KENNEDY SPACE CENTER, FLA. - Inside the Vehicle Assembly Building at NASA's Kennedy Space Center, a solid rocket booster segment is being lifted by a 325-ton crane for stacking. The booster is part of the shuttle stack for Discovery and mission STS-121. Launch of Discovery is scheduled for no earlier than May. Photo credit: NASA/Jack Pfaller

KENNEDY SPACE CENTER, FLA. - Inside the Vehicle Assembly Building at NASA's Kennedy Space Center, a solid rocket booster segment (upper left) is mated with the lower segment. The booster is part of the shuttle stack for Discovery and mission STS-121. Launch of Discovery is scheduled for no earlier than May. Photo credit: NASA/Jack Pfaller