  After a year of concentrated effort, in which the resources of NASA, the Jet Propulsion Laboratory, and American science and industry had been marshalled, Mariner II had probed secrets of the solar system some billions of years old. Scientists and engineers had studied the miles of data processed in California from the tapes recorded at the five DSIF tracking stations around the world. Two and a half months of careful analysis and evaluation yielded a revised estimate of Venus and of the phenomena of space. As a result, the dynamics of the solar system were revealed in better perspective and the shrouded planet stood partially unmasked. When the Mariner data were correlated with the data gathered by JPL radar experiments at Goldstone in 1961 and 1962, the relationships between the Earth, Venus, and the Sun became far clearer than ever before. Two experiments were carried on the spacecraft for a close-up investigation of Venus’ atmosphere and temperature characteristics—a microwave radiometer and an infrared radiometer. They were designed to operate during the approximate 35-minute encounter period and at a distance varying from about 10,200 miles to 49,200 miles from the center of the planet. Cosmic dust detector. Solar plasma spectrometer.
Magnetometer. High-energy particle detector.
Microwave and infrared radiometers.
Table 2. Mariner Experiments
Four experiments for investigation of interplanetary space and the regions near Venus employed: a magnetometer; high-energy charged particle detectors, including an ionization chamber and Geiger-Mueller radiation counters; a cosmic dust detector; and a solar plasma detector. These six scientific experiments represented the cooperative efforts of scientists at nine institutions: The Army Ordnance Missile Command, the Ewen-Knight Corp., the California Institute of Technology, the Goddard Space Flight Center of NASA, Harvard College Observatory, the Jet Propulsion Laboratory, the Massachusetts Institute of Technology, the State Universities of Iowa and Nevada, and the University of California at Berkeley. Table 2 lists the experiments, the experimenters, and their affiliations. At the Jet Propulsion Laboratory, the integration of the scientific experiments and the generation of a number of them were carried out under the direction of Dr. Manfred Eimer. R. C. Wyckoff was the project scientist and J. S. Martin was responsible for the engineering of the scientific experiments. DATA CONDITIONING SYSTEMMariner’s scientific experiments were controlled and their outputs processed by a data conditioning system which gathered the information from the instruments and prepared it for transmission to the Earth by telemetry. In this function, the data system acted as a buffer between the science systems and the spacecraft data encoder. The pulse output of certain of the science instruments was counted and the voltage amplitude representations of other instruments were converted from analog form to a binary digital equivalent of the information signals. The data conditioning system also included circuits to permit time-sharing of the telemetry channels with the spacecraft engineering data, generation of periodic calibration signals for the radiometer and magnetometer, and control of the direction and speed of the radiometer scanning cycle. During Mariner’s cruise mode, the data conditioning system was used for processing both engineering and science data. If the spacecraft lost lock on the Sun or the Earth during the cruise mode, no scientific data would be telemetered during the reorientation period. Engineering data were sampled and transmitted for about 17 seconds during every 37-second interval. The planetary encounter mode involved only science and COSMIC DUST DETECTORThe cosmic dust detector on Mariner II was designed to measure the flux density, direction, and momentum of interplanetary dust particles between the Earth and Venus. These measurements were concerned with the particles’ direction and distance from the Sun, the momentum with respect to the spacecraft, the nature of any concentrations of the dust in streams, variations in cosmic dust flux with distance from the Earth and Venus, and the possible effects on manned flight. Mariner’s cosmic dust instrument could detect a particle as small as something like a billionth of a gram, or about five-trillionths of a pound. This type of sensor had been used on rockets even before Explorer I. It had yielded good results on Pioneer I in the region between the Earth and the Moon. The instrument was a 55-square-inch acoustical detector plate, or sounding board, made of magnesium. A crystal microphone was attached to the center of the plate. The instrument could detect both low- and high-momentum particles and also provide a rough idea of their direction of travel. The dust particle counters were read once each 37 seconds during the cruise mode. This rate was increased to once each 20 seconds during the encounter with Venus. The instrument was attached to the top of the basic hexagonal structure; it weighed 1.85 pounds, and consumed only 0.8 watt of power. SOLAR PLASMA EXPERIMENTIn order to investigate the phenomena associated with the movement of plasma (charged particles of low energy and density streaming out from the Sun to form the so-called “solar wind”) in interplanetary space, Mariner carried a solar plasma spectrometer that measured the flux and energy spectrum of positively charged plasma components with energies in the range 240 to 8400 volts. The extremely sensitive plasma detector unit was open to space, consumed 1 watt of power, and consisted of four basic elements: curved electrostatic deflection plates and collector cup, electrometer, a sweep amplifier, and a programmer. The curved deflector plates formed a tunnel that projected from the chassis on the spacecraft hexagon in which the instrument was housed. The deflection plates were supplied by amplifier-generated voltages which were varied in 10 steps, each lasting about 18 seconds, allowing the instrument to measure protons with energies in the 240 to 8,400 electron volt range. The programmer switched in the proper voltage and resistances. HIGH-ENERGY RADIATION EXPERIMENTMariner carried an experiment to measure high-energy radiation in space and near Venus. The charged particles measured by Mariner were primarily cosmic rays (protons or the nuclei of hydrogen atoms), alpha particles (nuclei of helium atoms), the nuclei of other heavier atoms, and electrons. The study of these particles in space and those which might be trapped near Venus was undertaken in the hope of a better understanding of the dynamics of the solar system and the potential hazards to manned flight. The high-energy radiation experiment consisted of an ionization chamber and detectors measuring particle flux (velocity times density), all