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J. L. Margot (California Institute of Technology), P. D. Nicholson (Cornell U.)
There are currently about a dozen known near-Earth objects with well-determined orbits and semi-major axis between 0.99 and 1.01 AU (Ted Bowell's asteroid database, 2003). We examined their orbital trajectories using the Horizons integrator (Giorgini, 1996) in an effort to find asteroids on Earth horseshoe orbits. Two objects (2002 AA29 and 2000 PH5) displayed a recent abrupt reversal in the evolution of their ecliptic longitude with respect to that of Earth, indicating a classic horseshoe or tadpole behavior. In a Sun-centered frame co-rotating with Earth, their trajectory displays the horseshoe pattern with the expected libration period of ~100 years.
2002 AA29 was previously recognized as being on a horseshoe trajectory (Connors et al., 2002). Wiegert et al. (2002) suggested that 2000 PH5 and 2001 GO2 are on horseshoe orbits, although their claim rests on a single 4.5-day observational arc for 2001 GO2.
Although the mean longitude of 2000 PH5 always remains at least ~25 degrees away from the longitude of Earth, the asteroid makes very close Earth approaches, within a few lunar distances. This is due to its significant ~0.2 eccentricity and the corresponding epicycle-like motion that is superimposed on the libration in mean longitude. The fact that this object happens to have just the right eccentricity to bring it so close to Earth suggests that it may have been barely ejected from the Earth-Moon system into an heliocentric orbit.
Goldstone radar observations conducted by JLM and collaborators show that the object does not appear to be man-made. Higher resolution observations with the Arecibo radar will be conducted in an attempt to constrain its plausible source region. Because of its peculiar origin, repeating close approaches to Earth, and low delta-V, this object may be an attractive target for a sample return mission. A long-lived transponder on its surface would also provide interesting dynamical information.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.