The I.U.S. (Inertial Upper Stage) was manufactured by Boeing as a two-stage module to bring satellites from low Earth orbit to a geostationary or an interplanetary orbit.
The I.U.S. is a three axes stabilized solid twin-rocket motor, Orbus-21 and Orbus-6E (with extendable nozzle), for two burn times.
Originally intended only as an interim solution until the introduction of the Centaur-G stage on the Space Shuttle, the I.U.S. was after adjustment of the Centaur-G the standard for heavy geostationary payloads and deep space missions.
The I.U.S. was total of 24 times used both on launcher type Titan-34D, Titan-34D7, Titan-IVA and Titam-IVB as well as the Space Shuttle.
Payloads were TDRS communication satellites and military satellites DSP and NASA missions Magellan, Ulysses, Galileo and Chandra. For the launch of the Ulysses spacecraft was the I.U.S. equipped with an additional PAM-S upper stage.