Mariner probes had already orbited Mars, but NASA’s Viking program was designed specifically to land on the planet in search of, among other things, life.
Viking 1 went into orbit around Mars on June 19, 1976.
NASA planned to send a module to land on the surface on Independence Day, to celebrate the U.S. bicentennial, but complications delayed the landing until July 20.
Viking 2 landed on September 3 that same year.
Both probes were outfitted with biological experiments to determine whether there was any sign of life on Mars.
For friends of Martians, the results were disappointing.
The Viking probes did not discover any biological evidence of life on that planet, but they studied the soil, weather, and atmosphere, and sent back over 42,000 photographs.
NASA’s Viking program was highly successful, and also was the most expensive and ambitious mission ever sent to Mars.