Comets Leave a Trail of Information

CAMBRIDGE, Mass. (August 3, 1997 00:39 a.m. EDT) -- The recent visits of the largest and the closest comets of modern times have provided astronomers with a wealth of new information that redraws the picture of how comets are structured, scientists reported Saturday at a national astronomy meeting.

Among the most surprising findings from the visits of Comet Hyakutake last year and Comet Hale-Bopp this year were the discovery of a new type of tail made of sodium atoms that rapidly changed position, and the revelation that a comet's nucleus may be surrounded by a swarm of ice pellets that produce most of its visible tail.

Comet Hale-Bopp, one of the largest comets ever seen, was the first ever found to be accompanied by a third type of tail in addition to the two usual tails, which are composed of dust and electrically-charged gas. That discovery was made in April by a team of European astronomers.

Immediately after the European report, a team from Boston University found that pictures they had taken in March also showed the new type of tail, which is made of sodium atoms. But they found that its position was quite different from what the Europeans saw in April: While the European team found the new tail off to one side of the other two, in the BU pictures the sodium tail was right in the middle.

"This is the first comet in which we've taken images that show a sodium tail," said Jody Wilson of the BU observing team. And, he said, "the abundance of sodium is actually increasing as you get further from the nucleus," a mystery the team has still not been able to explain.

"The position of this sodium tail and its pattern of brightness away from the nucleus are very different from the normal comet tails," said Michael Mendillo, another BU astronomer. Mendillo, Wilson, and dozens of other astronomers reported their findings about Hyakutake and Hale-Bopp at the annual meeting of the American Astronomical Society's Division for Planetary Sciences, held this year at the Massachusetts Institute of Technology.

Mike A'Hearn, an astronomer at the University of Maryland, said because of its exceptionally large nucleus, about 18 to 25 miles across, comet Hale-Bopp "was producing the most gas and dust of any comet in modern times." As a result, he said, "we could make observations that were never possible before.

Besides the new kind of tail, the unprecedented observations also made possible the identification of more than a dozen chemical compounds never before seen in comets.

Meanwhile, a group of astronomers at the University of Wisconsin reported the discovery of another never-before-seen feature of a comet, this one surrounding last year's Comet Hyakutake. Their observations, also reported in the current issue of the journal Science, show that comet -- and, they say, presumably others as well -- was surrounded by several hundred billion chunks of ice about the size of marbles.

These tiny chunks, not the half-mile-wide "dirty snowball" of the comet's nucleus, were responsible for producing most of the dust and gas that form the comet's visible tail. It is this vast, thin cloud of dust and gas that gives comets their spectacular appearance.

The icy chunks surrounding Hyakutake's nucleus was inferred from the observation of a long bright arc behind the nucleus of Hyakutake, which the astronomers concluded must have resulted from collisions between the marble-sized ice pellets. "Nothing like this has ever been seen, except for Hyakutake," said Harold Weaver, an astronomer at Johns Hopkins University.

--By DAVID L. CHANDLER, The Boston Globe