Mariner 2
Mariner 2, an American space probe to Venus, was the first robotic space probe to report successfully from a planetary encounter. The first successful spacecraft in the NASA Mariner program, it was a simplified version of the Block I spacecraft of the Ranger program and an exact copy of Mariner 1. The missions of the Mariner 1 and 2 spacecraft are sometimes known as the Mariner R missions. Original plans called for the probes to be launched on the Atlas-Centaur, but serious developmental problems with that vehicle forced a switch to the much smaller Agena B second stage. As such, the design of the Mariner R vehicles was greatly simplified. Far less instrumentation was carried than on the Soviet Venera probes of this period—for example, forgoing a TV camera—as the Atlas-Agena B had only half as much lift capacity as the Soviet 8K78 booster. The Mariner 2 spacecraft was launched from Cape Canaveral on August 27, 1962, and passed as close as to Venus on December 14, 1962.
The Mariner probe consisted of a diameter hexagonal bus, to which solar panels, instrument booms, and antennas were attached. The scientific instruments on board the Mariner spacecraft were: two radiometers, a micrometeorite sensor, a solar plasma sensor, a charged particle sensor, and a magnetometer. These instruments were designed to measure the temperature distribution on the surface of Venus and to make basic measurements of Venus's atmosphere.
The primary mission was to receive communications from the spacecraft in the vicinity of Venus and to perform radiometric temperature measurements of the planet. A second objective was to measure the interplanetary magnetic field and charged particle environment.
En route to Venus, Mariner 2 measured the solar wind, a constant stream of charged particles flowing outwards from the Sun, confirming the measurements by Luna 1 in 1959. It also measured interplanetary dust, which turned out to be scarcer than predicted. In addition, Mariner 2 detected high-energy charged particles coming from the Sun, including several brief solar flares, as well as cosmic rays from outside the Solar System. As it flew by Venus on December 14, 1962, Mariner 2 scanned the planet with its pair of radiometers, revealing that Venus has cool clouds and an extremely hot surface.
Background
With the advent of the Cold War, the two then-superpowers, the United States and the Soviet Union, both initiated ambitious space programs with the intent of demonstrating military, technological, and political dominance. The Soviets launched the Sputnik 1, the first Earth orbiting satellite, on October 4, 1957. The Americans followed suit with Explorer 1 on February 1, 1958, by which point the Soviets had already launched the first orbiting animal, Laika in Sputnik 2. Earth's orbit having been reached, focus turned to being the first to the Moon. The Pioneer program of satellites consisted of three unsuccessful lunar attempts in 1958. In early 1959, the Soviet Luna 1 was the first probe to fly by the Moon, followed by Luna 2, the first artificial object to impact the Moon.With the Moon achieved, the superpowers turned their eyes to the planets. As the closest planet to Earth, Venus presented an appealing interplanetary spaceflight target. Every 19 months, Venus and the Earth reach relative positions in their orbits around the Sun such that a minimum of fuel is required to travel from one planet to the other via a Hohmann Transfer Orbit. These opportunities mark the best time to launch exploratory spacecraft, requiring the least fuel to make the trip.
The first such opportunity of the Space Race occurred in late 1957, before either superpower had the technology to take advantage of it. The second opportunity, around June 1959, lay just within the edge of technological feasibility, and U.S. Air Force contractor Space Technology Laboratory intended to take advantage of it. A plan drafted January 1959 involved two spacecraft evolved from the first Pioneer probes, one to be launched via Thor-Able rocket, the other via the yet-untested Atlas-Able. STL was unable to complete the probes before June, and the launch window was missed. The Thor-Able probe was repurposed as the deep space explorer Pioneer 5, which was launched March 11, 1960, and designed to maintain communications with Earth up to a distance of as it traveled toward the orbit of Venus. No American missions were sent during the early 1961 opportunity. The Soviet Union launched Venera 1 on February 12, 1961, and on May 19–20 became the first probe to fly by Venus; however, it had stopped transmitting on February 26.
For the summer 1962 launch opportunity, NASA contracted Jet Propulsion Laboratory in July 1960 to develop "Mariner A", a spacecraft to be launched using the yet undeveloped Atlas-Centaur. By August 1961, it had become clear that the Centaur would not be ready in time. JPL proposed to NASA that the mission might be accomplished with a lighter spacecraft using the less powerful but operational Atlas-Agena. A hybrid of Mariner A and JPL's Block 1 Ranger lunar explorer, already under development, was suggested. NASA accepted the proposal, and JPL began an 11-month crash program to develop "Mariner R". Mariner 1 would be the first Mariner R to be launched followed by Mariner 2.
