International Space Station repairs. Astronauts Piers Sellers (with red stripes) and Michael Fossum on the robotic arm of space shuttle Discovery during mission STS-121. They carried out repairs to the International Space Station (ISS). On the shuttle's 12 day mission the crew also tested new equipment and procedures to improve shuttle safety and deliver supplies to the ISS. This was the 115th shuttle flight and the 18th US flight to the ISS.