Snowy Dirtball, Through and Through

In 1950, astronomer Fred Whipple gave the world a popular description of comets that has endured for five decades. He called them dirty snowballs. It was a simple way to think of a primordial ball of frozen gas and dust, and it also had the presumed benefit of being accurate.

But scientists had never actually seen the inside of a comet, and now Comet LINEAR has soiled the popular definition slightly.

Before LINEAR's breakup, the SOHO spacecraft, which normally monitors the Sun, had been keeping an eye on the comet. A French-Finnish instrument called SWAN was measuring water in the comet's halo.

Then came the comet's swan song and the instrument found that the density of water in the inner chunks was between 15 and 30 kilograms (33 and 66 pounds) per cubic meter, far less than the figure of 500 often assumed for comets.

In comparing this with the comet's total mass before the breakup, researchers figure that the amount of ice was about 100 times smaller than the total mass.

"Comet Linear was more like a snowy dirtball than a dirty snowball," Weaver said.

The researchers who led the SOHO study said Comet LINEAR was probably "as flimsy and light as the expanded polystyrene used for packing fragile equipment."

But Jean-Loup Bertaux, who worked on the study, said the estimate was "tentative and controversial."

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