Spacecraft
Three Mariner R spacecraft were built: two for launching and one to run tests, which was also to be used as a spare. Aside from its scientific capabilities, Mariner also had to transmit data back to Earth from a distance of more than, and to survive solar radiation twice as intense as that encountered in Earth orbit.Structure
All three of the Mariner R spacecraft, including Mariner 2, weighed within of the design weight of, of which was devoted to non-experimental systems: maneuvering systems, fuel, and communications equipment for receiving commands and transmitting data. Once fully deployed in space, with its two solar panel "wings" extended, Mariner R was in height and across. The main body of the craft was hexagonal with six separate cases of electronic and electromechanical equipment:- Two of the cases comprised the power system: switchgear that regulated and transmitted power from the 9800 solar cells to the rechargeable 1000 watt silver-zinc storage battery.
- Two more included the radio receiver, the three-watt transmitter, and control systems for Mariner's experiments.
- The fifth case held electronics for digitizing the analog data received by the experiments for transmission.
- The sixth case carried the three gyroscopes that determined Mariner's orientation in space. It also held the central computer and sequencer, the "brain" of the spacecraft that coordinated all of its activities pursuant to code in its memory banks and on a schedule maintained by an electronic clock tuned into equipment on Earth.
The primary high-gain parabolic antenna was also mounted on the underside of Mariner and kept pointed toward the Earth. An omnidirectional antenna atop the spacecraft would broadcast at times that the spacecraft was rolling or tumbling out of its proper orientation, to maintain contact with Earth; as an unfocused antenna, its signal would be much weaker than the primary. Mariner also mounted small antennas on each of the wings to receive commands from ground stations.
Temperature control was both passive, involving insulated, and highly reflective components; and active, incorporating louvers to protect the case carrying the onboard computer. At the time the first Mariners were built, no test chamber existed to simulate the near-Venus solar environment, so the efficacy of these cooling techniques could not be tested until the live mission.
Scientific instruments
Background
At the time of the Mariner project's inception, few of Venus's characteristics were definitely known. Its opaque atmosphere precluded 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 hypothesis that the planet was tidally locked with respect to the Sun. No oxygen had been detected in Venus's atmosphere, suggesting that life as existed on Earth was not present. It had been determined that Venus's 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, but this had not yet been conclusively determined.The Mariner spacecraft would be able to verify this hypothesis by measuring the temperature of Venus close-up; at the same time, the spacecraft could determine if there was a significant disparity between night and daytime temperatures. 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.
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.
Due to the limited capacity of the Atlas-Agena, only of the spacecraft could be allocated to scientific experiments.
Instruments
- A two-channel microwave radiometer of the crystal video type operating in the standard Dicke mode of chopping between the main antenna, pointed at the target, and a reference horn pointed at cold space. It was used to determine the absolute temperature of Venus's 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 mm and 19 mm. The total weight of the radiometer was. Its average power consumption was 4 watts and its peak power consumption 9 watts.
- 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 μm and 10 to 10.8 μm. The latter corresponding to the carbon dioxide band. The total weight of the infrared radiometer, which was housed in a magnesium casting, was, and it required 2.4 watts of power. It was designed to measure radiation temperatures between approximately.
- A three-axis fluxgate magnetometer to measure planetary and interplanetary magnetic fields. 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 seconds. It had three analog outputs that had each two sensitivity scales: ± 64 γ and ± 320 γ. 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.
- An ionization chamber with matched Geiger-Müller tubes to measure high-energy cosmic radiation.
- A particle detector to measure lower radiation, also known as the Iowa detector, as it was provided by the University of Iowa. It was a miniature tube having a 1.2 mg/cm2 mica window about in diameter and weighing about. It detected soft x-rays efficiently and ultraviolet inefficiently, and was previously used in Injun 1, Explorer 12 and Explorer 14. It was able to detect protons above 500 keV in energy and electrons above 35 keV. The length of the basic telemetry frame was 887.04 seconds. During each frame, the counting rate of the detector was sampled twice at intervals separated by 37 seconds. The first sampling was the number of counts during an interval of 9.60 seconds ; the second was the number of counts during an interval of 0.827 seconds. 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.
- A cosmic dust detector to measure the flux of cosmic dust particles in space.
- A solar plasma spectrometer to measure the spectrum of low-energy positively charged particles from the Sun, i.e. the solar wind.
In addition to these scientific instruments, Mariner 2 had a data conditioning system and a scientific power switching 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 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.
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's cloud layer, but "that cameras could also answer questions that we were way too dumb to even pose".