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

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 .