© AP

Around 4 billion years ago, was this the far side of the moon?
A study of lunar craters suggests the side of the moon that now faces Earth was once facing in the opposite direction

The man in the moon may once have faced out to space, until a chance collision with a giant asteroid billions of years ago, scientists have found.

A study of the heavily cratered surface has revealed evidence of a huge impact early in the moon's history that may have been powerful enough to set it spinning, making its far side periodically point towards Earth.

Today, the orbit of the moon is such that it always shows the same face - the near side - to people looking up from Earth. While the moon does have a "dark side", the side in shadow is constantly moving, depending on the relative position of the sun.

In the study, scientists at the Paris Institute of Earth Physics in France analysed the age and locations of 46 craters formed by asteroids thumping into the moon's surface.

Computer simulations show that if the same face had always pointed towards us, the moon's western hemisphere as seen from Earth should be pocked with around a third more craters than the eastern hemisphere, because the west faces the moon's direction of travel - the same reason running in the rain will get your front wet more than your back.

But to their surprise, researchers Mark Wieczorek and Matthieu le Feuvre found that the craters in the western hemisphere were among the youngest on the moon, while those in the eastern hemisphere were much older. The finding, reported in New Scientist magazine, suggests that the eastern flank was once pummelled by asteroids far more than the west.

"The simplest interpretation of [our] observations is that the oldest lunar impact basins formed when the present-day 'nearside' of the moon was directed away from the Earth and that a single impact subsequently reoriented the moon about its spin axis by 180 degrees," the authors write in the journal Icarus.

The scientists believe that a large asteroid could have slammed into the moon and set it spinning, so that for tens of thousands of years it would have appeared to rotate in the night sky.

Craters on the moon that could have been formed by such a large collision have been dated to around 3.9bn years, soon after it formed from debris knocked off Earth by an even larger cosmic collision.