Cruithne, the Earth's Second Moon

Yes. Although many people may ignore it, the Moon is not the only celestial body that accompanies the Earth in its journey around the Sun. There exists a second body, an asteroid called Cruithne, that shares our orbit in a “non-stable” way. With a diameter of only 5 kilometers, this body discovered by Duncan Waldron on October 10, 1986, describes a trajectory that, seen from our planet, resembles a horseshoe, moving between the orbits of Mercury and beyond Mars, crossing Earth's orbit in each cycle.

Near-Earth Asteroids

Cruithne belongs to a group called “asteroids near Earth” (NEAs, for “Near-Earth asteroids”) which, as its name suggests, are our closest neighbors in the solar system. Some of them pass closer to Earth than the distance to the Moon. They vary in size from 30 or 40 kilometers to just a handful of meters. This is a very special group, since it allows us to speculate about the frequency of impacts that Earth has suffered in the past and, much more importantly, those it could suffer in the future.

A Co-Orbital Companion

The asteroid Cruithne stands out for having a rather unusual orbit, following what celestial mechanics specialists know as a “co-orbital path” with Earth. This means, in simple words, that it shares its orbit with our planet. For decades it was speculated that this type of trajectory was possible, but it was not until October 10, 1986, when Duncan Waldron discovered Cruithne on a photographic plate taken with the UK Schmidt telescope of the Siding Spring Observatory in Coonabarabran (Australia), that it was known that a real body was capable of moving in that way. In any case, only on June 12, 1997, in a scientific article published in the journal Nature, Paul Wiegert, Kim Innanen and Seppo Mikkola precisely described the trajectory followed by this asteroid.

It is not easy to describe the way this “moon” moves with respect to our planet. An orbital diagram in “co-rotation” can only be visualized if the observer moves along with the planet in question. A good way to do it is to imagine that you are “flying” over the north pole of our planet, moving with it along its orbit, while keeping your eyes on the asteroid's path. From that position you can appreciate more or less what you see in the previous video: Cruithne describes a dance around Earth that is much more complicated than the one performed by the Moon, being more similar to a horseshoe than to an ellipse.

Distance and Safety

Cruithne cannot be seen with the naked eye. Not only is it too small for that, but it is at an enormous distance from Earth. At its maximum approach it only gets to about 12 million kilometers from our planet, about 30 times farther than the Moon is. A reassuring fact is that despite the complexity of its trajectory and its long-term instability, the calculations of Wiegert and Innanen show that it will not impact us. Or at least, not for a couple of million years. As we said, seen from Earth, its orbit resembles a horseshoe or a kidney, and it changes cycle every 387 years. It is expected that in July 2289 Cruithne will make one of its maximum approaches to Earth, just as it did in 1902.

Other Orbital Companions

Within the group of Near-Earth asteroids there are, at least, three others that have orbits similar to Cruithne's. Other planets, like Mars, also have objects that accompany them in co-orbital paths, but as far as we know, none of them describes a trajectory as strange as that of Cruithne. Did you know?

Cruithne, on Wikipedia