Potentially dangerous alien bugs hitching a ride to Earth on returning spacecraft will never be allowed to escape, a leading scientist has said.
Rock and ice samples brought back from Mars or Jupiter's moon Europa would be contained for ever if necessary, said Dr John Rummel, senior scientist at the Seti Institute in Mountain View, California.
Many experts are worried about the risk of Earth microbes contaminating other worlds during space exploration.
But there are also concerns about extraterrestrial bacteria or viruses accidentally picked up from the surface of other planets invading the Earth.
Such pathogens are likely to be hardy survivors, and human immune systems may be powerless against them.
Dr Rummel is taking part in a talk on space contamination at the American Association for the Advancement of Science (AAAS) annual meeting in Boston.
He said: "If we bring samples back from either Europa or Mars, we will contain them until hazard testing demonstrates that there is no danger and no life, or continue the containment indefinitely while we study the material.
"It is assumed that such life would be hardy - to survive the trip to Earth; not easy - and precautions taken would provide a very high degree of containment." Microbes may conceivably be living to this day on Mars below the planet's radiation-baked surface.
And scientists believe Europa has a global salty ocean beneath its icy crust which may well harbour simple life.
It is "pretty easy" for bugs to survive space journeys as long as they are shielded from the sun's ultraviolet radiation, said Dr Rummel.
"Microbial life, as a whole, is pretty immune to cold, dry conditions," he said.
"Introduced to the intense radiation around Europa, exposed microbes should die off in hours to days to weeks, but organisms protected inside the spacecraft would still be alive as long as the silicon chips are functioning."
Systems to prevent contamination of pristine planets or moons by Earthly bugs were essential, Dr Rummel said.
Aside from the ethical considerations, an escape by terrestrial microbes could make it impossible to prove evidence of extraterrestrial life.
"Earth organisms in the Europan ocean could hide the origin of Europan life from science, forever," said Dr Rummel.
Mars was not likely to have been affected by bugs carried on spacecraft landing there due to the lack of a global ocean and the harsh surface conditions, which meant they would not survive for long.