Asteroid 2010 SO16 - Earth stalker in a stable horseshoe orbit


O-F Administrator,
News Reporter
Oct 30, 2009
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Astronomers from the Armagh Observatory in Northern Ireland have found that a recently discovered asteroid has been following the Earth in its motion around the sun for at least the past 250,000 years, and may be intimately related to the origin of our planet.

2010 SO16, hereafter referred to as “SO16”, was discovered on 2010 September 17 by the WISE Earth-orbiting observatory (Obs. Code C51) and subsequently followed up by ground-based telescopes.

"Its average distance from the sun is identical to that of the Earth, but what really impressed me at the time was how Earth-like its orbit was", said Apostolos Christou, an astronomer at the Armagh Observatory in the United Kingdom, who led the study that pinned down the asteroid's orbit.

Christou and study co-author David Asher found that asteroid 2010 SO16 takes about 175 years to travel from one end of its orbit to another before doubling back. In diagrams, the asteroid's orbit resembles a giant letter "C" with the Earth ticked between the endpoints.


Currently, the asteroid is at a point in its orbit that brings it near the horseshoe's tip that trails the Earth. But despite its apparent attachment to Earth's orbit, the asteroid poses no risk of smacking our planet.

"This asteroid is terraphobic", Christou said. "It keeps well away from the Earth. So well, in fact, that it has likely been in this orbit for several hundred thousand years, never coming closer to our planet than 50 times the distance to the moon."

Orbital elements of 2010 SO16 at JD2455400.5 (MJD 55400):
Element|Value|1-σ uncert.

a (au)|1.00039|9.961e-6


i (deg)|14.536|0.0008793

ω (deg)|108.283|0.003205

Ω (deg)|40.523|0.001306

M (deg)|137.831|0.004636

Currently, three other horseshoe companions of the Earth are known to exist but, unlike 2010 SO16, these linger for a few thousand years at most before moving on to different orbits. Also, with an estimated diameter of 200-400 meters, 2010 SO16 is by far the largest of Earth’s horseshoe asteroids. The object’s absolute magnitude (H = 20.7) makes this the largest object of its type known to-date.

It could be an ordinary asteroid coming from the Main Belt between Mars and Jupiter. In that case, the random gravitational pull of the different planets would be responsible for its present orbit.

It could also be a piece of the moon that escaped the gravity of the Earth-moon system and went into an independent orbit around the sun. However, the very stability of its orbit means that there is currently no way to transport it from the moon to where it is now.

Finally, 2010 SO16 could represent leakage from a population of objects near the so-called triangular equilibrium points 60 degrees ahead of and behind the Earth in its orbit. Such a population has been postulated in the past but never observed, because such objects are always near the sun in the sky. If they do exist, they may represent relic material from the formation of Earth, the moon, and the other inner planets 4.5 billion years ago.

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