When President John Kennedy set the goal of landing a human safely on the Moon, the US had a grand total of fifteen minutes' worth of manned spaceflight, and astronomers knew next to nothing about what the conditions on the Moon's surface were like. Would a landing craft be swallowed up in a deep layer of lunar dust? No one knew.
A Surveyor moon lander, on display at the Smithsonian Air and Space Museum.
To answer these questions (and also to help work out all the engineering problems involved in navigating a spaceship to the moon and landing there), NASA began two unmanned programs to explore conditions at the Moon. The first of these was already in the works--the Ranger program had been originally designed in 1959 as a scientific expedition to the Moon, but when the manned-landing program was begun, Ranger was altered to serve as a data-gathering reconnaissance for the planned Apollo missions. Ranger was not capable of landing on the Moon, but was intended to fly directly into it, transmitting data and photographs right up to the moment of impact. The first six Ranger missions, from 1961 to 1963, were all failures: they exploded on launch, missed the Moon completely, or had technical failures and returned no data. The string of failures prompted a Congressional investigation into NASA's management system and, for a time, placed the whole moon landing program in jeopardy. It wasn't until 1964 that the first Ranger successfully sent photos of the Moon's surface. Two more Rangers followed in 1965.
Once the kinks had been worked out in Ranger and reliable methods were developed for lunar navigation, it was decided to go ahead with the Surveyor program, which was intended to soft-land on the Moon's surface and send back detailed information about the conditions there. And since the Apollo missions would require the development of new technology for a landing, including throttle-able engines to control the rate of descent, Surveyor was designed to develop and test this necessary equipment.
The Surveyor landers carried TV and still cameras, as well as pressure gauges in the landing pads and instruments to measure the hardness, density and depth of the lunar soil and analyze its chemical composition. It also carried a radar system to help with landing. Launched atop an Atlas-Centaur booster rocket, the Surveyors were sent on a direct flight to the Moon, taking about 2.5 days to get there. Once it reached the Moon, Surveyor's descent engine would fire for about three minutes to bring it to a soft landing on the surface. A total of seven missions were planned, each one targeted at a different potential Apollo landing sight.
The Surveyor program received a sudden new urgency when the Soviet Union landed its own unmanned probe, Luna-9, on the Moon on February 3, 1966, just four months before Surveyor 1's planned mission. Not only was this the first successful soft-landing on the Moon, but it demonstrated to NASA that the Russians were at about the same technological level as the US, and that the Space Race to the Moon was a lot closer than previously thought.
Surveyor 1 was launched on May 30, 1966 and successfully landed on the Moon, in the Ocean of Storms, on June 2. It touched down about six miles away from its intended coordinates, and transmitted data for about a year. Over the next two years, six more Surveyors were launched. Two of them malfunctioned and crashed on the surface, but the remaining four sent back data and photos. Onboard instruments showed that the Moon's surface was hard rock covered with a very thin layer of fine dust, called "regolith", and that surface conditions were indeed suitable for a safe landing. Information from Surveyor 5 led to the selection of the Sea of Tranquility as the site for the first planned Apollo landing.
After their batteries ran out, all of the Surveyors remained on the Moon's surface, inert. When a landing site for the Apollo 12 mission in 1969 was being selected, it was decided to land as close as possible to the site of the Surveyor 3 probe and, if possible, obtain some pieces to bring back so the effects on the materials of long-term exposures to space could be studied. The Apollo 12 Lunar Module landed just a few hundred yards from the Surveyor, and pieces of the probe, including the camera housing, were returned to Earth.
When examined, the Surveyor camera housing revealed a surprise: it contained still-living Earth bacteria. Apparently when it was being assembled in 1966, one of the NASA workers had sneezed inside it, and the bacteria hitchhiked all the way to the Moon, where they survived in the radiation-bathed vacuum for almost three years.
The Surveyor program was a success. The controllable-thrust engines used in the Surveyor lander were the basis for the descent engine in the Lunar Module, which allowed astronauts to control their speed as they landed on the surface of the Moon. The methods used for navigation and course correction in the Surveyor program were adapted for Apollo. And after the Apollo missions, the Surveyor design was modified and used as the basis for the Mars Viking landers.
One of the engineering models of the Surveyor lander, used to plan missions, is on display at the Smithsonian Air and Space Museum.