Big dust-up in outer space

Comet collision raises powdery cloud

scientists wait for clearing


July 15, 2005|By Dennis O'Brien | Dennis O'Brien,SUN STAFF

What happens when a comet crashes into an 820-pound piece of copper at 23,000 miles per hour?

You raise a lot of dust - and less ice and gas than you might expect.

"It looks like a mix of ultrafine material, kind of like talcum powder," said Michael A'Hearn, the University of Maryland astronomer who is principal investigator for the Deep Impact mission.

NASA launched Deep Impact Jan. 12 to probe the interior of the comet Tempel 1 by pulling up within 300 miles and releasing an 820-pound copper probe, known as an impactor, directly into the comet's path.

Dozens of ground- and space-based cameras, including the Hubble Space Telescope, recorded the July 4 crash, which occurred some 83 million miles from Earth. Now scientists are beginning to talk about what they found.

For example, astronomers have long thought that comets are balls of rocks and ice. But A'Hearn said a preliminary analysis of the Tempel 1 impact indicates fewer huge chunks of ice and volatile gases than expected - at least where the impactor hit.

"We know it's not like an ice cube," A'Hearn said. "It may be that there's this mix of ultrafine dust with small crystals of ice, but what we know is that these small crystals were not joined together in a big block of ice."

A'Hearn remains mystified as to why the surface of Tempel 1 appears so different from other comets photographed in 2001 and 2004. Images from the impactor show that Tempel 1 has two rounded pits that look like impact craters and a large, smooth surface. The other comets had no such features.

The size of the new crater on Tempel 1 - and whether we will ever see images of it - will not be known for about two weeks, he said. The amount of dust raised by the impact made the crater difficult to photograph in the moments after the impact - particularly with the focusing problem discovered on the fly-by spacecraft's high resolution camera.

The glitch was discovered after launch, but tight NASA budgets had forced scientists to eliminate a mechanism that would have allowed the camera to refocus in flight. NASA hopes that reconstruction software now being installed will allow scientists to refocus the digital images after the fact.

The $330 million mission is designed to answer questions about the nature and composition of comets, which are thought to contain the same materials that formed the solar system 4.5 billion years ago.

Astronomers will spend months studying the flash of the impact, the crater formed, the gases emitted, and any changes detected in the comet's heat, brightness and chemistry.

Astronomers at the European Southern Observatory say dust raised by the impact is from the comet surface and is not the pristine, subterranean material researchers had hoped would be raised. The plumes of dust disappeared days after the impact. The comet now looks much as it originally did and has gone "back to sleep."

Scientists are surprised that the impact, which hit with a force equivalent to exploding 5 tons of TNT, resulted in no increase in emissions of water vapor and far lower levels of other gases, such as carbon dioxide, than anticipated.

"A lot of us thought that there might be more, but nature's in the driver's seat here," said Gary Melnick, principal investigator of the ground- and satellite-based telescopes operated by the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

The center's ground- and space-based telescopes showed the same amounts of water vapor released after the impact as before it - about 550 pounds of water per second.

"It [the impactor] could have hit at an end of the comet where there's rocky materials and not as many volatile gases," said Paul Feldman, a Johns Hopkins University astronomer who recorded the impact with the Hubble Space Telescope.

Feldman said that the ejected gases identified so far include ethane and methanol, two simple, carbon-based molecules that are known to be precursors to life on Earth.

Tempel 1 is now millions of miles from the fly-by spacecraft and is heading away from the sun. The fly-by craft stopped photographing the comet last week, A'Hearn said.

A'Hearn said he hopes NASA will approve a $30 million mission to use the fly-by to probe the Comet Boethin in 2008. Boethin, discovered in 1975, orbits the sun every 11 years.

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