Astronomers fear that organisms on an exploring spacecraft or a landing vehicle have the potential to contaminate Mars.
This would make it difficult for scientists to determine whether a life form existed on another planet or was introduced there by explorers.
Currently, spacecraft landing on Mars or other planets where life might exist must meet requirements for a maximum allowable level of microbial life.
"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," explained Kasthuri J. Venkateswaran, an Indian-American researcher with the biotechnology and planetary protection group at NASA's Jet Propulsion Laboratory.
That assumption may not hold up as recent research has shown that some microbes may use various protective mechanisms to survive interplanetary flights.
So it is important to know what types of micro-organisms from the Earth can survive on a spacecraft or landing vehicle.
Spore-forming bacteria are of particular concern. Spores can withstand certain sterilisation procedures and survive the harsh environments of outer space.
"To our surprise, some of the spores survived for 18 months," Venkateswaran said.
The spores could survive a trip on a spacecraft to Mars if these are sheltered against solar radiation - perhaps in a tiny pocket of the spacecraft surface or underneath a layer of other spores.
Future exploration missions can use the results of these investigations to help find ways to minimise the risk of contaminating another planet, the study that appeared in Astrobiology Journal said.