The Plasma Diagnostics Package (PDP), which had been previously flown on STS-3, made its return on the mission, and was part of a set of plasma physics experiments designed to study the Earth's ionosphere. During the third day of the mission, it was grappled out of the payload bay by the Remote Manipulator System and released for six hours. During this time, "Challenger" maneuvered around the PDP as part of a targeted proximity operations exercise. The PDP was successfully grappled by the RMS and returned to the payload bay at the beginning of the fourth day of the mission.