MadSci Network: Astronomy |
Once in motion, a body keeps moving unless an external force acts on it. In the case of a satellite, its orbit eventually decays because at the distance that the satellite circles the earth ( only 36000 km) , there are still enough molecules to gradually slow it down. Of course the satellite's small mass makes it vulnerable to this effect. But with a much larger mass such as a moon or planet, the effect is much smaller especially since they encounter far less particles in the near-vacuum of space. The occasional cometary and asteroid debris are exceptions, but they are still of insignificant mass compared to that of the earth. Enrico Uva
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