Two NASA astronauts worked hand-in-hand with a robotic arm during a six hour-plus spacewalk to attach a 1,700 pound ammonia tank to the International Space Station.

Making their second spacewalk in three days, Discovery's mission specialists Rick Mastracchio and Clay Anderson had trouble with one of four bolts designed to attach the ATA to the ISS's Starboard 1 truss. The pair began the spacewalk at 1:30am EDT (0530 GMT) on Sunday, about 45 minutes ahead of schedule. They spent seven hour and 26 minutes installing the ammonia tank on the International Space Station.

NASA noted that the spacewalk was the 235th conducted by US astronauts, and the fifth for both Mastracchio and Anderson.

The replacement ammonia tank was brought to the space station aboard Discovery, which was launched from the Kennedy Space Center early Monday morning.

The astronauts focused on lifting the tank out of the NASA shuttle's cargo bay. During the spacewalk, the space station's main robotic arm, Canadarm II, move the tank into position so it could be temporarily attached to the outside of the station.

The tank will be moved into its permanent position on the space station and attached there during the mission's next spacewalk on Tuesday, according to the space agency.

During the spacewalk, Mastracchio and Anderson have also retrieved an experiment that had been attached to the outside of the station's Japanese Kibo Laboratory, and removed a failed gyroscope that is part of the station's navigation system. The pair attached a replacement gyroscope.

The robotic arms onboard the space shuttles and the space station have long played key roles in various NASA projects.

There are three robotic arms on the space station - the Canadarm II, a newer device dubbed Dextre and a smaller robotic arm that's attached to the Japanese module.

The robots are regularly used to inspect the shuttle's heat shield for damage during liftoff. The devices are also used to move cargo out of the shuttle and attach it to the space station. Astronauts have even used the robotic arms to be tansported from one end of the space station to the other.

Canadarm II is expected to be used throughout Discovery's 13-day mission, which will continue into next week.

Sharon Gaudin covers the Internet and Web 2.0, emerging technologies, and desktop and laptop chips for Computerworld. Follow Sharon on Twitter at @sgaudin. Her email address is [email protected]