The first U.S. landings on the Moon were achieved by the automatic Surveyor spacecraft in a test of soft-landing technology for the coming Apollo missions. The Surveyors studied aspects of the lunar environment that might affect astronauts and carried out analyses of lunar surface material.

Surveyor 1, launched May 30, 1966, landed in the southwest portion of the Moon's Oceanus Procellarum on June 2. In seven months of operation it sent back 11,240 television pictures and transmitted data on the surface's radar reflectivity, temperatures, and bearing strength. The second Surveyor, launched Sept. 20, 1966, was lost when it crashed three days later because of a malfunctioning rocket.

Surveyor 3, launched Apr. 17, 1967, landed on April 20 in eastern Oceanus Procellarum. It was also equipped with a television camera and with a significant addition: a telescoping arm capable of digging trenches, moving rocks, and conducting other tests with surface material. The craft provided 6,326 pictures, and its arm excavated to a depth of 17.5 cm (7 in). Surveyor 4, launched July 14, 1967, was the only other casualty in the series. It presumably crashed on July 16, although its transmissions abruptly ceased a little more than two minutes before it was to land.

The first direct chemical measurements on the Moon were made by Surveyor 5. Launched Sept. 8, 1967, it landed September 11. Instead of a sampling arm it carried an alpha-backscatter experiment that irradiated the surface beneath it with alpha particles and monitored the return from the different elements in the soil. The experiment showed that lunar material is similar to terrestrial basalt and does not exhibit elements alien to Earth. In another test, the vehicle's engines were fired for 0.55 seconds to study the effects of high-velocity exhaust gases on the soil. The craft sent back 18,006 pictures during the five weeks of its primary mission and survived two weeks of inactivity during the cold lunar night, being reactivated to send back more than 1,000 additional pictures.

Surveyor 6, launched Nov. 7, 1967, landed on November 10 in Sinus Medii, essentially in the center of the Moon's visible hemisphere. With its mission nearly accomplished, including most of its total of 29,952 pictures, the craft then scored a first by firing its engines and taking off again for a 2-m-long (6.6-ft) hop. This displacement provided a baseline for stereoscopic viewing and photogrammetric mapping of the surrounding terrain. The craft obtained further alpha-backscatter data and served as a reference point for Doppler-tracking studies of the Moon's motions. Surveyor 6 also sent back views of the Earth, stars, and solar corona.

The four previous Surveyor successes freed the remaining probe, Surveyor 7, to be sent to a site of primarily scientific, rather than safe-landing, interest. The site, north of crater Tycho, was selected because it is in the lunar highlands. The craft, launched Jan. 7, 1968, and landing on January 10, used its sampling arm to move its alpha-backscatter experiment to different locations.

Jonathan Eberhart

Bibliography: Ryan, P., Invasion of the Moon, 1957-1970, rev. ed. (1971); U.S. Surveyor Program Office, Surveyor Program Results (1969).