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These very curious celestial bodies are in-between a moon and an asteroid. They are called trojans: they follow the same orbit as Earth (around the sun), their path is locked to Earth and they won't collide with us. Gravitation law dictates their choreographic orbits.
In the July 28 2011 issue of the journal Nature , astronomers announce that the as-yet-unnamed near-Earth asteroid with the temporary designation 2010 TK7, shares the Earth's orbit with it. As the asteroid and the Earth both orbit the Sun, 2010 TK7 remains in step with our planet, remaining ahead of us as seen from Earth.
Like one of a pair of dancers performing a complicated tango, the asteroid moves in an elaborate path that brings it sometimes closer and sometimes farther from our planet. The Earth and the asteroid remain in sync however, with the asteroid always preceeding the Earth as they both move around the Sun.
Another way to think of this horseshoe is to consider a three-lane, circular race track. The Earth is a large truck moving at a constant speed down the centre lane and the asteroid is a car. When in the outer lane, the car is going a bit slower than the truck, and the truck starts to catch up. But just when the truck is about to pass, the car switches to the inner lane and speeds up. It then starts to pull away from the truck, but because the track is circular, the car will eventually catch up with the truck from behind. When it gets close, the car again switches to the outer lane and slows down. Then the whole cycle repeats. This is what is happening in a simple horseshoe. Both vehicles share the same highway, but in a coordinated fashion so as to avoid collision. In reality, the delicate coordination of the asteroid and the Earth is performed by the laws of celestial mechanics, and requires just the right conditions.
Is it possible to find videos showing the trajectory of these trojans? If you have 1,000 frames (pictures) representing one year worth of orbiting around the sun, feel free to contact me, and I'll create a great video that shows simulations of their movements, as in a movie.