Friday, July 26, 2002

Eighteen minicomets trailing a comet in a mosaic across about 620,000 miles have been discovered by University of Hawaii astronomers. Letters indicate the eighteen minicomets trailing comet 57P.

UH astronomers
discover 18 minicomets

The scientists noticed the smaller
bodies trailing a larger comet

By Helen Altonn

University of Hawaii planetary astronomers have discovered 18 minicomets trailing a comet in a mosaic across about 620,000 miles.

"It's just a spectacular thing," said Dave Jewitt of the Institute for Astronomy, who found the "zoo of tiny minicomets" with postgraduate researcher Yan Fernandez and graduate student Scott Sheppard.

University of Hawaii

The minicomets are actually fragments from comet 57P/du Toit-Neujmin-Delporte.

Jewitt said his team "had a hunch" there might be more fragments after one was found associated with the comet about 10 days ago. They began searching the sky on the nights of July 17-18 with the UH 88-inch telescope on Mauna Kea, "the smallest and oldest one," Jewitt said. "It is still very competitive."

They made a digital map of the sky around the comet and captured a panoramic view of the fragments. They identified them by taking successive images of a field and detecting their motion against the background stars.

The discoverers are not sure of the sizes but estimate the brightest fragments are probably less than a few hundred yards across, smaller than Diamond Head Crater. The smallest ones are about the size of a house. They speculate that chunks of the comet broke off the nucleus after a violent event.

Comets consist of water, ice and rocky material formed in the early days of the solar system.

"Fundamentally, it's a mystery why comets split," Jewitt said in an interview. "It seems to suggest they are very fragile bodies. The best guess is they break apart when heated by the sun. Heat from the sun causes the ice, basically, to melt, then give off gas or steam, which may blow the nucleus apart."

The warming and evaporating ice would produce great thermal and physical stresses to the body of the nucleus.

"But we don't really know; that's only one idea," Jewitt said.

Another possibility is that material might be thrown off the nucleus because "it is spinning so quickly that gravity can't hold on to it any more," he said.

Normally, he said, only vapor and tiny dust grains fly off the nucleus and a comet with a long tail is seen on Earth, such as comets Hyakutake and Hale-Bopp in the late 1990s.

Comet 57P/du Toit-Neujmin-Delporte, named for the three people who discovered it in 1941, is too faint to be seen even with binoculars. The "57P" means its the 57th comet in the list of comets seen on two of their passages around the sun. The first one listed, "IP," is Halley's Comet.

The discovery of the 19 fragments, including the initial one reported, has been announced by the Central Bureau for Astronomical Telegrams, the official clearinghouse for reporting cometary discoveries.

The Hawaii astronomers will continue monitoring the fragments, which they said "provide a laboratory for us to witness major evolutionary events and can help us understand a comet's basic constitution."

They expect most of the pieces to fade and become invisible but said some may last for years.

Historically, split comets are quite rare, Jewitt said.

"Until recently, we knew of only a handful of split comets, but the more we look, the more we see.

"What's happening is we have better telescopes and detectors and are just more aware of the possibility they might be split, so we find them more efficiently."

The three astronomers are among other UH scientists attending an international meeting in Germany on asteroids, comets and meteors. A meeting is held every three years to discuss small objects of the solar system.

"The whole field is just exploding," Jewitt said before departure.

He expects a lot of discussion at the Berlin meeting on the newly discovered 1.2 mile-wide asteroid, 2002 NT7, and speculation that it could strike the Earth in 2019.

"The media has hyped it up a little bit," Jewitt said. "It has a very small probability of hitting the Earth -- one in a million."

UH Institute for Astronomy

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