SPACE : Mars Flights Stir Fears of Germ Threats : Scientists weigh how to deter extraterrestrial invaders and how to avoid tainting other planets.

When the first Apollo astronauts to walk on the moon returned to Earth, they entered quarantine chambers designed to stifle any lunar germ that might have come back with them.

With the first of an international armada of Mars-bound spacecraft set for launch late next year, scientists are now asking how they might deter a similar Martian invader. At the same time, they want to keep the spaceships from shattering pristine extraterrestrial ecosystems--if any exist.

In a time when astronomers are spotting planets in other solar systems, space may be the final frontier of environmentalism.

Fear of a true-life Andromeda strain wreaking havoc on Earth may seem farfetched, but no more so than the lunar-bug threat that kept the Apollo astronauts locked up for three weeks. At NASA’s request, the National Research Council will form an expert panel to weigh the hazard of interplanetary infection, and how to combat it.

“The likelihood is very small, but the risk is very great,” said Michael A. Meyer, who serves as what NASA calls its planetary protection officer. “When you’re talking about something that could affect Earth’s environment, it’s not hard to justify a conservative approach.”

Few revelations could tell humans more about the nature of life than finding it elsewhere. If there is no life besides Earth’s, there is nothing to protect but the raw chemistry of other planets.

But if there is, advises a 1967 Outer Space Treaty monitored by the International Council of Scientific Unions, earthlings are obliged to not disturb it, especially so scientists can study its natural state.

“Respect for other life forms--it’s the ultimate environmental issue,” said James Cantrell, an engineer in Utah State University’s space dynamics laboratory.

There is also the deeper quandary of whether humans can discern life we may not understand. As missions go deeper into space, we may not know what we bring back.

“That’s something that just scares the devil out of you,” said Kenneth H. Nealson, a University of Wisconsin environmental microbiologist who headed a National Research Council committee that considered ways to keep from tainting Mars with exotic Earth life. The new scientific panel will pick up where that one left off.

Most of the bodies that make up our solar system seem so hostile to life as we know it that they carry no risk. The only exceptions bear traces of water, the elixir of life. One is Europa, an icy moon of Jupiter. Another is Mars, where sinuous channels imply water was abundant 3 billion to 4 billion years ago, when life appeared in a similar climate on Earth.

Nobody wants spacecraft searching for life on Mars to detect stowaway Earth life they took there themselves. So NASA imposed strict cleaning standards on the two Viking landers that set down on Mars in 1976. Perhaps 10% of the billion-dollar missions went toward toughening instruments to withstand hours of baking inside a giant oven built to sterilize the spacecraft.

The landers found only a dry, unforgiving terrain fried by solar radiation and laced with harsh compounds that chew up the building blocks of life. So NASA eased its sterilization standards for the $150-million Mars Pathfinder, expected to lift off late next year and drop to Mars in 1997.

“We’ve gone from a paranoid position to a much more reasonable one,” said Cantrell.

NASA still does not want missions to contaminate other planets. For example, it plans to burn the last fuel in a future craft called Mars Global Surveyor in order to boost its orbit so it won’t fall to Mars for at least 50 years. By then, the question of Martian life should be answered.

“We just don’t have enough evidence to say for sure there is nothing there,” said Donald L. DeVincenzi, deputy chief of space science at NASA’s Ames Research Center in Mountain View, Calif.

Which raises the sticky specter of so-called back contamination of Earth by what NASA terms a “sample return mission.”

The possibility of Martian life “is so close to zero it can be considered to be zero,” said Norman H. Horowitz, a professor of biology at Caltech who worked on the Viking project.

Then again, others say, maybe Martian microbes found refuge deep in some hot-spring oasis. Extinct bacteria might have left hardy spores. Maybe a hunk of Earth seeded Mars with life, making alien beings enough like their earthly cousins to be dangerous.

Vessels returning from Mars would probably carry an outer layer designed to burn away on reentry, leaving them sterile to the touch, Meyer said.

“You can’t even think about bringing rocks from Mars without getting people panicky,” Nealson said. “It will be easier to put a man on Mars than to bring something back.”