The Viking program represents a major effort by the United States to explore Mars, with the particular goal of performing experiments on Martian soil to look for possible evidence of life. Four individual spacecraft were sent to Mars as part of the Viking project, two orbiters and two landers, launched as identical orbiter/lander pairs.
The Viking 1 spacecraft was launched from Cape Kennedy on August 20, 1975, using a Titan IIIE/Centaur rocket. The Titan launch vehicle consisted of a regular Titan missile with two large solid-fuel rockets attached to it, giving the total package a greatly increased launch thrust over that of the Titan by itself; the extra thrust was needed in order to lift the combined orbiter/lander spacecraft and the Centaur upper stage from the Earth. The Centaur upper stage then carried the spacecraft out of Earth orbit and onto its journey to Mars. The Viking 1 spacecraft was inserted into orbit at Mars on June 19, 1976, by the firing of a rocket motor on the orbiter that slowed the spacecraft enough to be captured in orbit around the Red Planet. The orbiter carried two cameras, an infrared mapping experiment, and an instrument that measured the amount of water vapor in the thin Martian atmosphere (intended to aide in identifying a landing site that had access to water vapor, which might have aided any life on the surface).
The Viking 1 lander was carried to Mars within a special covering case (the white object visible in the photo of the Viking spacecraft) after undergoing very extensive decontamination prior to launch. Since the mission was intended to search for possible life on Mars, the scientists wanted to make sure that no Earth organisms had been inadvertently carried to Mars on the lander spacecraft itself. On July 20, 1976, the Viking 1 lander successfully landed on Mars in the Chryse Plantia region, becoming the first spacecraft to return useful data from the surface of Mars. The lander included two scanning cameras, which provided stereo images of the landing site, and an extendable arm to collect samples and deliver them to the experiments inside the body of the spacecraft. Three separate experiments looked for evidence of life in the Martian soil: the Pyrolytic Release experiment (PR), the Labeled Release experiment (LR), and the Gas Exchange experiment (GEX). The lander also had a Gas Chromatograph/Mass Spectrometer (GCMS) that could make very precise measurements of the composition and abundance of any organic compounds in the soil. The PR, GEX, and GCMS all gave negative results, while the LP gave initial positive signals. However, long-term LR runs showed no subsequent labeled gas release (interpreted to indicate a positive biologic reaction) for the samples that gave initial positive responses, even when additional labeled liquid was repeatedly added to these soil samples. Most scientists now consider the LR results were produced by inorganic chemical reactions in the soil, but this conclusion is still actively contested by some researchers. The Viking 1 orbiter operated until August 17, 1980, after completing 1,485 orbits of the planet. The Viking 1 lander operated until November 11, 1982, after operating for 2,306 Earth days (more than six Earth years).