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Cruithne

Holy Cow, I never heard of this moon! Thanks for posting this. Here's some more info I found about Cruithne. Apparently, there may be a whole bunch of these things out there.

MANY MOONS
From Andrew Yee
[http://helix.nature.com/nsu/991007/991007-2.html]
Many moons
By PHILIP BALL
The Moon might have a whole clutch of hidden siblings, according to planetary scientists Carl Murray and colleagues of Queen Mary and Westfield College in London, UK. In a paper published in the 27 September issue of Physical Review Letters[1] they show that asteroids that pass close to the Earth can become trapped in weird orbits around our planet.
One such asteroidal companion to the Earth has already been discovered. In 1997, scientists in Canada and Finland reported that the asteroid Cruithne, a chunk of rock wandering between the orbits of Mercury (the innermost planet) and Mars, is following a path that is linked to the motion of the Earth (see Nature 387, 685; 1997).
Cruithne does not go around the Earth, like the Moon itself -- its trajectory is considerably more complex. Basically it travels in loops shaped like a kidney bean, which lie beyond the Earth. As the Earth circles the Sun, it drags Cruithne's loopy path with it. But the motion is even more complicated than this, because the loop runs slightly ahead of the Earth, completing almost a full circle until it approaches the planet from the other side -- whereupon it changes direction and creeps back again.
At its closest point (which it reaches every few hundred years), Cruithne comes within just 10 million miles of the Earth. Its path actually overlaps the Earth's position, but there is no risk of collision because the kidney-bean loops are tilted at an angle to the plane of the Earth's orbit -- so Cruithne passes over our head, as it were.
The only other known examples of such peculiar "horseshoe" orbits are those of two of Jupiter's satellites, Janus and Epimetheus. But neither has such a complex relationship to its mother planet as does Cruithne does to the Earth, and Murray and colleagues have examined the theoretical aspects of the asteroid's motion to develop a better understanding of how it arises.
What they found was that Cruithne's strange dance represents just one manifestation of a whole class of "co-orbital motions" -- that is, of asteroids whose orbits are tied to those of a planet. These motions become possible if the asteroids pass by a planet on orbits that are very elongated (rather than circular) and tilted with respect to the plane of the Solar System.
Under these conditions, the planet can capture the asteroid, forcing it into co-orbital motion for periods of several thousand years. But because the motion of many-body gravitationally bound systems like the Solar System is intrinsically chaotic, these periods of capture don't last forever -- the asteroid might escape to drift at random, before perhaps then being recaptured in a different kind of orbit.
The researchers identified at least one other known near-Earth asteroid that might share Cruithne's fate, becoming temporarily enslaved to the Earth. They say that both this asteroid, called Khufu, and Cruithne itself could in the future adopt orbits that do actually circle the Earth, like the Moon -- but going "backwards", in the other direction. Their calculations predicted that, in the past, Khufu could already have behaved in this way for 35,000 years. It may be that our planet even now has such "retrograde" moons, too small to be easily spotted.
See the York University and Tuorla Observatory site http://www.asteroid.yorku.ca/ to learn more about the asteroid Cruihne.
[1] Namouni. F, Christou A.A., & Murray C. D. Coorbital Dynamics at Large Eccentricity and Inclination. Physical Review Letters 83, 2506; (1999)
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