The Viking project was a successful effort by the US National Aeronautics and Space Administration (NASA) to land a pair of twin robotic probes on the surface of Mars in 1976. The Viking landers, which were paired with orbiters that photographed the planet from space, were the first successful Mars landers. They remained so (despite a number of attempted Soviet landings) until 1996, when NASA’s Pathfinder mission successfully touched down. The Viking landers took high-resolution digital images of the Martian surface in color and black-and-white, recorded years of weather data, and processed soil samples in miniaturized onboard laboratories to search for signs of life. Despite confusing results from the life experiments, most scientists today believe that Viking did not detect life.
efforts to send robotic probes to other planets in the solar system began as soon as rockets powerful enough to do so were developed in the early 1960s. The first probe to return data from Mars was NASA’s Mariner 4, which flew past the planet on July 15, 1965. Twenty-one pictures were taken by a television camera during the flyby, stored on a tape recorder, and radioed back to Earth the next day. The existence of numerous large craters on Mars was discovered for the first time, a surprise to scientists, who had thought Mars more Earthlike.
Planning for a robotic landing on Mars began at once. By 1966, the basic design of the Viking mission existed under the project name Voyager (later re-used for a pair of robot probes to the outer solar system). Funding was cut for the original “Voyager” mission in 1967 due to budget pressure on NASA resulting from the Apollo program and the Vietnam War, but was eventually restored. The program was re-named Viking, and a Viking land-orbiter spacecraft combination was designed by engineers at NASA’s Langley Research Center in collaboration with the Jet Propulsion Laboratory in Pasadena, California. After several more ups and downs, including budget overruns that eventually raised the mission cost to about $1 billion (about $3.4 billion in 2005 dollars), the Viking probes were finally launched in August and September 1975. They rode Titan III rockets into Earth orbit. Each was then launched toward Mars using a hydrogen-oxygen powered Centaur second stage.
Each Viking spacecraft consisted of an orbiter and a lander that traveled to Mars together. After arriving in Mars orbit, the lander remained dormant, its systems turned off, while the orbit took high-resolution photographs of the Martian surface, looking for a safe landing site.
Viking 1 arrived at Mars on June 19, 1976. NASA’s desire to not crash the lander on boulders
was intense, so mission planners debated photographic and Earth-based radar data on surface roughness for weeks before selecting a site—Chryse Planitia, a low-lying plain. After having orbited Mars for a month, the Viking spacecraft was ordered to split in two. The lander fired retro-rocks and descended into the Martian atmosphere, leaving the orbiter behind.
Protected by a saucer-shaped aeroshell, the lander slowed itself first by friction, plunging into the thin Martian atmosphere at about 10,000 miles per hour. At 19,000 feet, parachutes were deployed. At 4,000 feet, the parachutes and aeroshell were separated from the lander using small explosive charges. The lander rode its own rocket the rest of the way down to the surface, guided by on-board radar and computers with no help from Earth. Earth was so far away that radio control signals would have taken 19 minutes to get to Earth from Mars and another 19 minutes to get back again, making remote control of a landing impossible.
Finally, on July 20, 1976, the Viking 1 lander made an historic soft touch-down on fold-out legs. Nineteen minutes after touchdown, NASA controllers received radio signals confirming the event and exploded into cheers. As of 2006, the Viking landers were still the only craft to have ever landed successfully on Mars using this soft-landing approach; the three other successful landings (Pathfinder in 1996 and the Mars Exploration Rovers in 2004) used an airbag system that allowed them to bounce and roll upon touchdown.
A few minutes after landing, the lander began taking the first-ever close-up picture of the Martian surface: a view of one of the lander’s own footpads in black-and-white, with surrounding sand and rocks. This image was chosen because if the lander did not manage to transmit any more pictures, it was felt that a close-up of the Martian soil would provide more scientific data than any other one image could.
More pictures soon followed, including color photographs showing the Martian landscape and horizon. The Martian sky looked pink at first, which was considered an obvious error, so NASA adjusted the colors to make the sky blue before releasing the pictures to the press. Later it was realized that the Martian sky is in fact a light shade of pink.
Viking 2 arrived at Mars soon after Viking 1 had landed. On September 3, 1976, it landed successfully on the plain called Utopia Planitia.
The landers carried not only cameras but weather-sensing instruments and miniature biological laboratories. Each laboratory contained four experiments to test Martian soil samples for evidence of life.
In the pyrolitic release experiment, Martian soil was incubated with water, light, and carbon dioxide containing radioactive carbon (carbon-14). The idea was that Martian microorganisms might eat some of the carbon. After a few days of incubation, the gas was pumped out, leaving any possible microorganisms in the soil. Baking the soil at 1200° F (650° C) would drive out carbon from vaporized microorganisms, which could be sensed by its radioactivity. The result could be compared to results of soil baked without being incubated, to test whether the radioactive carbon in the first sample had been put there by living things.
In the labeled-release experiment, a Martian soil sample was mixed with a liquid nutrient mixture containing carbon-14. Gases from the sample were then tested to see if they contained carbon-14, possibly released by life chemistry in the soil.
In the gas-exchange experiment, Martian soil was mixed with liquid nutrients. Gases from the sample were then tested to see if they contained oxygen, methane, and other gasses that might be released by life.
Finally, in the gas chromatograph/mass spectrometer experiment, soil was examined directly to see if it contained organic (carbon-containing) compounds.
All the Viking 1 biology experiments except the labeled-release experiment returned negative results— no life. However, seven out of nine pyrolitic-release experiments gave dramatic positive results. Similar data were later returned by the Viking 2 lander. Scientists have proposed that a still-unknown Martian chemical reaction, not Martian life, caused the apparently positive results. Although the nature of this chemical reaction has not been identified, most scientists agree that the Viking landers did not detect life. A few disagree. Similar experiments will not be carried out until the European ExoMars lander arrives on Mars in 2014, as presently scheduled.
The mission ended in 1982, when an erroneous command to the Viking 1 lander erased antenna-positioning data and contact was lost. The Viking orbiters and landers radioed some 50,000 digital photographs back to Earth, and it would be 20 years before pictures comparable to Viking’s were again returned from the Martian surface. In the early 2000s, scientists were still using data from the Viking missions, in combination with data from later missions, to study Mars. For example, in 2003 researchers announced that Viking data had helped located exposed water ice near the south pole of Mars.
Hamilton, John. The Viking Missions to Mars. ABDO, 1998. National Aeronautics and Space Administration. The Martian Landscape. Washington, DC: US Government Printing Office, 1978.
National Aeronautics and Space Administration. On Mars: Exploration of the Red Planet. Washington, DC: US Government Printing Office, 1984.
Titus, Timothy N., et al. “Exposed Water Ice Discovered Near the South Pole of Mars.” Science. 299 (2003): 1048-1051.
National Aeronautics and Space Administration. “Viking: Mission to Mars.” July 20, 2006. <http://www.nasa.gov/mission_pages/viking/> (accessed November 27, 2006).