When you are in the business of sending spacecraft to other planets, it is probably wise to do everything you can to keep your space-probes sterile(无菌的). NASA, America’s space agency, certainly does so. After all, you would not want bugs from one planet to contaminate another where they might possibly thrive. But according to Curt Mileikowsky, of the Royal Institute of Technology in Stock-holm, this may already have happened naturally billions of years ago when the solar system was young. For Dr Mileikowsky has taken a century-old idea called panspermia (有生 源说), and shown that it is plausible.