(CNN) -- Unexpected space debris came flying close to the International Space Station on Tuesday, prompting its six astronauts to take shelter inside two Soyuz capsules, NASA said.
Russia's Interfax news agency said preliminary data on "the dangerous approach" shows that the "trash" came within about 250 meters (820 feet) of the station.
NASA spokesman Kelly Humphries said it was not immediately known how close the debris came, but that 820 feet was the closest it could have come, according to NASA's projections.
Officials at NASA are investigating what the debris was, NASA spokesman Joshua Buck said.
By the time it was spotted, it was "too late to make an avoidance maneuver," so NASA ordered the six crew members to "shelter in place," Buck said. About 7:30 a.m. ET, the crew members climbed into the two Soyuz capsules positioned at the station.
NASA determined that the debris would come closest to the station at 8:08 a.m. ET.
Three minutes later, at 8:11 a.m. ET, the all-clear was sounded and astronauts were allowed to exit the capsules, Buck said.
Buck described the debris as an "unknown object of unknown size."
-CNN's Josh Levs contributed to this report