ScienceShot: Newfound Asteroid on Earth's Tail

Don't panic, but Earth has a celestial stalker. An asteroid discovered last fall moves in roughly the same orbit as Earth does. However, there's no need for a restraining order. Computer models indicate that for the foreseeable future, the object (denoted with an arrow in the photo) will stay at least 19 million kilometers away from our planet and, therefore, doesn't threaten a collision. Right now, the asteroid, dubbed 2010 SO16, is making one of its 

closest approaches to Earth, researchers at the Armagh Observatory in the United Kingdom report in April's Monthly Notices of the Royal Astronomical Society. Relative to Earth, the asteroid, which likely ranges between 200 meters and 400 meters across, moves in a horseshoe-shaped path that sometimes carries it to the far side of the sun. Simulations suggest that unlike the paths followed by three other known asteroids in such orbits, 2010 SO16's orbit has been stable for at least 250,000 years and will likely remain so for at least 200,000 years into the future. 

A long-lived horseshoe companion to the Earth

Apostolos A. Christou, David J. Asher

(Submitted on 31 Mar 2011)

We present a dynamical investigation of a newly found asteroid, 2010 SO16, and the discovery that it is a horseshoe companion of the Earth. The object's absolute magnitude (H=20.7) makes this the largest object of its type known to-date. By carrying out numerical integrations of dynamical clones, we find that (a) its status as a horseshoe is secure given the current accuracy of its ephemeris, and (b) the time spent in horseshoe libration with the Earth is several times 10^5 yr, two orders of magnitude longer than determined for other horseshoe asteroids of the Earth. Further, using a model based on Hill's approximation to the three-body problem, we show that, apart from the low eccentricity which prevents close encounters with other planets or the Earth itself, its stability can be attributed to the value of its Jacobi constant far from the regime that allows transitions into other coorbital modes or escape from the resonance altogether. We provide evidence that the eventual escape of the asteroid from horseshoe libration is caused by the action of planetary secular perturbations and the stochastic evolution of the eccentricity. The questions of its origin and the existence of as-yet-undiscovered co-orbital companions of the Earth are discussed.