Boffinry FTW
Being able to detect microns of difference, at these distances, and in a hostile environment?
Boffins...you rock.
NASA has announced that it expects to place two formation-flying space probes into orbit around the Moon on New Year's Eve and New Year's Day. The Gravity Recovery And Interior Laboratory (GRAIL) spacecraft are intended to map the lunar gravitational field with unprecedented accuracy, giving the boffinry community many useful …
not as obvious as it would initially seem:
"GRAIL-A will arrive in lunar orbit at 1:21 p.m. PST (2100 GMT) on Saturday December 31, and GRAIL-B on Sunday January 1 2012 at 2:05 p.m. PST (2200 GMT)."
but:
"one specific spacecraft (GRAIL-A) to follow the other (GRAIL-B)"
So GRAIL-A gets there first but actually ends up behind GRAIL-B; presumably it (A) does a few orbits of the moon then slots in behind B, once it arrives.
I think I've seen a claim that Luna isn't a proper satellite either. Something about it not being pulled backwards relative to Earth's orbit around the Sun, and therefore it's properly a binary companion. Is that so?
However
http://www.universetoday.com/15019/how-many-moons-does-earth-have/
as of June 2008, admits Luna and excludes Cruithne and 2002 AA-29.
I'm not sure what the official definition of a moon actually is, but possibly it involves being formed from the same debris, and at the same time, as the host planet? The Moon is now thought to have been formed later that the Earth, as the result of a collision between the Earth and another planet-sized body, I think. It is also much larger than most moons.
GJC
If a tiny rock got landed in Earth orbit it would technically be a natural satellite I suppose.
As for definition of a moon, well we called Luna a moon before we had any theory about it being blown off from the earth. I doubt the reason it forms matters so much as the orbit and size/structure.