Launched 1975
Decommissioned 1983
The success of the Mariner 9 mission in orbiting Mars in 1972 led American scientists to conceive of the Viking program with the aim of landing a spacecraft on the Red Planet. The mission was named for Scandinavian explorers of the ninth and tenth centuries who discovered North America.
The fourth planet from the Sun with an orbit next in line after Earth, Mars has a period of revolution at 687 days, almost double that of Earth. When the Sun, Earth, and Mars are aligned, the distance of Mars from Earth is 35 million miles; this alignment takes place every 15 to 17 years. When Mars is at its greatest distance from the Sun, it is about 63 million miles from Earth. Scientists believed the dark-colored patches on the planet might be simple forms of life. This could be determined by collecting samples and conducting experiments.
The Viking probes were conducted to discover whether life did in fact exist on Mars. The spacecraft consisted of an orbiter—similar to those used in the Mariner program—and a lander. The orbiter carried two television cameras and infrared instruments for mapping the planet’s thermal characteristics and for detecting atmospheric pressure. The lander, which contained most of the scientific instruments, was a squat, three-legged spacecraft that fit inside a shell for protection while descending through the Martian atmosphere.
The spacecraft would make several orbits of the planet as scientists searched for the least dangerous landing site. Because Mars is strewn with rocks it presented obstacles; for instance, if the lander tipped over it would be useless. Once the lander separated from the orbiter, it would be slowed by both a parachute and a braking rocket. The landing speed was six and one-half miles per hour.
The landers received their power from two radioactive heat sources. Each lander had a robot arm with a scoop at the end that could reach down and pick up soil samples and then drop them into a mini-chemistry lab for analysis. The lander also had a small weather station at the end of a boom and an antenna for communication with Earth. The lander had to be sterilized in hot nitrogen gas to protect Mars from possible contamination by Earth life forms.
Vikings 1 and 2 were launched on August 20 and September 9, 1975, respectively. Viking 1 arrived at Mars on July 19, 1976; Viking 2 followed on August 7, 1976. As part of the celebrations for the bicentennial of the United States, Viking 1 broadcast a signal to a replica of its arm used to scoop soil samples that then cut the ribbon opening the new National Air and Space Museum in Washington, D.C., on July 1, 1976.
The original landing site was found to be too rough when surveyed by the Viking 1 orbiter, and another site was selected to the north. On July 20 the lander was set free from the orbiter and descended onto the surface of the Chryse Planitia (Golden Plains). Within minutes it began sending black-and-white photos back to Earth; the transmissions took 18 minutes and 18 seconds to reach Earth. Viking 2 landed about 3,100 miles farther north on the Utopia Planitia (Plains of Utopia). Scientists were amazed that they were able to land both craft successfully, especially when the first photos showed that the landers had both touched down in the middle of large rocks that could have toppled them or broken their legs. Viking 1 landed only 26 feet away from a large boulder.
The immediate aim of both landers was to search for signs of life. Their scoopers picked up soil samples to be analyzed for signs of carbon-14 and gases given off by live organisms. Two experiments turned out to be especially interesting. In one experiment nutrients that would have supported bacteria were mixed with Martian soil, which decomposed in the way bacteria would have done. In another test air from Earth was released and reacted with the soil the way it would have done in the presence of photosynthesis. However, these experiments were not conclusive and were not matched by others. The scientific consensus was that some kind of inorganic chemistry was imitating organic chemistry. The results were later duplicated by science labs on Earth. The two Viking spacecraft found no evidence of life but left open the question if life had once existed on Mars.
The weather stations of the Viking landers found results much like those that were expected. Winds were relatively light, reaching about 14 miles per hour. Because Mars is farther from the Sun than the Earth and has a much thinner atmosphere, the planet is much colder. The maximum temperature at the Viking 1 landing site was −24°F. Temperatures can get as high as 70°F at the equator; since the atmosphere does not retain heat, temperatures descend to −166°F at night. The atmosphere is made up largely of carbon dioxide with traces of nitrogen and argon.
Although the two Viking spacecraft were designed to operate for six months, they continued for a longer period of time. The two orbiters eventually ran out of thruster gas: Orbiter 2 after two years and Orbiter 1 after four years. Lander 2 suffered a power failure in March 1980, but Lander 1 continued sending back weather reports and photos until March 8, 1983. Plans for a Viking 3 mission were never carried out.
Viking was one of the most successful United States space programs. A major achievement of the Viking mission was an atlas of the planet, Viking Lander Atlas of Mars, which was compiled by the United States Office of Space Science and Applications.
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