( CNN ) -- Two astronauts conducted a spacewalk Saturday to address an ammonia leak in the International Space Station 's cooling system , a mission that ended with NASA optimistic the potentially major problem had been fixed . In addition to inspecting the site of the leak , NASA astronauts Chris Cassidy and Tom Marshburn removed and replaced a 260-pound pump controller box , NASA explained . Afterward , Cassidy and Marshburn could n't detect any sign of a leak -- which might have been manifest , in this case , as ammonia snow flakes -- when space agency officials back on Earth ran the new pump . Still , NASA noted that the area needs to be monitored long-term to prove that the problem has been fixed . `` No leaks ! '' tweeted Chris Hadfield of Canada , the space station 's commander , who choreographed the spacewalk . `` We 're bringing Tom & Chris back inside . ... This is an amazing place & time . '' The entire spacewalk took 5 hours and 30 minutes -- an hour less than expected . First detected early Thursday morning , the leak was causing ammonia to enter space at a rate of 5 pounds per day , Mike Suffredini , NASA 's space station program manager , told reporters Friday . Ammonia is used to cool the solar arrays that provide electricity to station systems . The discovery spurred teams at NASA , over a busy 24-hour stretch , to go into `` a full-court press to understand what the failure is '' and how to address it , NASA flight director Norm Knight said Friday . That set the stage for the spacewalk involving three of the space station 's six-man crew . The leak was in a cooling loop in a solar array that has leaked before , and astronauts tried to fix it in November . It 's unclear whether the leak detected this week was the same one or a new issue . The ammonia coolant for the power channel , one of eight used to supply electricity to the station , would likely have run out by late Friday morning had it not been shut down , NASA said . `` It is a serious situation , but between crew and experts on the ground , it appears to have been stabilized , '' Hadfield tweeted Thursday . Still , the space station 's crew -- which also includes three Russian cosmonauts -- were never in danger because of it , NASA said . Moreover , the agency has said the rest of the orbiter was otherwise operating normally . Both Marshburn and Hadfield are scheduled to leave the space station at 7:08 p.m . ET Monday with Russian cosmonaut Roman Romanenko , and NASA said Friday that the spacewalk would n't affect those plans . Cassidy and Russian cosmonauts Alexander Misurkin and Pavel Vinogradov will stay behind . They 'll eventually be joined by NASA astronaut Karen Nyberg , Russian cosmonaut Fiyodor Yurchikhin and European Space Agency astronaut Luca Parmitano , who are due to launch aboard a Russian Soyuz spacecraft on May 28 . CNN 's Laura Smith-Spark contributed to this report .
2 astronauts did a spacewalk to address ammonia leaking from the International Space Station . They inspected the site of the leak and replaced a pump controller box on the orbiter . Afterward , initial tests showed no sign of a continuing leak , which was spotted Thursday , . 3 of the 6 men involved in the 5Â½-hour spacewalk are set to leave for Earth on Monday .