This image obtained from NASA on April 14, 2010 shows dwarfed by space shuttle Discovery, NASA astronauts Rick Mastracchio(R) and Clayton Anderson working in Discovery's aft payload bay during the mission's third and final session of extravehicular activity on April 13, 2010 as construction and maintenance continue on the International Space Station. Earth's horizon and the blackness of space provide the backdrop for the scene. Astronauts on board the International Space Station (ISS) might have to do an unscheduled fourth space walk to fix a malfunctioning valve on a newly installed ammonia tank, NASA officials said April 14, 2010. The tank, which is a key part of the cooling system on the ISS, was installed Tuesday during what was supposed to be the last space walk by US shuttle astronauts.