UNTIL Simon Marius and Galileo Galilei discovered four of Jupiter鈥檚 moons 400 years ago, the only known moon was a rather prominent object in Earth鈥檚 night sky 鈥� one that is familiar even to today鈥檚 light-blighted city dwellers. Since then, scores of moons have been discovered within our solar system, yet Earth鈥檚 companion still stands out as one of the most remarkable members of this clan.
For one thing, it is a very big fish in a small pond. Moons are rare in the inner solar system: Venus and Mercury don鈥檛 have any, and Mars鈥檚 two moons are tiny pebbles in comparison with ours. Indeed, our moon would seem more at home in the outer solar system, among the massive satellites that orbit the gas giants.
The moon鈥檚 bulky dimensions may be a reflection of its unique origin. Moons are generally thought to form in one of two ways 鈥� either by coalescing from the same cloud of debris as their parent planet, or as wandering objects captured by the parent鈥檚 gravitational pull. Our moon had a more bloody birth, when a large protoplanet collided with the young Earth 4.5 billion years ago and cast out a glowing ring of melted and vaporised rock, some of which condensed to form our satellite. That ancient cataclysm might have been lucky for us, as the moon helps to stabilise the axial tilt of Earth, removing one source of extreme climate change that might otherwise have afflicted our planet.
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It looks as though the US will abandon crewed missions to the moon for the foreseeable future. The long-term prospects for human colonisation were given a boost last year, though, when NASA鈥檚 spacecraft LCROSS found the firmest evidence yet for water ice buried at the lunar south pole.
Read more: Weird worlds: The solar system鈥檚 10 strangest moons
