The aft booster segments to be used with Discovery on STS-121 are fitted with a crane in the Vehicle Assembly Building.

The right aft booster segments (aft skirt and aft motor segment) to be used with space shuttle Discovery on mission STS-121 are fitted with a crane in the Vehicle Assembly Building at NASA Kennedy Space Center. The crane will lift the mated segments and install them on the mobile launcher platform in Bay 3. Mission STS-121 is the second space shuttle mission in the return-to-flight sequence. The booster assembly is a major milestone in the processing sequence that leads to launch. The launch date is targeted no earlier than May. Photo credit: NASA/Jack Pfaller