Viking 1 and 2 were identical robotic spacecraft launched on August 20, 1975 and September 9, 1975 respectively. They were the first man-made spacecraft to land on another planet. Each Viking spacecraft consisted of an orbiter and a lander. Each orbiter and lander flew to Mars together, and then decoupled in the Martian atmosphere. The lander descended to the ground and the orbiter continued to orbit the planet. The entire mission was designed to continue for 6 weeks after landing, but all 4 components continued to be active long after this deadline had passed. The Viking 1 orbiter continued to fly over the Martian surface for a full three years, and the lander lasted 7 years on the surface of Mars. The Viking 2 orbiter and lander both lasted for four years.

The Viking 1 and 2 landers descended to two different parts of Mars, but they carried out the same types of experiments. While on the ground, they performed tests of the Martian soil to look for signs of life. However, no such signs were detected. Both the landers and the orbiters sent many hundreds of images of the surface of Mars back to Earth.