Mission Type: Flyby
Launch Vehicle: Atlas-Agena D (no. 23 / Atlas D no. 5401 / Agena D no. AD157 / 6933)
Launch Site: Eastern Test Range / launch complex 12, Cape Canaveral, USA
NASA Center: Jet Propulsion Laboratory
Spacecraft Mass: 244.9 kg at launch
Spacecraft Instruments: 1) ultraviolet photometer; 2) S-band occultation experiment; 3) dual-frequency occultation experiment; 4) solar plasma probe; 5) magnetometer; 6) trapped-radiation detector; and 7) celestial mechanics experiment
Total Cost: Total research, development, launch, and support costs for the Mariner series of spacecraft (Mariners 1 through 10) was approximately $554 million.
References:
Deep Space Chronicle: A Chronology of Deep Space and Planetary Probes 1958-2000, Monographs in Aerospace History No. 24, by Asif A. Siddiqi

National Space Science Data Center, http://nssdc.gsfc.nasa.gov/

Solar System Log by Andrew Wilson, published 1987 by Jane's Publishing Co. Ltd.

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. 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 8,165 kilometers, but the agency altered its plan in favor of a more modest 75,000-kilometer flyby in order to prevent the nonsterilized vehicle from crashing into the planet.

After a midcourse correction on 19 June, Mariner 5 began transmitting data about Venus during its encounter on 19 October. Closest approach was at 17:34:56 UT at a range of 4,094 kilometers.

Mariner 5 found no radiation belts trapped by Venus' magnetic field. The ultraviolet photometer detected a hydrogen corona (as did the Soviet Venera 4), but no oxygen emission. Mariner 5's instruments indicated that the planet's surface temperature and pressure were 527°C 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.

On 4 December 1967, NASA lost contact with the spacecraft, although controllers briefly regained contact on 14 October 1968. The spacecraft did not transmit any further telemetry, and NASA eventually stopped attempts to communicate with the vehicle, now in heliocentric orbit.