is the contamination of other worlds with Earth
microbes. The risk of forward-contamination is twofold: that human beings may accidentally seed a previously sterile world, thus creating "extraterrestrials
" that are really of terrestrial origin (and which might even make it impossible to determine whether the life later found is terrestric or local); or that an actual alien biosphere
could be devastated by Earth's bacteria.
Forward-contamination has been demonstrated by the apparent survival on the Moon of Streptococcus bacteria on Surveyor 3, which is however no longer thought to be true.
Given the impossibility of sending a bacteria-free human being into space, forward-contamination will remain an issue for as long as manned missions continue. Assuming a manned mission eventually reaches Mars, for example, it is inevitable that a microbial soup will be released into the environment of the red planet. The best known fictional portrayal of forward-contamination is in The War of the Worlds, where would-be alien conquerors are killed off by Earth microbes.