Earth's '2nd moon' is really an asteroid

Answer: An asteroid named Cruithne is sometimes referred to as Earth's second moon, although this epithet is not really accurate. Some stargazers call it Earth's kid sister.

The name Cruithne, we read, comes from that of an ancient Celtic tribe. It's pronounced "croo-een-ya."

Discovered in 1986, Cruithne is a "Near-Earth Asteroid" about 3 miles across. It's also known as Asteroid 3753, or 1986 TO. Astronomers say it is not a true natural satellite of Earth because it's not gravitationally bound to our planet in the same way the real moon is. Rather, it's in a state of "1:1 resonance" with Earth, which means it makes one trip around the sun for each trip the Earth makes. Because it circles the sun along roughly the same path as the Earth, it's also called a co-orbital near-Earth asteroid.

Although Cruithne is actually orbiting the sun, from the Earth it seems, sort of — this gets really complicated — to move in a horseshoe-shaped path around us. The weird geometry of Cruithne's orbit never brings it closer to Earth than about 40 times the distance of our real moon, so there's not much likelihood of a collision. (Unless, of course, something perturbs its orbit.)

Since Cruithne's discovery, a few other asteroids have been detected that appear to be in resonance with the Earth.

One possible candidate for an Earth-resonant asteroid, though, turned out to be a spent rocket booster.