NASA is planning on a smash hit with a comet

By Frank D. Roylance
The Baltimore Sun

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You can learn something about a rock by looking at it. But what most geologists really want is to smack it with a hammer.

That's just what planetary scientists will do July 4 when NASA's Deep Impact mission reaches the comet Tempel 1 after a trip of six months and 80 million miles.

If all goes well, an 820-pound copper "hammer" the size of a bathtub will separate from its mother ship and, 24 hours later, smash into the comet's icy nucleus at about 23,000 mph.

"It's bound to be a blast," said Lucy McFadden, a University of Maryland astronomer and member of the Deep Impact team.

The high-speed impact will wallop the pickle-shaped comet with energy equivalent to 4.8 tons of TNT, said Michael A'Hearn, another UM astronomer and principal investigator on the $311 million mission.

Nobody's sure what will happen next. There's a small chance the impactor will blow the 2-½-mile-long comet to smithereens, or simply bore through it like a bullet through a snowball. More likely, scientists say, it will blast open a crater the size of a football stadium. It all depends on what Tempel 1 is made of, and how sturdily it is composed.

Which is exactly what scientists hope to learn.

The blast also will reveal the comet's interior chemistry and nail down more precisely what conditions were like when it formed at the solar system's birth more than 4.5 billion years ago.

The Deep Impact spacecraft is undergoing final tests at Cape Canaveral, Fla. It will blast off atop a Delta 2 rocket at 2:40 p.m. Thursday, and if all goes well, rendezvous with Tempel 1 on Independence Day. There it will release the impactor and move off to watch the collision.
 
The copper hammer, equipped with a camera, guidance and maneuvering systems, will move directly into the comet's path, like a deer stepping in front of a speeding pickup truck.

"We will have Ranger-style images of the comet up until the point of impact," McFadden said, a reference to the National Aeronautics and Space Administration's three Ranger missions in 1964 and 1965. Each sent back dizzying photos as it fell toward the moon and crashed, providing the world's first close-up lunar images.

Several unmanned probes have made close comet fly-bys, beaming images and data to Earth. The most recent was the Stardust mission, which flew by Comet Wild 2 in January. It even captured a bit of comet dust for return to Earth in January 2006.

Astronomers say a more detailed knowledge of comets' composition would help to refine their theories about what conditions were like across the young solar system.

"The relative abundances [of different gases] tell you about the conditions when the planets were forming," A'Hearn said. "However, the gases that come out spontaneously are all coming from the surface."

Because those surface layers have been altered by heating during the comet's previous passages around the sun, scientists can't be sure they're seeing their primeval composition. That's the key point of Deep Impact, A'Hearn said — blasting a hole "to get down to a depth where the ices are relatively unprocessed."

Comet Tempel 1 was discovered in 1867 by German lithographer-turned-astronomer Ernst Wilhelm Tempel. It's a "short-period" comet that formed in the Kuiper Belt, outside the orbit of Neptune. It circles the sun once every 5-½ years, passing close enough to Earth on every other orbit to be easily reached and observed from Earth, McFadden said. That made it a perfect target for Deep Impact.

Tempel 1 normally is 10 times fainter than the dimmest object the naked eye can see, but it should brighten after the impact. "All of that ejecta will be reflecting sunlight," A'Hearn said.

Unfortunately, comet watchers will have to plant themselves somewhere between New Zealand and the southwestern United States to see it.