If you want to see what's inside a comet, you've got to break some spacecraft.
- By Tony Reichhardt
- Air & Space magazine, May 2005
(Page 2 of 3)
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.
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.