Asteroid Crash May Explain Mercury's Strange Spin
by Charles Q. Choi, SPACE.com Contributor
Date: 11 December 2011 Time: 01:00 PM ET
First high-resolution image of Mercury transmitted by the
MESSENGER spacecraft (in false color, 11 narrow-band color filters).
CREDIT: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
When one body orbits another — say, a moon around a planet or a planet around a star — the orbiting body often spins. Our planet experiences day and night because it spins on its axis, regularly changing which side it exposes to the sun.
However, the gravitational pull that orbiting moons and planets experience slows the rate of their spin. The most stable arrangement they can reach is to keep just one side always facing the body they are orbiting. Such "tidal locking" is why our moon always keeps the same face pointed toward Earth.
Glenn A. Walsh, Project Director,
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