Deep impact

I went to the Deep Impact invited talk this morning which was really good. By sitting in the path of the oncoming comet the impact had a total energy of 19 GigaJoules. That is an awful lot of kinetic energy!

The approach images taken from the impactor (can someone link it below as I'm short of time) show a little bit of jitter at the start due to course corrections, but the final jitter is due to dust impact. The impactor was hit by three 1-10 mg dust grains within the last 21 seconds and 1 dust grain of up to 100 mg with three seconds to impact. That last one is not thought to be typical of the rest of the comet though. The dust sizes are mostly greater than 10 micrometres and are a mix of rocky dust and ice.

The comet itself is 3 kilometres (+- 0.1) wide and is covered with craters that have a distribution of sizes consistent with impact craters on other objects in the solar system. The comet nucleus has an odd shape and as it tumbles it changes in brightness in a strange way - the "light curve is double peaked" is the technical term for it.

So what happened at impact. The movie shows and incredibly bright and fast flash of ejecta which is likely to be composed of liquid silicate drops (mixed with copper from the impactor) that were ejected by hydrodynamics (liquid flow) rather than just a mechanical process (solid objects). Following this very brief (less than 0.5 seconds) flash, there was a plume of particles smaller than 10 micrometres that was connected to the surface for several hours. A total of about 10 million kilograms was ejected from the comet compared to the impactor which was only 360 kg. The crater formation was gravity controlled which I think means that the material didn't escape from the comet but landed back on the surface. Somebody correct me if I'm wrong. Although the crater has not yet been seen - not from want of trying by the Deep Impact team - the site of impact is known to an accuracy of around 10m and it is thought that the crater will be about 100m in diameter.

I should just add that tomorrow, due to the infinite wisdom of my supervisor, I shall be going to a meeting in Reading. I shall be back in Cambridge for Friday though so check back then.

Finally, Slacker Astronomy are raising money for the Pontchartrain Astronomy Society. They have been affected by Katrina and Aaron suggestions helping them out. Why not make your own deep impact and contribute?

Posted in astro blog by Stuart on Wednesday 07th Sep 2005 (18:27 UTC) | Permalink
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