Spacewar set the scene for the future. Apart from a brief excursion into ping-pong, the first computer games many of us experienced were variants of Space Invaders.

    From arcade games and consoles, the space invader rapidly appeared on early home computers. Looking back at the blocky graphics, buzzy sounds and jerky motions can make a games player who wasn't around at the time wonder about the sanity of their elders. Today simulations of the original games are tedious after about five minutes, but back then they were exciting and fresh.

    Most of us are familiar with Space Invaders' totally unnatural image of wave after wave of ship descending the screen in jerky unison. In terms of gameplay and graphics this is several steps back from Spacewar, but the game was hugely popular. Asteroids , which

    		followed soon after, put you in charge of a ship in a field of floating space debris. Your mission was simply to survive. Closer in feel to Spacewar, though without a live opponent, Asteroids threw in the difficulties of controlling motion and inertia.

    Our most famous literary invaders appeared in an early scientific romance by H.G. Wells. Wells churned out books, from very successful romances to turgid future histories and political polemics. At his best, he kept things simple, and The War of the Worlds remains powerful today. This 1898 masterpiece triumphs by the way it plonks aliens into the very ordinary life of suburbia.
    Wells' Martians are genuinely frightening and the whole book remarkably atmospheric. I'll come back to Wells in other categories, but it isn't possible to leave the turn of the century without also referring to Jules Verne. Verne's books were more pure adventure and the science of a lower order. His From the Earth to the Moon, for instance, has its astronauts propelled by a giant gun that would have left them splattered across the back of the shell. Verne's writing does not read as well a century later as Wells', but even so his influence is considerable. Space invaders remain a common theme to this day. In skimming though the last century, it's impossible not to stop at John Wyndham's rather cosy invasions, which are probably all the more effective because of their attack on middle England. Day of the Triffids is the best known, with its unlikely mobile killer plants and a blinded humanity, but more effective is the Midwich Cuckoos , where the invasion takes place in the much more subtle context of alien impregnation of the women of a village. The golden-eyed children who result are treated with as much conspiratorial secrecy as anything in the X Files.