Donald Savage
Headquarters, Washington, DC                   August 7, 2000
(Phone:  202/358-1547)

Nancy Neal
Goddard Space Flight Center, Greenbelt, MD
(Phone:  301/286-0039)

Ray Villard
Space Telescope Science Institute, Baltimore, MD
(Phone: 410/338-4514)

RELEASE: 00-122

HUBBLE DISCOVERS MISSING PIECES OF COMET LINEAR

     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.

     The Space Telescope Science Institute is operated by the 
Association of Universities for Research in Astronomy, Inc., for 
NASA, under contract with NASA's Goddard Space Flight Center, 
Greenbelt, MD.  The Hubble Space Telescope is a project of 
international cooperation between NASA and the European Space 
Agency (ESA).

Note to Editors:   Electronic image files are available on the 
Internet at:
             http://oposite.stsci.edu/pubinfo/pr/2000/27
                  http://hubble.stsci.edu/go/news 
            http://oposite.stsci.edu/pubinfo/latest.html
           http://oposite.stsci.edu/pubinfo/pictures.html

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