The Atlas II family, which includes the Atlas II, IIA and IIAS, was developed in the mid-1980's to address the growing demand for large commercial geosynchronous satellites.  Over the past decade the Atlas II vehicle has been continually stretched and upgraded to improve payload performance.  In the early 1990's General Dynamics (now a part of Lockheed Martin) made the decision to upgrade the Atlas first stage propulsion system.  In the end, Martin Marietta (which had acquired General Dynamics before merging with Lockheed) chose a single RD-180 engine over two NK-33's.  The re-engined vehice, known as the Atlas IIAR, has since been renamed the Atlas III.

Currently, Atlas II vehicles are being flown from both US ranges and are marketed world wide by International Launch Services (ILS).  However, the days of the Atlas II are numbered.  The maiden flight of the Atlas III launch vehicle, the replacement for the Atlas II, occurred on May 24, 2000. 

The Atlas II family of vehicles use a uniqe two and a half stage to orbit concept.  The core booster uses three MA-5 engines, a booster and two sustainers.  In the case of the Atlas IIAS, four solids are used to augment payload performance.  All three MA-5 engines are ignited prior to liftoff.  Approximately 180 seconds into first stage ascent, the two larger MA-5 booster engines are shut down and jettisoned, reducing weight and improving payload performance.  The center MA-5 sustainer engine burns for an additional 100 seconds up to MECO and staging.  The Centaur second stage uses two RL-10A-4 engines to place up to 6,700 lb (Atlas IIA)  and 7,950 lb (Atlas IIAS) into a GTO orbit.