Nasa astronauts have completed the last spacewalk of the space shuttle era.The six-and-a-half hour excursion retrieved an ailing ammonia pump on the International Space Station (ISS), where shuttle Atlantis is docked.The spacewalk is part of a busy schedule for the last shuttle mission, which will also see four tonnes of supplies delivered to the ISS.Smaller supply ships developed by private companies are expected to supply the ISS when the shuttle programme retires. Nasa extended Atlantis' mission by one day on Monday and it will head homeward for the last time on 21 July.