It takes a vacuum to keep a clean floor, but the vacuum of space isn't good for cleaning much. Certainly not the bacteria that have been surreptitiously hitching a ride on the vessels that set out from Earth.
In spite of the low temperatures and high radiation that contribute to space's well-known hostility to life—and NASA's best efforts to construct spacecraft in sterile conditions—some bacterial spores seem built to survive anything. Even traveling through space. New research reveals that space agencies are going to need new methods to sanitize future planetary explorers, because the old ones were designed for old spaceships and old assumptions, and left the door open to the tiniest, most resilient forms of life.
The discovery that tardigrades, also known as 'water bears,' can survive in the vacuum of space gave rise to all sorts of old questions: if they can survive space, maybe that's where they came from. And if space is survivable, not only can something come to Earth and find a home, but whatever we send up there on shuttles or satellites could be unknowingly carrying life from Earth, too.
But, we already knew that, which is why NASA takes care to sterilize planetary probes. Not only does the United Nations Outer Space Treaty prohibit the contamination of potentially fertile worlds with our terrestrial bacteria, but space programs are trying hard to prevent any false positive reports from their probes. It'd be pretty embarrassing to claim you found life on Mars, only to have to later admit that it was life that you brought there in the first place. But what counts as space-level sanitary depends on assumptions about life in outer space—namely, that there can't be any.
"If you are able to reduce the numbers to acceptable levels, a proxy for cleanliness, the assumption is that the life forms will not survive under harsh space conditions," explains Kasthuri J. Venkateswaran, a researcher with the Biotechnology and Planetary Protection Group at NASA's Jet Propulsion Laboratory, to Keith Cowing at Astrobiology. However, Venkateswaran's own research has found bacteria that can spore up and can likely survive a trip to Mars, or even a five-year trip to Europa.
Spores of Bacillus pumilus SAFR-032 have shown especially high resistance to techniques used to clean spacecraft like as ultraviolet radiation and peroxide treatment, and it also survived in the vacuum of space. Spores were exposed for 18 months on the European Technology Exposure Facility (EuTEF) mounted outside the space station, and to Venkateswaran's surprise some spores with higher concentrations of proteins associated with UV resistance survived.
In another study, spores of Bacillus pumilus SAFR-032 and Bacillus subtilis 168 survived for 18 months on the EuTEF, when UV rays were filtered, as might happen for spores that are under other spores, or in a pocket of the spacecraft surface, Cowing wrote.
Extremophiles, bacteria accustomed to surviving for millions of years frozen in suspended animation in Earth's permafrost, were also found to be capable of surviving in space. The lived long enough even to survive a trip from Earth to Jupiter's moon Europa, where they could come back to life in Europa's speculated oceans.
Given that there hasn't been any confirmed discoveries of life beyond Earth yet, all of this contamination hand-wringing might seem unnecessary. But getting a sterile spacecraft is the first step in confirming life beyond, to say nothing of what introducing a heretofore unknown bacterium might to do an ecosystem.
But, in spite of these concerns, NASA's still looking for the best way to get its probes clean.
Working on Opportunity in the clean room. Image: NASA
Thing is, according to their planetary protection website, “NASA currently has only one approved method of spacecraft sterilization—dry heat microbial reduction.” But today's lighter, smaller spacecraft, which can survive in the vacuum of space, can't survive the dry heat microbial reduction. As the Jet Propulsion Laboratory's Karen Buxbaum told Space Daily in 2001, "We have spacecraft materials that are not designed to withstand that type of dry heat environment. So our electronics are incompatible with heating, certain adhesives that we use, some of the optical surfaces, and synthetic materials that we use for spacecraft are ill suited for that type of approach."
So NASA's been looking for a new way to clean spacecraft. Some astrobiologists, like Alberto Fairén of Cornell University and Dirk Schulze-Makuch of Washington State and the NASA Office of Planetary Protection's protocols and policies, claim that the out-going spacecraft cleaning budget is a ripe for cutting by the cash-strapped space agency. "As planetary exploration faces drastic budget cuts globally," they were quoted as saying in the Daily Galaxy, "it is critical to avoid unnecessary expenses and reroute the limited taxpayers' money to missions that can have the greatest impact on planetary exploration."
They estimate that bacterial exchange has been going on, at least between Earth and Mars, for billions of years and that this is unnecessary fretting, especially in the near term. But as we set our sights farther and farther out, to Europa or Saturn's moon Titan, one hopes that this becomes an issue that we actyually have to fix—and that we do fix.