环绕地球轨道的小行星

When you hear about asteroids close to the Earth, you probably have visions of collisions and extinctions and a postapocalyptic future. Or of brave space cowboys trying to knock them off course. You probably don’t picture a puppy that’s followed you home. But astronomers have found just that: an asteroid that’s been on Earth’s tail for at least a quarter of a million years. The observations were published by the Royal Astronomical Society. [Apostolos Christou and David Asher, "A long-lived horseshoe companion to the Earth"]

The asteroid was first spotted during a routine survey performed by a satellite. But it was the asteroid’s earthlike orbit that really got the astronomers’ attention. The object’s orbit is virtually the same distance from the sun as is the Earth’s. And unlike most near-Earth asteroids—whose eccentric, egg-shaped orbits take them straight through the solar system—this asteroid’s orbit’s almost circular. So it seems to be following in our footsteps, although it’s never come closer than about 50 times the distance to the moon.

Where this companion came from is anybody’s guess. It could be left over from the same material that made our planet. Which would make it less like a puppy and more like an annoying sibling. One we hope will never hit us.

—Karen Hopkin