Ranger was a series of U.S. spacecraft that were designed to provide, in the 20 minutes before impact on the Moon, about 300 close-up photos per minute of the lunar surface that could be used for increased accuracy in mapping, for studies of small-scale structure, and for the initial stage of selecting landing sites for the manned missions to follow. The best Ranger photos of the Moon, taken by Ranger 9 just before impact, had a resolution of approximately 0.25 m (10 in).
The prime contractor for the Ranger program was the Jet Propulsion Laboratory of the California Institute of Technology. The spacecraft, which weighed about 365 kg (810 lb), had a hexagonal base about 1.5 m (5 ft) in diameter. A 2.5-m-high (8.25-ft) telescopelike cone structure, which contained 6 television cameras and an omnidirectional antenna, extended above this base. Two panels, containing 9,800 solar cells that provided 200 W of electricity, unfolded in flight and gave the spacecraft a wingspan of 4.6 m (15 ft).
All of the Ranger spacecraft were launched by an Atlas-Agena rocket from Kennedy Space Center. After Earth orbit was attained, the Agena second stage was briefly restarted (prior to separation from the spacecraft) in order to inject Ranger into a lunar trajectory. After midcourse corrections the spacecraft were to crash into the Moon at about 9,700 km/h (6,027 mph), some 68 hours after launch.
Only the last three Ranger missions met their objectives, but they did so with great success. Rangers 1 and 2 (launched Aug. 23 and Nov. 18, 1961) were test models in which the Agena engine failed to restart. Ranger 3 (launched Jan. 26, 1962) missed the Moon and entered solar orbit, as did Ranger 5 (launched Oct. 18, 1962). Ranger 4 (launched Apr. 23, 1962) curved around the Moon and crashed on its far side, while the television system aboard Ranger 6 (launched Jan. 30, 1964) failed before it impacted on the Moon.
Ranger 7 (launched July 28, 1964) impacted successfully at 10.7° S, 20.7° W in the Sea of Clouds, sending back 4,316 photos. Ranger 8 (launched Feb. 17, 1965) struck the Moon at 2.7° N, 24.8° E in the Sea of Tranquillity, taking 7,137 photos at an approach angle that swept a large portion of the lunar surface and provided the first close-up views of its highlands. Ranger 9 (launched Mar. 21, 1965) impacted at 12.9° S, 2.4° W inside the crater Alphonsus, yielding 5,814 photos that were the first close-up views of a lunar crater's interior.
The improved resolution of the Ranger photos over their Earth-based counterparts revealed several previously unknown aspects of the Moon, such as the small-scale topography. The photos showed striking similarities between mare and crater floors, a smoothed appearance in numerous shallow depressions, and a relative absence of rubble over substantial portions of the surface. Ranger 9's target, the crater Alphonsus, gave signs of a complex history influenced by both internal and external forces. The mare in the photos showed an absence of large mountain ranges as well as of impact-saturated areas containing numerous overlapping craters. This discovery influenced the selection of mare regions as preferred sites for the Surveyor and Apollo landings.
Bibliography: Breuer, W. B., Race to the Moon (1993); Jet Propulsion Laboratory, The View from Ranger (NASA EP-38; 1966); Ley, Willy, Ranger to the Moon (1965); Wilhelms, D. E., To a Rocky Moon (1994).