending the mission.
00:53:43Still, as Giotto plunges toward Halley's, scientists hope its camera will be able
00:53:48to capture the first pictures ever taken of a comet nucleus, unobscured by the coma.
00:54:01Well, my excitement, honestly, is mostly to see what a nucleus looks like,
00:54:07because I've been speculating on this for the last 35 years.
00:54:10And what does it really look like?
00:54:22As long as I've been aware of the sky, it's always been a source of mystery and awe to me.
00:54:30And to see that fuzzy object out there that only comes back once in a person's lifetime,
00:54:36that I was told about by my grandparents, it's just fascinating.
00:54:42This is a time of firsts.
00:54:44The first color pictures, the first time that spacecraft are actually going out to view the comet up close,
00:54:51to see what the nucleus actually looks like.
00:54:53It's something that I've always been looking forward to,
00:54:55and I hope that my grandkids will be around to see it in 2061.
00:55:04In each pass around the sun, Comet Halley loses about a meter of its surface to form the coma and tail.
00:55:11Scientists think that it will pass this way again 3,000 more times before it becomes inactive or disintegrates.
00:55:18Like a clock ticking off every 76 years, Comet Halley has returned as it has for centuries.
00:55:25It is the same comet that has been observed and recorded for over a million years.
00:55:31It is the same comet that has been observed and recorded since 240 B.C.
00:55:36It is the same comet that many future generations will gaze upon.
00:55:40It unites us with the past.
00:55:42It joins us with the future.
00:55:49Halley's comet has returned, and it won't be back until 2061.
00:56:30NASA Jet Propulsion Laboratory, California Institute of Technology
00:56:38NASA Jet Propulsion Laboratory, California Institute of Technology
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