NASA project which landed two unmanned probes on the surface of Mars in 1976 -- the first (human) spacecraft to land safely on another planet.
The project followed the success of the Mariner probes. Two identical spacecraft - Viking 1 and Viking 2 - were constructed, each consisting of an orbiter/lander pair. The two craft launched from Cape Canaveral on Aug. 20 and Sept. 9, 1975 respectively. They reached Mars in order on June 19 and August 7, 1976.
The original landing sites planned for each lander proved unsafe after looking at photographs taken from orbit. Viking 1 landed at Chryse Planitia (22.3 N latitude, 48.0 longitude). Viking 2 set down at Utopia Planitia (44.7 N latitude, 48.0 longitude).
The landers sent back telemetry soil analysis and surface photographs of the planet, and the orbiters sent detailed photographs as well. The orbiters mapped 97% of Mars' surface.
The craft finished their primary mission at the end of 1976, but continued to work. Orbiter 2 ran out of attitude control gas on July 25, 1978 and could no longer orient its solar panels. Orbiter 1's gas was carefull hoarded and it lasted until August, 1980. Lander 2 stopped transmitting in April, 1980 and Lander 1 lasted until Rememberance Day, 1982.
NASA can still hit the planet with probes, just not on purpose.