Mariner 5 (Mariner Venus '67) was a spacecraft of the Mariner program that carried a complement of experiments to probe Venus' atmosphere by radio occultation, measure the hydrogen Lyman-alpha (hard ultraviolet) spectrum, and sample the solar particles and magnetic field fluctuations above the planet. Its goals were to measure interplanetary and Venusian magnetic fields, charged particles, plasmas, radio refractivity and UV emissions of the Venusian atmosphere
Mariner 5 was actually built as a backup to Mariner 4, but after the success of the Mariner 4 mission, it was modified (by reversing and reducing the size of the solar panels and adding a thermal shield) for the Venus mission.
It was launched toward Venus on June 14, 1967 from Cape Canaveral Air Force Station Launch Complex 12 and flew by the planet on October 19 that year at an altitude of 3990 kilometres (2480 miles). With more sensitive instruments than its predecessor Mariner 2, Mariner 5 was able to shed new light on the hot, cloud-covered planet and on conditions in interplanetary space.
Radio occultation data from Mariner-5 helped to understand the temperature and pressure data returned by the Venera-4 lander, which arrived at Venus shortly before it. After these missions, it was clear that Venus had a very hot surface and an atmosphere even more dense than expected.
The operations of Mariner 5 ended in November 1967 and it is now defunct in a heliocentric orbit.
Two-Frequency Beacon Receiver S-Band Occultation Helium Magnetometer Interplanetary Ion Plasma Probe for E/Q of 40 to 9400 Volts Celestial Mechanics