Making sure the spacecraft are as biologically clean and contamination-free as possible before they leave Earth is NASA's planetary protection (PP) policy. It protects other solar system bodies from Earth life and protects Earth from extraterrestrial life that may be brought back by returning space missions. NASA's policy is based on the most recent understanding of planetary conditions and biology, and regular recommendations from the US National Academy of Science.

"Keeping the spacecraft as clean as possible before, during and after launch is very important for any science instruments searching for organic compounds on the surface of other planets," said Laura Newlin, Jet Propulsion Laboratory (JPL) engineer and Planetary Protection (PP) Lead for the MER missions. JPL's Biotechnology and Planetary Protection Group seeks to advance spacecraft cleanliness, sterilization and validation technologies for NASA's solar system exploration missions.

"Up to 300,000 spores are allowed on the exposed surfaces of the landed spacecraft," said Newlin. "That many spores would fit on the head of a large pin."

A companion requirement to this is the average spore density on the surfaces must be less than 300 spores per square meter (28 spores per square foot). There are approximately 4500 square meters (approximately 48,000 square feet) of surface on each MER spacecraft, including the cruise stage.

When the spacecraft arrived at KSC from JPL in February and March, they were transported to the Payload Hazardous Servicing Facility in KSC's Industrial Area. Prior to that, the highbay and ground support equipment were cleaned, sampled and recleaned to reduce further biological contamination when the spacecraft arrived.

Both spacecraft have since undergone extensive alcohol-wipe cleaning and bio-testing processes. They were disassembled and cleaned to remove any contamination that may have occurred during the cross-country transport. During reassembly, JPL PP team members sampled surfaces of both spacecraft to check for microbial spores.

Culturing of the samples was performed in several KSC life sciences labs using equipment from JPL or provided by KSC including media claves, sonicators, water baths, incubators, microscopes, bio-safety hoods, and a large magnified colony counter.

"Currently our total spore count on the surface of both MER vehicles is comfortably under 200,000. So we are below the allowable level," Newlin said.

Other PP strategies exist for MER surfaces that are inappropriate for the traditional cleaning method. These include dry heat microbial reduction of the hardware in a dry environment at 125 degrees Celsius (257 F.) for five hours. The process is performed piece by piece on large surface areas that can tolerate the temperature, such as thermal blankets, airbags, honeycomb structures and parachutes in their cans.

A High Efficiency Particulate Arrestor (HEPA) filter is also used to filter out 99.97 percent of particles that are 0.3 microns or larger on MER's electronic boxes and the rover body. These permanent fixtures will also help filter out the Martian dust when the MER spacecraft land on Mars. Spacecraft propellant lines were also precision cleaned.

According to Newlin, the PP team worked with the spacecraft design engineers to determine PP strategies, what hardware should be cleaned and what hardware would require other PP approaches, all of which were integrated into the design, fabrication, and assembly of the spacecraft.