Enter your email address:

The Astro Logic Series - Earth's Second Moon

Have astronomers discovered Earth's second moon? What do you think? The object being referred to is called Cruithne, which is a 3 mile (5 km) object in a horseshoe orbit around the Erath, having a period of 770 years. It was discovered in 1986, but it took a lot of observations in order to figure out its complicated orbit, which was determined in 1997.

In the press release, one of the scientists involved with the study called the object "a moon", because it shares Earth's orbit, however, it's definitely not a moon like our moon. First, a horseshoe orbit is much different from the elliptical orbit that the moon makes around the Earth. The Moon actually orbits the planet Earth, while Cruithne just shares the Earth's orbit around the Sun. The orbit of Cruithne is also very inclined with respect to Earth's orbit around the Sun, so it moves in and out of the plane that the most of the planets orbit in. This large inclination is part of the reason that Cruithne won't collide with Earth.

Second, object trapped in orbits like Cruithne are only expected to remain in the orbit for a few thousand to tens of thousands of years which may sound like a long time, but it's actually fairly short in the time scale of solar system history. After Cruithne escapes from its present orbit it may become a near earth asteroid on a different close-to-earth orbit, or move on to an orbit more similar to our Moon's orbit, in which case it would be more like a real moon. No one seems quite sure which scenario will happen.

So, at present, I would call Cruithne as more of a Near Earth Asteroid that's trapped by Earth's gravity. And in fact, it is classified by astronomers as an Aten Asteroid, which is a group of near earth asteroids on similar orbits. But Cruithne is a good example of the fact that Earth's gravity can interact with nearby asteroids, bringing them closer to Earth or forcing them onto different, strange orbits.