Forward contamination is the accidental contamination of other worlds with microbes, such as viruses or bacteria, brought from Earth. That such contamination is at least possible was demonstrated by the apparent survival for many months on the Moon of Streptococci bacteria on Surveyor 3. The risk is that on a planet or moon that was not sterile the release of terrestrial microorganisms, to which the indigenous life-forms would have no immunity, could prove devastating. This risk is particularly great where crewed missions are involved. The ship, air, and occupants of the first human mission to Mars, for example, will inevitably release into the martian environment a microbial menagerie. Thomas Gold hypothesized that it might even be possible to seed life accidentally on a previously virgin world in this way (see "garbage theory," of the origin of life).
Fred Hoyle and Chandra Wickramasinghe, in a modern version of panspermia, have suggested that life originated on Earth from prebiotic chemicals and possibly even microbes delivered on meteorites or the dust from comets (see life, in space), and furthermore, that such "contamination" continues today giving rise to epidemics (see epidemics, from space).
The theme of life from one world being unable to resist infection from "alien" microbes has been employed in fiction. In The War of the Worlds, H. G. Wells eventually kills of his Martian invaders with Earthly bugs. Michael Crichton's The Andromeda Strain (1964) envisages the genetic alteration of terrestrial microbes by ionizing radiation high in the atmosphere and their return to Earth as deadly mutants.1, 2, 3, 4, 5
1. Geiger, P. J., Jaffe, L. D., and Mamikunian, "Biological Contamination
of the Planets." In G. Mamikunian and M. H. Briggs, eds., Current
Aspects of Biology. Oxford: Oxford University Press (1965).
2. Horowitz, N. H., et al. "Planetary Contamination I: The Problem and the Agreements," Science, 155, 1501 (1967).
3. Murray, B. C., et al. "Planetary Contamination II: Soviet and U.S. Practices and Policies," Science, 155, 1505 (1967).
4. Sagan, Carl, ed. Communication with Extraterrestrial Intelligence. Cambridge, Mass.: MIT Press (1973).
5. Sagan, C., et al. "Contamination of Mars," Science, 159, 1191 (1968).