The most spectacular celestial viewing event of 1997 was the arrival of comet Hale-Bopp in the northern hemisphere. Its closest approach to Earth was on March 22nd, and its closest approach to the Sun (when it was at its brightest) was on April 1st—not to return to Earth again until the year 4397. The comet, designated C/1995 O1, was discovered independently on July 23, 1995, by Alan Hale, New Mexico, and Thomas Bopp, Arizona. It was the farthest comet ever discovered by amateurs, and appeared 1,000 times brighter than comet Halley did at the same distance.
THE GREAT COMET OF 1997. Above, the bright head of comet Hale-Bopp, called the coma, is pointed towards the Sun. The coma is composed of dust and gas, masking the solid nucleus of the comet made up of rock, dust and ice. Photo taken by Jim Young at NASA's Jet Propulsion Laboratories Table Mountain Observatory in March 1997.
An unprecedented year-long study was made of Hale-Bopp by two NASA observatories—the Hubble Space Telescope and the International Ultraviolet Explorer. Astronomers estimated that it had a monstrous nucleus about 19 to 25 mi in diameter. The average comet is thought to have a nucleus of about three miles in diameter, or even smaller. By comparison, the comet or asteroid that struck Earth 65 million years ago, possibly causing the extinction of the dinosaurs, was probably six to nine miles across.
Scientists were surprised to find that the different ices in its complex nucleus seemed to be isolated from each other. They reported seeing unexpectedly brief and intense bursts of activity from the nucleus during the monitoring period, suggesting that the nucleus must be an incredibly dynamic place. Astronomers using spectroscopic instruments were also amazed to discover that the comet had a thin, third tail composed of sodium atoms, a type never seen before.
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