THE EARTH HAS A COMPANION IN ADDITION TO THE MOON. The object, asteroid 3753, is in orbit not around the Earth but in concert with it in a horseshoe-shaped trajectory that co-rotates with the Earth in its orbit around the sun; with respect to the sun, the trajectory is an eccentric ellipse somewhat inclined to the ecliptic plane. The discovery in 1906 of such an asteroid, 588 Achilles, near Jupiter, confirmed Joseph-Louis Lagrange's theory that planet-sun systems have points (now called Lagrangian points) where a third object of negligible mass could reside in stable equilibrium. Many such captive asteroids have been since found near our planetary neighbors, but only two of them, Janus and Epimetheus (companions of Saturn) have horseshoe orbits. Asteroid 3753, 5 km in diameter, was first spotted in 1986, but its trajectory was not understood until now with the numerical modeling research of astronomers at York University (Ontario) and the University of Turku (Finland). (Paul Wiegert et al., Nature, 12 June 1997.)