Horseshoe shape of how the orbit appears from Earth

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…

The asteroid first caught the eye of the scientists, Apostolos “Tolis” Christou and David Asher, two months after it was found by the WISE infrared survey satellite, launched in 2009 by the United States. “Its average distance from the Sun is identical to that of the Earth”, says Dr Christou, “but what really impressed me at the time was how Earth-like its orbit was”. 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.

… So while on the one hand its orbit is remarkably similar to Earth’s, in fact “this asteroid is terraphobic”, explains 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”. This is where it is now, near the end of the horseshoe trailing the Earth…

… 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 as such objects are always near the Sun in the sky. If they do exist, they may represent relic material from the formation of Earth, Moon and the other inner planets 4.5 billion years ago.

For the time being, the astronomers would like to see the physical properties of the object studied from the ground, especially its colour. “Colour, a measure of an asteroid’s reflectivity across the electromagnetic spectrum, can tell you a lot about its origin”, they explain. “With this information we can start testing possible origin scenarios with hard data. If it proves to be unique in some way, it may be worth sending a probe to study it up close, and perhaps bring back a sample for laboratory scrutiny.”

Probably at least as interesting as, say, who might appear on Dancing With The Stars next season. And a lot more useful.