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'Deep Impact' Probe to Try to Puncture a Comet

By Guy Gugliotta
Washington Post Staff Writer
Monday, January 10, 2005; Page A05

When it comes to space exploration, where scientists often measure their needs in milli-this and micro-that, Deep Impact, as its name suggests, has all the subtlety of a punch in the mouth.

Barring unforeseen delays, NASA will launch on Wednesday a 1,325-pound spacecraft on a one-way trip to the comet Tempel 1. On July 3, the spacecraft will jettison an 820-pound copper projectile in the comet's path and get out of the way as comet and projectile meet at a relative speed of 23,000 mph.

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This, perhaps not surprisingly, will happen on July 4, and if you are somewhere in the Pacific between Australia and the United States, you might be able to see it in the constellation Virgo, within a few degrees of Spica, for this ordinarily dreary and invisible-to-the-naked-eye comet is going to light up like a Roman candle.

During a recent news conference to introduce Deep Impact, Project Manager Rick Grammier predicted "great fireworks," and telescopes on the spacecraft, in space (Hubble, Spitzer, Chandra) and on the ground are going to be watching closely and recording every second of the event and its aftermath.

Deep Impact is the first human-made probe designed to hit a comet and penetrate to its nucleus. The size of the crater and the material that spews from it will give scientists their best-ever information about what comets are made of and how they formed.

"The biggest uncertainty is what [will happen] at the time of impact," said University of Maryland astronomer Michael A'Hearn, the mission's lead scientist. "It is this uncertainty that makes it important to do this conceptually simple experiment."

Comets date from the origin of the solar system about 4.5 billion years ago, forming from the whirling detritus of dust and gas that spread outward from the young sun to the chill nether reaches of space beyond the planets.

Because of their relatively pristine state, comets offer scientists the opportunity to glean new clues about the formation of the solar system itself. Volcanic activity, erosion and sunshine have given Earth and the other rocky planets and moons multiple geological makeovers, but comets, except for the surface layer, are pretty much the same as they were at the dawn of time.

Comets reenter the inner solar system when their orbits are perturbed, perhaps by impacts with other comets or by celestial disturbances that bring them under the gravitational influence of the large outer planets, which in turn fling them toward the sun to be transformed into the tailed fireballs pictured in textbooks.

Tempel 1 is a potato-shaped comet about 5 1/2 miles long and perhaps two miles across, according to Don Yeomans, a senior research scientist at NASA's Jet Propulsion Laboratory and a member of the Deep Impact team.

It is a "short period" comet that has migrated in from the Kuiper Belt, a ring of objects that orbit the sun beyond Pluto, between 2.8 billion and 9.3 billion miles away. Tempel 1 remains mostly between Mars and Jupiter in a 5.5-year elliptical orbit about the sun.

A'Hearn described Tempel 1 as a "typical" comet, believed to be composed mostly of dark carbon and hydrogen compounds, rocky material and water ice -- the prototypical "dirty snowball." It rotates once every 41 hours.

The basic question that Deep Impact hopes to address is how Tempel 1 is put together. The size and shape of the projectile crater and the debris trail that streams from it will provide important clues.

"We've got a betting pool in the science team, and the smart money says it's a rubble pile held together mostly by gravity," Yeomans said in a telephone interview. If it is, the projectile will make a relatively shallow but spectacular gouge in the comet "about the size of the Rose Bowl," Yeomans said.

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