The Lunar Excursion Module (LEM), better known as the Lunar Module (LM), was the transport vehicle used in the Apollo program to ferry two astronauts between the Command Module in lunar orbit and the surface of the Moon. The lunar-orbit rendezvous eliminated the need to land the entire Apollo spacecraft on the Moon on a direct ascent flight path from Earth orbit, and thus made possible a lighter load of fuel. Nine LMs were flown during the Apollo program, and six landed on the Moon.


The two-stage LM was designed and built by the Grumman Aircraft Engineering Corporation under a $1.6-billion contract with the National Aeronautics and Space Administration. Because little was known about the lunar surface when construction began in 1962, engineers designed the cantilever landing gear — consisting of four sets of legs, each ending in a dish-shaped pod — so that the vehicle could land safely and remain upright on a variety of surfaces. With its legs extended, the LM was 7 m (23 ft) high and 4.3 m (14 ft) across. The descent stage, which stood slightly more than 3 m (10 ft) high, had an engine whose thrust could be controlled within a range of 44,500 to 4,600 newtons (4,530 to 475 kg, or 10,000 to 1,050 lb). To ensure a soft landing, the engine was fired continuously during descent from lunar orbit. The crew rode in the ascent stage, which was equipped with an engine of 15,550 newtons (1,600 kg, or 3,500 lb) thrust. Both descent and ascent were made with the aid of a sophisticated guidance and navigation system that included a radar altimeter.

At launch, the LM was carried atop the third stage of a Saturn 5 rocket. As the vehicle system left Earth orbit on a translunar trajectory, the pilot of the Apollo Command and Service Module turned the spacecraft around and docked it nose to nose with the LM. In lunar orbit, two crew members transferred from the Apollo Command Module to the LM ascent stage through a docking tunnel, undocked the LM, and landed on the Moon. To leave the Moon, the crew fired the ascent-stage engine (using the descent stage as a launch platform), ascended to lunar orbit, and docked with the Apollo spacecraft. The descent stage was left on the Moon and the ascent stage was jettisoned in lunar orbit.

Richard Lewis

Bibliography: Baker, David, The History of Manned Space Flight (1981); Hallion, R.P., and Crouch, T.D., eds., Apollo (1979); Lewis, Richard, Appointment on the Moon (1969).