NASA astronaut Chris Cassidy performs a spacewalk in May to inspect and replace a pump controller box on the International Space Station. On Saturday, two astronauts will perform the first in a series of similar spacewalks to fix a broken cooling line on the ISS. AP hide caption

itoggle caption AP

Canadian Space Agency astronaut Chris Hadfield strums his guitar, a Larrivee Parlor, on the International Space Station in December. NASA hide caption

itoggle caption NASA

The Falcon 9 SpaceX rocket lifted off from space launch complex 40 at the Cape Canaveral Air Force Station in Cape Canaveral, Fla., early Tuesday. John Raoux/AP hide caption

itoggle caption John Raoux/AP