Planetary protection

The rules that stop us contaminating other worlds, and them contaminating us.

Spacecraft going anywhere that might host life are cleaned to a specified bioburden, assembled in filtered rooms and sometimes baked. The rules are graded: a Mars orbiter is cleaned less than a lander, and a lander bound for somewhere with liquid water is cleaned hardest. It is why Galileo and Cassini were deliberately destroyed rather than left to drift into Europa or Enceladus. The return direction is regulated too, which is a large part of why bringing a Mars sample back is difficult and expensive. The standards are written in spores per square metre, and the awkward part is that they are checked by culturing swabs — which finds the organisms that grow in a laboratory dish and misses whatever does not. Everybody in the field knows the assay is the weak link, and the argument about how much cleanliness is enough has been running since the Viking landers were baked in 1975.