Lunar Orbiter - astronomy.

Lunar Orbiter - astronomy. I INTRODUCTION Lunar Orbiter, series of United States spacecraft that orbited the moon between August 1966 and January 1968. The primary mission of the Lunar Orbiters was to provide detailed photographs of the Apollo Zone, the region of the moon where the Apollo astronauts planned to land (see Apollo program). The first three Lunar Orbiters completely mapped the Apollo Zone, so the remaining two spacecraft went on to photograph the rest of the moon. The Lunar Orbiters and the concurrent Surveyor program were the final U.S. spacecraft to prepare the way for the Apollo program. The United States had sent spacecraft to the moon during the Pioneer, Ranger, and Surveyor programs, and the Union of Soviet Socialist Republics (USSR) sent Luna and Zond probes to the moon before, during, and after the Lunar Orbiter program. The USSR launched Luna 10, the first spacecraft in lunar orbit, a few months before the U.S. launched Lunar Orbiter 1; the U.S. Lunar Orbiters rapidly surpassed the Soviet orbiters and prepared the United States for the first piloted moon landing. II SPACECRAFT The Lunar Orbiter spacecraft were all identical. Each had a pressurized camera housing suspended in a frame that also supported solar panels and a dish-shaped antenna. Lunar Orbiter's solar panels turned sunlight into electricity that powered the camera and scientific instruments (see Solar Energy), and the antenna allowed the spacecraft to communicate with the earth. On the other side of the camera, fuel tanks and a rocket motor allowed the spacecraft to change its orbit. The solar panels and the antenna remained folded next to the spacecraft body while the orbiter was en route to the moon. With the antenna and solar panels deployed, the spacecraft measured 5.6 m (18 ft) wide and 3.7 m (12 ft) high. In orbit around the moon, the Lunar Orbiter spacecraft had a mass of about 385 kg (about 849 lb). The container that housed Lunar Orbiter's camera was sealed and pressurized to protect the camera and allow the film to be developed onboard the spacecraft. A clear window in the camera housing allowed the camera's two lenses--an 80-mm medium-resolution lens and a 610-mm high-resolution lens--to gather light from the moon. The 80 mm lens exposed an area of film adjacent to an area simultaneously exposed by the 610 mm lens to produce two images of the same area at different resolutions. Lunar Orbiter could make more than 210 of these combination medium- and high-resolution photographs. After the film was exposed, it ran through an automatic developer that produced a negative. A device similar to a modern optical scanner passed a focused beam of light through the negative to a sensor that read changing light levels. The sensor converted the information into an electronic signal and sent the signal back to the earth, where it was recorded on magnetic tape and later converted back into images. Making and transmitting each picture required 45 minutes. All of the Lunar Orbiters carried instruments to detect micrometeoroids (particles of dust in space), and a radiation detector. The micrometeoroid detectors were small containers of helium attached to the outer edge of the spacecraft. If a micrometeoroid broke the skin of the container, the helium escaped and a pressure-sensitive switch in the detector counted the event. Scientists were interested in finding out how many particles were in the space around the moon to determine how strong the Apollo capsules would have to be to protect the astronauts inside and to determine whether it would be safe for an astronaut to go on a spacewalk while in orbit around the moon. Tracking stations on the earth sent radio signals to the Lunar Orbiter while the spacecraft was on the near side of the moon to study the moon's gravitational field. The spacecraft received the signal and sent it back to the tracking stations. Computers at the tracking stations compared the two signals to get accurate measurements of the spacecraft's position and speed. By studying these measurements, scientists could determine how the moon's gravity affected the spacecraft's orbit. The radiation detector aboard the Lunar Orbiter spacecraft monitored conditions primarily to protect the sensitive film from bursts of radiation from the sun. If the detector detected excessive amounts of radiation, it could command the flap over the camera window to close and the spacecraft to orient itself to protect the camera. III MISSIONS All of the Lunar Orbiter missions made it into orbit around the moon successfully and they all returned at least some photographs. Lunar Orbiter 1 left the earth on August 10, 1966, and reached the moon four days later to become the first U.S. spacecraft to orbit an object other than the earth. After some engineering tests, the spacecraft's rocket lowered the probe into an orbit suitable for mapping the moon. Lunar Orbiter 1 found mascons, dense masses buried within the moon that exerted increased gravitational pull. This knowledge was important for Apollo mission planning. On August 23 the spacecraft took the first picture of Earth from the moon. A malfunction in the shutter of the high-resolution lens made all but a few of the high-resolution images unusable, but Lunar Orbiter 1 returned more than 200 good medium-resolution images. On October 29 engineers from the National Aeronautics and Space Administration (NASA) intentionally ordered Lunar Orbiter 1 to crash into the moon so that it would not interfere with Lunar Orbiter 2, which launched on November 6, 1966. Lunar Orbiter 2 snapped a close-up picture of the lunar crater Copernicus that newspapers called 'the picture of the century.' Both the high-resolution and mediumresolution lenses of the Lunar Orbiter 2 produced more than 200 images, for a total of more than 400 images. The spacecraft was deliberately made to crash on October 11, 1967. Lunar Orbiter 3 left the earth on February 5, 1967. On February 21 it photographed the Surveyor 1 spacecraft on the lunar surface. Surveyor 1 had ended its mission and shut down the year before. The film motor failed on March 2, but not before Lunar Orbiter 3 photographed the site where the first piloted moon landing would occur in 1969. Lunar Orbiter 3 only returned about 50 combination medium- and high-resolution images, but with Lunar Orbiter 3's images, scientists had images of the entire region of the moon in which Apollo spacecraft could land. Lunar Orbiter 3 was deliberately made to crash on October 9, 1967. Lunar Orbiter 4 launched on May 4, 1967. It entered lunar polar orbit, allowing it to pass over parts of the moon other than the Apollo Zone. Moisture inside the camera housing fogged the camera window for part of the mission, but almost all of Lunar Orbiter 4's images were usable. Controllers lost contact on July 17 after Lunar Orbiter 4 had transmitted all its pictures. Scientists believe that the spacecraft crashed into the moon in October 1967. Lunar Orbiter 5 reached lunar polar orbit after an August 1, 1967, launch. Mission controllers used it to practice Apollo spacecraft tracking. All of Lunar Orbiter 5's 213 combination medium- and high-resolution images were useable. The spacecraft was crashed on January 31, 1968, to clear lunar orbit for the first piloted Apollo spacecraft. In all, the five Lunar Orbiters mapped 99 percent of the moon. Contributed By: David S. F. Portree Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.