Two US astronauts stepped out on a brief spacewalk Wednesday to install a backup computer at the International Space Station after one failed earlier this month.

Astronauts Rick Mastracchio and Steve Swanson climbed out of the space station starting at 9:56 am (1356 GMT).

Although they were about a half hour behind schedule in getting outside the space station, the spacewalk was completed without incident in just an hour and 36 minutes, well ahead of the expected 2.5 hour timeline.

The two astronauts worked together to quickly loosen the bolts that held the broken computer to the space station and then to re-attach the replacement within less than an hour.

Shortly afterward, NASA command control in Houston declared the replacement mission "successful."

"We have a good MDM. In diagnostic mode as expected," one of the engineers on the ground told the astronauts, as the mission streamed live on NASA TV.

The computer is known as a multiplexer/demultiplexer (MDM), one of 45 aboard the orbiting laboratory.

The 10-year-old backup MDM failed after a routine restart operation on April 11 -- the primary computer continued to work without issues, however.

NASA said it provides commands to certain space station systems, including the external cooling system, solar alpha rotary joints and mobile transporter rail car.

The replacement had been stored at the space station in the Destiny lab since 2001, when it was delivered aboard the Endeavour space shuttle. The broken model was brought back into the space station, where tests would be conducted to find out what went wrong.

Swanson also performed an unrelated "get-ahead" task to cut off two wire ties from the doors of a secondary power distribution assembly -- a wiring hub -- to allow the space station's robotic arm to open and close the module's doors if needed in the future.

The astronaut initially ran into trouble as he tried to open one of the doors to test the force necessary and was initially unable to get it fully open, but ultimately was able to maneuver the hatch.

The spacewalk was the 179th in support of the orbiting space station, a global collaboration that includes Europe, Canada, Japan, the United States and Russia.

Just prior to the spacewalk, a Russian cargo capsule undocked from the space station for a two-day test of a new system to automatically reattach itself, NASA said.

The Progress 53 resupply ship separated from the space station at 4:58 am (0858 GMT), and was to retreat a distance of 311 miles (500 kilometers).

"It will return back to the Zvezda docking port Friday morning after Russian flight controllers have tested its new Kurs automated rendezvous system," the US space agency said.

A similar operation was done in July 2012 when another Progress cargo freighter pulled away from the station and redocked, also for a trial run of its automated return system.