Astronauts aboard the International Space Station were forced to take safety precautions in the wee hours of the morning on Monday when a cloud of space junk zipped by in orbit. The cause of the debris is a Russian missile test that destroyed a satellite in orbit.

Russian news agency TASS reported the junk cloud flew past the ISS at 2:06 a.m. ET, and again from 3:38 a.m. to 3:44 a.m., citing information from the Russian space agency Roscosmos.

Astronauts on the ISS were directed to retreat into vehicles docked to the space station to take shelter from a possible impact. Russian astronauts took refuge in the Soyuz MS-18 capsule, while American astronauts went to the Space Crew Dragon capsule. With objects moving at speeds of up to 17,500 mph in orbit, even a collision with something the size of a marble could be potentially devastating.

All astronauts have been allowed to return back into the ISS. TASS reported that the debris cloud’s orbit has moved out of the orbit of the ISS, and the space station is now in the “green zone,” apparently no longer under threat.

According to NASA, the space station is still passing through the debris cloud every 90 minutes, but there is no urgency to move astronauts back into shelter positions.

The debris cloud originated from the destruction of a defunct Russian satellite named Kosmos-1408, first launched into orbit in 1982. The UK space tracking firm Seradata tweeted on Monday that 14 new pieces of orbital debris had been identified and tracked and that an anti-satellite missile strike on the satellite was the prime suspect. LeoLabs also confirmed the existence of new objects in the area where Kosmos-1408 is supposed to be.

The State Department released a statement Monday afternoon slamming Russia for its anti-satellite test, saying it had created over 1,500 pieces of new orbital debris. On Monday evening, NASA confirmed the ISS safety precautions were a direct result of the anti-satellite test.

“Earlier today, due to the debris generated by the destructive Russian Anti-Satellite (ASAT) test, ISS astronauts and cosmonauts undertook emergency procedures for safety," NASA administrator Bill Nelson said in a statement. "With its long and storied history in human spaceflight, it is unthinkable that Russia would endanger not only the American and international partner astronauts on the ISS, but also their own cosmonauts." The agency said it would continue to monitor the situation and ensure the safety of the space station crew.

Kosmos-1408 had an orbit above the ISS, and the missile strike that destroyed the satellite could conceivably produce a debris cloud that could fall in altitude and eventually threaten the ISS. Harvard University astronomer Jonathan McDowell tweeted Monday morning that Kosmos-1408 was a “plausible candidate” for the debris cloud, and added that given the massive size of the satellite, he would expect "thousands of pieces of cataloged debris" to emerge from its destruction.

The U.S. Space Command said in a statement that it was “aware of a debris generating event in outer space,” and that it would work with NASA and the State Department to assess these reports.

On November 11, the ISS was forced to move its altitude by 1.2 kilometers to avoid another close call with space junk. Most experts are worried that increased military activity in space could cause space junk to bloom and render Earth’s orbit unsafe for satellites and spacecraft.

This story has been updated with a statement from NASA.