Long before we learned that Mars is a barren, inhospitable planet, popular fiction envisioned little green men there and even Martians invading Earth.  In truth, it is we Earthlings who have invaded Mars with our landing craft and robots roaming around to study the surface. Now, a panel of scientists worries that spacecraft might bring microorganisms from Earth and confound the search for life on the red planet. The experts are asking the U.S. space agency, NASA, to improve the way it cleans missions bound for Mars.

Recent U.S. robotic findings that Mars once had a shallow saltwater sea have increased the hope of someday finding microbial life forms there, either something still alive or a fossil from the past when Mars was supposedly wetter and warmer.

But planetary scientist David Paige of the University of California at Los Angeles wonders how we could be sure a microbe were truly Martian in origin if the spacecraft observing it had not been completely sterilized of Earthly germs.

"Mars in its current state, which we hope is pristine, potentially has something very valuable for us to find there, which is either positive evidence for or against the presence of life," he said.  "By contaminating Mars, we reduce our ability to study this very important problem."

Mr. Paige is a member of a panel of private experts who studied the issue for the National Academy of Sciences, a body that advises the U.S. government and Congress on scientific matters. The panel concludes that NASA's existing spacecraft cleaning techniques are outdated and typically eliminate only a fraction of microorganisms.

David Paige says it is important not to contaminate Mars with a microbe that could spread.

"Contamination by microbes that go there and die or even microbes that stay alive, but don't propagate isn't nearly as much a concern as if a microbe were put into a place where it could start to grow and literally start taking over the planet," he explained.

NASA currently detects microbes on a spacecraft by wiping the vehicle and putting the cloth into a laboratory dish with nutrients to let them replicate. But the National Academy of Sciences panel says the procedure does not give a comprehensive tally of the microbes present. It calls on the space agency to sponsor research to detect biological molecules that do not require laboratory growing time.

To eliminate such molecules, the space agency cleans its spacecraft and in some cases, bakes components with dry heat. But scientists have learned that some organisms can withstand extreme temperatures. Even if the heat does kill many, it can be applied only to spacecraft designed to withstand high temperatures.  So the experts say NASA should investigate alternative cleaning methods, such as the use of radiation or vapor disinfectants. NASA could also tailor sterilization methods by applying tools that determine the genetic sequences of the microorganisms.

David Paige acknowledges that Earth microbes might already have landed on Mars during past missions.

"We can't undo the past, but we can set policies for the future that give us the best chance of not contaminating the planet while still at the same time permitting very exciting exploration at the same time," he added.

Mr. Paige's group calls on NASA to begin testing new spacecraft sterilization techniques within three years and put them into effect in 11 years.