# Earth's Moon origin

I have been reading and listening to discussions on origins and again today heard a scientist advance the idea that a meteor collided with the earth and the collision resulted in the Moon being ejected and finding its way to its current orbit.

Is this plausible?

Would it not be the same as a simple ballistic trajectory and quickly just return back to the earth?

Manmade satellites typically have an initial propulsion and later have another "burn" in order to insert it into its orbit.

Any help explaining this would be appreciated.

I have been reading and listening to discussions on origins and again today heard a scientist advance the idea that a meteor collided with the earth and the collision resulted in the Moon being ejected and finding its way to its current orbit.
This is a fairly well established theory, and in my experience people have a lot of confidence in the theory---while some of the details are fuzzy (continued below).

Is this plausible?
Would it not be the same as a simple ballistic trajectory and quickly just return back to the earth?
Its a very difficult problem to simulate, at least to get the exact "right" (that is: consistent) results. The overall picture is that a large impact (a body within an order of magnitude the size of the earth) collided with the earth, ejected a significant amount of surface material which conglomerated into the moon. I might actually remember something about the ejected material re-colliding with the earth, and the second round of ejecta forming the moon (but I might be making that up). In any case, the ejecta consolidated into the moon at a much much closer distance (at least a factor of 10), and it has progressively moved further away.

Manmade satellites typically have an initial propulsion and later have another "burn" in order to insert it into its orbit.
The details of stability of an orbit depend greatly on the specific features of the situation. Most man-made satellites require additional attention because they're in such close orbits (e.g. low-earth orbits) which dissipate orbital energy via friction with the atmosphere. An object as large and far away as the moon doesn't have this problem.

Do you know what postulated earth orbital elements have been suggested or used in calculations, prior to the impact; what the elements for the meteor were prior to the impact and if one looks at the resultant earth moon pair with their respective orbital elements afterward, is it possible to "connect the dots"?

This is a fairly well established theory, and in my experience people have a lot of confidence in the theory---while some of the details are fuzzy (continued below).

Its a very difficult problem to simulate, at least to get the exact "right" (that is: consistent) results. The overall picture is that a large impact (a body within an order of magnitude the size of the earth) collided with the earth, ejected a significant amount of surface material which conglomerated into the moon. I might actually remember something about the ejected material re-colliding with the earth, and the second round of ejecta forming the moon (but I might be making that up). In any case, the ejecta consolidated into the moon at a much much closer distance (at least a factor of 10), and it has progressively moved further away.

The details of stability of an orbit depend greatly on the specific features of the situation. Most man-made satellites require additional attention because they're in such close orbits (e.g. low-earth orbits) which dissipate orbital energy via friction with the atmosphere. An object as large and far away as the moon doesn't have this problem.

About when would this collision possibly have occured? I have seen dating information on moon rocks of about 3.5 bya. Presumably the moon would have been formed prior to the formation of its rocks??

Do you know what postulated earth orbital elements have been suggested or used in calculations, prior to the impact; what the elements for the meteor were prior to the impact and if one looks at the resultant earth moon pair with their respective orbital elements afterward, is it possible to "connect the dots"?
I can guarantee that the orbital elements of the impactor (which was more likely a planetoid) are unknown; as far as the earth before vs. after I'm really not sure---but a literature search would tell you. Try searching the arxiv, I think planetary science people are good about posting their stuff their.

About when would this collision possibly have occured? I have seen dating information on moon rocks of about 3.5 bya. Presumably the moon would have been formed prior to the formation of its rocks??
Again, your best bet would be to check the literature. I'm certain its between 3.5 and 4.5 byr, and my guess would be right in the middle.

Edit: see http://en.wikipedia.org/wiki/Giant_impact_hypothesis

Could we stop referring to the impactor as a meteor. It was a planet approximately the size of Mars. As HeadScratcher suspected a substantial proportion of the ejected material fell back to Earth. Some was ejected into further orbits and a small amount coalesced to form the moon.

The impact occured shortly after the formation of the proto-Earth, probably less than 50 million years after the formation of the solar system. The debris ring created by the impact probably took less than 100,000 years to collect together to form the moon.

The hypothesis is not wholly accepted, but the majority of planetologists would view it as the most plausible explanation. Most early objections realting to details of orbits and orientations have been addressed by more detailed FEA simulations.