1.   By international agreement, any spacecraft sent to Mars must be sterilized to avoid contaminating that planet with terrestrial microbes.

2.   Insights into terrestrial microbes and their effects, scientists say, can only aid Martian explorations that seek to find extraterrestrial life or evidence of its workings long ago.

3.   The higher reaches of the sky may have been contaminated by terrestrial microbes riding on satellites or rockets.

4.   NASA opted to destroy Galileo to keep it from slamming into Europa, contaminating it with terrestrial microbes stowed away on the probe.