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Shuttle Discovery astronauts have made the first of two spacewalks this week to carry out maintenance tasks on the International Space Station (ISS).

Steve Bowen and Alvin Drew spent six-and-a-half hours outside the orbiting platform, preparing it for the installation of a new store room and moving a faulty coolant pump.

Discovery arrived at the ISS on Saturday.

It is the ship's final mission into space before being retired to a museum.

Bowen is making a small piece of history, himself. He was called up late for the mission following a bicycle crash injury to the assigned lead spacewalker Tim Kopra.

It means Bowen is on consecutive shuttle trips having flown on Atlantis in May last year - the first time that has ever happened.

The US space agency (Nasa) said Monday's spacewalk was the 243rd by American astronauts and the 154th in support of ISS assembly and maintenance.

Bowen and Drew relocated a broken ammonia pump, and they also moved some cabling to allow the permanent attachment to the station on Tuesday of the Italian-built logistics module known as Leonardo.

The cylinder, which has been used down the years as a packing box for supplies in the orbiter's payload bay, would normally return to Earth with every shuttle mission, but for Discovery's flight it is being left on the station to provide extra storage space.

Bowen and Drew ended their walk by carrying out a Japanese request to "bottle" a sample of space vacuum in a metal container.

The light-hearted project will see the capsule, signed by many astronauts, returned to Earth to go on public display.

Nasa's mission control played the song Message in a Bottle by the British rock group The Police as the two Americans captured their sample of nothing.

Discovery is due back on Earth on Monday, 7 March. The orbiter is expected eventually to go to the Smithsonian Institution's Air and Space Museum.