When you are in the business of sending spacecraft to other planets, it is probably wise to do everything you can m keep your space-probes sterile (无菌的). NASA, America's space agency, certainly does so. After all, you would not want hugs from one planet to contaminate another where they might possibly thrive. But according to Curt Mileikowsky, of the Royal Institute of Technology in Stockholm, this may already have happened naturally billions of years ago when the solar sys- tem was young. For Dr Mileikowsky has taken a century-old idea called panspermia (有生源说), and shown that it is plausible. 57. Panspermia is the theory that life does not start independently on each planet that has it (assuming that other planets do). Rather, it hops from place to place, 'infecting' new worlds as it goes. Supported by experts in biology, geology and celestial mechanics, Dr Mileikowsky argued to the American Astronomical Society meeting in Atlanta that this is not as outlandish as it sounds. 58. Bungling (笨手笨脚) space organizations apart, the only mode of travel open to microbes seems to be meteorites (流星). Most of these are small bits of junk from the asteroid (小行星) belt that have gone off course. But some are rocks that have been flung into space from the surfaces of planets as a result of those planets having been struck by even larger bits of rock--decent-sized asteroids or comets. 59. If there is life on such a planet, microscopic forms of it will probably live deep in- side rocks, as they do on earth. The acceleration of lift-off would not kill something that size. 60. If a rock is large enough, the heat generated as it is thrown clear will be negligible except at its surface--where, ii anything, melting may even produce an airtight skin to protect any microbes deeper down from the unpleasant vacuum of space. (56)