Image: NASA and H. WEAVER

Although late for Independence Day celebrations, comet LINEAR put up fireworks of its own this year, scientists report. Between July 5th and 7th, the normally quiet cosmic drifter unexpectedly blew off a part of its crustan eruption not unlike a volcanic explosion, only at much colder temperatures. "We lucked out completely," says Harold Weaver of the Johns Hopkins University. "In one surge of brilliance, this underperforming comet showed us what it could have been."

Weaver and his colleagues caught comet LINEAR's show thanks to the Hubble Space Telescope. Ground-based instruments missed the event entirely. Using Hubble's Imaging Spectrograph, the astronomers first spotted LINEAR 74 million miles from Earth and then watched as it grew 50 percent brighter over the course of four hours. Only days later, when the comet had returned to its normal luminosity, did they see fragments in the object's tail. Their conclusion? The fragments made a violent departure from the comet's core, producing the bright flash.

During the past decade, scientists have come to suspect that a comet's core is actually little more than a rubble pile of dust and ice, and larger chunks called cometesimals. And any number of factors might wrest one of these rocks loose in an explosion like LINEAR's. "Observations of comet Hyakutake by the Hubble telescope and other observatories also showed fragments traveling down its tail, and some French researchers showed that these fragments might be these house-size cometesimals," notes team member Paul Feldman, also of Johns Hopkins.

Alternatively, Weaver and his group propose that perhaps a volatile region of the core was exposed to sunlight for the first time and vaporized, or that a buildup of pressure from ice converting to gas blew off part of the comet's surface. Whatever the cause, they should have the opportunity to figure it out in the future. This chance glimpse means that such outbursts probably occur fairly often, Weaver adds, because it's unlikely that Hubble just happened to spot one. Unless, as he said, they were lucky.