The booster segments cross over into the high bay where the mobile launcher platform is waiting in Bay 3.

In the Vehicle Assembly Building at NASA Kennedy Space Center, the right aft booster segments (aft skirt and aft motor segment) cross over into the high bay where the mobile launcher platform is waiting in Bay 3. The segments will be stacked for use with space shuttle Discovery on mission STS-121. 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