[FONT=ARIAL, OPTIMA, HELVETICA]he idea in a nutshell:[/FONT]
At the time Earth formed 4.5 billion years ago, other smaller planetary bodies were also growing. One of these hit earth late in Earth's growth process, blowing out rocky debris. A fraction of that debris went into orbit around the Earth and aggregated into the moon.
[FONT=ARIAL, OPTIMA, HELVETICA]What were some earlier ideas?[/FONT]
One
early theory was that the moon is a sister world that formed in orbit around Earth as the Earth formed. This theory failed because it could not explain why the moon lacks iron.
A second early idea was that the moon formed somewhere else in the solar system where there was little iron, and then was captured into orbit around Earth. This failed when lunar rocks showed the same isotope composition as the Earth.
A third early idea was that early Earth spun so fast that it spun off the moon. This idea would produce a moon similar to Earth's mantle, but it failed when analysis of the total angular momentum and energy involved indicated that the present Earth-moon system could not form in this way.
http://www.psi.edu/projects/moon/moon.html
Theia is thought to have struck the Earth at an oblique angle, destroying Theia and ejecting most of Theia's
mantle and a significant portion of the Earth's mantle into space, while Theia's
core sank into Earth's core."the original mass of Theia ended up as an orbiting ring of debris, about
half of which coalesced into the
Moon between one and 100 years after the impact.
http://www.answers.com/topic/giant-impact-theory
The other half is MIA!!! Told ya so.