Space invaders have colonized the International Space Station.
When astronaut Peggy Whitson moves into the orbiting laboratory today for a six-month stay, she'll have two human roommates — as well as countless ones invisible to the naked eye, from microbes that can corrode metal to germs that can cause serious infections in people.
Outer space is a cold and sterile place, but spaceships are not. As the 9-year-old space station ages, it's likely to grow more micro-organisms that could pose a risk to its human residents and the station itself. Adding an extra worry, scientists have seen signs that the human immune system weakens during space trips.
"Wherever man goes, microbes go," says Cheryl Nickerson of Arizona State University, who studies disease-causing micro-organisms. Most of the bugs in orbit aren't dangerous, she says, but "there's absolutely a risk … to the crew."
In a study published Tuesday in the Proceedings of the National Academy of Sciences, Nickerson found that salmonella bacteria turned deadlier after a few weeks in space. The bacteria rode into orbit as an experiment aboard space shuttle Discovery in 2006.
No astronaut knows more about the risks of life on the station than Whitson, who'll be the first NASA astronaut to stay there twice. She is scheduled to float into the station's door today at roughly noon ET.
On the bright side for Whitson and her crewmates, space germs have never made an astronaut on the station seriously ill. Thanks to anti-microbe measures, the outpost is squeaky-clean compared with homes on Earth. Even so, the station's microscopic occupants can make life for the astronauts more difficult and less sanitary:
•In 2003, bacteria helped clog the cooling-system pumps in each of three spacewalking suits kept aboard the station. That forced the crew to make a spacewalk in backup suits that are harder to work in. The culprit bacteria lived in the water used as a coolant.
•Studies of space station water samples found that bacteria levels in the station's own cooling system were rising far sooner than expected, raising fears that the bugs could corrode the system's most delicate structures. The bacteria level has since stopped rising, and NASA will add a new kind of chemical to kill the germs in the system.
•Astronauts had to splash disinfectant on a large patch of mold that oozed its way across one of the station's walls. The cause: wet towels that brushed the wall when the crew hung them up to dry.
The space station isn't nearly as germ-ridden as the Russian space station Mir, which was home to dust mites, E. coli bacteria and dozens of other unexpected microbes.
The space station is dust mite-free, but it does harbor a varied zoo of microscopic residents. Scientists have found stowaways ranging from staph bacteria, which can cause abscesses in people with weakened immune systems, to bacteria that break down disinfectants.
A few times, the space station's walls and other surfaces have been dotted with enough microbes to violate the limit set by NASA. The toilet, handles and grates covering ventilation ducts are especially hospitable to bacteria and fungi.
One of the station's water supplies exceeded NASA's microbe standards by 18 times. More disinfection of the water and replacement of system tubing has vanquished the bacteria, says Duane Pierson, chief microbiologist at NASA's Johnson Space Center.