A cosmonaut raised the possibility that astronauts conducting space walks outside the International Space Station had gathered bacteria off its surface that may not have come from Earth, according to an interview published Monday by the Russian state news agency TASS.

"And now it turns out that somehow these swabs reveal bacteria that were absent during the launch of the ISS module. That is, they have come from outer space and settled along the external surface. They are being studied so far and it seems that they pose no danger," Anton Shkaplerov said, according to an English-language article the agency published after the interview originally ran.

Shkaplerov appears to be referring to the work of the Test and Biorisk projects, which monitor the exterior of Russian modules of the International Space Station. In May, TASS announced that Roscosmos, the Russian space agency, was studying 19 dust samples for potential signs of extraterrestrial life.

The International Space Station in orbit. NASA

Based solely on Shkaplerov's comments, it's too early to say we have found extraterrestrial life: the station has been in orbit for almost two decades and there are plenty of ways microbes could have snuck up since the launch. NASA did not respond to a request for comment about the statement, but in 2014, when Russian officials announced that a similar project had found Earthen bacteria on the space station's exterior, NASA was quick to point out they had received no such information.

Even the best sterilization techniques can't necessarily remove all traces of life on Earth when an uncrewed mission launches. That's why, for example, NASA was careful to destroy the Cassini spacecraft by sending it plummeting into Saturn, rather than risk it touching the planet's potentially habitable moon Enceladus. It's also why when NASA selects target landing and exploration sites for its Mars missions, it rules out places it thinks bacteria could flourish.

Read more: NASA Put Dangerous Bacteria on a Satellite to Find Treatments for Future Space Travelers

But sterilization gets even trickier at the International Space Station, which has been in orbit since 1998. The space agencies that participate in the project do what they can to decontaminate supplies and quarantine astronauts, but like any other building that serves as a home for humans, its interior is known to be brimming with bacteria, which NASA monitors. Bacteria are also known to hang out in the upper reaches of Earth's atmosphere, as high as 20 miles above the surface.

Shkaplerov, who has spent a year in orbit, will be returning to the International Space Station via a rocket launch currently scheduled to take place next month.

This article was first written by Newsweek

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