Professor now has date with a comet


A University of Maryland, College Park scientist has won NASA's approval to lead a $279 million space mission that any 10-year-old boy would understand and applaud.

Astronomer Michael A'Hearn will lead a team that's planning to find out what's inside comet Tempel 1 by smashing into it with a 771-pound copper "hammer" -- the biggest they could loft into space.

"It's a guy thing," said College Park astronomer Lucy McFadden, co-investigator on the project being led by A'Hearn. "It's going to be a blast, that's for sure."

The mission to put the hammer into orbit is scheduled for launch in January 2004. Impact -- scheduled for July 4, 2005 -- is expected to blow a seven-story-deep hole in the comet.

As debris from the comet's interior is ejected, sensors on board the main spacecraft -- 300 miles above the comet -- will analyze the chemistry of the debris and the crater walls, and radio the results back to Earth.

Scientists will also get video images of the impact and make them available for display on the Internet and broadcast television.

The event will also be studied from ground-based observatories. It might produce a sudden brightening of the comet visible to amateur astronomers with small telescopes.

The 3-mile-wide comet will not be destroyed or knocked from its orbit, McFadden said.

"It's like if you throw a pebble at a moving car," she said. "You're not going to knock the car off course, unless you frighten the driver."

Even so, the mission had to pass scrutiny from the National Aeronautics and Space Administration for violations of the space agency's "planetary protection" rules. The conclusion, McFadden said, was that the impact would destroy nothing unique.

"There are millions of comets," she said.

The $279 million mission, called Deep Impact, is one of two space science missions selected in 1998 for final design review under NASA's Discovery series of "better, faster, cheaper" space science missons.

Still awaiting NASA's final approval is Messenger, a Discovery mission to map and photograph the surface of the planet Mercury. The spacecraft would be built and managed by the Johns Hopkins University Applied Physics Lab in Laurel.

"What fascinated me personally about [Deep Impact] is, as the design went through, it became clear how little we truly know about comet interiors," said Tom Morgan, program scientist for Deep Impact at NASA headquarters. He was a member of the group that reviewed the proposal and last week cleared the team to start building the spacecraft.

Comets, he said, "are important constituents of the outer solar system and keys to understanding the origins of all solar systems."

Comets are composed of dust and frozen gases that scientists believe are little changed since the formation of the solar system 4.5 billion years ago.

The precise composition, and relative proportions of that dust and gas, hold clues to the materials and physical conditions present in the frigid outer regions of the solar system where comets formed.

But astronomers aren't certain that what they see in their telescopes -- the relatively large halo, or "coma" of gas and dust that has escaped from the comet's tiny nucleus -- is "pristine."

Some suspect that the material in the coma comes from surface ices that have been changed chemically and physically from their primordial composition by repeated passages around the sun.

If they want to draw reliable conclusions about the composition of the early solar system, they need a look beneath the surface, at the interior.

"Our idea was to disturb the top surface and expose the pristine material," said Alan Delamere, an engineer at Ball Aerospace, the Colorado contractor where Deep Impact will be built.

He and Michael Belton, of the National Optical Observatory in Tucson, originated the idea of a comet impact mission.

Images of Halley's comet taken in 1987 showed its surface to be quite black, suggesting it was covered by dust instead of its primordial ices, Delamere said.

"Ever since that point, I was really disturbed about what the surface properties were of the comet, and how little we really knew about the mechanism that made it as black as it is."

Deep Impact also could shed light on how fragile comets are. Although several recent comets have been seen to break apart, little is known about the forces needed to trigger the breakups.

Comet Tempel 1 was discovered in 1867, and it orbits the sun once every 5.5 years. It was chosen because it has had plenty of time for its surface materials to have been changed by the sun.

After its launch, Deep Impact will orbit the sun for a year, then cruise out to Tempel 1's orbit for its rendezvous. On July 3, 2005 -- the day before impact -- it will release its copper cannonball.

That object will navigate on its own to the comet's surface. It carries no explosives, but its mass and speed relative to the comet -- 22,300 mph -- will deliver energy equivalent to 4.5 tons of TNT.

Pictures and spectroscopic data on the blast will be gathered by instruments on board the main spacecraft, which will fly past the comet at a safe distance of more than 300 miles.

"We don't want to get any closer," McFadden said. Small dust particles could fog Deep Impact's camera lenses. Bigger debris could destroy the spacecraft.

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