Why isn the moon headed to the earth?
Satellites close to the earth experience orbital decay because they are actually still (just) in the earth’s atmosphere, and so there is a little drag affecting them. The space-station stays up because we use rockets to keep it in position. The moon and our geo-syncrounous satellites (and interplanetary missions, etc) are sufficiently far away that they are not experiencing any drag, and so move according to Newton’s laws of motion. Except that the moon is receding and will eventually depart the earth-luna system. Not sure on the reason, but it is probably just that the moon has some extra energy left over from its creation. This energy translates to an ever-larger orbit.
Ivars Peterson, in his very entertaining Newton’s Clock, reports work which asserts that our solar system as a whole is not stable, and will eventually come apart. As I recall, this was deduced from a claim that Pluto’s orbit is chaotic. Also as I recall, ignoring the changes in the Sun mentioned above, once the Moon begins moving back toward the Earth, it won’t stop before it reaches Roche’s Limit, at which point it will be torn apart by tidal forces, producing something like the rings of Saturn.
(I’m not a scientician, so this may be bullshit, but I’m surprised no one else has said it yet) The moon is in orbit because when it was captured by Earth it had roughly the right amount of momentum (and in exactly the right direction) to balance Earth’s gravity. If it had had significantly more, it would have gained atittude and eventually disappeared into space. If it had significantly less, it would have fallen to Earth long ago. But no, it had near enough to the right amount for neither to have happened yet.
