Surface and orbital observations in one mission
Viking 2 was the second Mars probe in the US National Aeronautics and Space Administration's Viking programme, consisting of an orbiter and lander. It combined surface sampling, meteorology and investigations of possible signs of life with orbital observations of topography, water vapour and temperature, studying both a specific location and the planet as a whole.
Langley Research Center managed the overall project, while the Jet Propulsion Laboratory was responsible for the orbiter. Project manager James Martin, primary mission director Thomas Young and orbiter scientist Conway Snyder supported development of the imaging system. Flight operations and data-analysis personnel worked in Pasadena, California, receiving data and transmitting commands through ground stations at Goldstone, Canberra and Madrid.
Launch in 1975 and a renewed search for a landing site in 1976
Originally designated Viking-A, Viking 2 had been scheduled to launch first, but battery problems requiring repairs reversed the order. It lifted off from Launch Complex 41 at Cape Canaveral on 9 September 1975 aboard a Titan III–Centaur rocket. During the long cruise, the orbiter supplied power to the lander and handled attitude control, communications and checks. The combined spacecraft entered Mars orbit on 7 August 1976.
The planned landing site was not simply retained in the final flight sequence. Viking 1, which had arrived earlier, photographed candidate landing areas for the second probe and found the terrain too rough. In searching for a safe site, orbital imaging covered nearly half of the area between 40 and 50 degrees north on Mars, and Utopia Planitia was finally selected. After arrival, Viking 2 continued inspecting the landing area and landed on 3 September. The imaging team's mission table records touchdown at 22:37:50 Universal Time.
Utopia Planitia was less smooth than expected
The panorama after landing surprised the team. Earlier orbital data had led researchers to expect a relatively thick sandy cover, but the lander's cameras showed numerous rocks and boulders. The horizon curved markedly downward in the panorama, partly because of the lander's tilted body. This imaging effect cannot directly be interpreted as major changes in the actual ground elevation.
NASA's 1977 results report compared the two landing sites. Viking 2's surroundings contained more rocks, which looked relatively uniform and often had vesicles. No exposed bedrock or ripple-shaped dunes like those at Viking 1's site were seen nearby. Shallow troughs interconnected to form polygons, with one passing beside the lander. These differences made observations at the two sites valuable for comparison, rather than allowing the first landing area to be treated as a simple sample of all Mars.
A functioning seismometer and wind noise
Viking 1's seismometer failed to deploy properly, whereas Viking 2's instrument operated for about nineteen months, providing data different from images and soil chemistry. The difficulty was that the seismometer was mounted on the lander itself. Wind moving the spacecraft and mechanical activities also generated vibrations, so many records could not be used directly to search for marsquakes.
The 1981 final report counted about 2,100 hours of low-noise data suitable for detecting seismic events, only 16 percent of total operating time. A candidate event occurred on the eightieth Martian day, but wind-speed measurements were unavailable within twenty minutes of it, leaving a gust of wind as a possible explanation. The team consequently recommended isolating future seismometers from lander activity and wind-induced interference. This engineering experience was itself a practical result of the mission.
References: [4]
The orbiter turns towards the poles
After landing, Viking 2's orbiter continued changing its orbit to expand coverage. Its inclination increased to 75 degrees on 30 September 1976, then to 80 degrees on 20 December, when periapsis altitude was also lowered to 800 kilometres. The higher inclination brought the cameras closer to high-latitude regions, while the northern hemisphere, where periapsis lay, received more detailed imaging.
All the polar images in Viking Orbiter Views of Mars came from the Viking 2 orbiter. One north-polar mosaic combined about 300 photographs, showing curved frost-free dark bands and an outer belt of dunes. Layered deposits on exposed slopes also became identifiable. Researchers used these to discuss how accumulations of windblown material and volatiles might preserve records of past climate changes.
References: [1]
The two components end operations at different times
The life-detection experiments did not produce a conclusive discovery of life. Viking 2's results resembled those of Viking 1: chemical reactions in the soil could mimic some biological processes and had to be interpreted alongside other analyses. The mission's value therefore also included geological comparisons, polar imagery and long-term records of instrument operating conditions.
The orbiter and lander had different operational lifetimes. The imaging team's report records that the Viking 2 orbiter was shut down on 25 July 1978 after returning nearly 16,000 pictures of Mars and its moons. The lander continued working until April 1980. From launch in 1975, through orbital insertion and landing in 1976, to the successive cessation of the two components, this was an observing mission extended in stages.