The Agena was a very successful second-stage rocket booster made by Lockheed Missiles & Space Company of Sunnyvale, California. It was 26 feet long and five feet in diameter and produced 16,100 pounds of thrust. It used unsymmetrical dimethyl hydrazine as fuel, with inhibited red fuming nitric acid as its oxidizer. It had a three-axis gyro guidance and control system with correction inputs from horizon sensors.
It was paired with several first-stage boosters in the early days of US space exploration, such as the Thor and Atlas. In particular, the Agena booster sent many of the Mariner probes on their way and boosted early communications satellites into geo-synchronous orbit.