Two astronauts went for a spacewalk outside the International Space Station to reposition two spare solar array ammonia coolant pump assemblies.

NASA astronauts Drew Feustel and Ricky Arnold quickly swapped the positions of two spare ammonia pumps that are part of the space station’s critical cooling system.

One pump got too cold because of a power shutdown 17 years ago and is called Frosty; flight controllers plan to test it in the coming days to see if it still works. The other, a failed unit dubbed Leaky, spewed out ammonia five years ago. The units are designed to help push coolant through solar array batteries to keep them from overheating.

“We’ve been doing a ton of work to play musical chairs with all these (pumps) so we can have good available spares,” flight controller Alex Apyan said from Houston during the spacewalk.

Each 235-pound pump is about 3 feet by 2.5 feet by 1.5 foot.

This was the 210th spacewalk in support of the International Space Station. It was Feustel’s eighth spacewalk and the fourth spacewalk of Arnold’s career.