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Observations continue of an unusual, small asteroid, 2010 SO16, discovered by the infrared satellite WISE. Because of its large proper motion (left), the magnitude 20.7 object was immediately classed as a near-Earth asteroid, the class of objects, normally with quite elliptical orbits, that can pass close to our planet.
But an observing campaign carried out by Apostolos "Tolis" Christou and David Asher, of the Armagh Observatory (Northern Ireland) with the Faulkes telescope of Las Cumbres Observatory Global Telescope Network, has revealed that 2010 SO16 has an orbit very similar to that of the Earth, with a mean distance from the Sun almost identical.
This is not the only object with such an orbit, three others are known, but they are all much smaller than 2010 SO16, which has a diameter estimated to be 200-400 metres. However, there is another fundamental difference: computer simulations that followed the object from 2 million years in the past to 2 million years into the future, show that the orbit is more stable than those of its smaller "brothers", and is stable over timescales of 250 thousand years.
As seen from a point of reference that rotates with the Earth, the asteroid traces a horseshoe path (right image) with the Sun at its centre, over a period of 175 years. The Earth lies between the two ends of this figure, and so the asteroid never gets any closer than about 20 million kilometres to Earth. At the moment it is located right at one of the extremes of the horseshoe and so is as close to us as it ever gets.
It's Earth-like orbit makes it especially interesting to understand from where it came. According to Christou and Asher it is unlikely to have ended up in its present orbit if it were once part of the main belt of asteroids between Mars and Jupiter, and it is also unlikely to be a fragment of the Moon which escaped the gravitational pull of the Earth-Moon system.
The most likely hypothesis is that it is actually material left over from the formation of the Earth and that it is the only member so far observed of a population of bodies, located at the L4 and L5 Lagrangian points of the Earth's orbit (at +60 and -60 degrees). Such objects have long been searched for but would be very difficult to see given that they would always lie close to the Sun in the sky. 2010 SO16 could have been removed from such a group after a collision with another asteroid.
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