A mission from the mid-1970s provides the benchmark. When Nasa sent two robotic Viking landers to Mars, both had been sterilised to a level comparable to the best operating theatre. The rules to be applied to each space mission fall into a discrete set of categories. These reflect scientific knowledge of the nature of the celestial bodies, and the conditions under which life exists on Earth. Thus, Venus is not judged to be in need of phenomenal standards of protection, because its surface temperature of about 500 C is far above that at which any life has been found on Earth, and the chemical bonds necessary for the maintenance of such life break down at about 160 C. Bugs on board a spacecraft sent to land there would soon be killed.