Six astronauts took shelter inside two Soyuz capsules when a piece of space debris came close to the International Space Station on Tuesday.

The unidentified object came within 1,100 feet of the space station — closer than any piece of space junk ever, said NASA’s space operations chief, Bill Gerstenmaier.

“Officials at NASA are investigating what the debris was,” CNN reported quoting NASA spokesman Joshua Buck.

By the time it was spotted, it was too late to make an avoidance plan, so NASA ordered the six crew members to shelter in place, Buck said.

Around 7:30 a.m. ET, the crew members took shelter into the two Russian Soyuz capsules positioned at the station.

NASA determined the closest approach of the debris at 8.08 a.m. ET. Around 8.11 a.m. ET Mission Control sounded all-clear. The two Americans, three Russians and one Japanese astronaut floated back into the space station, Buck said.

Buck described the debris as an unknown object of unknown size.