Newly discovered asteroid could be Earth's companion

Astronomers from the Armagh Observatory in Northern Ireland have found that a recently discovered asteroid may be intimately related to the origin of our planet.

They found that the asteroid, discovered by the WISE infrared survey satellite launched in 2009 by the United States, has been following the Earth in its motion around the Sun for at least the past 250,000 years.

"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 "Tolis" Christou.

Most near-Earth asteroids -- NEAs for short-- 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 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.

Christou and David Asher created 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.

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.

"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," he added.

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.

Their work appeared in the journal Monthly Notices of the Royal Astronomical Society.