Cruithne is a comparatively near-Earth asteroid which, although it does not technically orbit the Earth, does – because of its location in the sky – seem to semi-circle the Earth in a rough horseshoe shape. However, horseshoe orbits are effectively optical illusions created by our own relative position in space, rather than true orbits; for this reason, Cruithne is not truly the Earth’s second moon.
– Orbit and Composition –
Cruithne is about an asteroid about 3 miles wide. Currently it comes closest to Earth every November, at which time it is still several dozen times as far away from us as the Moon is – far enough away that, given its size, it can never be seen with the unaided eye. It is inclined and slightly offset relative to ours, which means that it never actually crosses the precise orbital path of the Earth. For that reason, the likelihood that Cruithne would ever collide with Earth is considered negligible. However, it will also never become Earth’s second moon – because it never actually orbits the Earth, simply appearing to do so because its orbit is very close to our own.
The asteroid’s orbit around the Sun is actually 364 days, not 365 days like Earth’s, so that there is a centuries-long pattern which astronomers believe it will follow. Right now, Cruithne is spiralling away from Earth a little farther with every orbit. In a few centuries’ time, we will have fallen behind far enough that Cruithne’s orbits begin to approach ours from the other direction, as though it were spiralling closer and closer. However, current calculations indicate that it will never collide: after the closest approach orbit, it will begin to spiral away again, as it is doing now. Each time this cycle occurs, Earth’s gravity pulls Cruithne a few hundred thousand miles off course. Cruithne’s gravity also pulls Earth, although its effect on our orbit is calculated to be just a centimeter or so.
– Discovery –
Cruithne was found in 1986 by the Siding Spring Observatory in Australia. It was named after the Cruithne, an early Irish ethnic group related to the Scottish Pict people. However, it took about a decade for its strange orbit to be fully charted, by a team of Canadian and Finnish astronomers.
– Searching for Second Moons –
Since the dawn of modern astronomy, astronomers have searched for a body worthy of the name “Earth’s second moon.” It is not implausible that such an object could exist: all it would take is a small asteroid getting trapped in a slow, distant orbit, nowhere near as luminous as our Moon but certainly fitting the scientific criteria for being Earth’s second moon.
The first generation of moon searchers, in the 1800s, quickly identified a number of these asteroid candidates. In 1918, Walter Gornold, who in his career as an astrologer went by the pseudonym Sepharial, even argued that he had found Earth’s second moon, and named it Lilith. Subsequent searchers have never found Lilith again, and the claim is considered to have been bogus.
Other interesting near-Earth objects that, for brief periods of time, were thought to meet the qualifications for being Earth’s second moon include 1998 UP1, 2002 AA29, and 2003 YN107, which also orbit the Sun close to Earth and occasionally seem to follow horseshoe orbital paths. Another, 2006 RH120, orbits just beyond the range at which Earth’s gravity could truly capture it, so that every few years it actually gets caught, spun into Earth orbit for a few rotations, and then escapes again. If it ever did get truly captured, RH120, not Cruithne, would become the best candidate for Earth’s second moon (and would presumably deserve a new name, as well).
