Icy Split: Comet fragments into 19 pieces

For comets, breaking up isn’t hard to do. When these relics of ice and rock from the formation of the solar system pass close enough to the sun, the ice evaporates, creating a dramatic dusty tail. Sometimes, the stresses are strong enough to break off chunks of the comet’s nucleus.

BIG BREAK. The newly discovered fragments (circled) of Comet 57P/du Toit-Neujmin-Delporte. Fernandez, Sheppard, Jewitt

Still, astronomers were surprised last month to discover that a comet had recently split into at least 19 pieces. The fragments of the comet, named 57P/du Toit-Neujmin-Delporte for the three astronomers who discovered the body in 1941, are strung along a 1-million-kilometer chain. The pieces appear to vary in length from a few tens of meters to a few hundred meters. Only one other known comet, Shoemaker-Levy-9, which crashed into Jupiter in 1994, has shown more fragments: 23.

Yan R. Fernandez, Scott S. Sheppard, and David C. Jewitt of the University of Hawaii in Honolulu became interested in the comet after another team reported that the icy body had a companion–a broken-off fragment. The Hawaii researchers observed the comet on July 17 and July 18 and discovered many more fragments. They report their findings in a July 20 circular of the International Astronomical Union.

The diversity of the fragments demonstrates that comets may just as easily eject large chunks as vent small amounts of gas and dust, says Fernandez.

“We notice the gas and dust of the ‘typical’ comet when it is active and the big obvious splitting events, but we don’t generally notice when the comet shoots off little faint minicomets,” he notes.

From the distances that the pieces have traveled from the nucleus, Fernandez calculates that the breakup happened at least several months ago. Another clue about the timing is the unexpected brightening of the comet shortly after March 1996, its last close approach to the sun. The brightening might have been a sign of a breakup, says Donald K. Yeomans of NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

“The breakup of comets for no obvious reason, or due to even very gentle [gravitational] forces, suggests that some comets are wimpy, fragile objects with very little strength,” says Yeomans.

Most comet fragments previously observed have lasted only days. But Fernandez and his colleagues suspect the du Toit-Neujmin-Delporte fragments may persist for several more months. Besides keeping a close watch on these chunks, the team will examine the comet’s remaining nucleus. “It is possible that more of the nucleus has been exposed, and some fresh, previously underground material” can now be observed, Fernandez notes.

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