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In the 1969 film Doppelganger, scientists discover and then visit an Earth-like planet sharing our orbit but on exactly the other side of the Sun.

Since then, astronomers have ruled out the possibility of such a planet on the grounds that its gravitational effects on other planets and spacecraft would be easy to see.

But that doesn’t rule out the possibility of smaller objects sharing Earth’s orbit and today, Apostolos Christou and David Asher at the Armagh Observatory in Northern Ireland say they’ve found one–an asteroid called 2010 SO16

Near-Earth asteroids are common but SO16 is in a category of its own. First and foremost, it has an exotic horseshoe-shaped orbit (see diagram above) which astronomers believe to be very rare.

Its worth taking a few moments to think about horseshoe orbits. Two points are worth bearing in mind. First, objects further from the Sun than Earth, orbit more slowly. Second, objects that are closer to the Sun orbit more quickly than Earth.

So imagine an asteroid with an orbit around the Sun that is just a little bit smaller than Earth’s. Because it is orbiting more quickly, this asteroid will gradually catch up with Earth.

When it approaches Earth, the larger planet’s gravity will tend to pull the asteroid towards it and away from the Sun. This makes the asteroid orbit more slowly and if the asteroid ends up in a orbit that is slightly bigger than Earth’s, it will orbit the Sun more slowly than Earth and fall behind.

After that, the Earth will catch up with the slower asteroid in the bigger orbit, pulling it back into the small faster orbit and process begins again.

So from the point of view of the Earth, the asteroid has a horseshoe-shaped orbit, constantly moving towards and away from the Earth without ever passing it. (However, from the asteroid’s point of view, it orbits the Sun continuously in the same direction, sometimes more quickly in smaller orbits and sometimes more slowly in bigger orbits.)

For SO16, the period of this effect is about 350 years

Horseshoe orbits are thought to be very unstable, since any small gravitational tug can destroy the fragile resonance that has been set up. However, SO16’s orbit is surprisingly robust.

Christou and Asher simulated its orbit with slightly different values for parameters such as its semi-major axis. In these simulations, SO16 remained in a horseshoe-shaped orbit for at least 120,000 years and sometimes for more than a million years.

Astronomers know of three other horseshoe companions for Earth but these are all much smaller (SO16 is a few hundred meters across) and none have orbits that are likely to survive for more than a few thousands years.

That makes SO16 kind of special. For anybody willing and able to look, it is currently near one of its points of closest approach, with an absolute magnitude of about 20, lagging the Earth by 0.13 AU, like a stray puppy.

And it will be there for some time, say Christou and Asher. “It will remain as an evening object in the sky for several decades to come.”

Ref: arxiv.org/abs/1104.0036  A Long-Lived Horseshoe Companion To The Earth

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