Editing Mariner 2 (section)

=== Scientific instruments ===

==== 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}}

==== Instruments ====
* A two-channel '''[[microwave radiometer]]''' of the crystal video type operating in the standard [[Robert H. Dicke|Dicke]] mode of chopping between the main antenna, pointed at the target, and a reference horn pointed at cold space.{{r|nasa9}} It was used to determine the absolute temperature of Venus' surface and details concerning its atmosphere through its microwave-radiation characteristics, including the daylight and dark hemispheres, and in the region of the terminator. Measurements were performed simultaneously in two frequency bands of 13.5&nbsp;mm and 19&nbsp;mm.{{r|report}}{{rp|198–204}}{{r|Barath_Feb64}} The total weight of the radiometer was {{convert|22|lb|kg|-1|order=flip}}. Its average power consumption was 4&nbsp;watts and its peak power consumption 9&nbsp;watts.{{r|Barath_Mar63}}
[[File:Mariner 2 infrared radiometer.png|thumb|Labeled diagram of the infrared radiometer design]]
* A two-channel '''[[infrared]] [[radiometer]]''' to measure the effective temperatures of small areas of Venus. The radiation that was received could originate from the planetary surface, clouds in the atmosphere, the atmosphere itself or a combination of these. The radiation was received in two spectral ranges: 8 to 9&nbsp;μm (focused on 8.4&nbsp;μm) and 10 to 10.8&nbsp;μm (focused on 10.4&nbsp;μm).{{r|report}}{{rp|205–213}} The latter corresponding to the [[carbon dioxide]] band.{{r|Chase_Mar63}} The total weight of the infrared radiometer, which was housed in a magnesium casting, was {{cvt|1.3|kg|lb}}, and it required 2.4&nbsp;watts of power. It was designed to measure radiation temperatures between approximately {{convert|200|and|500|K|C F}}.{{r|Chase_Nov63}}

* A three-axis '''[[Magnetometer#Fluxgate magnetometer|fluxgate magnetometer]]''' to measure planetary and interplanetary magnetic fields.{{r|report}}{{rp|213–218}} Three probes were incorporated in its sensors, so it could obtain three mutually orthogonal components of the field vector. Readings of these components were separated by 1.9&nbsp;seconds. It had three analog outputs that had each two sensitivity scales: ±&nbsp;64&nbsp;γ and ±&nbsp;320&nbsp;γ (1&nbsp;γ = 1&nbsp;[[Tesla (unit)|nanotesla]]). These scales were automatically switched by the instrument. The field that the magnetometer observed was the super-position of a nearly constant spacecraft field and the interplanetary field. Thus, it effectively measured only the changes in the interplanetary field.{{r|Coleman}}

* An '''[[ionization chamber]]''' with matched '''[[Geiger-Müller tube]]s''' (also known as a [[cosmic ray]] detector) to measure high-energy cosmic radiation.{{r|report}}{{rp|218–219}}{{r|Anderson}}

* A '''particle detector''' (implemented through use of an Anton type 213 Geiger-Müller tube) to measure lower radiation (especially near Venus),{{r|report}}{{rp|219–223}}{{r|VanAllen_Frank}} also known as the Iowa detector, as it was provided by the [[University of Iowa]].{{r|Anderson}} It was a miniature tube having a 1.2&nbsp;mg/cm<sup>2</sup> mica window about {{cvt|0.3|cm|in}} in diameter and weighing about {{cvt|60|g|oz}}. It detected soft [[x-ray]]s efficiently and [[ultraviolet]] inefficiently, and was previously used in [[Injun (satellite)|Injun 1]], [[Explorer 12]] and [[Explorer 14]].{{r|VanAllen_Frank}} It was able to detect protons above 500&nbsp;keV in energy and electrons above 35&nbsp;keV.{{r|nasa2}} The length of the basic telemetry frame was 887.04&nbsp;seconds. During each frame, the counting rate of the detector was sampled twice at intervals separated by 37&nbsp;seconds. The first sampling was the number of counts during an interval of 9.60&nbsp;seconds (known as the 'long gate'); the second was the number of counts during an interval of 0.827&nbsp;seconds (known as the 'short gate'). The long gate accumulator overflowed on the 256th count and the short gate accumulator overflowed on the 65,536th count. The maximum counting rate of the tube was 50,000 per second.{{r|VanAllen_Frank}}

* A '''[[cosmic dust]] detector''' to measure the flux of cosmic dust particles in space.{{r|report}}{{rp|223–224}}
[[File:Mariner 2 plasma probe.png|thumb|Instrument for studying plasma]]
* A '''solar plasma [[spectrometer]]''' to measure the spectrum of low-energy positively charged particles from the Sun, i.e. the [[solar wind]].{{r|report}}{{rp|224–228}}

The magnetometer was attached to the top of the mast below the [[omnidirectional antenna]]. Particle detectors were mounted halfway up the mast, along with the cosmic ray detector. The cosmic dust detector and solar plasma spectrometer were attached to the top edges of the spacecraft base. The microwave radiometer, the infrared radiometer and the radiometer reference horns were rigidly mounted to a {{cvt|48|cm|in}} diameter parabolic radiometer antenna mounted near the bottom of the mast. All instruments were operated throughout the cruise and encounter modes except the radiometers, which were only used in the immediate vicinity of Venus.

In addition to these scientific instruments, Mariner 2 had a data conditioning system (DCS) and a scientific power switching (SPS) unit. The DCS was a solid-state electronic system designed to gather information from the scientific instruments on board the spacecraft. It had four basic functions: analog-to-digital conversion, digital-to-digital conversion, sampling and instrument-calibration timing, and planetary acquisition. The SPS unit was designed to perform the following three functions: control of the application of [[alternating current|AC]] power to appropriate portions of the science subsystem, application of power to the radiometers and removal of power from the cruise experiments during radiometer calibration periods, and control of the speed and direction of the radiometer scans. The DCS sent signals to the SPS unit to perform the latter two functions.{{r|report}}

Not included on any of the Mariner R spacecraft was a camera for visual photos. With payload space at a premium, project scientists considered a camera an unneeded luxury, unable to return useful scientific results. [[Carl Sagan]], one of the Mariner R scientists, unsuccessfully fought for their inclusion, noting that not only might there be breaks in Venus' cloud layer, but "that cameras could also answer questions that we were way too dumb to even pose".{{r|space.com1}}