The Space Shuttle Discovery is rolled out of the Vehicle Assembly Building aboard its Mobile Launch Platform at the Kennedy Space Center in Florida. Photo: Philip Scott Andrews

Two astronauts walk across the gantry and away from the space shuttle Discovery during a routine practice evacuation in March 2010 for the STS-131 mission. Seven astronauts would ultimately lift off aboard Discovery from NASA's Kennedy Space Center in Florida the following month. Photo: Philip Scott Andrews

Work lights illuminate the Space Shuttle Discovery as it awaits its flight. Photo: Philip Scott Andrews

Sean King installs covers on one of Discovery's engines in Orbiter Processing Facility 3 shortly after the shuttle landed in Florida last year. Photo: Philip Scott Andrews

Astronaut Rick Mastracchio sits on the edge of Discovery's hatch during training in the launchpad's "white room," the last stop for astronauts before they enter the shuttle. Photo: Philip Scott Andrews

The Space Shuttle Endeavour is lifted inside the Vehicle Assembly Building before being mated to its external fuel tank and Solid Rocket Boosters. The VAB, built to house the Saturn V rocket that took Americans to the moon, is one of the largest buildings in the world. It sits on eight acres and rises to 525 feet. Photo: Philip Scott Andrews

Two Solid Rocket Booster segments sit, waiting for installation, in the Vehicle Assembly Building. Photo: Philip Scott Andrews

Three technicians photograph Atlantis as it is towed into the Orbiter Processing Facility, or OPF, after its final flight into space. Photo: Philip Scott Andrews

The "stack," two solid rocket boosters mounted to the external fuel tank, sits in the Vehicle Assembly Building at Kennedy Space Center in Florida in February 2010. The boosters would eventually be joined to Discovery. Photo: Philip Scott Andrews

Aerospace technician Al Schmidt on the flight deck of Discovery in 2010. Technicians had to check and double-check all of the switches to make sure everything worked on launch day. Photo: Philip Scott Andrews

Autographs of astronauts, employees, family members and specially cleared visitors, such as senators and sports figures, are scribbled on the wall of the white room in OPF 3. In this image, the white room is connected to the open hatch of Discovery. Photo: Philip Scott Andrews

Astronaut Alvin Drew posed for a picture on the launchpad at Kennedy Space Center during Terminal Countdown Training. A month later he flew the Space Shuttle Discovery into orbit. Photo: Philip Scott Andrews

A view of Discovery's open cargo bay from inside the flight deck of the orbiter. Photo: Philip Scott Andrews

A shuttle technician crawls through the hatch of Discovery while preparing it for flight. Photo: Philip Scott Andrews

Solid rocket boosters fire as the space shuttle Atlantis lifts from Pad 39A at the Kennedy Space Center on November 16, 2009. Photo: Philip Scott Andrews

The Final Space Shuttle, Atlantis, heads for the International Space Station after 30 years of Space Shuttle Missions. Photo: Philip Scott Andrews

The Space Shuttle Endeavour approaches the runway at the Kennedy Space Center to conclude its final mission into space. Photo: Philip Scott Andrews

The Space Shuttle Discovery lands at Kennedy Space Center in Florida after returning from the International Space Station. Discovery delivered a multi-purpose logistics module filled with science racks that were transferred to laboratories on the International Space Station. Photo: Philip Scott Andrews

The Space Shuttle lifts off from the pad. Photo: Philip Scott Andrews

Spectators watch as Atlantis arcs through the sky. Five shuttles — Columbia, Challenger, Discovery, Atlantis, Endeavour — flew 135 missions over 30 years and were the first reusable orbital spaceships. Photo: Philip Scott Andrews

Smoke clears from the launchpad after a successful flight. Photo: Philip Scott Andrews