Scientists at NASA's Jet Propulsion Laboratory, Pasadena, Calif., helped to piece together what happened when Comet LINEAR (C/1999 S4) disintegrated in July 2000, and their results will appear today in a special issue of Science featuring studies of the comet.

Scientists watched the comet break up when it was nearly 115 million kilometers (72 million miles) from the Sun. NASA's Hubble Space Telescope and the Very Large Telescope took pictures at different resolutions and different times. From the pictures, scientists learned the details of how the comet broke up. The team was led by Dr. Hal Weaver, an astronomer at the Johns Hopkins University in Baltimore, Md. The fragments have spread out, to disappear forever into deep space. The mini-comets that the scientists saw ranged from about some 50 to more than 100 meters (165 to more than 300 feet) across. Today, the pieces will be roughly 600 million kilometers (400 million miles) from Earth.

"One question we tried to answer was, 'Did everything happen at one time, or did the pieces of the comet slowly fragment off?'" said Dr. Zdenek Sekanina of JPL, the paper's second author. He identified some of the fragments in the pictures from Hubble and the Very Large Telescope, determined their sizes and relative motions and the times they separated. "We found that the comet's breakup was gradual but episodic. Also, the distances among the mini-comets grew as time went by, and we wanted to find out how rapidly."

There are two forces working on the different distances between the mini-comets, Sekanina said. One is that the fragments broke off at different times. The other is that gases flowing from the broken chunks of dust and ice were propelling them to different speeds depending on their size.

Sekanina predicted that the tail would become a narrow, bright band, made from the sunlight-reflecting dust released as the comet crumbled. While the new tail was relatively bright at first, the comet's original head disappeared, confusing calculations of the orbit. The last pictures of the tail were taken in the second half of August 2000, about four weeks after the event. Then the comet's remains vanished forever.

Dr. Michael Keesey of JPL calculated the comet's orbit, its distance from the Sun, its probable origin and its angle to Earth. It was a long period comet, born in the Oort cloud, which is postulated to extend from outside the orbit of the farthest planet, Pluto, to about 30 trillion kilometers (20 trillion miles) from the Sun. It took comet LINEAR about 60,000 years to travel once around the Sun.

The comet, popularly called LINEAR for the site of its discovery, the Lincoln Near Earth Asteroid Research, Lexington, Mass., was one of several dozen comets discovered in this way.

Another comet discovered by LINEAR, C/2001 A2, recently broke up as it was nearing the Sun. It was observed to undergo an outburst in late March 2001, which may have signalled the splitting. Breaking up may be a common end for comets, Keesey said.

JPL is managed by the California Institute of Technology in Pasadena for NASA. A picture of the comet is available at .

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