Exploration of Mars
The Viking program launched "Viking 1" and "Viking 2" spacecraft to Mars in 1975; The program consisted of two orbiters and two landers – these were the first two spacecraft to successfully land and operate on Mars.
The primary scientific objectives of the lander mission were to search for biosignatures and observe meteorologic, seismic and magnetic properties of Mars. The results of the biological experiments on board the Viking landers remain inconclusive, with a reanalysis of the Viking data published in 2012 suggesting signs of microbial life on Mars.
The Viking orbiters revealed that large floods of water carved deep valleys, eroded grooves into bedrock, and traveled thousands of kilometers. Areas of branched streams, in the southern hemisphere, suggest that rain once fell.
List of Mars orbiters
The Soviets' Mars program and NASA's Mariner program became the two first successful space programs that intended to explore Mars through orbiters. Mars 2, Mars 3 and Mariner 9 were all launched into space in May 1971, and all entered Mars’ orbit that same year. NASA's Mariner 9 reached the planet's orbit first on November 14, narrowly beating the Soviet's spacecraft amid the space race, and subsequently became the first spacecraft to orbit another planet.