To the surprise and delight of astronomers, NASA's Hubble Space Telescope has discovered a small armada of "mini- comets" left behind by what some astronomers had assumed was a total disintegration of the explosive comet LINEAR.

Hubble's powerful vision has settled the fate of the mysteriously-vanished solid nucleus of the comet, which seemed to disappear after it moved around the Sun.

On July 27, ground-based observers lost sight of the bright core of the comet and suggested that the nucleus disintegrated into a pile of dust. Astronomers at the Space Telescope Science Institute (STScI) in Baltimore, MD, quickly reprogrammed Hubble to search for the missing nucleus. Johns Hopkins University astronomer Hal Weaver said he was stunned when the Hubble image popped up on his computer screen. "My first thought was Hubble Space Telescope does it again! We caught the fish! This is amazing, very exciting, very neat."

Though comets have been known to break apart before, this is the first time astronomers have a close-up view of the dismantling of a comet's nucleus due to the Sun's heat. Since the 1950s, researchers assumed comet nuclei were loose clusters of ice and dust, called cometesimals, held together by gravity. Solar heat causes the ices to sublimate and violently release gas as explosions and garden hose-style jets. The pressure of the solar radiation blows away particles like debris caught in a gale.

Some astronomers think that the fragments now being seen in LINEAR may be the primordial building blocks of the original nucleus, the so-called cometesimals, which theory predicts should be several tens of feet across. The breakup of a comet tells scientists how it was put together in the first place. The cometesimals were built up from micron-sized grains of dust as it collected in the early solar system, roughly 4.6 billion years ago.

On Weaver's screen were at least a half dozen "mini-comets" with tails, resembling the shower of glowing fireballs from fireworks. They were clustered in the lance-head tip of an elongated stream of dust and an isolated brighter piece in front of the cluster may be the parent nucleus for the smaller fragments. Hubble's exceptional resolution and sensitivity allowed it to reveal the nuclei as separated bodies at a level of detail never before seen in a disintegrating comet.

Some astronomers find it hard to imagine how an object the size of a mountain could totally disintegrate in only two weeks. "Actually, I would have been more amazed if Hubble saw no pieces," adds co-investigator Carey Lisse, of STScI. "The comet's breakup was too violent and fast for it to completely vaporize. How do you pulverize something the size of a mountain?"

Weaver says it will be important for the largest ground-based telescopes to try and see the mini-comets as they spread apart. This may yield further clues on the structure of the original nucleus and the sizes of the remaining fragments.

Some astronomers believe this was Comet LINEAR's first visit to the inner solar system, after traveling for nearly the distance of one light-year (six trillion miles) from the vast comet storehouse called the Oort cloud. Other astronomers suggest that LINEAR may have been a fragile piece that broke off of a larger comet that visited our solar system more than 10 million years ago.

It's estimated that 20-30 percent of comets are so fragile they completely disintegrate when they pass the Sun.