Saturday, August 18, 2007

Viking 1

Credit: NASA

Viking 1 was launched on August 20, 1975 using a Titan/Centaur rocket. Ten months later, the orbiter began taking photos of Mars five days prior to Mars orbit insertion, which occurred on June 19, 1976. The landing of the Viking 1 Lander was originally planned for July 4, 1976, the United States Bicentennial, but photos of the primary landing site showed that it was too rough for a safe landing. The landing was delayed until July 20th, when a safer site had been found.

The Lander and its aeroshell separated from the orbiter at 8:51 (UT) on July 20th. After a few hours, at about 300 km altitude, the Lander was reoriented for atmospheric entry. The aeroshell with its ablative heat shield slowed the craft as it plunged through the atmosphere. During this time, entry science experiments were performed. At 6 km altitude, traveling at about 250 meters per second, the Lander's parachutes were deployed. Seven seconds later the aeroshell was jettisoned, and eight seconds after that the three lander legs were extended. At 1.5 km altitude, retrorockets on the lander itself were ignited and, 40 seconds later, at about 2.4 meters per second, the Lander arrived with a relatively light jolt. The Viking 1 Lander is located in the western Chryse Planitia at 22.697° North, 48.222° West.

The landing rockets used an 18 nozzle design to spread the hydrogen and nitrogen exhaust over a wide area. It was determined that this would limit surface heating to no more than 1 degree Celsius and that no more than 1 mm of the surface material would be stripped away. Since most of Viking's experiments centered on this surface material a more straightforward design would not have served. This system was never used again on a Martian landing. It was relatively heavy and cumbersome, and the system used now - surrounding the probe with airbags for a bouncing landing - has been in use ever since.

Transmission of the first surface image began 25 seconds after landing. The seismometer failed to uncage, and a sampler arm locking pin was stuck and took 5 days to shake out. All the other experiments functioned nominally. In January 1982, the Viking 1 Lander was named the Thomas Mutch Memorial Station in honor of the leader of the Viking imaging team.

The Orbiter's primary mission ended at the beginning of the solar conjunction on November 5, 1976. The extended mission commenced on 14 December 1976 after the solar conjunction. Operations included close approaches to Phobos in February 1977. The periapsis (lowest point of the orbit) was reduced to 300 km on March 11, 1977. Minor orbit adjustments were done occasionally over the course of the mission, primarily to change the walk rate — the rate at which the planetocentric longitude changed with each orbit, and the periapsis was raised to 357 km on 20 July 1979. On August 7, 1980, the Viking 1 Orbiter was running low on altitude control gas and its orbit was raised to prevent the Orbiter impacting with Mars (and possible contamination of the planet) until the year 2019. Operations were terminated on August 17, 1980 after 1,485 orbits.

The Lander operated for 2,245 sols, until November 13, 1982, when a faulty command sent by ground control resulted in loss of contact. The command was intended to uplink new battery charging software to improve the Lander's deteriorating battery capacity, but the software inadvertently overwrote data used by the antenna pointing software. Attempts to contact the lander during the next four months, based on the presumed antenna position, were unsuccessful. In 2006 the Viking 1 lander was imaged on the Martian surface by the Mars Reconnaissance Orbiter.

The above picture is the first panoramic view by Viking 1 from the surface of Mars. The out of focus spacecraft component toward left center is the housing for the Viking sample arm, which was not yet deployed. Parallel lines in the sky are an artifact and are not real features. However, the change of brightness from horizon towards zenith and towards the right (west) is accurately reflected in this picture, taken in the late Martian afternoon. At the horizon to the left is a plateau-like prominence much brighter than the foreground material between the rocks. The horizon features are approximately three km (1.8 miles) away. At left is a collection of fine-grained material reminiscent of sand dunes. The dark sinuous markings in the left foreground are of unknown origin. Some unidentified shapes can be perceived on the hilly eminence at the horizon towards the right. A horizontal cloud stratum can be made out halfway from the horizon to the top of the picture. At left is seen the low gain antenna for the receipt of commands from the Earth. The projections on or near the horizon may represent the rims of distant impact craters. In the right foreground are color charts for the calibration of the Lander camera, a mirror for the Viking magnetic properties experiment, and part of a grid on the top of the Lander body. At upper right is the high gain dish antenna for direct communication between landed spacecraft and Earth. Toward the right edge is an array of smooth, fine-grained material which shows some hint of ripple structure and may be the beginning of a large dune field off to the right of the picture, which joins with dunes seen at the top left in this 300° panoramic view. Some of the rocks appear to be undercut on one side and partially buried by drifting sand on the other.

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