The Inertial Upper Stage (IUS), originally designated the Interim Upper Stage, was a two-stage, solid-fueled space launch system developed by Boeing for the United States Air Force beginning in 1976[4] for raising payloads from low Earth orbit to higher orbits or interplanetary trajectories following launch aboard a Titan 34D or Titan IV rocket as its upper stage, or from the payload bay of the Space Shuttle as a space tug.

During the development of the Space Shuttle, NASA, with support from the Air Force, wanted an upper stage that could be used on the Shuttle to deliver payloads from low earth orbit to higher energy orbits such as GTO or GEO or to escape velocity for planetary probes.

[3] The first launch of the IUS was in 1982 on a Titan 34D rocket from the Cape Canaveral Air Force Station shortly before the STS-6 Space Shuttle mission.

[9] After the Shuttle separated from the payload to a safe distance, the IUS first stage ignited and, as on a Titan booster mission, entered a "transfer orbit".

In addition to the communication and reconnaissance missions described above, which placed the payload into stationary (24-hour) orbit, the IUS was also used to boost spacecraft towards planetary trajectories.