Whether it's Martians in War of the Worlds or a telekinetic visitor in E.T. the Extra-Terrestrial, science fiction is filled with aliens making their way to Earth.
In science fact, though, there's greater concern that Earth-dwellers—specifically bacteria and microorganisms—could arrive at extraterrestrial destinations.
As NASA sends rovers to Mars, plans a trip to Jupiter's icy moon Europa, and looks for an ocean on Saturn's moon Enceladus, the hope is to find life-forms on those interplanetary bodies. To ensure that doesn't include forms that originated on Earth—and that the new environment isn't compromised the way Earth ecosystems can be by invasive species or infectious diseases—NASA is now thoroughly cleaning its space-bound vessels.
The vacuum and radiation of space, plus the heat given off when a spacecraft enters another planet's atmosphere, can be lethal to many microbes. But scientists have discovered that a few hardy ones can stick around.
"A surprisingly large variety of organisms are able to survive a trip through space," says NASA Planetary Protection Officer Catharine Conley. "Experiments to put microbes, lichens, plant seeds, and even small animals on the outside of the International Space Station have demonstrated survival for years."
To avoid the spread of microorganisms, NASA and other international space agencies have agreed, via planet protection protocols set by the Committee on Space Research (COSPAR), to strict cleaning protocols for all spacecraft.
Yet even after a thorough cleaning, ships still have some microorganisms attached. COSPAR has set levels for how many remaining organisms are acceptable, though those standards will surely change as scientists learn more about how Earth's organisms survive elsewhere.
As for how spaceships are cleaned, says Conley, most current sanitation methods are based on treatments used for medical devices.
Kasthuri Venkateswaran, a senior research scientist in the Biotechnology and Planetary Protection Group at the California Institute of Technology's Jet Propulsion Laboratory, says it's important to draw a distinction between organisms that can "survive" and ones that can "colonize."
"To colonize any place, one has to 'reproduce,'" he said. Spores that can't reproduce can't colonize, so these aren't a major concern.
Alberto G. Fairén, a planetary scientist working at Cornell University's department of astronomy, says some areas on Mars—those "special regions" where life may be able to exist today—should be kept especially well protected from Earth's microbes.
"Those are regions [where] terrestrial organisms could readily propagate—or ones thought to have [a high] potential for Martian life forms—because they have liquid water and [temperatures above] freezing at least some days per year," Fairén said. "Those regions of Mars should be avoided for landing."
President Barack Obama has proposed a budget for NASA that's $435 million less than the one approved last week by the House Appropriations Committee. It's not clear how that will affect planet protection.
Whatever happens, says Conley, continued funding is necessary. Just imagine an unclean spacecraft contaminating a new environment with Earth life, she says, to the degree that it's hard to tell what's actually indigenous.
"It would be very unfortunate to go looking for life on Europa," she said, "only to find life from Florida."