On the fourth floor of the National Aeronautics and Space Administration's Washington, D.C., headquarters, a lone figure works to protect the planets of our solar system from invasion.

Michael Meyer is NASA's Planetary Protection Officer."Being in charge of protecting all the planets has a laugh factor associated with it," he admitted in a recent telephone interview with Reuters. "It does sound like a very big job."

But Meyer does not scan the heavens for an alien invasion force - the threat he worries about is microscopic in size. He is charged with ensuring that micro-organisms from Earth do not hitch rides on NASA space probes and colonize neighboring planets, and when robot probes scoop up samples of Martian soil and return them to Earth for study, he is responsible for ensuring that organisms from Mars do not infect our planet.

"The idea of bringing things back to Earth causes some real concern, and it's appreciated that there is actually someone who spends time making sure that NASA does the moral thing," the 43-year-old microbiologist said.

The space agency has tentative plans to launch a mission in 2005 to collect samples from the surface of Mars, but the recent discovery of possible early life in a Martian meteorite on Earth may bring that forward to 2003. The prospect raises a number of challenges, Meyer said.

"How do you put your sample in a container, make sure the outside is clean and whatever's inside is sealed, and then how do you get it to Earth? Once you have it, how do you handle a sample so you don't have an accidental release? Engineering-wise its do-able but it won't be easy."

Meyer's immediate concern is a pair of spacecraft now undergoing final launch preparations at Cape Canaveral. Mars Global Surveyor, scheduled for launch in November, and Mars Pathfinder, due to blast off a month later, are the first of a new wave of robotic explorers headed for the Red Planet.

Efforts to establish whether indigenous life exists on Mars could be ruined if micro-organisms carried by the probes reproduce on the Martian surface. "Basically you have made it very difficult to do any future studies," Meyer said. "We don't want to accidentally measure Earth microbes."

The Global Surveyor will map the planet's desolate surface from high above and is considered little threat to the Martian environment. The Pathfinder, however, will parachute a package of instruments including a six-wheeled remote-controlled rover to the Martian surface.

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Elaborate precautions have been taken to ensure Pathfinder does not harbor bacterial spores from Earth. The craft is kept under surgically clean conditions and is regularly swabbed with alcohol to sterilize its surfaces. In addition, some of the probe's more hardy components were sterilized in ovens.

Despite these precautions, NASA accepts it is near impossible to rid Pathfinder of all microscopic hitchhikers and will tolerate a maximum of 10,000 bacterial spores on the 1,917 lb spacecraft. That figure may sound high, but Meyer points out that, by comparison, a teaspoon of soil contains about one million micro-organisms.

Any organisms that reach the Martian surface would find it hard going. Average temperatures are -81 degrees F and it is impossible for liquid water to exist in the low atmospheric pressure.

"There's almost no chance of an Earth organism growing on the sur-face of Mars," Meyer said. "Spores can survive on the surface for quite some time, but they can't reproduce."

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