Mission controllers aborted yesterday’s spacewalk from the International Space Station after one astronaut’s suit malfunctioned.

Carbon dioxide levels in Chris Cassidy’s spacesuit began to rise during the maintenance mission he was undertaking with Dave Wolf. NASA suspects the problem may trace to a lithium hydroxide canister which is supposed to remove the gas to maintain breathability inside the suit.

“It seems like the canister itself is experiencing some problems,” Aki Hoshide told the team from Houston (Reuters, CNET). “It’s not an imminent failure. We just wanted to make sure that you guys are back in the airlock.”

The problem meant Wolf and Cassidy were unable to finish their planned replacement of several huge batteries on the outside of the space station.

Image: Cassidy in an early shot from the same mission, with the checklist for extravehicular activity floating in front of him / NASA