The right aft booster rolls into the transfer aisle of the Vehicle Assembly Building it will be lifted onto the mobile launcher platform.

The right aft booster, comprised of the aft skirt and the aft motor segment, rolls into the transfer aisle of the Vehicle Assembly Building where it will be lifted onto the mobile launcher platform. 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