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Viking (spacecraft)

I INTRODUCTION

Viking (spacecraft), first space probe to survive landing on the surface of the planet Mars. Launched by the National Aeronautics and Space Administration (NASA), Viking was the most extensive and complex mission ever to explore Mars and included several experiments designed to test for life on Mars.

The Viking mission used two identical spacecraft to orbit and land on Mars. NASA launched Viking 1 on August 20, 1975, and Viking 2 on September 9, 1975. Each Viking spacecraft consisted of an orbiter, carrying a variety of imaging and remote sensing instruments to study Mars from orbit, and a lander, designed to operate on and intensively study the planet's surface. It took each spacecraft almost a year to reach Mars—Viking 1 went into orbit around Mars on June 19, 1976, and Viking 2 on August 7, 1976. Viking 1 landed on Mars on July 20, 1976, on the western slope of Chryse Planitia. Viking 2 set down September 3, 1976, at Utopia Planitia, 6460 km (4014 mi) from Viking 1.

The Viking spacecraft continued to function long past their planned 90-day mission. The Viking 2 orbiter ran out of fuel for its attitude-control system (the system that keeps the craft's solar panels pointed at the sun) and shut down on July 25, 1978; Viking's controllers on the earth were able to keep the Viking 1 orbiter functioning until August 7, 1980. The last data from the Viking 2 lander were received on the earth on April 11, 1980, and the Viking 1 lander made its final transmission on November 11, 1982.

II SPACECRAFT

The Viking orbiters were new versions of the Mariner series of probes that flew by Mars in the late 1960s and early 1970s. At 2320 kg (5115 lb), they were substantially heavier than the Mariner crafts since each had to carry enough fuel to power engines that would decrease both their speed and the speed of their attached landers when the spacecraft entered the planet's orbit.

The Viking landers owed their heritage to the lunar-landing Surveyor series used in the late 1960s, but they had many significant modifications. Their three legs were spring-loaded to assure a soft touchdown—the lander's three descent engines were designed to turn off slightly above the planet's surface so as not to unduly contaminate the landing area with fuel exhaust. The landers were powered by radioisotope thermoelectric generators (RTGs), which use the heat produced by the normal decay of plutonium dioxide to make electrical energy. Dish-shaped high-gain antennas communicated with the earth, while smaller antennas were used to communicate with the Viking orbiters. The landers were equipped with a variety of scientific instruments, including meteorology instruments mounted on extendable booms and two cameras mounted on the body of the landers. Extendable booms and scoops could reach out, dig in the soil, and deliver samples to the miniature analytical laboratories housed on board the lander. The entire ensemble of each lander folded up to fit neatly inside a heat shield for protection during atmospheric entry. In turn, the lander and heat shield fit inside a bioshield, which was designed to prevent Earth's microbes from getting inside and possibly contaminating Mars and the sterilized equipment.

III MISSION HIGHLIGHTS

The Viking orbiter cameras mapped almost the entire Mars surface to a resolution of only a few hundred meters in addition to scrutinizing potential landing sites for the Viking landers. The photographic images contain a wealth of detail, showing volcanoes, lava plains, immense canyons, cratered areas, wind-formed features, and evidence that water once flowed on the surface. These data surpassed the earlier information obtained from Mariner missions and replaced it as the standard.

The Viking orbiters showed Mars to have two main global regions: northern low plains and southern cratered highlands. Rising above these regions are the volcanic Tharsis and Elysium bulges, and carved into the equatorial terrain is Valles Marineris, a system of giant canyons. The infrared thermal mappers (heat detectors) and the atmospheric water detectors on the orbiters showed that the northern and southern hemispheres have very different climates. The scant available water is transported seasonally between the equator and the polar ice caps, and global dust storms originate in the south in summer. Water vapor is relatively abundant only in the far north during the summer, and permafrost covers much of the planet.

As the landers descended toward the surface, onboard instruments measured the composition and physical properties of the Martian upper atmosphere; the meteorology instruments made the same measurements at the surface. The element nitrogen, never before detected on Mars, was found in the extremely thin Martian atmosphere, which is composed mostly of carbon dioxide. The ratios of the isotopes (atoms of the same element but differing in numbers of neutrons) of the elements nitrogen and argon suggested that atmospheric density had been much greater in the distant past. Atmospheric temperatures ranged from a summer midday at -14 C (5.4 F) to a winter nighttime at -120 C (-196 F). Barometric pressure varied markedly through the year as carbon dioxide, the major constituent of the atmosphere, froze out to form the immense polar caps at the north and south poles and light covers of frost elsewhere. The Martian winds were found to blow more slowly than expected, with the highest gusts during dust storms only 119 km/h (74 mph) and average speeds far lower.

The two landers kept their landing sites under nearly continuous surveillance for three Martian years. The seismometer (a device to detect earthquakes) on the Viking 2 lander detected only one event, suggesting that Mars is geologically quiet. The sampling equipment measured physical and magnetic properties of the soil, finding it to be an iron-rich clay. Color images from the landers showed scenes reminiscent of the deserts of the southwestern United States.

An important aspect of the Viking missions was the search for life on Mars. The landers carried several instruments that had been designed to detect the barest trace evidence of life. Although the lander instruments found no indication of organic compounds, three biology experiments discovered unexpected and puzzling chemical activity in the Martian soil. But no clear evidence for the presence of living microorganisms in soil near the landing sites was found. According to some biologists, Mars soil remains too sterile, due to ultraviolet radiation from the sun and the soil's extreme dryness and oxidizing nature, to sustain life. Nevertheless, the possibility that life once existed on Mars in the distant past remains open.

Contributed By:

Joseph N. Tatarewicz

Mars Landing Site

Two U.S. Viking landers landed on the surface of Mars in 1976. The spacecraft collected and analyzed samples of the Martian soil, rock, and atmosphere, and took pictures of the surface of Mars.

NASA