An image of the asteroid 2010 SO16 taken by astronomers using the Faulkes Telescope North. The asteroid has an odd horseshoe-shaped orbit and has been trailing Earth for nearly 250,000 years, astronomers say.
Credit: Faulkes Telescopes/Las Cumbres Observatory
But as seen from Earth, 2010 SO16 gradually traces out a horseshoe shape in space, taking 175 years to go from one end of the horseshoe to the other. In this image, the Sun [S] is at the centre, the asteroid [diamond] approaches Earth [E] and then recedes from it.
Credit: The Armagh Observatory
ARMAGH: Earth has a stalker: a recently discovered asteroid which has has been tracking us around the Sun for at least 250,000 years, and may be intimately related to the origin of our planet.
The asteroid first caught the eye of the scientists, Apostolos ‘Tolis’ Christou and David Asher from the Armagh Observatory in Northern Ireland, two months after it was found by the U.S. Wide-Field Infrared Survey Explorer (WISE) satellite, launched in 2009.
"Its average distance from the Sun is identical to that of the Earth," said Christou, "but what really impressed me at the time was how Earth-like its orbit was."
How stable is the orbit?
Most near-Earth Asteroids (NEAs) have very eccentric, or egg-shaped, orbits that take the asteroid right through the inner solar system.
But the new object, designated 2010 SO16, is different. Its orbit is almost circular so that it cannot come close to any other planet in the solar system except possibly the Earth.
The researchers set out to investigate how stable this orbit is and how long the asteroid has occupied it. To do that, they first had to take into account the current uncertainty in the asteroid's orbit.
Simulating the clones' evolution
"Not knowing precisely the location of a newly-discovered NEA is quite common," explained Asher. "The only way to eliminate the uncertainty is to keep tracking the asteroid for as long as possible, usually months or years."
But the two scientists overcame that problem by creating virtual ‘clones’ of the asteroid for every possible orbit that it could conceivably occupy.
They then simulated the evolution of these clones under the gravity of the Sun and the planets for two million years into the past and in the future.
Several hundred thousand years in orbit
They found that all the clones remained in a so-called ‘horseshoe’ state with respect to the Earth. In this configuration, an object mimics very closely the orbital motion of our planet around the Sun, but as seen from Earth it appears to slowly trace out a horseshoe shape in space.
Asteroid 2010 SO16 takes 175 years to make the trip from one end of the horseshoe to the other. So while on the one hand its orbit is remarkably similar to Earth's, in fact "this asteroid is terraphobic," said Tolis, co-author of the paper in the Monthly Notices of the Royal Astronomical Society.
"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." This is where it is now, near the end of the horseshoe trailing the Earth.
