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The Deep Impact mothership will capture information on the comet's freshly exposed primordial interior. (Pat Rawlings/NASA)

Comet Cracker

If you want to see what's inside a comet, you've got to break some spacecraft.

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MIKE A’HEARN THINKS BINOCULARS should be enough. When his 800-pound, copper-tipped spacecraft collides at six miles a second with an unsuspecting comet called Tempel 1 in July, no one, not even A’Hearn, knows exactly what will happen. There almost certainly will be a smash, and a splash, and a flash, as tons of icy grit freed from the heart of the comet spray outward into sunlight. The whole drama, he reckons, shouldn’t take more than 800 seconds. A’Hearn expects the brightening will be visible on Earth with a small telescope or binoculars, perhaps even the naked eye, and that millions of people will be watching. God knows he will. So will half the telescopes on Earth, amateur and professional alike.

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Closer to the action, the impactor’s mothership will be observing with its cameras and spectrometers from a safe distance of 310 miles, having separated and veered off from the smaller impactor 24 hours earlier. A’Hearn, a planetary scientist at the University of Maryland, and the rest of the team that designed the mission, which they call Deep Impact, could have placed the mothership even closer. But they chose the distance partly to protect it from dust impacts and partly to ensure that the instruments take in everything that comes flying out from the comet.

There are many ways to study comets, and scientists have tried most of them. The U.S.-European Solar and Heliospheric Observatory—SOHO—routinely watches comets fall into the sun from its vantage point a million miles from Earth. In the 1980s, European, Russian, and Japanese spacecraft flew close to Halley’s Comet, taking pictures and sampling the dust and gas boiling off the nucleus as the comet rounded the sun. Deep Space 1 visited Comet Borrelly in 2001, and Stardust came within 149 miles of Comet Wild 2 last year, grabbing dust samples that will return to Earth next January.

Deep Impact will be the first spacecraft to crack open a comet’s nucleus to see what’s inside. But if that makes it sound all big and bad, it’s not. The comet runs into the spacecraft, not the other way around. The TV-size impactor will wait in Tempel 1’s path like a bug on a highway, and when the four-mile-wide comet comes crashing into it, the spacecraft will vaporize instantly. Then, if all goes as planned, a 100- to 150-foot-deep crater will form, exposing the comet’s pristine interior—material that has been sealed up since it formed at the edge of the solar system billions of years ago. For A’Hearn and his team, this is the payoff of the mission: finding out precisely which elements make up Tempel 1’s nucleus.

As appealing as most people find the idea of smashing up a multimillion-dollar spacecraft, A’Hearn says that when the impactor was first proposed more than a decade ago, “the instinctive reaction was ‘That’s dumb—why would you want to do that?’ ” In fact, there’s no better way to see deep inside a comet; no present drill could go into space and make a hole that deep. Anyway, the destructive element never bothered A’Hearn: “If you think about what you know and what you don’t know, this is a way to find out things you don’t know.”

This isn’t just fun, people. It’s science.

Peter Schultz, a co-investigator for Deep Impact, has spent much of his career thinking about the physics of cratering. As a planetary geologist at Brown University in Rhode Island, he has investigated impact sites from Argentina to Canada, trying to reconstruct in detail what happens when a small body like an asteroid slams into a planet at ten times the speed of a rifle bullet. To predict the effect of Deep Impact’s collision, he’s been making his own craters at the Vertical Gun Range at NASA’s Ames Research Center near San Francisco. There, inside a bland, industrial-looking building, he shoots at different target materials with projectiles traveling at velocities up to four miles per second—not as fast as Tempel 1 will be moving, but fast enough so the results can be used to predict what might happen to the comet.

Schultz has experimented with pumice, a lightweight volcanic rock, as the target. He’s also tried pumice dust. He’s used perlite, a crushed rock commonly found in gardens; perlite with ice; and silica microbeads, the material used to make stop signs reflective. He varies the speed and angle of impact; he varies the porosity and density of the target. He’s done hundreds of test firings to master the nuances of cratering.

Comet nuclei are not easy to imagine. Having seen pictures of what look like solid objects, we think of them as rocks. But some, says Schultz, may be as fluffy as cotton candy. A’Hearn likens their insides to “very good powder snow for skiing.” The degree of fluffiness, porosity, lumpiness or smoothness—all these factors affect how deep a crater will form, and what shape it will take.

The speed of the impactor also makes a difference. If you stood over a deep pile of pumice dust, the kind Schultz sometimes uses to simulate cometary material, and dropped a metal rod pointing down into the pile, the rod would fall straight through. But at four miles a second, a rounded impactor forms a crater instead, shattering and melting in the process.

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