York University astronomers, working with a Finnish colleague, have discovered a mysterious "companion" to the Earth, shedding more light on the origins of our solar system.

York University postdoctoral researcher Dr. Paul Wiegert, York University astronomy professor Dr. Kim Innanen, and Dr. Seppo Mikkola of Turku University in Finland, have discovered that an asteroid accompanying the Earth in its orbit is the only natural companion to the Earth other than the Moon.

Discovered in 1986, Asteroid 3753 measures between one and ten kilometers in diameter, and has an inclined and eccentric path that crosses the orbits of both Earth and Venus. Although discovered 11 years ago, no one had tracked its path thoroughly enough to detect its rare "horseshoe" orbit. Wiegert and Innanen conducted computer simulations over the past eight months to support their research. Mikkola independently verified their findings.

The asteroid circles the Sun in just a bit under 1 year. Over time this slight mismatch allows it to overtake Earth, every 385 years to be exact. But as it comes near, the Earth's gravity adds a little energy, expanding its orbit. Suddenly circling the Sun in just *over* 1 year, the asteroid never catches Earth and moves away. Then, 385 years later, Earth catches up, this time *robs* 3753 of a little energy, and drops it into a smaller, faster orbit.