Mariner 5 (Mariner V or Mariner Venus 1967) was a spacecraft of the Mariner program that carried a complement of experiments to probe Venus's 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, plasma, radio refractivity and UV emissions of the Venusian atmosphere. Mariner 5 was built as a backup to Mariner 4, but after the success of the Mariner 4 mission, it was modified to be used for a Venus flyby mission to take place during the 1967 Venus launch window.

Wikipedia: https://en.wikipedia.org/wiki/Mariner_5. Related NASA reference: https://science.nasa.gov/mission/mariner-5/. Description adapted from Wikipedia contributors (CC BY-SA 4.0; https://creativecommons.org/licenses/by-sa/4.0/).