The Viking program consisted of a pair of American space probes sent to Mars, Viking 1 and Viking 2. Each spacecraft was composed of two main parts: an orbiter designed to photograph the surface of Mars from orbit, and a lander designed to study the planet from the surface. The orbiters also served as communication relays for the landers once they touched down. The project cost roughly US$1 billion. It was highly successful and formed most of the body of knowledge about Mars through the late 1990s and early 2000s.