PASADENA, Calif. The issue of contamination is a serious one for NASA, which takes great pains to ensure its interplanetary spacecraft are as spick-and-span as possible before launch, lest they sully any other world with terrestrial bugs.

Its also the law: The 1967 Outer Space Treaty requires spacefaring nations to avoid "harmful contamination" of other celestial bodies.

Most probes, like the recent Mars Polar Lander, must meet stringent policies that allow no more than 300 spores per square yard (meter). An average floor, by means of comparison, could teem with as many as 100,000 spores in an equal area.

Workers in clean "bunny suits" at Lockheed Martin testing the failed Mars Climate Orbiter before launch.

"At places of interest, like Mars and Europa, places where the temperatures are moderate, where water in some physical form is likely to exist, places where there might be life, no matter what like, you have to do something about cleaning up the spacecraft you are going to send there," said Jack Barengoltz, a senior physicist at NASAs Jet Propulsion Laboratory, who was planetary protection manager for several Mars missions, including 1997s Pathfinder.

The key is to keep things clean as the spacecraft are being built. Typically, that job goes on in clean rooms peopled with workers clad in coveralls informally known as "bunny suits."

"The people doing this are dressed up to the nines in sterile garb," Barengoltz said.

Large areas of the spacecraft that can handle dry heat get baked at a temperature of around 260 degrees Fahrenheit (125 degrees Celsius). On Mars Pathfinder, for example, the spacecrafts parachute and airbags were both baked.

Other areas get wiped down, often repeatedly, with a 70 percent solution of alcohol.

"The problem with spacecraft is the materials tend to be sensitive and the engineers who work on them tend to be even more sensitive," Barengoltz said.

For the twin Viking landers, sent to Mars in 1975 in a hunt for life, engineers baked the entire probes for nearly two days straight. Even minimal traces of terrestrial life could have skewed test results.

For the upcoming Europa Orbiter mission, the issue of contamination will also be a tricky one. Although the spacecraft will circle the icy moon of Jupiter at a safe distance, one day its mission will end, sending the probe crashing down onto the surface.

How to dispose of the satellite, which could carry microbial stowaways, without contaminating what scientists believe may be the most likely spot to find extraterrestrial life poses a perplexing problem.

NASA hopes to send future missions to probe Europas oceans for life: What it doesnt want to find is a terrestrial version it inadvertently introduced on a previous mission. The Space Studies Board, which advises NASA, will release a report on the issue in the next few weeks.

"We dont want to introduce Earth life into an environment we are trying to study," said John Rummel, who is NASAs planetary protection officer.