Surveyor (spacecraft) - astronomy.

Surveyor (spacecraft) - astronomy. I INTRODUCTION Surveyor (spacecraft), series of seven unpiloted probes that were the first American spacecraft to soft-land, or land without crashing, on the moon. Surveyor probes gathered data about the moon's surface composition to help scientists plan the Apollo piloted lunar landing missions. The Surveyor program was the American response to the Soviet Luna 9 spacecraft, which became the first craft to soft-land on the moon on February 3, 1966. II SPACECRAFT The Surveyor probes had a strut framework with three legs measuring 3.05 m (10 ft) tall and 4.27 m (14 ft) across. They weighed about 270 kg (about 600 lb) once they reached the lunar surface. Each had two conical omnidirectional antennas (antennas that could receive signals from all directions) on booms and a flat, square, high-gain antenna on a mast. The mast also supported a solar panel measuring 3 sq m (9 sq ft) that generated electricity to power the spacecraft (see Solar Energy). At the base of the mast was a television camera for imaging the moon's surface. Controllers on the earth sent commands through the omnidirectional and high-gain antennas. The high-gain antenna was also used for television transmission of lunar images back to the earth. The probe's three feet were equipped with hydraulic shock absorbers and with aluminum pads that crumpled to absorb shock and cushion Surveyor's landing. Electronic systems were covered with reflective foil and placed near heaters to protect them from temperature extremes. A star sensor enabled Surveyor to orient itself in space by sighting on the star Canopus. Propellant tanks mounted to Surveyor's frame supplied the nitrogen-powered rockets. Nitrogen-powered thruster rockets and three supplementary vernier rockets were used for steering the craft. Surveyor used a radar system that measured altitude to control its descent (see Altimeter). A 655-kg (1444-lb) solid rocket motor, called a retrorocket, slowed Surveyor as it approached the moon. The Surveyors also carried science equipment: Surveyor 3 and 7 carried a scoop on an arm to collect soil samples and Surveyor 5, 6, and 7 carried an alpha-particle back-scatter device, which shot alpha particles at the moon's surface and measured the way they reflected to determine the soil's composition. III MISSIONS The Surveyors were launched on a two-stage Atlas-Centaur rocket. Thirty minutes before landing, controllers on the earth commanded Surveyor to point its retrorocket at the moon. About 100 km (about 60 mi) above the moon the retrorocket and vernier rockets fired, slowing Surveyor. When Surveyor landed it was moving at 13 km/h (8 mph). After landing, controllers turned the high-gain antenna toward the earth and the solar array toward the sun and sent a command to the spacecraft to activate the science equipment. The Surveyors were shut down for hibernation during lunar nights, when their solar panels did not receive sunlight and temperatures sank to levels that would have been hard on the equipment if it had been operating. Surveyor 1 was launched on May 31, 1966, and landed near the crater Flamsteed on June 2. The probe's camera returned 11,150 images before the spacecraft went into its second hibernation, from which it failed to revive. Surveyor 2 was launched on September 20, 1966, and tumbled out of control on the way to the moon. The spacecraft transmitted data until it crashed on the moon near the Copernicus crater on September 22. Surveyor 3 was launched on April 17, 1967, and landed in eastern Oceanus Procellarum. Surveyor 3's altitude control system failed to shut off the thrusters when the craft landed, causing it to hop around the landing site for about 30 seconds before controllers manually stopped the thrusters. The footprints that Surveyor 3's hops left helped engineers learn more about the composition and texture of the moon's surface. Surveyor 3 carried a digger that tested the surface by digging trenches and collected soil for analysis by onboard instruments. Surveyor 3's camera returned 6315 photos before the craft failed to revive after its first hibernation. The second piloted moon mission, Apollo 12, landed near Surveyor 3 in November 1969. Astronauts retrieved the digger and other parts and returned them to the earth. Surveyor 4 was launched on July 14, 1967. When the craft was 11 km (7 mi) above the moon controllers lost contact with it. Surveyor 5 was launched on September 8, 1967, and landed in Mare Tranquilitatis, the site of the first Apollo moon landing. Scientists analyzed the surface composition using an alpha-particle backscatter device and transmitted 19,049 photos. Surveyor 6 was launched on November 7, 1967, and landed in Sinus Medii near the moon's equator on November 9. The lander returned 30,027 television pictures to the earth and employed the alpha-particle backscatter device to analyze surface composition. Surveyor 7 was launched on January 7, 1968, and landed near Tycho crater on January 9 carrying both the alpha-particle backscatter device and digger. This last Surveyor lander returned 22,001 images. Surveyor cameras returned more than 65,000 photos in all. Using the alpha-particle backscatter device and the digger, scientists determined that the moon's surface consists of aluminum, oxygen, and silicon and is strong enough to support exploring astronauts, setting the stage for the Apollo lunar missions. Contributed By: David S. F. Portree Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.