possibility that Martian life, if it has a common origin with Earth life in the distant past, could swap DNA with Earth microorganisms in the same way. Another reason for the 0.01 μm limit is because of the discovery of ultramicrobacteria as small as 0.2 μm across. Robotic advocates consider that humans represent a significant source of contamination for the samples, and that a BSL-4 facility with robotic systems is the best way forward. See also References Astrobiology Biosafety level 4 laboratories Planetary science Sample return missions