Earth’s quasi-satellites

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.

1 of 2 people learned something from this entry.

  1. Tyestin said,

    September 17, 2011 at 5:39 pm

    (Knew it already.)

    I learned about them from watching the British panel show show Quite Interesting, which I highly recommend. :)

  2. Sean said,

    September 27, 2011 at 11:24 am

    (Learned something new!)

    Fascinating! However, the orbit in the little animation you posted, isn’t that the orbit of an object which is a satellite of the sun, rather than Earth?

  3. Kiri said,

    September 27, 2011 at 11:34 am

    Yes, exactly — the object orbits the Sun just like the Earth does. But because of its proximity and its particular orbit, which has the same period as the Earth, it still exerts a gravitational effect on the Earth (and vice-versa). Like the Moon, the quasi-satellite is sometimes between the Earth and the Sun and sometimes on the far side of the Earth from the Sun (i.e., it “orbits” the Earth with a very long period — equal to the period of its orbit around the Sun).

  4. Sean said,

    September 27, 2011 at 11:55 am

    Ah, which I could have read by clicking on the quasi-satellite link you provided. Hahahaha… I should read thoroughly before posting. :) You have a wonderful, fascinating blog.

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