Surveyor program

• 1. Introduction

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  The Surveyor program launched its first spacecraft toward the Moon on the 30th of May 1966. The most pressing question it carried was one nobody could answer from Earth: how deep was the lunar dust? If the dust was too deep, no astronaut could safely land. Seven robotic probes were sent to find out, and to prove that a safe landing was even possible. The program ran from June 1966 through January 1968, and its total cost was officially recorded at $469 million. When it was over, five of those spacecraft had landed successfully, and all seven were still sitting on the Moon.

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• 2. The Descent Architecture

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  Each Surveyor launched on an Atlas-Centaur rocket, which placed it directly on a trans-lunar path with no parking orbit. The journey from Earth to the Moon took between 63 and 65 hours. On arrival, the spacecraft had just over three minutes to slow from 2.6 kilometers per second to a standstill.

  At 75.3 kilometers above the lunar surface, the main solid-fuel retrorocket fired and burned for 40 seconds. It reduced the spacecraft's speed to about 110 meters per second. At 11 kilometers from the surface, the retrorocket and its radar unit were jettisoned.

  Three vernier engines running on liquid fuel then took over for the remaining two and a half minutes of descent. At 3.4 meters above the ground, those engines cut off, and the craft dropped in free fall to the surface. The landing speed was approximately 3 meters per second. The free fall was deliberate, designed to prevent rocket exhaust from contaminating the landing site before the onboard instruments made contact.

  Each craft weighed between 995.2 and 1040 kilograms at the moment of trans-lunar injection. By the time of landing, after fuel was burned and hardware jettisoned, the weight had dropped to between 294.3 and 306 kilograms.

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• 3. Beyond the Landing

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  Luna 9, the Soviet probe that touched down in February 1966, landed four months before Surveyor 1. The Surveyor 1 landing confirmed that the two national programs had reached this milestone at nearly the same time.

  The robotic shovels on several Surveyor spacecraft measured how the lunar soil responded to mechanical force. Others carried alpha scattering instruments and magnets to analyze the surface's chemical composition. The landers also exercised mid-course correction maneuvers during the trans-lunar flight, a technique the Apollo missions would require.

  Three engineering systems were tested for the first time in the environment near the Moon. These were closed-loop terminal descent guidance, throttleable engines, and the radar units that measured altitude and velocity during the final approach. None had previously operated in the Moon's thermal and radiation environment.

  A helium leak shortly after Surveyor 5's launch forced engineers to shorten its final descent sequence, but the spacecraft landed safely on the 11th of September 1967.

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• 4. Two That Failed

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  Surveyor 2 lifted off on the 20th of September 1966. A corrective burn during the trans-lunar flight failed, leaving the spacecraft without attitude control. Contact was lost at 9:35 UTC on the 22nd of September. The craft crashed into the Moon at high velocity.

  Surveyor 4 launched on the 14th of July 1967, on a trajectory that was otherwise without fault. Telemetry went silent 2.5 minutes before the scheduled touchdown over Sinus Medii. Engineers suspected that the solid-fuel retrorocket had exploded during its final burn. No signal was ever recovered.

  Sinus Medii was the target Surveyor 4 never reached. Surveyor 6 would land there successfully four months later, on the 10th of November 1967.

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• 5. A Human Visit

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  Surveyor 3 launched on the 17th of April 1967. Three days later it touched down in a small crater within the Mare Cognitum section of Oceanus Procellarum. That crater was later named Surveyor.

  The landing radar failed to shut the vernier engines down at touchdown, keeping them firing through the impact. The spacecraft lifted off the surface twice before finally coming to rest. Both liftoffs were unintentional. The TV and telemetry systems sustained damage from the repeated impacts. Despite that, Surveyor 3 transmitted 6,315 television images, including the first views of Earth as seen from the surface of another world.

  In November 1969, Apollo 12 astronauts Pete Conrad and Alan Bean set down nearby. They walked to the spacecraft, removed several components including the television camera, and brought them back to Earth for study. Surveyor 3 remains the only probe on another world ever visited by humans.

  Its camera is now on display at the National Air and Space Museum in Washington, DC.

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• 6. First Lunar Liftoff

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  Surveyor 6 launched on the 7th of November 1967, and landed three days later in Sinus Medii, near the site where Surveyor 4 had gone silent. Its successful landing formally satisfied the Surveyor program's obligation to the Apollo project.

  On the 17th of November 1967, at 10:32 UTC, Surveyor 6's engines were restarted. They burned for 2.5 seconds and generated 150 pounds of thrust. The spacecraft rose 12 feet off the lunar surface and moved 8 feet to the west. It then settled back down in a second clean soft landing. This was the first controlled liftoff and relanding performed on another world.

  The craft was shut down on the 24th of November for the two-week lunar night. Contact was made again on the 14th of December, though no useful data was obtained.

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• 7. Lasers From Earth

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  Surveyor 7 launched on the 7th of January 1968, and landed three days later on the outer rim of the crater Tycho. Surface operations began shortly after touchdown.

  On the 20th of January, the spacecraft's television camera detected two laser beams aimed at it from Earth. One originated at Kitt Peak National Observatory in Tucson, Arizona. The other came from Table Mountain at Wrightwood, California. The beams were visible against the night side of a crescent Earth.

  Operations on the second lunar day ran from the 12th to the 21st of February 1968. Battery damage from the first lunar night had made transmissions sporadic, and contact with Surveyor 7 was lost on the 21st of February. The mission objectives were fully satisfied.

  Among the data Surveyor 1 had returned was information on the Moon's orbital motion, used to refine charts of its path around Earth and to sharpen estimates of the distance between the two worlds.

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