Viking was the culmination of a series of missions to explore the planet Mars; they began in 1964 with Mariner 4, and continued with the Mariner 6 and 7 flybys in 1969, and the Mariner 9 orbital mission in 1971 and 1972.

Viking was designed to orbit Mars and to land and operate on the planet's surface. Two identical spacecraft, each consisting of a lander and an orbiter, were built.

NASA's Langley Research Center in Hampton, Virginia, had management responsibility for the Viking project from its inception in 1968 until April 1, 1978, when the Jet Propulsion Laboratory assumed the task. Martin Marietta Aerospace in Denver, Colorado, developed the landers. NASA's Lewis Research Center in Cleveland, Ohio, had responsibility for the Titan- Centaur launch vehicles. JPL's initial assignment was development of the orbiters, tracking and data acquisition, and the Mission Control and Computing Center.

NASA launched both spacecraft from Cape Canaveral, Florida -- Viking 1 on August 20, 1975, and Viking 2 on September 9, 1975. The landers were sterilized before launch to prevent contamination of Mars with organisms from Earth. The spacecraft spent nearly a year cruising to Mars. Viking 1 reached Mars orbit on June 19, 1976; Viking 2 began orbiting Mars on August 7, 1976.

After studying orbiter photos, the Viking site certification team considered the original landing site for Viking 1 unsafe. The team examined nearby sites, and Viking 1 landed on July 20, 1976, on the western slope of Chryse Planitia (the Plains of Gold) at 22.3°N latitude, 48.0° longitude.

The site certification team also decided the planned landing site for Viking 2 was unsafe after it examined high-resolution photos. Certification of a new landing site took place in time for a Mars landing on September 3, 1976, at Utopia Planitia, at 47.7°N latitude, and 225.8° longitude.

The Viking mission was planned to continue for 90 days after landing. Each orbiter and lander operated far beyond its design lifetime. Viking Orbiter 1 exceeded four years of active flight operations in Mars orbit.

The Viking project's primary mission ended November 15, 1976, 11 days before Mars's superior conjunction (its passage behind the Sun). After conjunction, in mid-December 1976, controllers re-established telemetry and command operations, and began extended-mission operations.

The first spacecraft to cease functioning was Viking Orbiter 2 on July 25, 1978; the spacecraft had used all the gas in its attitude-control system, which kept the craft's solar panels pointed at the Sun to power the orbiter. When the spacecraft drifted off the Sun line, the controllers at JPL sent commands to shut off power to Viking Orbiter 2's transmitter.

Viking Orbiter 1 began to run short of attitude-control gas in 1978, but through careful planning to conserve the remaining supply, engineers found it possible to continue acquiring science data at a reduced level for another two years. The gas supply was finally exhausted and Viking Orbiter 1's electrical power was commanded off on August 7, 1980, after 1,489 orbits of Mars.

The last data from Viking Lander 2 arrived at Earth on April 11, 1980. Lander 1 made its final transmission to Earth Nov. 11, 1982. Controllers at JPL tried unsuccessfully for another six and one-half months to regain contact with Viking Lander 1. The overall mission came to an end May 21, 1983.

With a single exception -- the seismic instruments -- the science instruments acquired more data than expected. The seismometer on Viking Lander 1 would not work after landing, and the seismometer on Viking Lander 2 detected only one event that may have been seismic. Nevertheless, it provided data on wind velocity at the landing site to supplement information from the meteorology experiment, and showed that Mars has very low seismic background.

The three biology experiments discovered unexpected and enigmatic chemical activity in the Martian soil, but provided no clear evidence for the presence of living microorganisms in soil near the landing sites. According to mission biologists, Mars is self-sterilizing. They believe the combination of solar ultraviolet radiation that saturates the surface, the extreme dryness of the soil and the oxidizing nature of the soil chemistry prevent the formation of living organisms in the Martian soil. The question of life on Mars at some time in the distant past remains open.

The landers' gas chromatograph/mass spectrometer instruments found no sign of organic chemistry at either landing site, but they did provide a precise and definitive analysis of the composition of the Martian atmosphere and found previously undetected trace elements. The X-ray fluorescence spectrometers measured elemental composition of the Martian soil.

Viking measured physical and magnetic properties of the soil. As the landers descended toward the surface they also measured composition and physical properties of the Martian upper atmosphere.

