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Hale-Bopp and Hyakutake

Comet Hale-Bopp

Nature Magazine cover of Hale-Bopp The image of Comet Hale-Bopp blazing across the night sky of 1997 has delighted casual stargazers and astronomers alike. It was first spotted in July 1995 by Alan Hale and Thomas Bopp, who observed it the same night in neighboring states (New Mexico and Arizona, respectively). Astronomers immediately became excited about the comet because it appeared to be unusually bright—even when it was still well outside Jupiter's orbit. Careful analysis of a Hubble Space Telescope image indicated that reason for Hale-Bopp's brightness was its exceptionally large size. The nuclei of most comets are about 1-2 miles across. The nucleus of Halley's comet, which is considered to be fairly large, was approximately 10 miles across in its largest dimension. Hale-Bopp's nucleus is estimated to be 25 miles across.

Even though Hale-Bopp is farther from the Earth than the Earth is from the sun, it has set an all time visibility record—remaining discernible to the naked eye longer than any other comet in recorded history. First glimpsed in Australia in May 1996, Hale-Bopp should remain visible until the end of 1997. This 18-month spectacle will last twice as long as the previous record holder, Comet Flaugergues, which was visible to the naked eye for nine months in 1811.

Hale-Bopp also may have set a record for the amount of time it has remained at high brightness or "magnitude 0," but astronomers caution that brightness is a difficult feature to measure and that historical records are not always reliable in this regard.

Perhaps the most exciting feature of Hale-Bopp is its unique tail. Like most comets, it has two tails that can be seen by the naked eye under the right conditions: a bright dust tail, which is created by the reflection of sunlight on the dust streaming from the comet, and a fainter ion tail, which is composed of electrically charged atoms swept from the comet by the solar wind. But in April 1997, a group of European astronomers using one of the world's most powerful telescopes at La Palma in the Canary Islands, found a third tail. Setting spectroscopic instruments to exclude all light from the comet except the yellow light emitted by sodium atoms, the scientists were surprised to see a narrow third tail located near the ion tail. They estimate that this new sodium tail, which is not visible to the naked eye, is about 373,000 miles wide and about 31 million miles long.

Throughout history and across cultures, people have viewed the passing of a comet as an omen: a sign of something good, or bad, to come. The ancient Greeks believed that comets foretold war, or an outbreak of disease. The ancient Chinese believed that a comet sighting might be followed by disease and political upheaval, but they also believed that a comet might indicate a good harvest. The number of tails the comet had was important; a three-tailed comet was a bad sign, but a two-tailed comet was good one. What the new revelation about Hale-Bopp's tail would mean to the ancients is anybody's guess, but it undoubtedly provides an exciting new area of study for astronomers.

Comet Hyakutake

Astronomer Hyakutake In the spring of 1996, stargazers were treated to a smaller, but closer, comet. Discovered on January 30, 1996 by an amateur astronomer from southern Japan, Comet Hyakutake (pronounced "hyah-koo-tah-kay,") came 13 times closer to the Earth than Hale-Bopp.

There were two unusual aspects to comet Hyakutake's orbit around the Sun. First, the plane containing its motion was almost perpendicular to the plane that contains the Earth and other planets. This meant that the comet probably originated from the Oort Cloud rather than from the Kuiper Belt (see Comets 101). Secondly, and more importantly, Hyakutake passed VERY close to the Earth on March 25, 1996. Although Hyakutake's miss distance was only the fifth closest of this century (comet IRAS-Araki-Alcock came three times closer in 1983), the other comets that came closer were all considerably less active, and therefore less bright, than Hyakutake. In fact, not since 1556 has a comet as active as Hyakutake passed as close to the Earth as Hyakutake.

Careful studies of Hyakutake's orbit indicate that it last passed this close to the Sun about 8,000 years ago and will make its next passage about 14,000 years from now. Unlike the case of comet Halley, there's no chance that any of us will ever see Hyakutake again.

X-Ray image of Hyakutake In the spring of this year, shortly after comet Hyakutake was discovered, the scientific community swung into action. Hyakutake was the most intensively studied comet since Halley, and astronomical observatories all over the world Hubble Image of Hyakutake focused their sights on this now famous celestial visitor. The result was a bonanza of exciting new discoveries. Among the most surprising was the discovery of X-rays coming from the comet.

The Hubble Space Telescope produced the most detailed images of the comet, revealing a very different picture than what could be seen with binoculars or wide-field telescopes. The Hubble images emphasized the ``heart'' of the comet, the region near the nucleus, while wide-field pictures were dominated by light from the outer coma and tails.

Processed image of Hyakutke This processed Hubble image shows the bright jets of dust emanating from the nucleus more clearly.

A portion of the Hubble image shows three companions to comet Hyakutake. Scientists believe that the surface of Hyakutake's nucleus is Hyatuke Image covered with a "crust," and that a piece of this crust was lifted off and blown into the tail as it disintegrated.

Several new molecules were discovered in comet Hyakutake that should provide important insights into the understanding of the origin of our solar system. Infrared observations revealed that Hyakutake's nucleus is loaded with hydrocarbon ices, like methane, acetylene, and ethane. Hydrocarbons are the basis of life, so scientists were excited to find so many of them in a comet. Current thermo-chemical models of the solar nebula (the gas and dust cloud from which the solar system formed) suggest that ethane and acetylene should be much less abundant than methane, but this was not seen in Hyakutake. Scientists may need to revise their understanding of the early solar system or entertain the idea that comets consist primarily of interstellar material—the matter between the stars that eventually collapses into the clumps that become new stars.





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