For the first time, millions of bacterial spores have been purposely exposed to outer space, to see how they are affected by solar radiation. The results support the idea that life could have arrived on Earth in the form of bacteria carried from Mars on meteorites. The idea that life started elsewhere and spread through space is called panspermia. It was first proposed in 1903 by the Swedish chemist Svante Arrhenius, who suggested that solar radiation might propel single spores across solar systems. Recent discoveries of Martian meteorites that have reached Earth have raised the possibility that bacterial spores could have hitched a ride on these rocks. Most meteorites spend millions of years in space, but meteorites taking a direct route would make it from Mars to Earth in just a few years – too short a time to experience much damage from deadly cosmic rays.