All spacecraft such as orbiters, landers and rovers must be built in cleanrooms that meet International Organization for Standardization specifications for particulate control and air flow as well as gowning protocols for protective clothing.
Planetary Protection requires workers to wear full cleanroom garments. These include head coverings, coveralls and shoe covers. Workers wear these garments over their street clothes to prevent contaminants such as clothing fibers and hair and skin particles from entering the cleanroom and potentially transferring to sensitive spaceflight hardware.
Planetary Protection also requires clean assembly practices, as well as preventing recontamination on flight hardware up to the last time of access prior to launch.
NASA uses multiple cleaning methods to reduce the number of viable organisms on a spacecraft and vaporize chemical remnants of terrestrial biology. Dry Heat Microbial Reduction is a process that heats hardware under controlled humidity, temperature and time conditions. This method not only kills microbes on the surface or encapsulated within hardware, but the heating process can bake off organic molecules from hardware surface.
If spacecraft parts or scientific instruments are susceptible to damage from heating, then there are other methods for microbial reduction, such as exposure to vapor hydrogen peroxide or ultraviolet light. Each cleaning method has benefits and drawbacks to be considered when deciding on the best approach.
Exposed hardware surfaces are cleaned during the assemble process by alcohol wiping. Alcohol does not destroy bacterial spores in general, rather, they are removed by the mechanical action of physical cleaning. Hardware is wiped clean before final assembly and installation. Workers routinely wipe down flight hardware surfaces that remain accessible to maintain cleanliness.
Finally, ground support equipment is subject to rigorous cleaning and testing for biological cleanliness throughout the assembly process.