Re: the 490K MPH
That velocity is of course relative. It would probably be more accurate to state that it is relatve to some fixed coordinate in the universe around us than the center. It is however the shared velocity of everything in this particular orbital distance from the center.
A hypothetical body wandering in from interstellar space would most likely be moving far slower especially to be slow enough to either be captured in solar orbit or to not impart a solar escape velocity to an impacted early Earth. (This raises the question of why such a high velocity collision did not impart a highly elliptical orbit after combining their inertia.) The length of time necessary to reach Earth over stellar distances is further increased... by a lot. This decreases the candidates for its source even more. This wandering rock would have to come from somewhere close to be at the required velocity to meet the criteria for our Moon maker.
The Galactic Environment of the Sun
http://www.americanscientist.org/template/AssetDetail/assetid/21173/page/2
“Our sun is also in motion. Relative to the average motion of the most commonly measured nearby stars, the sun moves with a speed of about 16.5 kilometers per second, or nearly 50 light-years per million years... The sun oscillates through the plane of the galaxy with an amplitude of about 230 light-years, crossing the plane every 33 million years. However, the sun’s motion relative to the local stellar neighborhood should not be confused with its movement around the center of the galaxy, since the whole solar neighborhood (including the sun) orbits the galactic center once every 250 million years. Just as we do not include the earth’s velocity around the sun when calculating the speed of an airplane (we are only interested in the ground-speed), astronomers do not include the sun’s galactic orbital velocity when describing its local motion.”
Could it have been another planet still forming between Mars and Jupiter (in the asteroid belt), that perhaps got knocked out of orbit by other chunks still there before it was completed, or flung out by the push/pull of Jupiter's gravity on the one side, and the sun and inner planets on the other?
-PJ