MadSci Network: Astronomy |
If asteroids pass close how will tides be affected or how will the orbit of the moon be affected at various points in its orbit? Or with a direct hit on the moon what will happen to its orbit at various points?
The Moon masses about 7.35 x 1022 kg. Its average distance is 3.844 x 105 km from the Earth, and it orbits the Earth in about 700 hours, or 2.52 x 106 sec, for an average orbital velocity (around the Earth!) of about 1 km/sec. The Moon's kinetic energy relative to the Earth, then, is about 3.5 x 1022 joules, equivalent to 8 million megatons of TNT or at least several thousand times the total destructive force of the entire world's nuclear arsenal.
To substantially effect the orbit of the Moon around the Earth, an asteroid would have to strike the Moon with a substantial fraction of that energy, let's say 1 million megatons. Since objects at the Earth's distance from the Sun are moving at about 30-50 km/sec relative to the Sun, and kinetic energy is given by E = ½mv2, we can quickly calculate that we would need an asteroid massing more than 1012 kg, or about a billion tons. This is large, but within the range of masses of known near-Earth asteroids such as 253 Mathilde or 243 Ida.
The effect of such a huge strike would depend on the exact details, including the direction of impact. But the odds are low, so low that nobody really knows them exactly (compare the likelihood of a strike by an object large enough - from one to five tons - to reproduce the 1908 Tunguska explosion, estimated as 0.25 to 2 per century).
Dan Berger |
Bluffton College |
http://cs.bluffton.edu/~berger |
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