The Surveyor spacecraft were a series of seven lunar soft-landing vehicles launched by the USA in the period 1966-1968. They were a second generation of lunar spacecraft, following the Ranger series that ran from 1961 to 1965, and paved the way for the later soft landings required for Apollo. The main aims of the Surveyor project were to accomplish a soft landing on the Moon, provide basic data in support of Apollo, and perform scientific operations on the lunar surface for an extended period. The Ranger 3, 4, 5 soft landing attempts having failed, Surveyor was to achieve the USA's first soft landings on another world. Orbital surveys by the Lunar Orbiter spacecraft complemented the in situ investigations by Surveyor.
Industrial studies for the project that became Surveyor began in mid 1960, with the Hughes Aircraft Company being chosen as prime contractor, under NASA JPL. The first launch was initially planned for late 1963 but a series of technical and programmatic issues forced an accumulated delay of nearly three years, by which time development of the Apollo landers was already well under way, and the Soviet Union had already made the first successful soft landing with Luna 9.
The main challenge for Surveyor was designing one of the first systems for performing a soft landing on another planetary body, with the associated terminal guidance and control problems of braking the spacecraft to land intact, and the then great uncertainty regarding the lunar surface's physical properties. The spacecraft was also required to survive the cold of the fortnight-long lunar night.
The launch vehicle chosen was the Atlas Centaur, which itself was still under development. This resulted in changes to the mass available for Surveyor and its payload. In the event, the first four Surveyors were classed as engineering test models. The final three carried more payload, though still somewhat less than had originally been envisaged (see Corliss, 1965 for information on other instruments developed). The mass on separation from the Centaur upper stage ranged from 995 to 1040 kg, and that at touchdown from 294 to 306 kg, of which up to 32.2 kg was scientific payload. After separation, the spacecraft locked onto the Sun and Canopus and performed a mid-course correction (using its vernier engines) before descending to the lunar surface directly (i.e. without first entering lunar orbit). The total flight time from launch to landing ranged from 63.5 to 66.5 h.
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