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.
Atlas-Agena
The Mariner spacecraft were built by NASA's Jet Propulsion Laboratory. Mariner 1 and Mariner 2 were twins, launched on July 22 and August 27, 1962, to fly by the planet Venus. The first two craft used the same spacecraft bus as the Block I Rangers, each weighing and instrumented to perform radiometric temperature measurements of the planet, and to measure interplanetary magnetic fields and particles.
Mariner 1's Atlas-Agena malfunctioned and went off course, requiring its destruction approximately 5 minutes after liftoff. Mariner 2 successfully made the 3½-month flight, becoming the first spacecraft to fly by another planet. It carried microwave and infrared radiometers, and sensors for cosmic dust, solar plasma and high-energy radiation, and magnetic fields.
Mariner 3 and Mariner 4 used a redesigned spacecraft bus weighing , and were launched on November 5 and November 28, 1964 to fly by the planet Mars. Mariner 3 failed after a successful launch when its payload shroud failed to open. These Mariners carried cameras, and Mariner 4 successfully returned pictures of Mars as it flew by.