September 17th, 2011 at 10:29 am (Astronomy)
How many moons does the Earth have? Just one, of course. But I recently learned that right now the Earth also has five natural quasi-satellites. The companion bodies orbit the Sun with the same period that the Earth does, but with a different eccentricity. Our five quasi-satellites are 3753 Cruithne, 2002 AA29, 2003 YN107, 2004 GU9, and 2010 SO16.
Here is a depiction of 3754 Cruithne’s orbit:
These orbits are not stable over the long term (with respect to the Earth) because they lie outside the Earth’s Hill sphere, its region of moon attraction. Eventually they’ll move on to other orbits. But for now, we have these five extra companions in our journey around the Sun.