Have astronomers discovered Earth's second moon?
I was recently watching a program about popular misconceptions which stated that the Earth has two moons. Apparently the second moon was discovered in 1994, has a diameter of 3 km and orbits the Earth once every 770 years. I would like to know if this is true and if so, why is there no information about it in modern amateur astronomy books.
I'm really glad you sent this email, because we've received several questions about a second moon of Earth, and I wasn't sure what people were referring to or where they had heard it, but the details you included made it possible to figure it out!
Anyway, to answer your question, I think the object that you're referring to is called Cruithne, which is a 3 mile (5 km) object in a horseshoe orbit around Earth that has a period of 770 years. You can read a press release about it at Space.com. 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 Earth. The Moon actually orbits the planet Earth, while Cruithne just shares the Earth's orbit around the Sun. You can read more about its motion and about horseshoe orbits at a question previously answered by Dave, or at this page about Cruithne. 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, objects trapped in orbits like Cruithne's 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 timescale 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 onto 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 I guess I would think of Cruithne as more of a Near Earth Asteroid that's trapped by Earth's gravity, not as a moon. And in fact, it's 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.
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