Unfamiliar sight: The far side of the Moon as captured by one of NASA's Apollo missions in the 1970s. The Moon is locked in a synchronous rotation with the Earth, meaning that we always see the same side of it.

Credit: NASA

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LONDON: The far side of the Moon may have once faced Earth, says a new study which argues that a massive asteroid impact billions of years ago could have flipped it through 180º.

From Earth we only ever see one side of the Moon, as its rotation is locked with ours and it spins once for every orbit around the Earth.

Moon flip

However, crater patterns on the lunar surface suggest this orientation may not have always been the case, say scientists Mark Wieczorek and Mathieu Le Feuvre from the Paris Institute of Earth and Physics in France.

The idea of a Moon flip is not totally new. In 1975 researchers in the U.S. proposed that if a large enough asteroid slammed into the satellite, it would wobble back and forth like a pendulum before settling down in a locked rotation with only one face visible from Earth.

Until now, though, there has been no evidence to back up the theory.

For the new study, detailed this month in the journal Icarus, Wieczorek and Le Feuvre scoured images of the Moon's cratering patterns.

Recent work has suggested that as the Moon orbits Earth, its leading side – the western hemisphere – is 30 per cent more likely to get hit by asteroids that the other. "It's really like a car moving through the rain, there'd be more rain falling on the front windscreen than the back," explained Wieczorek.

Dating craters

The researchers compared the ages of craters across the surface. As expected, that the younger asteroid impacts were on the western side. But to their surprise, the oldest craters were on the eastern side – the back windscreen – suggesting that the Moon's orientation may once have been reversed.

The age of the craters was estimated by measured by looking at layers of debris thrown out from multiple impacts, and also by the number of craters superimposed on top of one another.

But where's the evidence for a massive Moon-flipping asteroid impact? Wieczorek claims there are six giant basins that could mark the site of the blow.

The current favourite is the Smythii basin that lies on the Moon's equator "so it would have more ability to spin up the Moon than, say, if it was on the north pole." The experts estimates the direct hit at over 3.9 billion years ago.