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FROM THE MOON TO THE OORT CLOUD Part One - Part Two - Part Three
by Bruce Moomaw Cameron Park - April 4, 2000 - A more recent puzzle comes from the photos that NEAR took of the big asteroid 253 Mathilde when it flew by it in 1997, which revealed startlingly huge craters on Mathilde -- some of them wider than the asteroid's radius -- which nevertheless show no sign of cracking the asteroid or scattering chunks of rock across its surface. 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.
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.
LUNAR AND PLANETARY SCIENCE PART ONE
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