mounted in a box measuring 6 × 6 × 2 inches and weighing just under 3 pounds. The box was attached halfway up the spacecraft superstructure in order to isolate the instruments as much as possible from secondary emission particles produced when the spacecraft was struck by cosmic rays, and to prevent the spacecraft from blocking high-energy radiation from space. The ionization chamber had a stainless steel shell 5 inches in diameter, with walls only 1/100-inch thick. The chamber was filled with argon gas into which was projected a quartz fibre next to a quartz rod. A charged particle entering the chamber would leave a wake of ions in the argon gas. Negative ions accumulated on the rod, reducing the potential between the rod and the spherical shell, eventually causing the quartz fibre to touch the rod. This action discharged the rod, producing In order to penetrate the walls of the chamber, protons required an energy of 10 million electron volts (Mev), electrons needed 0.5 Mev, and alpha particles 40 Mev. The particle flux detector incorporated three Geiger-Mueller tubes, two of which formed a companion experiment to the ionization chamber; each generated a current pulse whenever a charged particle was detected. One tube was shielded by an 8/1,000-inch-thick stainless steel sleeve, the other by a 24/1,000-inch-thick electron-stopping beryllium shield. Thus, the proportion of particles could be determined. The third Geiger-Mueller tube was an end-window Anton-type sensor with a mica window that admitted protons with energies greater than 0.5 Mev and electrons, 40,000 electron volts. A magnesium shield around the rest of the tube enabled the instrument to determine the direction of particles penetrating only the window. The three Geiger-Mueller tubes protruded from the box on the superstructure of the spacecraft. The end-window tube was inclined 20 degrees from the others and 70 degrees from the spacecraft-Sun line since it had to be shielded from direct solar exposure. THE MAGNETOMETERMariner carried a magnetometer to measure the magnetic field in interplanetary space and in the vicinity of Venus. Lower sensitivity limit of the instrument was about 5 gamma. A gamma is a unit of magnetic measurement and is equal to 10?5 or 1/100,000 oersted, or 1/30,000 of the Earth’s magnetic field at the equator. The nails in one of your shoes would probably produce a field of about 1 gamma at a distance of approximately 4 feet. Housed in a 6- × 3-inch metal cylinder, the instrument consisted of three magnetic core sensors, each aligned on a different axis to read the three magnetic field components and having primary and secondary windings. The presence of a magnetic field altered the current in the secondary winding in proportion to the strength of the field encountered. The magnetometer was attached near the top of the superstructure, just below the omni-antenna, in order to remove it as far as possible from any spacecraft components having magnetic fields of their own. An auxiliary coil was wound around each of the instrument’s magnetic sensor cores to compensate for permanent magnetic fields existing in the spacecraft itself. These spacecraft fields were measured at the magnetometer before launch and, in flight, the auxiliary coils carried currents of sufficient strength to cancel out the spacecraft’s magnetic fields. The magnetometer reported almost continuously on its journey and for 20 days after encounter. During the encounter, observations were made each 20 seconds on each of the three components of the magnetic field. MICROWAVE RADIOMETERA microwave radiometer on board Mariner II was designed to scan Venus during encounter at two wavelengths: 13.5 and 19 millimeters. The radiometer was intended to help settle some of the controversies about the origin of the apparently high surface temperature emanating from Venus, and the value of the surface temperature. The equipment included a 19-inch-diameter parabolic antenna mounted above the basic hexagonal structure on a swivel driven in a 120-degree scanning motion by a motor. The radiometer electronics circuits were housed behind the antenna dish. The antenna was equipped with a diplexer, which allowed it to receive both wavelengths at once without interference, and to compare the signals emanating from the two reference horns with those from the planet. The reference horns were pointed away from the main antenna beam so they would look into deep space as Mariner passed Venus. This feature allowed the antenna to “bring in” a reference temperature of approximately absolute zero during encounter. The microwave radiometer was to be turned on 10 hours before the encounter began. An electric motor was then to start a scanning or “nodding” motion of 120 degrees at the rate of 1 degree per second. Upon radiometer contact with the planet, this scanning rate would be reduced to 1/10 degree per second as long as the planetary disk was scanned. A special command system in the data conditioning system would reverse or normalize the direction of scan as the radiometer reached the edge or limb of the planet. The signals from the antenna and the reference horns were to be processed and the data handled in a receiver, located behind the antenna, which measured the difference between the signals from Venus and the reference signals from space. The information was then to be telemetered to the Earth. The microwave radiometer was automatically calibrated twenty-three times during the mission by a sequence originating in the data conditioning system, so that the correct functioning of the instrument could be determined before the encounter with Venus. INFRARED RADIOMETERThe infrared radiometer was a companion experiment to the microwave instrument and was rigidly mounted to the microwave antenna so that both radiometers would look at the same area of Venus with the same scanning rate. The instrument detected radiation in the 8 to 9 and 10 to 10.8 micron regions of the infrared spectrum. The infrared radiometer had two optical sensors. As the energy entered the system, it was “chopped” by a rotating disk, alternately passing or comparing emissions from Venus and from empty space. The beam was then split by a filter into the two wavelength regions. The output was then detected, processed, and transmitted to the Earth. The infrared radiometer measured 6 inches by 2 inches, weighed 2.7 pounds, and consumed 2 watts of power. The instrument was equipped with a calibration plate which was mounted on a superstructure truss adjacent to the radiometer. MARINER’S SCIENTIFIC OBJECTIVESEquipped with these instruments and with the mechanism for getting the measurements back to Earth, Mariner II was prepared to look for the answers to some of the questions inherent in its over-all mission objectives:
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