A remote manipulator system installed on the orbiter Atlantis is being installed in Discovery's payload bay.

In NASA Kennedy Space Center’s Orbiter Processing Facility Bay 3, a remote manipulator system, or space shuttle arm, previously installed on the orbiter Atlantis, is being installed in Discovery’s payload bay. The arms were switched because the arm that was installed on Atlantis has special instrumentation to gather loads data from the second return-to-flight mission, STS-121. Discovery is the designated orbiter to fly on STS-121. scheduled to launch no earlier than May 2006.