According to a August 14, 2014 article in Physics World, a researcher in Italy is proposing a mission to an asteroid called 3753 Cruithne, a five kilometer wide near Earth object that shares an orbit with the Earth, but maneuvers about it in what is referred to as a “horseshoe” orbit.
According to a page on Cruithne, a horseshoe orbit can be described thus:
“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.”
Cruithne’s actual orbit is a little more complicated than that as it tends to spiral from side to side, sometimes moving faster than Earth, sometimes slower. Its closest approach to Earth is 12,500,000 kilometers away.
The Italian plan is to send a small satellite propelled by an electric ion engine to rendezvous with Cruithne. Then it would deploy two even smaller CubeSats to perform closer examination of the asteroid.
Cruithne is of scientific interest because its makeup is thought to have persisted from what it was during the origin of the solar system. Visiting it could tell much about how the solar system was formed.