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Cryo Chamber

The Ultimate Target Shoot

All the brightest and booming-est pyrotechnic shows on Earth combined couldn’t for a nanosecond distract Mike A’Hearn from watching the Fourth of July ice works event that he’s planning for the year 2005.

On that day, while the rest of America oohs and aahs over rockets’ red glare, the University of Maryland astronomer and professor will watch a comet traveling at 28,000 mph smash into an unmanned NASA spacecraft that’s been hurtling toward its target for six months.

The explosion should be spectacular.

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At least that’s what he hopes to see while hunkered down at NASA’s Jet Propulsion Laboratory with his team of scientists, engineers and educators, no doubt keeping his fingers crossed that Comet Tempel 1 and the Deep Impact spacecraft perform as expected.

The scientific purpose of the collision: to carve a crater into the comet that will let scientists analyze the debris and peek at its interior, perhaps gathering clues to the beginnings of our solar system.

Comets are very old and very cold. Some 4.56 billion years ago, when our solar system took shape, the comets formed at a great distance from the sun.

Unlike planets, which have been melted and mixed up and remixed and turned over in an endless series of catastrophic processes that all but obliterated knowledge of the beginning composition of the solar system, comets (at least those, like Tempel 1, that are small enough that internally generated heat does not change things) have spent most of their time at a great distance from the sun. There, in a long, deep freeze, their innards have theoretically remained intact.

“Unprocessed ices, ices way below the surface that haven’t been warmed and refrozen, that’s what we’re after,” A’Hearn says. “Because they’re pristine. That’s the ultimate thing we’re after.”

As principal investigator for Deep Impact, A’Hearn is champing at the bit for a look-see at exactly what that material is and to get some sense of its relative abundance. He hypothesizes that it might be primarily carbon dioxide ice, the released vapor of which can’t be seen by the human eye, but can be detected by a spectrometer flying onboard the Deep Impact spacecraft.

Deep Impact is actually two connected spacecraft powered by one rocket: a collision spacecraft and an observer spacecraft, flying together through space until separation, just before the collision. It is scheduled to launch from Cape Canaveral, Fla., on Dec. 30, 2004.

“Astronomy is a passive science, not experimental,” says A’Hearn. “As astronomers, we look. We watch God do the experiments and we see what happens.”

But Deep Impact is different: “It’s a bit of the boy in the sandbox playing, throwing something at something and seeing what happens,” A’Hearn says. “The uncertainty of what’s going to happen makes the experiment worth doing.”

The experiment is elegantly simple. Two spacecraft — an impactor and a fly-by — will piggyback for six months of circuitous flight into deep space. Ultimately, the two will part company. The impactor will position itself in the path of the oncoming comet, snapping pictures and communicating with the fly-by until it vaporizes as a result of the collision. The fly-by, outfitted with cameras and a spectrometer, will speed off to a safe distance for taking pictures of both the collision and the creation of the crater.

All of this will happen about 82 million miles from Earth. A $279 million, seven-year-long project from proposal-to-collision, Deep Impact will culminate in a 15-minute-long show. Then it’s all over. The observer spacecraft will continue on its orbit, becoming a satellite of the sun.

From Earth-bound telescopes, the collision just might be able to be witnessed as a brightening of the comet, says University of Maryland astronomer Lucy McFadden, a co-investigator for Deep Impact. But only if the weather is clear. The chances of actually seeing something going on will be far better for those who log onto the Deep Impact Website, where images of the collision and the creation of the crater will be posted as near real-time as possible, McFadden says.

It will take seven minutes for the data to get from the comet to Earth, and about an hour for pictures of the collision to be posted on the Web.

There’s a 50 percent chance, A’Hearn predicts, that the comet’s crater will form in the same way that a crater forms when Earth is struck by an meteorite. Then again, a piece of the comet could break off, or it could break into many, many pieces. Or, the impactor spacecraft could shoot a hole straight through the comet. Nobody knows what will happen because the nature of the stuff that makes up the nucleus of the comet is unknown: It could be dense; then again, it could be fluffy.

Experimental and computer models predict the collision will result in a crater that’s 328 feet in diameter and 92 feet deep. But these measurements can be off by a factor of two in either direction, cautions A’Hearn, whose own best guess is a bigger crater than predicted. And that would be a good thing as far as the scientists are concerned, because it means there’ll be a better chance to witness the phenomenon.

The force of the impact is likely first to cause a column of the comet’s innards to shoot straight up; and next, to cause layers of the comet’s surface to peel back, like an onion, according to McFadden. Scientists will be looking at the nature of this stuff that shoots up and peels back on the surface as well as hoping to see deep inside the fresh crater hole.

“We’re not sure how well we’ll be able to see it,” she says. “As we’re clicking pictures of the debris coming out of the crater, we’ll learn about its mass. If it comes out and goes far, with a big splash, it’s not very dense. If there’s little splash, the stuff is strong and welded together.”

Of course, no one will see much of anything if the spacecraft doesn’t hit the side of the comet that’s facing the sun — or if the spacecraft and comet don’t collide at all.

“I am nervous about missing the comet,” A’Hearn says. “Our prediction is that the chances of missing it are less than 1 percent. But we’ve been surprised by comets. Cometary nuclei can be boomerang shaped. If you come at something bent from the wrong orientation, you might miss it, depending on your navigation.”

By analyzing the temperature and density of the innards of this comet, scientists hope to have a better idea about the nature of the protoplanetary disc that our planets formed in at the beginning of the solar system.

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