An American astronaut from the space shuttle Discovery has successfully completed a risky repair mission on the underside of the shuttle. Astronaut Steve Robinson made the unprecedented spacewalk to the belly of an orbiting shuttle attached to a robotic arm at the International Space Station. He pulled-out two dangling pieces of foam protruding from between the shuttle's fragile thermal tiles. NASA engineers were concerned the protrusions could disrupt the flow of hot gases as the shuttle re-enters Earth's atmosphere. Discovery's astronauts may have to make one more repair before for the shuttle is cleared to land next week. NASA says a piece of thermal insulation fabric came loose during launch, and they fear it could fly off during re-entry, striking the shuttle. NASA engineers say they are taking every possible precaution before Discovery lands -- more than two years after the shuttle Colombia disintegrated during re-entry.