In the movies, humans often fear invaders from Mars. These days, scientists are more concerned about invaders to Mars, in the form of micro-organisms from Earth. Three recent scientific papers examined the risks of interplanetary exchange of organisms using research from the International Space Station. All three, "Survival of Rock-Colonizing Organisms After 1.5 Years in Outer Space," "Resistance of Bacterial Endospores to Outer Space for Planetary Protection Purposes" and "Survival of Bacillus Pumilus Spores for a Prolonged Period of Time in Real Space Conditions," have appeared in Astrobiology（天体生物学） Journal. Organisms hitching a ride on a spacecraft have the potential to contaminate（污染） other celestial bodies, making it difficult for scientists to determine whether a life form existed on another planet or was introduced there by explorers. So it's important to know what types of micro-organisms from Earth can survive on a spacecraft or landing vehicle.

Currently, spacecraft landing on Mars or other planets where life might exist must meet requirements for a maximum allowable level of microbial life, or bioburden. These acceptable levels were based on studies of how various life forms survive exposure to the rigors（严格，寒颤） associated with space travel.

"If you are able to reduce the numbers to acceptable levels, a proxy for cleanliness, the assumption is that the life forms will not survive under harsh space conditions," explains Kasthuri J. Venkateswaran, a researcher with the Biotechnology and Planetary Protection Group at NASA's Jet Propulsion Laboratory and a co-author on all three papers. That assumption may not hold up, though, as recent research has shown that some microbes are hardier than expected, and others may use various protective mechanisms to survive interplanetary flights.

Spore-forming bacteria are of particular concern because spores can withstand certain sterilization（消毒，杀菌） procedures and may best be able to survive the harsh environments of outer space or planetary surfaces. Spores of Bacillus pumilus SAFR-032 have shown especially high resistance to techniques used to clean spacecraft, such as ultraviolet (UV) radiation and peroxide treatment. When researchers exposed this hardy organism to a simulated Mars environment that kills standard spores in 30 seconds, it survived 30 minutes. For one of the recent experiments, Bacillus pumilus SAFR-032 spores were exposed for 18 months on the European Technology Exposure Facility (EuTEF), a test facility mounted outside the space station.

"After testing exposure to the simulated Mars environment, we wanted to see what would happen in real space, and EuTEF gave us the chance," says Venkateswaran. "To our surprise, some of the spores survived for 18 months." These surviving spores had higher concentrations of proteins associated with UV radiation resistance and, in fact, showed elevated UV resistance when revived and re-exposed on Earth.

The findings also provide insight into how robust microbial communities are able to survive in extremely hostile（敌对的） regions on Earth and how these microbes are affected by radiation.