More (nearly) moons than you'd expect....

One piece of news that I've been meaning to write about is that Earth has acquired yet another moon. Yet another? I hear you cry, but John, we've got only the one! It's that huge great grey silver thing lurking a light second or so away. Some American fella went there in the sixties and found he couldn't take small steps, only giant leaps.

Well if you were thinking that then you're only half right. Earth has one big, full time moon. It also has several small part time ones.
Here's how it works: these part timers are asteroids whose orbits happen to take roughly the same amount of time to orbit the sun as earth. Hence, they orbit at the same distance, follow earth closely, and earn the term co-orbital. They wander back and forth along their orbits relative to earth, and when they enter the region of earth gravitational influence the can become temporarily caught up with earth, and begin to follow it through space, along a spiral path. This isn't a true Earth orbit, but it looks a lot like one, so objects that do this are sometimes called quasi satellites.

Our newly found friend is called 2009BD. It's about ten meters wide and has come within 400,000 miles of earth. That’s a tribute to how well and sensitively the skies are watched these days, although I expect that its remarkably similar orbit to ours made detection easier.
For the record this one’s no threat to earth. It could pack a punch similar to the Hiroshima bomb, but even if at some point in the future it winds up on a collision course, the overwhelming odds are that it would explode to far above the ground (like this ten meter meteorite) to do any real damage; ten meters is not large enough to punch through our atmosphere very well.
Larger, although still not a danger for millions of years at least, is 3753 Cruithne, which is about five KM across.
We actually know little about these small companions, although they have histories and characters all their own. It has been speculated that one day they might be useful for in space manufacturing, but right now they hold the same interest as all asteroids: they are likely to be remnants from the solar systems formation, and could teach us more about how earth and its companion planets came to be.
Also, given their proximity to earth, they might be places to look for material blasted off of earth during big impacts, and so might tell us something about our planets history directly.

So hopefully someone will take the opportunity to do some science and earn some bragging rights, and be the first to send a probe or even an person to one of earths other moons.....