Editing Mariner 5 (section)

==History==
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. Mariner 5 omitted several experiments from Mariner 4, including the TV camera, the ionization chamber/geiger counter, the cosmic ray detector, and the cosmic dust detector. It retained the helium-vector magnetometer, solar plasma probe, and trapped radiation detector from Mariner 4. Unlike Mariner 4, Mariner 5 needed to face away from the Sun to keep its high-gain antenna pointed at Earth because of its trajectory. As a result, the solar panels were reversed to be aft facing so they could remain pointed at the Sun. Additionally, since its mission to Venus brought it in closer proximity to the Sun, fewer solar cells were needed to achieve the necessary power generation, and as a result the solar panels were reduced in size to save mass as well as to make room for two 50&nbsp;MHz dual-frequency receiver (DFR) antennas that were mounted on the frame of two of the solar panels. Since the aft side of the spacecraft faced the Sun, the solar plasma probe was relocated to the aft-facing side of Mariner 5.<ref name=Layman/>

[[File:Mariner 5 diagram.png|thumb|Mariner 5 diagram]]

The mounting for the high-gain antenna also needed modification. Unlike Mariner 4, where the geometry of the transfer orbit allowed for the high-gain antenna to be inclined at a relatively simple 38 degrees from the bottom plane, Mariner 5's trajectory required the high-gain antenna to be skewed at a more awkward angle. The high-gain antenna also included a single-use mechanism that allowed the high-gain antenna to make a shift in its angle as part of the radio occultation experiment.<ref name=Layman/> Mariner 5 also included some additional equipment that was not flown on Mariner 4, such as its Ultraviolet Photometer, two 50&nbsp;MHz DFR antennas, a 423&nbsp;MHz DFR antenna mounted on the end of one of the solar panels, and a deployable Sun-shade on the aft of the spacecraft for thermal control. The UV Photometer was originally supposed to fly on Mariner 4 and would have been mounted to its TV Camera scan platform. However, it was removed (allowing it to be flown on Mariner 5) and swapped out for a thermal/inertial mass simulator late in the assembly of Mariner 4 as it was discovered to create electrical arcing problems that would have jeopardized the TV Camera.<ref name=Layman/>

Prior to the choice of Venus as the target, proposals had been made to send it to either the comet [[7P/Pons–Winnecke]] or [[10P/Tempel]].<ref name=Ulivi57/>