A spacewalk by two U.S. astronauts was called off on Tuesday morning due to the risk of space debris, NASA announced in a statement.

NASA astronauts Kayla Barron and Thomas Marshburn were scheduled to leave through the International Space Station’s Quest airlock at 5:30 a.m. EST to replace a part on the space station, according to a NASA blogpost.

“Marshburn and Barron will work at the Port 1 truss structure, where the antenna is mounted. The antenna recently lost its ability to send signals to Earth via NASA’s Tracking and Data Relay Satellite System,” NASA said in the blog.

However, four hours before the scheduled spacewalk, the ISS tweeted that the repair would be delayed until more information was available.

The source of the debris hasn’t been confirmed. Two weeks ago Russia had conducted an anti-satellite test that created a “dangerous” debris field in the orbit.

The spacewalk was scheduled to last 6 1/2 hours and be Barron’s first and Marshburn’s fifth spacewalk, NASA said.