Comet Hale–Bopp

• 1. Introduction

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  By the 9th of April 1997-69 percent of Americans had seen Comet Hale-Bopp, formally designated C/1995 O1. The comet left a far deeper mark on the public than Halley's Comet had in 1986. Astronomers came to call it the Great Comet of 1997. Behind that crowd of watchers was an enormous ball of ice and dust, found by accident, by two people, on the same night. This documentary asks how two observers with very different equipment spotted the same faint smudge, and why scientists refused to trust early excitement about it. It asks what the comet's light revealed about the birth of the Solar System. It asks why some people looked up and saw a spaceship. And it asks where the comet has been, and where it is going.

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• 2. Two Discoverers

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  Just after midnight on the 23rd of July 1995, Alan Hale stood in his driveway in New Mexico, tracking comets that were already known. He had spent many hundreds of hours hunting for a new one without success. That night he stumbled on a faint object near the globular cluster M70 in the constellation Sagittarius, shining at apparent magnitude 10.5. He checked that no other deep-sky object sat nearby. He consulted a directory of known comets and found nothing in that patch of sky. Once he saw the object drift against the background stars, he emailed the Central Bureau for Astronomical Telegrams, the clearing house for such discoveries, at around 6:50 UT.

  Thomas Bopp did not own a telescope. He was at Vekol Ranch, about 90 miles south of Phoenix, near Stanfield, looking at star clusters and galaxies with friends. His friend Jim Stevens owned a homemade 17.5 inch Dobsonian telescope, and at its eyepiece Bopp came across the same object. Like Hale, he checked his star maps for anything known near M70 and found nothing. About two hours after Hale, he alerted the bureau with a Western Union telegram. Brian G. Marsden, who had run the bureau since 1968, laughed about it afterward: "Nobody sends telegrams anymore. I mean, by the time that telegram got here, Alan Hale had already e-mailed us three times with updated coordinates."

  The next morning, astronomers confirmed a new comet, and International Astronomical Union circular 6187 announced the discovery. Its position was the real shock. At 7.2 astronomical units from the Sun, between Jupiter and Saturn, it was the most distant comet amateurs had ever found. Comets that far out are usually faint and show no sign of activity, yet this one already wore a coma, the glowing cloud around its nucleus.

  An older photograph soon confirmed how unusual it was. An image taken in 1993 at the UK Schmidt Telescope held the unnoticed comet at about 13 astronomical units from the Sun. At that distance Halley's Comet would have been more than 100 times fainter. Later analysis suggested the nucleus was roughly six times the size of Halley's. Such promise had fooled astronomers before, and they knew it.

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• 3. Spring of 1997

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  Comet Kohoutek in 1973 had been touted as a comet of the century and then turned out to be unspectacular. That memory made astronomers cautious about Hale-Bopp. Comets are unpredictable, and many erupt at great distances only to dim later.

  Hale-Bopp became visible to the naked eye in May 1996. Its brightening slowed in the second half of that year, but scientists stayed cautiously hopeful. In December 1996 it sat too close to the Sun to be seen. When it reappeared in January 1997, anyone who looked could find it, even from big cities with light-polluted skies.

  The Internet was a growing phenomenon, and websites posting daily images from around the world became hugely popular. They played a large role in the unprecedented public interest. By February 1997 the comet shone at second magnitude. On the astronomers' scale, lower numbers mean brighter objects. It now showed two tails: a blue gas tail pointing straight away from the Sun, and a yellowish dust tail curving along its orbit.

  On the 9th of March 1997, a solar eclipse crossed China, Mongolia and eastern Siberia. Observers there could see the comet in the daytime sky. The comet came closest to Earth on the 22nd of March 1997, at a distance of 1.315 astronomical units, about 196.72 million kilometres.

  Perihelion, the point nearest the Sun, came on the 1st of April 1997. The comet outshone every star in the night sky except Sirius, and its dust tail stretched 40-45 degrees across the heavens. It was visible before the sky had fully darkened. Many great comets pass perihelion very close to the Sun, but Hale-Bopp stayed in view all night for Northern Hemisphere observers.

  Its true peak came a little later, between the 2nd and the 4th of April 1997, at about magnitude -1.8. The delay came from a solar heating lag, in which heat from the closest approach kept boosting the release of gas and dust. The Northern Hemisphere would not see a comet this bright again until Comet Tsuchinshan-ATLAS in October 2024.

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• 4. What It Was Made Of

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  Dust poured off Hale-Bopp at up to 2.0 kilograms per second, perhaps thick enough to make the inner coma optically thick. The dust grains were hot, reflective, and showed a strong silicate emission feature at 10 micrometres. From those properties, astronomers concluded the grains were smaller than in any other comet studied. The comet also showed the highest linear polarization ever detected in a comet. That is the effect of sunlight scattering off dust, and it pointed to the same conclusion about tiny grains.

  A third kind of tail was among the most remarkable finds. Besides the familiar gas and dust tails, Hale-Bopp carried a faint sodium tail, visible only through powerful instruments with dedicated filters. It was made of neutral atoms rather than ions, and it stretched about 0.33 astronomical units. It lay between the other two tails, which implies that radiation pressure from sunlight pushed the sodium away from the comet's head. Astronomers have not settled whether colliding dust grains or ultraviolet light knocking sodium loose is the main cause.

  The heavy water in Hale-Bopp held about twice as much deuterium, a heavy form of hydrogen, as Earth's oceans do. If that is typical, comets cannot be the only source of Earth's water, though impacts probably delivered a good share. The deuterium ratio varied between different compounds in the comet. Astronomers took that as a sign that its ices formed in interstellar clouds rather than in the solar nebula, at temperatures of around 25-45 kelvin.

