It’s mission accomplished for astronauts who plugged up a leak on the International Space Station during a hastily-arranged spacewalk on Saturday.

Astronauts Chris Cassidy and Tom Marshburn replaced a pump that had been leaking ammonia since Thursday -- taking just three hours to do the job.

Shortly after the repairs, ISS Commander Chris Hadfield tweeted: “No leaks! We're bringing Tom & Chris back inside. In two days Tom, Roman & I return to Earth in our Soyuz. This is an amazing place & time.”

Joel Montalbano, NASA's deputy space station program manager, said it could take four or five weeks to determine if the this is a permanent fix.

"I will tell you that we're happy. We're very happy," Montalbano said. "We didn't see any obvious signs of a leak, but it's going to take some time ... for us to look at the system, evaluate the system and make sure we did, indeed, stop the leak."

U.S. astronauts Cassidy and Marshburn ventured out into space and travelled 45 metres to the worksite shortly after the hatch of the ISS opened at 8:45 a.m. ET. on Saturday.

The hastily-arranged spacewalk was announced on Friday after a radiator leak was detected in the ISS power system.

Expedition 35 crew members Cassidy and Marshburn removed a 260-pound pump controller box that may be the source of the leaking ammonia coolant, and replaced it with a spare about three hours after the mission began.

NASA had said the leak was serious but never posed a threat to the six-member crew aboard the space station.

Even if the space station lost one of its eight power channels, it would still be fine, according to Canadian astronaut Jeremy Hansen.

"Now, if it went down to six, we'd have to start shutting down some science projects and experiments,” Hansen told CTV News. “If it went down to five, it would be even harder to keep the space station running. And at some point, we'd have to abandon the space station."

Of course, things never got to that point.

Early in the spacewalk, NASA reported the men spotted nothing unusual at the pump and flow subassembly box, which was suspected of leaking the coolant. The crew replaced the pump package as a precaution.

Shortly after the repairs, ISS Commander Chris Hadfield tweeted: “No leaks! We're bringing Tom & Chris back inside. In two days Tom, Roman & I return to Earth in our Soyuz. This is an amazing place & time.”

The crew first noticed small white flakes of ammonia leaking out of the ISS on Thursday. Ammonia is used to cool the power channels that provide electricity to the station’s systems.

Hadfield, the Canadian on the team, is scheduled to return to Earth Monday morning following five months in space.

LIVE UPDATES:

12:44 p.m. ET: Hadfield tweets: “Houston just sent the command to start flowing ammonia through the newly-installed pump. Gloved fingers crossed.”

12:08 p.m. ET: ISS reports that the spare pump, one of three spares on the space station, is functioning normally in initially start-up process. Crew says it will take some “to declare victory.”

11:46 a.m. ET: New pump up and running, no leakage is being reported. Accumulator quantities are increasing by the minute, NASA reports, adding it's "all good news". Old pump may have been source of ammonia leak detected on Thursday.

11:19 a.m. ET: Hadfield adds his insights into the spacewalk: “The reason they regularly check their gloves is for damage. Even though multi-layer, even a tiny leak requires immediate haste to airlock,” he tweets.

10:51 a.m. ET: More than two hours into spacewalk and no evidence of source of leakage or damage. Mystery mounts as NASA analyzes the data, or lack thereof, of ammonia leak that was first reported Thursday.

10:27 a.m. ET: NASA reports that Cassidy and Marshburn are working “like two CSI investigators” in their attempt to locate the source of the ammonia leak. Spacewalk is about 30 minutes ahead of schedule.

10:07 a.m. ET: ISS crew removes pump and flow control system out of box; reports that no ice or ammonia flakes have been found.

9:50 a.m. ET: Cassidy and Marshburn report that they’re in good shape, flying 255 miles over San Diego.

9:28 a.m. ET: Expedition 35 commander Chris Hadfield tweeted that both Cassidy and Marshburn were safely outside the ISS, adding: “THAT was a busy AM inside.”

9:20 a.m. ET: Upon reaching the suspected site of the leak, Cassidy reports that he didn’t see ammonia flakes or anything out of the ordinary before beginning the process of unbolting the box.

8:45 a.m. ET: The hatch of the airlock opened and Cassidy and Marshburn venture out after going through a checklist of tasks.