WATCH: Astronauts Troubleshoot Ammonia Leak

Two NASA astronauts replaced a 260-pound pump outside the International Space Station on Saturday in hopes of stopping the ammonia leak in the cooling system for one of the station's massive solar arrays, NBC News reported. 

Two veteran spacewalkers, Tom Marshburn and Chris Cassidy, made an impromptu spacewalk to troubleshoot the problem that was detected Thursday, when station crew members alerted Mission Control that they saw "snowflakes" of frozen ammonia floating away from an area around the pump box, according to NBC News. 

Saturday's operation ranks as one of the fastest turnarounds ever for a space station repair. The spacewalk began at 8:44 a.m. ET, about a half-hour later than originally planned, but astronauts made quick progress.

"No evidence of any ammonia leakage whatsoever. We have an airtight system — at the moment," Mission Control reported after the swap was completed.

NASA said the leak was significant, but it never jeopardized the safety of the crew.

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