WASHINGTON -- More needs to be done to make sure that microbes from earth aren't carried to other planets or contaminate samples brought back from outer space, researchers from NASA's Pasadena, California-based Jet Propulsion Lab reported here on Tuesday.

JPL scientist Kasthuri Venkateswaran presented his research at the American Society for Microbiology general meeting. He and his colleagues tested spacecraft for contamination after manufacture and before flight. They tested only unmanned spacecraft scheduled for landing on other planets. Such a mission is planned to Mars in June.

Decontamination requirements aren't as stringent for unmanned spacecraft that orbit earth or other planets, or for manned missions that won't be landing anywhere, such as the International Space Station.

But for ships that will set down, the concern is that new pathogens or species could be introduced to another planet, or that soil or life-form samples could become contaminated en route back to earth.

The United Nations Committee on Space Research, or COSPAR, has set cleanliness standards for international spacecraft.

Venkateswaran and colleagues wanted to see if current spacecraft met that standard. They found multiple species of bacteria on interior spacecraft surfaces, even after they had been subjected to dry heat or hydrogen peroxide, Venkateswaran told Reuters Health.

The amount of surviving bacteria was surprising, he said, adding that NASA was working on technology to further reduce what he called the "biological burden" inside a spacecraft.

Just because there are surviving bacteria "doesn't mean we're going to colonize other planets, but we have to reduce those burdens," he said.

Until NASA can further decontaminate spacecraft, manned landings on other planets will be ruled out, he said.

"A human is a fermenter," he said, noting that humans carry a wide variety of microbes that will be harder to contain than those on spacecraft.