WASHINGTON, July 31 (UPI) -- Astronomers have used observations from Earth's 1996 close encounter with Comet Hyakutake ("HEE-yah-koo-TAH-keh") to confirm an idea first suggested in the 1970s -- that much of the gas produced by a comet comes from an ice particle halo.

The team of scientists, led by University of Wisconsin at Madison astronomer Walter M. Harris, reported their findings in the journal Science.

Comet Hyakutake passed within 9.5 million miles (15.25 million km) of Earth early in 1996. Harris and his colleagues used a new 11.5-foot (3.5 m) telescope atop Kitt Peak, Ariz., to capture some of the most detailed ground-based optical readings ever performed on a comet.

Comets, which astronomers describe as "dirty snowballs," are essentially large balls of ices -- including ammonia, methane and water -- left over from the formation of the solar system. Harris says swarms of ice particles are blown off a comet's nucleus as it sweeps through the inner solar system and radiation boils away its surface.

Although these particles make up only a small fraction of the comet's mass, Harris says they could be responsible for up to 90 percent of the gas observed in the head of the comet. Although the nucleus of Comet Hyakutake is only about 1.5 miles (2.5 km) in diameter, the head of the comet grew to tens of thousands of miles in diameter and the comet's tail grew to about 10 million miles (16 million km) in length.

The researchers also report a previously unobserved and possibly unique feature of Comet Hyakutake -- bright arcs in the tail of the comet that may be the result of interacting gas flows.

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Copyright 1997 by United Press International.
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