AN OBSCURE asteroid, just five kilometres across, is the Earth鈥檚 only known natural companion other than the Moon.
Asteroid 3753 was discovered in 1986 and looked entirely ordinary until Paul Wiegert of York University in Ontario and two colleagues ran computer simulations of the orbits of many asteroids that cross the Earth鈥檚 orbit, including 3753. They found that 3753 is trapped in a complex orbit by the Earth鈥檚 gravity.
鈥淣o one had anticipated this sort of thing,鈥� says Wiegert. Several asteroids have orbits locked to the gravitational pull of Jupiter, and one was recently discovered locked to Mars, but they follow simple patterns.
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The Earth鈥檚 new companion wanders between the orbits of Mercury and Mars. But the centre of its orbit drifts back and forth to trace a horseshoe pattern relative to the Earth. The full orbital cycle takes 770 years, the researchers report in this week鈥檚 Nature (vol 387, p 685).