On target. A fireball (brightest splotch) expands above comet Tempel 1 as a vertical column of debris (shadow cast toward top) rises from the collision with Deep Impact.

Deep Impact Makes a Lasting Impression

The two-part Deep Impact spacecraft--a bulletlike impactor and its watchful mothership--performed flawlessly on 4 July. Mission scientists have no doubt that Deep Impact gathered much of the raw data they need to understand at least some of the secrets hidden for 4.5 billion years within the 14-kilometer-long chunk of primordial ice and dust at the heart of comet Tempel 1. With the best close-up images of a comet nucleus and tons of comet crud flying into space, "Who would have thought cratering would be so much fun?" asks team member Peter Schultz of Brown University in Providence, Rhode Island.

First came the death-plunge pictures snapped by the incoming impactor. The nucleus of Tempel 1 "looks very different from Wilt 2's or Borrelly's," says Deep Impact principal investigator Michael A'Hearn of the University of Maryland, College Park. Those are the other two comet nuclei closely imaged by spacecraft. Unlike on those nuclei, "a lot of things on Tempel 1 look like [impact] craters," he says.

Then came the main event. Before the smashup, scientists "didn't have a clue" what was going to happen, says A'Hearn. In the images returned the first day, the initial sign of contact was a very small, faint dot of a flash, says Schultz. That was the impactor, a clothes-washer-size, copper-laden bullet, penetrating the surface. Then there was a "really bright flash" after the impactor penetrated the nucleus and vaporized, creating a ball of incandescent comet vapor.

At the same time, material shooting out of the penetration hole like a Roman candle cast a shadow across the nucleus, says Schultz. And a curtain of ejecta zoomed upward and outward. "It looks so similar to the experiments" in the lab, says Schultz. That implies to him that Tempel 1 is not armored by a thick, hard crust, as some had imagined, but wrapped in a soft, dusty layer.

Still, team members have yet to identify the much-anticipated crater hidden beneath suspended impact dust. Schultz thinks it will be big, that is, bigger than a house. Also yet to come is the analysis of the spectroscopic data that should reveal the composition of freshly exposed primordial material--presumably the same stuff that made up the planets.

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