Space junk threatened the seven astronauts aboard the International Space Station on Monday and forced them to seek shelter in their docked capsules.
The US Space Command said it was tracking a field of orbiting debris, the apparent result of some type of satellite break-up.
The astronauts retreated into their docked capsules early Monday, after being told of the last-minute threat. Mission Control had them close the hatches between the space station compartments again later in the day, as a safety precaution.
"We are actively working to characterize the debris field and will continue to ensure all space-faring nations have the information necessary to maneuver satellites if impacted," the Space Command said in a statement.
NASA officials did not provide any immediate comment.
The Russian Space Agency said via Twitter that the astronauts were ordered into their docked capsules earlier in the day, in case they had to make a quick getaway. Later, the crew was routinely performing operations, the agency noted.
Friends, everything is regular with us!" tweeted the space station's commander, Russian Anton Shkaplerov.
But the cloud of debris seemed to be posing a threat on each passing orbit. Mission Control informed the crew that the latest pass was expected to last seven minutes, and had them interrupt their science research to once again take safety precautions.
Some 20,000 pieces of space junk are being tracked, including old and broken satellites. Last week, a fragment from an old Chinese satellite the target of a missile-strike test in 2007 came uncomfortably close. While it later was shown to be no threat, NASA had the space station move anyway.
The space station is currently home to four Americans, one German and two Russians.