The RM-81 Agena was an American rocket upper stage and satellite support bus which was developed by Lockheed initially for the canceled WS-117L reconnaissance satellite program. Following the split-up of WS-117L into SAMOS and Corona for image intelligence, and MIDAS for early warning, the Agena was later used as an upper stage, and an integrated component, for several programs, including Corona reconnaissance satellites and the Agena target vehicle used to demonstrate rendezvous and docking during Project Gemini. It was used as an upper stage on Atlas, Thor, Thorad and Titan IIIB rockets, and considered for others including the Space Shuttle and Atlas V. A total of 365 Agena rockets were launched between February 28, 1959 and February 1987, when the last Agena D was launched.
On some missions, the payload was built directly into the Agena, which provided it with electric power, communications and three-axis stabilization. Payload components were usually located ahead of the Agena's standard bulkhead. On missions where the payload was not built into the Agena, and instead separated after launch, the Agena was known as an Ascent Agena. The Agena was upgraded twice from the original Agena-A in order to support heavier and more sophisticated satellites, such as Corona spacecraft with multiple and more powerful cameras.
The final Agena launch was of an Agena-D on 12 February 1987, configured as the upper stage of a Titan 34B. In all, 365 Agena vehicles were launched by NASA and the US Air Force