The right aft booster is ready to be lifted onto the mobile launcher platform inside the Vehicle Assembly Building.

The right aft booster, comprised of the aft skirt and the aft motor segment, is ready to be lifted onto the mobile launcher platform inside the Vehicle Assembly Building. The booster is being assembled for the space shuttle Discovery on mission STS-121, 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