Editing Mariner 2 (section)

==== Background ====
At the time of the Mariner project's inception, few of Venus' characteristics were definitely known. Its opaque [[atmosphere of Venus|atmosphere]] precluded [[telescope|telescopic]] study of the ground. It was unknown whether there was water beneath the clouds, though a small amount of [[water vapor]] above them had been detected. The planet's [[rotation rate]] was uncertain, though JPL scientists had concluded through [[radar]] observation that Venus rotated very slowly compared to the Earth, advancing the long-standing{{r|beyond1}} (but later disproven){{r|nasa8}} hypothesis that the planet was [[Tidal locking|tidally locked]] with respect to the Sun (as the Moon is with respect to the Earth).{{r|avweek1961}} No oxygen had been detected in Venus' atmosphere, suggesting that life as existed on Earth was not present. It had been determined that Venus' atmosphere contained at least 500 times as much [[carbon dioxide]] as the Earth's. These comparatively high levels suggested that the planet might be subject to a [[runaway greenhouse effect]] with surface temperatures as high as {{cvt|600|K}}, but this had not yet been conclusively determined.{{r|report}}{{rp|7–8}}

The Mariner spacecraft would be able to verify this hypothesis by measuring the temperature of Venus close-up;{{r|avweek1960a}} at the same time, the spacecraft could determine if there was a significant disparity between night and daytime temperatures.{{r|report}}{{rp|331}} An on-board [[magnetometer]] and suite of charged particle detectors could determine if Venus possessed an appreciable magnetic field and an analog to Earth's [[Van Allen Belts]].{{r|avweek1960a}}

As the Mariner spacecraft would spend most of its journey to Venus in interplanetary space, the mission also offered an opportunity for long-term measurement of the [[solar wind]] of charged particles and to map the variations in the Sun's [[magnetosphere]]. The concentration of [[cosmic dust]] beyond the vicinity of Earth could be explored as well.{{r|treasury}}{{rp|176}}

Due to the limited capacity of the Atlas-Agena, only {{convert|40|lb|kg|order=flip}} of the spacecraft could be allocated to scientific experiments.{{r|report}}{{rp|195}}