Earth & Sky

Cruithne was briefly called Earth's second moon

Source : Near-Earth asteroid 3753 Cruithne main page. © Copyright 1997-2000 by Paul Wiegert

In 1986, astronomers discovered an asteroid that they named Cruithne.

This asteroid is five kilometers – about three miles – wide. It’s one of several thousand asteroids whose orbits cross Earth’s orbit. Further study of Cruithne caused a stir, though – because, from Earth’s surface, Cruithne appears to orbit Earth. That’s why some people have mistakenly called Cruithne “Earth’s second moon.”

In reality, if you could look down from above the solar system, you’d see that Cruithne orbits the sun. But the orbit of Cruithne is locked to Earth’s orbit. In this case, that means that Cruithne is affected by Earth’s gravity, so that Earth and this asteroid return every year to nearly the same place in orbit relative to each other.

What’s more, Cruithne’s orbit isn’t stable. Computer models indicate that it’ll spend another 5,000 years in its current orbit. The asteroid might then move into true orbit around Earth for a time – at that time it would be a second moon – but not for long. After 3,000 years orbiting Earth, Cruithne would escape back into orbit around the sun.

Written by Earth & Sky