Sunday, October 23, 2005

Smashing Comets For Fun, Education, And Profit.

Asteroids and comets are likely to be mined for their resources in a future space-based economy. Knowing the structure and composition of those objects beforehand will help to improve the odds of success. There were two speakers at the Space Frontier Foundation's annual conference this weekend who talked about that topic. One was Michael A'Hearn, the principal investigator for the Deep Impact mission.* (NASA mission website here.)

Ahearn reported on the preliminary findings from Deep Impact's collision last July 4th with the comet Tempel 1. He said the mission obtained so much data he could study it until retirement.

Here are some of his comments:

The mission flew for only 6 months in space from January to July of 2005. During that flight the spaceship encountered one "space weather event" in which the ship was saturated with radiation from the sun.

There are significant outbursts from the surface of the comet. A'Hearn showed a film clip of the impactor's approach to the comet. During the flight the view noticeably jiggled three times. These jiggles were caused by high speed impacts with milligram-sized dust particles coming off the comet.

There does not appear to be any uniformity of shape for comets. Comet Tempel 1 looks completely different from the other three comets that have been observed up close. So far the team can't explain how the surface terrain of the comet was formed. The bottom half of the comet is pocked with impact craters. (A'Hearn said that's what they were but he said in print he'll only call them craters with the characteristics of impact craters.) There is an upper portion of the comet that is also cratered but the terrain is more smooth as if the craters are older in that region. There are several distinct elevations. The bottom portion was lower than a flat plain that adjoined it, which in turn was lower than the upper portion of the comet. How the comet's flat plain was formed, he couldn't say.

The observed temperatures on the comet rule out the presence of ice on the surface of the comet. There is a region observed in shadows with lighter colored areas, which could be ice mixed in with dust but there was not enough data to make a conclusion. These lighter colored areas are more reflective than the rest of the comet, however, even those areas are pretty non-reflective. The comet is even less reflective than soot.

The heat from the sun does not appear to penetrate beneath the surface and there is ice down there fairly close to the surface. The ejecta from the impactor's collision included lots of water ice, lots of organics, all in very fine particles.

The comet's core is not very dense. The interior appears to be highly porous. The estimate is that throughout the entire core it is probably 70 percent empty space. So the comet's structural strength is very weak. Despite this, the impactor's collision did not affect the structural integrity of the comet or move the comet significantly in its orbit. The estimate is that the maximum change would be to push the comet's perihelion out about 100 meters. The collision did leave a typical gravity-formed crater.

What's the significance of this for commercial exploitation? Any prospector seeking to mine a comet will have to expect outbursts from the comet. The comet's low structural strength will complicate docking and operations on the surface. However, the presence of lots of organics and ice just below the surface improves accessibility to the resources.


* The other speaker on this topic was Thomas Matula. He proposed a program for making investigation of Near Earth Asteroids profitable for private companies. More on that in a subsequent post.

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