The Voyager configuration also included a solid rocket motor as an additional stage. Viking and Voyager interplanetary spacecraft, illustrated in stowed configuration, mounted on Centaur D1-T upper stages. Viking and Voyager Spacecraft with Centaur D-1T Stages Centaur, 50 years after its first flight in 1963, still sees frequent use on Atlas 5 boosters. The Agena stage, first flown in 1959, would fly over 360 missions before being retired in 1987. Although periodically upgraded, upper stage families tend to remain in service for long periods of time. Centaur and IUS also served on multiple families of launchers. For instance, Agena was used on various Thor, Atlas and Titan rockets. Several upper stages have seen service on more than one booster. The Agena would provide the spacecraft with power, communications, and attitude control, as well as providing orbital maneuvering as needed. Many early American intelligence spacecraft, for instance, remained attached to their Agena upper stage throughout their operational life. Some upper stages remain attached to their payload and provide services long after reaching their initial orbit.

Often, upper stage engines can be restarted several times while in space.

Upper stages propel payloads on interplanetary trajectories, or into orbits higher than could otherwise be reached using a rocket booster. Illustration showing several upper stages.

Atlas agena rocket paper model