The Earth may have had two moons.

Interesting little development on the Moon. All this talk of the creation of planets and the elements whirring around space has me dreaming of a life in astrophysics. Maybe the childhood dream job at NASA isn't out of the question after all!

From BBC News and the Irish Times, August 3, 2011.

A new theory suggests the Earth once had a small second moon that perished in a slow motion collision with its "big sister".

Researchers suggest the collision may explain the mysterious mountains on the far side of our Moon.

The scientists say the relatively slow speed of the crash was crucial in adding material to the rarely-seen lunar hemisphere.

For decades, scientists have been trying to understand why the near side of the Moon - the one visible from Earth - is flat and cratered while the rarely-seen far side is heavily cratered and has mountain ranges higher than 3,000m.

The researchers argue that the Earth was struck about four billion years ago by another planet about the size of Mars. This is known as the global-impact hypothesis. The resulting debris eventually coalesced to form our Moon.

Slow-motion impact

But the scientists say that another, smaller lunar body may have formed from the same material and become stuck in a gravitational tug of war between the Earth and the Moon.

Moons colliding?

"And one outcome of our research is that the new theory goes very well with the global impact idea."Dr Martin Jutzi from the University of Bern, Switzerland, is one of the authors of the paper. He explained: "When we look at the current theory there is no real reason why there was only one moon.

After spending millions of years "stuck", the smaller moon embarked on a collision course with its big sister, slowly crashing into it at a velocity of less than three kilometres per second - slower than the speed of sound in rocks.

Dr Jutzi says it was a low velocity crash: "It was a rather gentle collision at around 2.4km per second; lower than the speed of sound - that's important because it means no huge shocks or melting was produced.

At the time of the smash, the bigger moon would have had a "magma ocean" with a thin crust on top.

The scientists argue that the impact would have led to the build-up of material on the lunar crust and would also have redistributed the underlying magma to the near side of the moon, an idea backed up by observations from Nasa's Lunar Prospector spacecraft.

In a commentary, Dr Maria Zuber from the Massachusetts Institute of Technology (MIT) in Cambridge, US, suggests that while the new study "demonstrates plausibility rather than proof", the authors "raise the legitimate possibility that after the giant impact our Earth perhaps fleetingly possessed more than one moon".

"Hopefully in future, a sample return or a manned mission would certainly help to say more about which theory is more probable."

The Far Side of the Moon

• It takes the Moon about the same amount of time to rotate on its axis as it does to complete an orbit of the Earth
• This is known as "synchronous rotation" and explains why the Moon always presents its familiar near side to Earth
• The near side is covered in smooth, dark lunarmaria (Latin for "seas") created by magma erupting on the surface
• The far side is more rugged, with a thicker crust pock-marked by impact craters; the highest point on the Moon is located on the far side.