"The first hint of trouble for Comet LINEAR came from ground-based observations at the Lowell Observatory from June 10 to June 12, when significant variations in the comet's brightness were first detected," says Hal Weaver, a research scientist in the physics and astronomy at The Johns Hopkins University.

These variations were originally attributed to rotation of the comet's nucleus, a common phenomenon known to change the brightness of comets. But when Weaver recently looked back at the data he began to suspect the change had links to the comet's eventual demise.

"Although no fragments were detected near the comet at that time, we now believe that this was the first indication that the comet was coming apart," says Weaver, who is reviewing the results on the recently deceased Comet LINEAR at the Division of Planetary Sciences Meeting in Pasadena, Calif., on October 26.

Comet LINEAR had been eagerly anticipated by astronomers, who were predicting that it might become visible to the naked eye in the night skies during July. While the comet's brightness never lived up to its advance billing, LINEAR nevertheless thrilled observers by its erratic behavior and eventual demise.

Observations early in July had suggested the comet was growing less stable. On July 7, Weaver and his collaborators used the Hubble Space Telescope to study the comet and captured pictures of a large chunk of the comet breaking away and moving down its tail, presumably being pushed away by jets of gas emanating from its surface. These jets are produced as sunlight boils ice on the comet directly into water vapor. The gas jets also eject small particles of dust into the coma, or atmosphere of the comet. Radiation pressure from the sun then sends this dust streaming behind the comet to form the comet's tail.

Extreme variations in the comet's brightness were detected by optical and radio telescopes during July 20-24, and astronomers observing the comet over the next 12 days complained that it looked like little more than a cloud of dust. Puzzled by what appeared to be a rapid disintegration of the nucleus, Weaver and colleagues decided to look more carefully at the comet using the Hubble Space Telescope. The Hubble images revealed a spectacular field of about a dozen mini-comets near the edge of the broad tail of dust seen in the earlier ground-based images. Each of the fragments had its own comet-like tail.

After a scramble to arrange time for follow-up observations, Weaver and his team observed the comet again on Aug, 6 using the Very Large Telescope in Chile. Although its resolution was not quite as good as Hubble's, the VLT's mirror has 10 times the collecting area, and it was able to detect about 17 mini-comets.

When they went back to the comet on Aug. 9 with the VLT, they were surprised to find that the mini-comets had virtually disappeared. Poor atmospheric conditions made it difficult to determine if a real change had taken place, or if atmospheric turbulence was hindering their view. But observations on Aug. 14 under excellent conditions confirmed that the mini-comets had faded dramatically.

Weaver and his colleagues are continuing to analyze the data they gathered to see if they can find clues to how LINEAR came apart. A better understanding of the comet's breakup could lead to a better understanding of how it came together 4.6 billion years ago in the early days of the solar system.

"If the comet broke up by shedding small pieces, then it's possible that the most massive object remaining in the field of mini-comets could be identified as its original nucleus," Weaver says. "On the other hand, it may be that the destruction of the comet was so complete that it's pointless to search for the 'original' object, much like you wouldn't call any particular piece of a badly shattered glass the 'original glass.'"

Astronomers have seen many other comets fragment, Weaver says, but very few have done so as dramatically as LINEAR. Current cometary theory suggests a range of forces that could have torn the comet apart, most of which should manifest more strongly as the comet gets closer and closer to the sun. These may include sharp temperature and pressure differences between the sunward and dark sides of the comet, and sudden vaporization of internal pockets of ice.

"We still do not understand what caused this comet to come apart, and don't generally understand what causes fragments to break off comets," says Weaver. "By continuing to investigate the data from Comet LINEAR, and folding in everything we know about other comets as well, maybe somewhere downstream we can explain what happened with a detailed physical model."

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