The solution may lie, paradoxically, in the fact that Mathilde -- as NEAR's measurements of its density indicate -- is not a solid chunk of rock but a "rubble pile" of small rock fragments stuck weakly together by their gravity.

Erik Asphaug and K.R. Housen reported that their lab simulations confirmed earlier beliefs that a fairly small impact on such an asteroid can blast a huge crater, and that most of the rocky debris -- being loose to start with -- is either blasted completely and permanently away from the asteroid or compressed down into the bottom of the hole.

The big question now is how many asteroids are such rubble piles (or clumps), rather than solid chunks of rock. Density measurements by Galileo and NEAR indicate that Eros and Ida really are solid rocks, but many other asteroids (including Mars' tiny moons Phobos and Deimos) may not be.

Contour will collect and analyze gas and dust to reveal a comet's makeup, greatly improving our knowledge of the key characteristics of comet nuclei and providing an assessment of their diversity.

Finally, while the comets -- the icy (and far commoner) analogues of the asteroids in the outer Solar System -- have so far drawn much less attention at the Conference, J.A. Nuth did propose the intriguing idea that the observed differences in the overall compositions of different comets, instead of being due to how close they formed to the sun, may actually be more a product of when they formed, due to the changes over time in the makeup of the initial "solar nebula" out of which the planets and smaller bodies formed in orbit around the Sun.

He predicts that "the average comet formed late in in nebular history will contain more hydrocarbons, ammonia and annealed dust than one formed earlier" -- an idea which, if it works out, may allow us to gauge the original formation ages of comets.

31st Lunar and Planetary Science Conference

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Satellite Launch To Boost DTH In India
Calcutta, India (SPX) Dec 28, 2005
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