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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.

In one of Schultz’s scenarios for Deep Impact—the most likely one, he thinks—material will spray out from the crater in a nice conical pattern. In others it also shoots straight back out the hole like sparks from a Roman candle. Some scenarios have the impactor getting embedded in the comet, and in one it goes right through the nucleus and comes out the other side. The last outcome, says Schultz, is so improbable that it is mentioned “almost tongue-in-cheek. But it shows you what we know about comets.”

Schultz conducts his gun tests with a projectile made of Pyrex; that material shatters at the slower velocity of the simulations, just as the copper impactor will shatter when Tempel 1 hits it at a higher speed. When A’Hearn first started working on Deep Impact, some people, no doubt hoping for the biggest possible boom, suggested the impactor be made of the heaviest materials they could think of, including uranium. But to dig a crater most effectively, says A’Hearn, you really want something about the same density as the comet. In fact, the engineers have carved little pockets from the copper projectile to reduce its density in order to more closely match the density estimated for Tempel 1.

And because the projectile will vaporize on impact, it has to be made of an element that won’t chemically combine with water from the comet, confusing the spectrometer readings taken by the mothership. That requirement ruled out aluminum, for example. Ball Aerospace, which built the spacecraft, had gotten a good deal on electronics boxes made of magnesium, but A’Hearn had to nix that deal. The best materials turned out to be noble metals, like gold, silver, platinum, and copper. Having only $267 million to spend on their mission, the team went with copper.

Whatever transpires when copper strikes comet, it will happen in slow motion. When an asteroid smashes into Earth, a crater forms in a few seconds of unimaginable violence. On a tiny comet nucleus, with its extremely weak gravity—you could jump off the surface and never come back down—you’d expect the explosion to go faster. But exactly the opposite happens. “It is very counterintuitive, and it took me a long time to think my way through it,” says A’Hearn. Right after impact, displaced material starts coming out from the interior. The more time passes, the slower the material exits. The crater stops growing only when the stuff from the interior is moving so slowly that gravity pulls it back before it reaches the rim. But in low gravity, even stuff moving very slowly can make it to the rim, so the whole process takes longer.

Schultz predicts that Deep Impact’s crater will take 200 seconds to form, maybe longer, though not more than 500 seconds. To give themselves some margin, the science team has planned to have the mothership’s cameras and spectrometers observe closely for 800 seconds. “We don’t want to fly by until it’s all over,” says A’Hearn.

Low gravity also makes the crater end up much bigger. If the Deep Impact projectile hit an airless body with the mass of Earth, it would gouge a hole maybe 20 feet wide. Schultz thinks the hole in the comet nucleus will be 10 or even 20 times larger.

The drama may not end with cratering. One important question about comets, particularly old ones like Tempel 1, is whether centuries of swinging in toward the sun has caused their volatile components, like water, to have boiled away. If not, reservoirs may be bottled up inside that will vent once the hard crust is breached. If Deep Impact opens such a vent, says A’Hearn, “my guess is that it will come within minutes. It could certainly be hours. Days I think is unlikely.”

The venting could be violent, with large jets of gas spewing into space. And if the nucleus contains lots of water vapor, Schultz says, “we may cause an explosion inside the comet,” one powerful enough to break Tempel 1 apart. That’s unlikely, says Schultz, “but as an experimentalist, you never say never.”

Some of Schultz’s simulations show big plates of crust flying off after the impact. By tracing the ballistic arc of the plates, the scientists could determine the comet’s gravity, and therefore its mass—a fundamental property that has never been measured for a comet.

During and after the explosion, the mothership’s cameras and spectrometers will be busily scanning the crater and the icy dust that comes flying out. The pristine material A’Hearn hopes to see—ices that haven’t been crunched, melted, or altered by sunlight since they first formed—could be dozens of feet deep, or right below the surface. The important clues about the early solar system will be the relative abundances of water, carbon monoxide, and carbon dioxide. From the proportions, the scientists will be able to deduce the temperature at which the compounds formed. That in turn will help them understand the conditions under which the solar system was created.

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