The solar wind is a violent burst of charged particles that creates magnetic storms in the Earth's upper atmosphere. But the sun's light, too, has pressure: Nasa scientists calculated 30 years ago that in the vacuum of space, the impact of sunlight on a gossamer-thin mirror would be enough to get it moving with an acceleration of a millimetre per second per second. This doesn't sound much, but within 24 hours a space sailing ship could be travelling at 100 metres a second, and still be accelerating. It could reach Mars in 400 days. The US Planetary Society has twice tried to launch a prototype space sailing ship; in May this year the Japanese space agency sent its experimental "kite" Ikaros floating towards Venus. And today the European Planetary Science Congress in Rome will hear the case for a fleet of "data-clippers" – robot windjammers laden with instruments, scudding out on the gentle breeze from the sun to Europa or Titan and then tacking back to Earth to download vast quantities of scientific data stored in flash memory. The biggest costs in space are rocket fuel and data transmission across epic distances. A fleet of automaton solar-wind-powered ships would greatly cut the costs and thereby multiply the returns. But beyond this ultimate economy drive, think of the beauty: of a fleet of silent windjammers, riding on radiation alone, literally making light work of charting the planets and their many satellites – all the way to the very last outposts of the empire of the sun.