Although sharing an orbit with a planet may seem an unlikely way for an asteroid to avoid collisions, being placed at the eye of the dynamical storm does offer some form of protection. The discovery by Wiegert et al. that the unnamed asteroid 3753 shares its orbit with the Earth is the latest and most unusual example of a co-orbital resonant configuration that has been known for more than two hundred years.

    The Italian-French mathematician Joseph-Louis Lagrange (1736-1813) studied the problem of three bodies moving under their mutual newtonian gravitational attraction. In 1772 he showed that a system consisting of the Sun and a planet had two equilibrium points located in the same orbit as the planet, one leading by 60 degrees and the other trailing by 60 degrees, each forming an equilateral triangle with the two masses. Lagrange showed that objects placed at these points were stable to small displacements provided the planet-Sun mass ratio was less than 0.04, and he predicted that Jupiter might harbour material at its leading and trailing points. It was not until the detection of 588 Achilles in 1906 that Lagrange's prediction was finally verified. There are now known to be several hundred such objects moving around Jupiter's triangular points, referred to as the Trojan asteroids. Not only was the type of orbital behavoir envisaged by Lagrange mathematically possible, but nature had obligingly provided actual physical examples.
    Asteroid 3753 won't hit the Earth soon, and we may well avoid physical contact with this 2 mile wide companion indefinitely. Many mysteries remain, however, about how these shared orbit systems evolve. At least for now there is a growing realization amoung planetary scientists that an improved understanding of these intimate planet-asteroid relationships is essential to studies such as the formation of planets to the dynamics inside ringed planets like Saturn.

LINK TO: Artemis Data Book - M.3.12 Orbital Mechanics - Lagrange Points.

LINK TO: Asteroid Research Bill Bottke's web site on asteroids, with pictures and interesting facts.