NASA uses multiple methods to reduce the risk of forward contamination of the Solar System by US-launched spacecraft. These methods will be outlined in the following subsections, and include (but are not limited to) pre-launch microbial reduction, bio-shielding to prevent re-contamination up to and including launch, and trajectory and orbit biasing post-launch.
Pre-launch Microbial Reduction. Preventing forward biological contamination begins first and foremost with reducing the quantity and potency of any microbial entities (referred to as a spacecraft’s “bioburden”) on NASA spacecraft hardware before launch (i.e. reducing the value of the quantity “R” in equation 15). This is accomplished through a number of methods. All spacecraft are cleaned, assembled, and tested in clean-room
environments by specially trained and outfitted personnel, which provides an initial series of opportunities to reduce the bioburden on the surfaces of the craft (NRC, 2000).
Depending on the level of Planetary Protection required, clean-room assembly may be followed by an additional sterilization technique at adjacent facilities known as dry-heat cycling (NRC, 2006). As the name would imply, dry-heat cycling sterilization subjects hardware to temperatures reaching 230 degrees Fahrenheit for extended periods of time (≥ 30 hours), and this process is repeated (NASA, 2008). The spacecraft components are essentially “baked” in a large oven, and this process kills a sufficiently high number of active microbes on and in the spacecraft to satisfy the mission-specific Planetary Protection requirements. However, because certain highly sensitive spacecraft components cannot survive such treatment, other forms of sterilization such as hydrogen-peroxide cycling, alcohol wiping, and exposure to beta/gamma radiation are also used depending on the types of components and/or types of microbial spores involved (though these methods require additional verification to ensure that proper sterilization has taken place) (NRC, 2006). An example of dry-heat sterilization is provided in Figure 69:
Figure 69: Dry-heat sterilization of a Viking Lander (photo credit: NASA, 2008)
Due to the previously mentioned sensitivity of certain components to high temperature environments, spacecraft rarely undergo complete dry-heat sterilization cycling in their fully assembled state (NRC, 2000). This poses challenges from a Planetary Protection certification standpoint, as it necessitates post-sterilization integration and testing (which increases the likelihood of re-contamination).
Verification of Post-Sterilization Bioburden Reduction. It is necessary to verify the effectiveness of sterilization procedures on bioburden reduction of a given spacecraft prior to launch. To do so, biological assays are conducted to measure the microbial density on all accessible spacecraft surfaces (samples are heat shocked, and “the surviving cells are cultured to determine the number of colony-forming units”) (NRC, 2000). Surfaces that are found to be insufficiently sterilized are flagged for additional cleaning, which would then be followed by additional assays for verification.
While this method is adequate for assessing the effectiveness of sterilization techniques on the internal and external surfaces of the spacecraft, it cannot be used to determine the amount of “encapsulated” bioburden (active microbes embedded inside components of the spacecraft that are not directly accessible). Although these microbes pose a less-significant threat to planetary contamination, as they would only interact directly with the surface of the target body under some form of off-nominal circumstance (a destructive landing for example), they must still be accounted for when certifying the cleanliness level of a sterilized spacecraft. To estimate the level of encapsulated bioburden, portions of the spacecraft likely to be harboring microbial material are assigned parameter values based on standards developed by the Planetary Quarantine Advisory Panel (NRC, 2006).
Prevention of Re-contamination. Once assembled and sterilized, NASA spacecraft are maintained in a clean-room environment until immediately before launch to prevent re-contamination, assuming the clean-room environment itself is sufficiently sterile. For missions that require higher levels of sterilization than can be provided by a standard clean-room environment, spacecraft are placed within a protective bio-shield casing (which may or may not contain additional countermeasures like positive-pressure barriers and high-efficiency particulate air [HEPA] filtration) that will protect the craft prior to and during launch (NRC, 2006). In some cases this positive-pressure defense barrier will be maintained through the full launch phase, and the casing will only be jettisoned once the craft has reached sufficient altitude to guarantee no further contamination risk from atmospheric gases. Additional countermeasures to prevent re-contamination, like
mandatory sterilization of all internal surfaces of launch fairings, are scheduled to be implemented for future robotic exploration missions (NRC, 2006).
Trajectory Biasing. In addition to sterilization techniques undertaken prior to spacecraft launch, it is possible to reduce the bioburden of a vehicle after launch by modifying its interstellar trajectory and final orbit (i.e. reducing the value of the quantity “PS” in equation 15). This is possible because the space environment is hazardous to Earth-based life – in particular, ionization caused by Solar radiation is known to damage DNA, meaning that extended exposure to such radiation will lead to a reduction in viable microbial life carried by a spacecraft. It is estimated that on a single interstellar trip to Mars, a spacecraft would experience 500 times as much Solar radiation as it would for a comparable time spent on the surface of the Earth (Buckey, 2006). Thus, by extending the length of time it takes a spacecraft to travel to its destination, or by altering its interstellar trajectory such that it experiences higher doses of Solar radiation, it is possible to passively sterilize a spacecraft during its transit phase (COSPAR, 2002).
However, intentionally biasing a spacecraft trajectory or orbit can have several negative consequences – for example, exposing sensitive equipment to excessive radiation can cause irreversible damage, and delaying the spacecraft’s arrival at the target body can shorten the useful lifespan of the vehicle. Using such a strategy thus requires careful consideration of these tradeoffs. Further, because it is not possible to verify or certify the effectiveness of this type of sterilization after the fact, it would be unwise to solely rely on this method to satisfy bioburden reduction requirements.