In December 1965, NASA approved a project to modify the Mariner 4 backup spacecraft to conduct a closer flyby of Venus than the only other NASA probe to fly past Venus, Mariner 2.
The primary goal of the mission was to conduct a radio-occultation experiment (much like Mariner 4 at Mars) to determine atmospheric properties of Venus. Unlike Mariner 4, however, Mariner 5 did not carry an imaging instrument.
Initially, NASA had planned to send Mariner 5 on a flyby at a miss distance of about 5,074 miles (8,165 kilometers) to the surface, but the Agency altered its plan in favor of a more modest 46,603-mile (75,000-kilometer) flyby in order to preclude the non-sterilized vehicle from crashing into the planet.
After a mid-course correction on June 19, 1967, Mariner 5 began transmitting data on Venus on October 19, 1967 during its encounter. Closest approach was at 17:34:56 UT at a range of 2,544 miles (4,094 kilometers), much closer than expected due to the course correction.
Mariner 5 found that although Venus does not have a magnetic field, the dense daylight ionosphere produces a bow shock that deflects the solar wind around the planet. The ultraviolet photometer detected a hydrogen corona (as found by the Soviet Venera 4) but no oxygen emission.
Mariner 5’s instruments indicated that the planet’s surface temperature and pressure were 981 degrees Fahrenheit (527 degrees Celsius) and 75 to 100 atmospheres, respectively -- which countered the Soviet claim that its Venera 4 spacecraft had managed to transmit from the planet’s surface.
The encounter with Venus deflected the spacecraft toward the Sun, and Mariner 5 entered solar orbit with parameters ranging from 0.579 AU and 0.735 AU.
On Dec. 4, 1967, NASA lost contact with the spacecraft, although controllers briefly regained contact on Oct. 14, 1968. The spacecraft did not transmit any further telemetry and NASA eventually stopped attempts (on Nov. 5, 1968) to communicate with the vehicle, now in heliocentric orbit.
Planetary scientists reviewed data from both Mariner 5 and the Soviet Venera 4 at a conference at Kitt Peak National Observatory in March 1968, one of the first major international meetings to discuss the results of planetary exploration. Scientists concluded that neither Mariner 5 nor Venera 4 had been entirely successful in communicating data about conditions at the planet’s surface.
Siddiqi, Asif A. Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016. NASA History Program Office, 2018.