The observation that some anhydrobiotic organisms can survive exposure to the conditions of space (see Chapter 3) raises the possibility that they may be able to transfer between planets either naturally (in meteorites) or artificially (in spacecraft). Towards the end of the nineteenth century, the Swedish chemist Svante Arrhenius suggested that life arrived on Earth as microbes or spores wafting through space from elsewhere. This theory (called panspermia) merely transfers the problem of the origin of life on Earth to elsewhere in the solar system or universe. However, the realisation that material can transfer between planets in the form of meteorites does suggest a mechanism by which organisms could travel between them. Life originating on one planet could then seed other habitable planets in its solar system.
In 1969, the second manned mission to the Moon (Apollo 12) visited the site of Surveyor 3, a robotic probe which landed on the Moon in 1967. The astronauts recovered parts of the probe, including its camera, and returned them to Earth, under sterile conditions which would have prevented contamination. Scientists were astounded to discover viable colonies of bacteria in the foam insulating the camera's circuit boards. The bacteria were Streptococcus mitis, a common inhabitant of the mouth and throat of humans. Perhaps one of the technicians assembling Surveyor 3 sneezed and deposited these passengers which then survived the launch of the probe, its journey through space and more than two and a half years on the Moon. This is the only known survivor of unprotected space travel to another body in our solar system. The Apollo 12 Commander Pete Conrad realised the significance of this finding:
I always thought the most significant thing that we ever found on the whole . . . Moon was that little bacteria who came back and lived and nobody ever said [anything] about it.
Only with our present knowledge of the survival abilities of cryp-tobiotes and extremophiles can we realise the significance of the bacteria's journey. It also provides us with a warning. We must be careful not to contaminate other planets accidentally with organisms from Earth or vice versa.
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