  Following the discovery that asteroid (3753) Cruithne was a coorbital
companion of the Earth, a new theory of coorbital motion has been developed
whereby planets or satellites can maintain companion objects in the same orbit
as themselves. This has led to the prediction of hitherto unknown types of
stable motion, all of which are seen in the evolution of specific near-Earth
asteroids. The slow diffusion of such objects through the Earth's coorbital
region is shown to lead to temporary capture, suggesting the existence of
undiscovered retrograde moons of the Earth.
