Mariner 3 was one of two identical deep-space probes designed and built by the Jet Propulsion Laboratory (JPL) for NASA's Mariner-Mars 1964 project that were intended to conduct close-up (flyby) scientific observations of the planet Mars and transmit information on interplanetary space and the space surrounding Mars, televised images of the Martian surface and radio occultation data of spacecraft signals as affected by the Martian atmosphere back to Earth.

Mariner 4 was the fourth in a series of spacecraft intended for planetary exploration in a flyby mode. It was designed to conduct closeup scientific observations of Mars and to transmit these observations to Earth. Launched on November 28, 1964, Mariner 4 performed the first successful flyby of the planet Mars, returning the first close-up pictures of the Martian surface. It captured the first images of another planet ever returned from deep space; their depiction of a cratered, dead planet largely changed the scientific community's view of life on Mars. Other mission objectives were to perform field and particle measurements in interplanetary space in the vicinity of Mars and to provide experience in and knowledge of the engineering capabilities for interplanetary flights of long duration. Initially expected to remain in space for eight months, Mariner 4's mission lasted about three years in solar orbit. On December 21, 1967, communications with Mariner 4 were terminated.