<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>I read somewhere on the internet that saturn or two of jupiters moons change the direction of their orbit often, as though they bounce from each other or repel each other-this seems insane! Which moons ones are they and how is this done?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>
That is not quite what happens. When any two objects have very similar orbits, the laws of dynamics prevent their ever colliding. If one of them tries to catch up with the other from behind, the added gravity from the leading body accelerates the trailer, which increases its orbital period. Because it now takes longer to complete its orbit, it ends up falling behind. This type of motion is called "libration". Certain moons and "Trojan asteroids" do it.
As you see, nothing ever reverses direction in its orbit. But things that were once gaining can fall back, and vice versa. From a distant observer's perspective, it might appear that one of the bodies had "bounced off" the other. But actually, close approaches are forbidden.
This phenomenon greatly frustrated early astronauts trying to rendevous the Space Shuttle with a space station. When they wanted to catch up to the station, their instincts were to fire thrusters to go faster. But this increases their orbital period and causes them to fall back. Eventually, they had to let computers do the Shuttle rocket control. -|Tom|-
