Comet's crater hidden, but plume tells story.

Deep Impact was a success! (But if we are going to name NASA projects after movies, we should have started with Ishtar.)

Seeing the crater produced by the Deep Impact mission's violent encounter with Comet Tempel 1 on Monday – one of the mission's key goals – could now be impossible.

The plume of gas and dust kicked up by the impact was much bigger, brighter and less transparent than expected. As a result, the crater itself, hidden behind the plume, will be very difficult to detect in the images taken by the flyby spacecraft.

But the science team has already figured out some indirect ways of determining the crater's dimensions, if the optical images cannot provide enough information. In any case, any problem with getting data on the crater challenge is far outweighed by the wealth of information returned from the first-ever deliberate comet impact.

And the show may go on for a while yet. Measurements by the Hubble space telescope and other observatories show the comet continued to brighten - and its new plume of ejected material continued to expand - for at least several hours after the impact.

And if the impact exposed a lot of fresh, volatile material at the bottom of the crater, the growing plume "could go on for weeks", according to the mission's chief scientist, Mike A'Hearn, at mission control at NASA's Jet Propulsion Laboratory in Pasedena, California, US.

Clearly, the science team has plenty to work on. Analysis of infrared images should soon reveal the precise temperatures of every part of the comet’s surface. This, in turn, will reveal how solid or porous the surface is, by showing how quickly it heats in response to changes in the amount of sunlight it receives.

As for the crater, it should be cooler than the surface, meaning it could be revealed by the infrared images.

And because the ejected plume casts a very clear shadow on the comet’s nucleus in optical images, it should be possible to determine the size of the plume's base - and that should correspond exactly to the size of the crater.

Analysis of the exact shape of the plume as it developed could also show just how deeply the impactor penetrated into the nucleus, and even the consistency of the materials, and whether it has a layered structure.
The plume will be crucial in revealing the composition of material in the comet's hidden interior, via spectroscopic analysis which is still being processed.

There are also the spectacularly detailed images of the comet's mountain-sized nucleus, taken before impact, which reveal a highly varied surface. The science team will try to interpret crater-like rings, dark linear scarp-like features, flat areas, and scattered bright patches.

Until that analysis is completed the science team is sticking to food analogies. A'Hearn said the nucleus "does not look like a pickle or a cucumber, it's closer to a loaf of bread or a muffin". And Jay Melosh, another Deep Impact team member, said that its internal porosity may make it "weaker than the weakest soufflé".
(Thanks to NewScience.com for the article above and the links below.)

NASA has produced a series of videos showing:
• The flash when the comet ran over the probe, as seen by the high-resolution camera.
• The flash, as seen by the medium-resolution camera.
• The approach of the probe to the comet, right up to seconds before impact.