'earth Has A Second Moon' - How Can That Be True?

Look up at the Moon on a dark night - it's THE Moon for good reasons: For most of our history it's been the only one we've known about, it has been the subject of countless legends and it's by far the biggest thing orbiting Earth. The Moon was humanity's prototype for what a 'moon' is: A natural object orbiting a planet.

So it's definitely the biggest, most important Moon of Earth.

But, in case you haven't spotted where I'm going with this yet, it isn't the only one (sort of).

The Moon's sister:

Once our sky might well have had a second Moon - the one that astrophysicists think existed 4500,000,000 years ago. Both THE Moon and the other moon formed when the new born Earth was hit by a protoplanet - a growing world the size of Mars. The resulting explosion was devastating: It turned planet Earth into a 300 km deep sea of lava, and trillions of tons of molten rock were blown into space. That cloud of debris clumped together under its own gravity to form.. two moons. Why do astronomers think it was two? Well, like all science, it's an educated guess, but one side of the Moon is definitely different than the other: It has fewer mountains, it has lots of low lying plains, and it has more of certain chemical elements. That's a pretty good fingerprint of a huge, huge collision - and the most likely candidate for the thing the Moon collided with is a second, slightly smaller, Moon. Moon 2 would have been born in a Lagrange point - a kind of gravitational 'neutral zone' - but would have drifted away from it. Eventually it would have had a sort of slow motion 'splatter' collision with the bigger Moon, taking several hours. Which makes our sky less interesting, but saves astrologers a lot of headaches.

The 'not quite a moon' moons:

The asteroid 3571 Cruinthe has caused astronomers a few headaches. 3571 Cruinthe doesn't actually orbit Earth, but it's own orbit around the Sun follows Earths. That kind of puts it in limbo - it's a permanent fixture of Earths neighbourhood, but it's not strictly a moon. The technical name for it is a co-orbital asteroid, and it's one of several near Earth. Sometimes they get referred to as 'moons' which isn't true, but is totally understandable - other planets have moons that small, and Cruinthe type asteroids are fairly permanent next door neighbours, just like the Moon.

The co-orbital asteroids are probably all pieces of protoplanets that blew up billions of years ago - although it has been suggested (not by me!) that some of them might be a cunningly disguised alien space probes, observing us!

The casual Moons:

Until a few years ago we didn't realise this, but Earth is constantly picking up hitchhiking space rocks. And, unlike Cruinthe, they don't just lurk nearby: They get well and truly caught in our planets gravity and start to orbit it - which makes the real moons. We don't see them up there because they're tiny (the most recent one was a teensy 5 meters across), but they could be very useful for exploring space on the cheap: They come from all across the solar system right to Earths doorstep, and could carry clues to how our solar system formed. But they don't hang about forever: Because they're so small their orbits are easily broken, and after a few years at most they go wandering back into deep space, carrying their stories with them..

So, sort of, Earth has lots of moons.