56. 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.