Horseshoe Orbit

A horseshoe orbit is a type of co-orbital motion of a small orbiting body relative to a larger orbiting body (such as Earth). The orbital period of the smaller body is very nearly the same as for the larger body, and its path appears to have a horseshoe shape in a rotating reference frame as viewed from the larger object.

The loop is not closed but will drift forward or backward slightly each time, so that the point it circles will appear to move smoothly along Earth's orbit over a long period of time. When the object approaches Earth closely at either end of its trajectory, its apparent direction changes. Over an entire cycle the center traces the outline of a horseshoe, with the Earth between the 'horns'.

Asteroids in horseshoe orbits with respect to Earth include 54509 YORP, 2002 AA₂₉, and 2010 SO₁₆, and possibly 2001 GO₂. A broader definition includes 3753 Cruithne, which can be said to be in a compound and/or transition orbit, or (85770) 1998 UP₁ and 2003 YN₁₀₇.

Saturn's moons Epimetheus and Janus occupy horseshoe orbits with respect to each other (in their case, there is no repeated looping: each one traces a full horseshoe with respect to the other).