Remember how NASA first asked those biologists at Fort Detrick to develop effective methods for decreasing the number of microorganisms on outbound spacecraft -- what insiders refer to as bioburden reduction? Well, as more missions came online, we got better at planetary protection. For example, NASA officials implemented strict crew quarantine rules for the early Apollo missions because they didn't know whether or not lunar microbes existed. After early testing of lunar samples, however, scientists determined that the moon never harbored life, so crew quarantine procedures were out the window after the third Apollo voyage.
The Viking missions of the mid-1970s were just as important for planetary protection as the Apollo ones, and led to the development of many techniques still used today.
- Cleanrooms and microbial barriers. NASA workers built Viking components in bug- and dust-free rooms known as cleanrooms. These rooms live up to their name by way of laminar airflow systems, which keep air moving in one direction along parallel flow lines and at uniform velocity. As the air moves, superfine filters trap dust, bacteria and other debris that might otherwise settle on the surface of equipment. All cleanrooms receive ratings based on how well they do their jobs. The lower the rating, the cleaner the facility. Class 10 rooms, for example, have fewer than 10 particles per cubic foot. NASA required Viking components to be built in Class 100 cleanrooms [source: NASA Office of Planetary Protection].
- Protective clothing. Before workers can step in a cleanroom, they must don special clothing from head to toe. These garments include hoods, masks, gloves and bunny suits, full-body suits like those made famous by Intel in the late 1990s. The clothing prevents workers from depositing hair or bacteria into the cleanroom environment.
- Sterilization. After the Fort Detrick experiments, NASA selected dry-heat sterilization as the preferred technique for the Viking landers. In essence, dry-heat sterilization requires putting the fully assembled spacecraft in a giant oven and baking it at 233 degrees Fahrenheit (112 degrees Celsius) for 30 hours. Before workers bake the vessel, they encase it in a large ceramic sheath -- something resembling CorningWare -- to help protect delicate components. An alternate method, used since Viking, relies on vaporized hydrogen peroxide, which can be applied at lower temperatures, yet still kills microbes effectively.
Of course, the techniques we've covered so far only decrease the bioburden on a spacecraft's metallic surfaces. NASA also worries about something known as encapsulated burden -- bacteria buried deep inside nonmetallic spacecraft material. If an orbiter or lander accidentally strikes its target, something known as an inadvertent impact in NASA-speak, these encapsulated microbes could be released, foiling the mission's planetary protection efforts.
To safeguard against this happening, mission planners employ a technique called trajectory biasing. Here's how it works: First, flight engineers aim the spacecraft so it will miss its target by hundreds or even thousands of kilometers. Then, after launch, they track the vessel carefully and, as they get more confident that it's on course and responding well, they begin correcting the trajectory slowly over time. If they ever lose contact with the spacecraft and can no longer control it, they know it will be far less likely to make an inadvertent impact with the target body.
Earth-return missions use all of these techniques for the outbound trip. The inbound trip requires a couple of steps to make sure returning astronauts or samples don't contaminate Earth's biosphere.