Planetary Protection
In the study of whether Mars has had environments conducive to life, precautions must be taken to avoid introduction of microbes from Earth. Consistent with this, the United States is a signatory to an international treaty stipulating that exploration must be conducted in a manner that avoids harmful contamination of celestial bodies. ?Planetary protection? is the collection of rules and practices used to avoid biological contamination in the process of exploration. NASA has a planetary protection officer responsible for establishing and enforcing planetary protection regulations. Spacecraft missions are responsible for implementing measures to comply with the regulations. In compliance with the treaty and NASA regulations, the Phoenix flight hardware has been designed and built to meet planetary protection requirements.
NASA?s primary strategy for preventing contamination of Mars with Earth organisms is to be sure that all hardware going to the planet is clean. One of the requirements for the Phoenix mission is that the exposed interior and exterior surfaces of the landed system, which includes the lander, parachute and back shell, must not carry a total number of bacterial spores greater than 300,000, with the average spore density not exceeding 300 spores per square meter (about 11 square feet) so that biological load is not concentrated in one place. Spore-forming bacteria have been the focus of planetary protection standards because these bacteria can survive harsh conditions for many years as inactive spores.
The standard of cleanliness is much higher for hardware that will touch parts of Mars thought to have potential for sustaining life. This applies to the soil containing water ice, which scientists believe lies just below the surface where Phoenix will land. The robotic arm is the only part of the spacecraft that will touch Mars? icy subsurface layer. Consistent with the higher cleanliness standards for subsurface contact, the robotic arm was designed and built to stricter cleanliness requirements than the rest of the lander. It must comply with a standard that allows less than one spore per square meter on the arm?s total surface area.
The three primary methods used for reducing the number of spores on the spacecraft are precision cleaning, dry heat microbial reduction and protection behind high-efficiency filters. For the arm, an innovative biological barrier wrapping was also developed to supplement the dry heat treatment. This biobarrier is essentially a customized enclosure to protect the arm from recontamination before it is put to use at Mars.
Technicians who assembled the spacecraft and prepared it for launch routinely cleaned surfaces by wiping them with alcohol or other solvent. Components tolerant of high temperature were heated to reduce spore burden according to NASA specification, at temperatures ranging from 110 to 146 degrees Celsius (230 to 295 degrees Fahrenheit) for durations up to 50 hours. The planetary protection team carefully sampled the surfaces and performed microbiological tests to demonstrate that the spacecraft meets requirements for biological cleanliness. Whenever possible, hardware was contained within a sealed container vented through high-efficiency filters.
The most distinctive feature for planetary protection on Phoenix is the biobarrier sealing the robotic arm, which is constructed of a film that holds up to baking, like a turkey basting bag. The film is made from Tedlar, a trademarked polyvinylflouride material with commercial uses ranging from durable surfaces on airline cabin furnishings to backing sheets for photovoltaic panels. The biobarrier film is supported by a skeleton of spring-loaded, aluminum-tube ribs to maintain its shape. Workers sealed the arm inside the biobarrier before beginning heat treatment to reduce spores on the arm. This prevented any new spores from getting onto the arm during final preparations before launch, and in the subsequent launch and cruise environments. The biobarrier will remain sealed until the lander reaches the Martian surface. On the Martian surface, the springs will retract the ribs and the film, allowing the arm to deploy.
Another way of making sure Phoenix doesn?t transport Earth life to Mars is to ensure that any hardware not meeting cleanliness standards does not go to Mars accidentally. When the Delta launch vehicle?s third stage separated from the spacecraft, the two objects were traveling on nearly identical trajectories. To prevent the possibility of the third stage hitting Mars, the shared flight path was deliberately set so that the spacecraft would miss Mars if not for its first two trajectory correction maneuvers. By design, the third stage is never aimed at Mars. For hardware expected to impact Mars, such as the cruise stage after lander separation, a detailed thermal analysis was conducted to make sure that plunging through Mars atmosphere gets it sufficiently hot that few to no spores survive.