1 {scene: NASA spacecraft on intercept course to prevent an asteroid destroying Earth}
1 {a rat runs across the control panel}
2 Ishmael: A rat! There's a rat on board! How on Earth did a rat get on a strictly quarantined and sterilised NASA spacecraft?!
3 {a white cat chases the rat}
4 Loren: One could ask the same of your cat...
4 Ishmael: It's lucky I brought him!

NASA are of course very careful to sterilise their spacecraft - especially ones destined to end up on another celestial body.

When the Apollo 12 mission landed on the moon in November, 1969, the lunar descent module landed within walking distance of the 1967 Surveyor 3 probe lander. Astronauts Charles "Pete" Conrad and Alan Bean retrieved the camera assembly from Surveyor 3 (while Richard Gordon waited for them in lunar orbit in the Command Module) and returned it to Earth. The intention was to analyse the effects of almost three years of exposure to the lunar environment on the equipment.

Lab analysis back on Earth turned up traces of the bacterium Streptococcus mitis inside the camera assembly. When cultured, they grew. For many years this has been interpreted as the result of contamination of the camera unit during assembly (perhaps a technician sneezed while assembling it), followed by the bacteria surviving dried out but in a viable state of hibernation during the long stay on the moon.

Since then, the evidence has been re-examined and it's not so clear that that is what actually happened. In particular, there are claims that the camera could have been contaminated back on Earth after retrieval, during the analysis. Unfortunately, much of the original evidence has been lost to time, and we may never know the truth about this.

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2021-01-09 Rerun commentary: Since writing this original annotation, there have been more recent experiments and findings that indicate that microbes can survive in space much longer than we expected. In an experiment performed on the International Space Station (academic journal article), bacteria were placed outside, in space, for three years. They formed clumps in which the outer bacteria died, but formed a protective shell around the inner bacteria, which lived on, and grew once returned to Earth.

Given this is shorter than the transit time of probes we send to Mars, the experimenters conclude that it's possible that we have already contaminated Mars with terrestrial bacteria. And that given material can be ejected into space from terrestrial planets by meteorite collisions, it's plausible that microbes have been travelling from Earth to Mars (and possibly vice versa) for billions of years already.

Cool!!