When the United States Air Force began its space reconnaissance programs in the 1960’s, it needed a way to stabilize its camera and signal interception payloads on orbit. Under normal circumstances of the day, satellites were either spin stabilized, or left to drift along on their orbit, maybe, if things were working right, the satellite would be oriented towards the sun for power. Nothing much was directed towards the planet as it orbited, though; a major requirement for taking photos of the Soviet Union as it passed under the camera system.
The answer was in a stage developed under the name “Hustler” later to be renamed “Agena.” The Agena stage would fly up until 1987, and would go through several evolutionary stages as time progressed. This video however, focuses on the early days of the CORONA and MIDAS programs and their use of the somewhat stubby, but very capable Agena A stage, used on top of the Thor IRBM to produce an early and very impressive payload orbital insertion and maneuvering capability, letting us know quite a bit, in the long term, about what was going on in the Soviet Union during those tense cold war years.
As always, enjoy the video!
https://www.youtube.com/watch?v=_DXZk7hN58Q