Duncan Forgan reports that what you might not know is that the Moon is not the only natural satellite of the Earth. As recently as 1997, his group discovered that another body, 3753 Cruithne, is what is called a quasi-orbital satellite of Earth. This simply means that Cruithne does not loop around the Earth in a nice ellipse in the same way as the moon, or indeed the artificial satellites we loft into orbit. Instead, Cruithne scuttles around the inner solar system in what is called a horseshoe orbit.
So Cruithne is our second moon. It is only about five kilometres across, which is not dissimilar to the dimensions of the comet 67P/Churyumov-Gerasimenko, which is currently playing host to the Rosetta orbiter and the Philae lander.
The surface gravity of 67P is very weak, walking at a spirited pace is probably enough to send you off into the wider cosmos. This is why it was so crucial that Philae was able to use its harpoons to tether itself to the surface, and why their failure meant that the lander bounced so far away from its landing site.
If Cruithne struck the Earth, though, that would be an extinction-level event, similar to what is believed to have occurred at the end of the Cretaceous period. Luckily, it is not going to hit Earth anytime soon, its orbit is tilted out of the plane of the solar system, and astrophysicists have shown using simulations that while it can come quite close, it is extremely unlikely to hit Earth. The point where it is predicted to get closest is about 2,750 years away.
Cruithne is expected to undergo a rather close encounter with Venus in about 8,000 years, however. There is a good chance that that will put an end to the Earths spare moon, flinging it out of Earths path, and out of the Solar System.
The Earth plays host to many wayward lumps of rock captured by Earth. And, astronomers have actually detected several other quasi-orbital moons that belong to the Earth, all attached for only a little while.
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