The two landers continuously monitored weather at the landing sites. Weather in the Martian midsummer was repetitious, but in other seasons it became variable and more interesting. Cyclic variations appeared in weather patterns (probably the passage of alternating cyclones and anticyclones). Atmospheric temperatures at the southern landing site (Viking Lander 1) were as high as -14°C (7°F) at midday, and the predawn summer temperature was -77°C (-107°F). In contrast, the diurnal temperatures at the northern landing site (Viking Lander 2) during midwinter dust storms varied as little as 4°C (7°F) on some days. The lowest predawn temperature was -120°C (-184°F), about the frost point ofcarbon dioxide. A thin layer of water frost covered the ground around Viking Lander 2 each winter.

Barometric pressure varies at each landing site on a semiannual basis, because carbon dioxide, the major constituent of the atmosphere, freezes out to form an immense polar cap, alternately at each pole. The carbon dioxide forms a great cover of snow and then evaporates again with the coming of spring in each hemisphere. When the southern cap was largest, the mean daily pressure observed by Viking Lander 1 was as low as 6.8 millibars; at other times of the year it was as high as 9.0 millibars. The pressures at the Viking Lander 2 site were 7.3 and 10.8 millibars. (For comparison, the surface pressure on Earth at sea level is about 1,000 millibars.)

Martian winds generally blow more slowly than expected. Scientists had expected them to reach speeds of several hundred miles an hour from observing global dust storms, but neither lander recorded gusts over 120 kilometers (74 miles) an hour, and average velocities were considerably lower. Nevertheless, the orbiters observed more than a dozen small dust storms. During the first southern summer, two global dust storms occurred, about four Earth months apart. Both storms obscured the Sun at the landing sites for a time and hid most of the planet's surface from the orbiters' cameras. The strong winds that caused the storms blew in the southern hemisphere.

Photographs from the landers and orbiters surpassed expectations in quality and quality. The total exceeded 4,500 from the landers and 52,000 from the orbiters. The landers provided the first close-up look at the surface, monitored variations in atmospheric opacity over several Martian years, and determined the mean size of the atmospheric aerosols. The orbiter cameras observed new and often puzzling terrain and provided clearer detail on known features, including some color and stereo observations. Viking's orbiters mapped 97 percent of the Martian surface.

The infrared thermal mappers and the atmospheric water detectors on the orbiters acquired data almost daily, observing the planet at low and high resolution. The massive quantity of data from the two instruments will require considerable time for analysis and understanding of the global meteorology of Mars. Viking also definitively determined that the residual north polar ice cap (that survives the northern summer) is water ice, rather than frozen carbon dioxide (dry ice) as once believed.

Analysis of radio signals from the landers and the orbiters -- including Doppler, ranging and occultation data, and the signal strength of the lander-to-orbiter relay link -- provided a variety of valuable information.

Other significant discoveries of the Viking mission include:

• The Martian surface is a type of iron-rich clay that contains a highly oxidizing substance that releases oxygen when it is wetted.

• The surface contains no organic molecules that were detectable at the parts-per-billion level -- less, in fact, than soil samples returned from the Moon by Apollo astronauts.

• Nitrogen, never before detected, is a significant component of the Martian atmosphere, and enrichment of the heavier isotopes of nitrogen and argon relative to the lighter isotopes implies that atmospheric density was much greater than in the distant past.

• Changes in the Martian surface occur extremely slowly, at least at the Viking landing sites. Only a few small changes took place during the mission lifetime.

• The greatest concentration of water vapor in the atmosphere is near the edge of the north polar cap in midsummer. From summer to fall, peak concentration moves toward the equator, with a 30 percent decrease in peak abundance. In the southern summer, the planet is dry, probably also an effect of the dust storms.

• The density of both of Mars's satellites is low -- about two grams per cubic centimeter -- implying that they originated as asteroids captured by Mars's gravity. The surface of Phobos is marked with two families of parallel striations, probably fractures caused by a large impact that may nearly have broken Phobos apart.

• Measurements of the round-trip time for radio signals between Earth and the Viking spacecraft, made while Mars was beyond the Sun (near the solar conjunctions), have determined delay of the signals caused by the Sun's gravitational field. The result confirms Albert Einstein's prediction to an estimated accuracy of 0.1 percent -- 20 times greater than any other test.

• Atmospheric pressure varies by 30 percent during the Martian year because carbon dioxide condenses and sublimes at the polar caps.

• The permanent north cap is water ice; the southern cap probably retains some carbon dioxide ice through the summer.

• Water vapor is relatively abundant only in the far north during the summer, but subsurface water (permafrost) covers much if not all of the planet.

• Northern and southern hemispheres are drastically different climatically, because of the global dust storms that originate in the south in summer.

Space History Viking Spacecraft Introduction

Views of the Solar System Copyright © 1997 by Calvin J. Hamilton. All rights reserved.