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 are carrying out repairs to the International Space Station (ISS). On the shuttle's 12 day mission the crew will also test new equipment and procedures to improve shuttle safety and deliver supplies to the ISS. This is the 115th shuttle flight and the 18th US flight to the ISS.