Re: Why are satellites usually in the same orbital plane?

There are two reasons most moons orbit their parent planets in the same plane: one is that they formed that way, and the other is that tides from the parent body change the orbit over a long time to be more along the planet's equator.

As current theory holds, planets and their moons form out of a swirling disk of gas and dust. This tends to flatten out due to angular momentum, which is why the planets in the solar system all have approximately the same orbital plane. On a smaller scale, the moons will orbit in the same plane as the planet's equator because of this as well.

However, there is a theory that the Earth's Moon was formed when a large body, about the size of Mars, slammed into the Earth. The resulting impact blew off a lot of matter which coalesced into the Moon. In this case, the Earth exerts tides on the Moon, which tends to line up the Moon over the Earth's equator (in our case, the Sun exerts a hefty tide on the Moon as well, which is why the Moon does not orbit directly over the Earth's equator).

A detailed discussion about how tides affect the orbit and spin of the Earth and Moon can be found at my Bad Astronomy web site. Another good place to look is Bill Arnett's The Nine Planets, which also has info about the Moon.