Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Fozzie on 03/02/2011 10:58:15
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With so many near-earth asteroids passing us over the millions of years since the Earth formed, and several recent ones closer than one lunar distance, it seems strange that none of them seem to have been captured by the Earth and gone into orbit around it. Why is this?
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An asteroid passing near Earth would normally travel in an elliptical orbit and therefore depart as quickly as it arrived. To be captured something would have to slow it down, but there is nothing that is likely to do that (the moon is too small and distant to do it easily).
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The gravitational capture of one object by another is quite difficult because on the assumption that the object is approaching from a considerable distance it requires the involvement of other material or objects to happen. This is because if two bodies approach each other from a distance, unless they hit each other, they will just perform a hyperbolic orbit around each other and go on their way. To convert a hyperbola into an elliptical orbit the object needs to be slowed down either by dust and gas to cause friction or the gravitational deflection in the right direction by another body to the effect that it slows down. As far as hitting each other is concerned, this also is very unlikely because the approaching asteroid has to be travelling in a direction exactly towards the body like a rifle shot. Any slight error and the effect of the conservation of angular momentum will cause it to miss. On the scale of the solar system the earth is a very small object.
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Mars appears to have captured two small asteroids, are there any theories as to how this happened
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Mars appears to have captured two small asteroids, are there any theories as to how this happened
Their origin is uncertain, but they may have been formed as a result of impacts, in a similar way to our moon.
http://en.wikipedia.org/wiki/Phobos_%28moon%29#Origin