  Spectroscopy also found many organic chemicals, several never before seen in a comet. Argon turned up for the first time in any comet. Krypton, which needs very cold conditions to freeze, was depleted more than 25 times relative to the Sun, while argon was enriched. Together these clues say the interior was always colder than 35-40 kelvin but was once warmer than 20 kelvin. Unless the solar nebula was colder and richer in argon than believed, the comet formed beyond Neptune in the Kuiper belt and later migrated outward to the Oort cloud.

  The activity came from several specific jets rather than evenly across the surface. By watching material stream away from them, astronomers timed the nucleus's rotation at about 11 hours 46 minutes.

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• 5. A Possible Companion

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  In 1997 a scientific paper proposed that Hale-Bopp's nucleus might have a smaller partner. The idea came from the pattern of dust seen in October 1995, which was hard to explain with a single body. The authors made no claim to have seen a satellite. They estimated it would be about 30 kilometres across, orbiting a main nucleus about 70 kilometres wide, once every three days at a distance of about 180 kilometres.

  The Hubble Space Telescope offered support. Its Wide-Field Planetary Camera 2 took images of the comet in 1996, and analysis of them revealed the proposed satellite. Adaptive optics observations in late 1997 and early 1998 then showed a double peak in the nucleus's brightness.

  The case never closed. Controversy remains over whether a binary nucleus is the only explanation for that double peak. Other observations did not confirm the satellite. Astronomers had seen comets break apart before, but no stable binary nucleus was found until a discovery in 2016.

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• 6. Spacecraft Rumors

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  In November 1996, amateur astronomer Chuck Shramek of Houston, Texas, took a CCD image of Hale-Bopp that showed a fuzzy, slightly elongated object nearby. His sky-viewing software did not recognize it, so he phoned Art Bell's radio program Coast to Coast AM and described a "Saturn-like object" following the comet. Courtney Brown, a political science professor at Emory University and a believer in remote viewing, soon concluded that an alien spacecraft was trailing it.

  Alan Hale and other astronomers had a plainer answer. The object was SAO141894, a star of magnitude 8.5. It was missing from Shramek's program only because the user preferences had been set incorrectly. Art Bell then claimed to hold an image of the object from an anonymous astrophysicist about to confirm it. Olivier Hainaut and David Tholen of the University of Hawaii said the photo was an altered copy of one of their own comet images.

  The rumor had a tragic end. In March 1997, thirty-nine members of the Heaven's Gate cult died in a mass suicide, hoping to reach a spaceship they believed was flying behind the comet.

  Nancy Lieder, who says she receives messages from aliens through an implant in her brain, offered another story. She said Hale-Bopp was a fiction meant to distract the public from the coming arrival of "Nibiru" or "Planet X", a giant planet that would disrupt Earth's rotation. Her original date for the catastrophe was May 2003, which passed without incident. Conspiracy websites kept the claim alive and tied it to the 2012 phenomenon, and scientists have repeatedly debunked it.

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• 7. Orbit Through Time

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  Around 2215 BC, roughly 4,200 years ago, Hale-Bopp probably made its previous pass through the inner Solar System. Its closest approach to Earth was then estimated at 1.4 astronomical units. It may have been seen in ancient Egypt during the reign of Pharaoh Pepi II, who ruled from 2247 to about 2216 BC. Pepi's pyramid at Saqqara contains a text that calls an "nhh-star" a companion of the pharaoh in the heavens. The hieroglyph nhh means long hair. The comet may also have come close to Jupiter that year, which would have changed its orbit sharply, and 2215 BC may have been its first visit from the Oort cloud.

  In April 1996, Hale-Bopp passed within 0.77 astronomical units of Jupiter. That was near enough for the giant planet's gravity to measurably reshape the comet's path. Its orbital period shrank to roughly 2,399 years, and its farthest distance from the Sun fell from about 525 astronomical units to about 354.

  Astronomers kept following it as it faded. In October 2007, at 25.7 astronomical units from the Sun, a coma driven by carbon monoxide showed the comet was still active. Herschel Space Observatory images from 2010 suggested a fresh layer of frost covered it. The comet was found again in December 2010 at 30.7 astronomical units, and in 2012 at 33.2. In 2022 the James Webb Space Telescope observed it at 46.2 astronomical units.

  The next return to the inner Solar System is due around the year 4385. The comet's orbit sits almost perpendicular to the plane of the planets, so close approaches to planets will be rare. The chance of an Earth impact is remote, about 2.5 in a billion per orbit. Weissman's conservative estimate puts the nucleus at 35 kilometres across, and an impact would release about 44 times the energy of the K-T impact event.

  Over many orbits, gravitational tugs tend to shrink the perihelion distance of comets like this one. Hale-Bopp has about a 15 percent chance of eventually becoming a sungrazing comet. It could lose huge amounts of mass or break apart, as the Kreutz sungrazers did. It would also blaze, perhaps outshining Halley's Comet in the year 837.

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• 8. Records and Remembrance

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  Hale-Bopp stayed visible to the naked eye for 569 days, about 18 months. The Great Comet of 1811 held the previous record at about nine months. In the Southern Hemisphere, people watched the comet fade through the second half of 1997. The last naked-eye sightings were reported in December 1997. It was also brighter than magnitude 0 for eight weeks, longer than any other recorded comet. Its nucleus is the largest well-measured one known after 95P/Chiron.

  Composer Dmitry Kayukin turned his memories of watching the comet into an album called Comet 97.

  Carolyn Shoemaker and her husband Gene, co-discoverers of comet Shoemaker-Levy 9, were in a car crash after photographing Hale-Bopp. Gene died in the crash. His ashes were sent to the Moon aboard NASA's Lunar Prospector mission, along with an image of Hale-Bopp, "the last comet that the Shoemakers observed together".

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