Mold spores commonly found aboard the International Space Station (ISS) turn out to be radiation resistant enough to survive 200 times the X-ray dose needed to kill a human being. Based on experiments by a team of researchers led by Marta Cortesão, a microbiologist at the German Aerospace Center (DLR) in Cologne, the new study indicates that sterilizing interplanetary spacecraft may be much more difficult than previously thought.

If science fiction films are anything to go by, one popular image of spacecraft is that they are so spotlessly clean that they make a hospital surgical theater look like a sewage treatment plant. However, real manned craft that have been occupied for any length of time, like the ISS, turn out to be anything but pristine.

The ISS is a collection of sealed cans inhabited by people who spend every minute of the day sweating, touching things, and exhaling moist air. Even with regular cleaning and a life support system designed to keep things under control, the result is a constant battle against mold and bacteria.

That's already bad enough and poses health hazards to astronauts as they breathe in spores and the like, but Cortesão's team's work on Aspergillus and Pennicillium, which are the two most common fungi on the ISS, indicate that getting rid of them is even tougher than thought – and that they may even be able to live outside the station.

The researchers exposed samples of Aspergillus and Pennicillium spores to X-rays, heavy ions, and high-frequency ultraviolet light of the kinds and intensities found in space. Such radiation damages DNA and breaks down cell structures, but the spores survived X-rays up to 1,000 gray, heavy ions at 500 gray, and UV rays up to 3,000 joules per meter squared.

Gray is a measurement of radiation exposure based on the absorption of one joule of radiation energy per kilogram of matter. To place the results into perspective, five gray will kill a person and 0.7 gray is how much radiation the crew of a Mars mission would receive on a 180-day mission.

Since mold spores can already survive heat, cold, chemicals, and drying out, being able to take on radiation as well poses new challenges. It means that not only will manned missions have to put a lot of effort into keeping the ship clean and healthy, it also means that unmanned planetary missions, which must be free of terrestrial organisms to prevent contaminating other worlds, will be harder to sterilize.

But according to Cortesão there is a positive side to this resiliency. Since fungal spores are hard to kill, they'd be easier to carry along and grow under controlled conditions in space, so they can be used as raw materials or act as biological factories.

"Mold can be used to produce important things, compounds like antibiotics and vitamins, says Cortesão. "It's not only bad, a human pathogen and a food spoiler, it also can be used to produce antibiotics or other things needed on long missions."

Since the present study only looked at radiation, orbital experiments are scheduled for later this year that will test their ability to withstand the combination of radiation, vacuum, cold, and low gravity found in space.

The results of the team's study were presented at the 2019 Astrobiology Science Conference.