Science fiction has traditionally looked to space to find metaphors for otherness. Aliens of every conceivable shape, hue and limb- count populate the history of a genre "fascinated by the encounter with difference", in the words of author Adam Roberts.

Consider the Martians. For a century, contemporary hopes and fears have been projected onto them. They have been a weather vane for the mood of successive eras. To H.G. Wells, Martians were a conquering foreign power. In The War of the Worlds (1898) they are analogous to Russia or Germany, but equipped with terrifyingly superior technology. They're imperial Britannia's worst nightmare, a foe whose weapons make her mighty navy and artillery look like toy boats and popguns.