Mariner 3 (together with Mariner 4 known as Mariner-Mars 1964) 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. It was the third of ten spacecraft within the Mariner program.
Mariner 3 was launched on November 5, 1964 from Cape Canaveral Air Force Station Launch Complex 13, but the shroud encasing the spacecraft atop its rocket failed to open properly, and Mariner 3 did not get to Mars. Unable to collect the Sun's energy for power from its solar panels, the probe soon died when its batteries ran out and is now derelict in a solar orbit.
Three weeks later, on November 28, 1964, Mariner 4 was launched successfully on a 7½-month voyage to Mars.
Payloads are separated by bullets ( · ), launches by pipes ( | ). Manned flights are indicated in bold text. Uncatalogued launch failures are listed in italics. Payloads deployed from other spacecraft are denoted in brackets.