The Mariner spacecraft contains six scientific experiments representing the efforts of scientists at nine institutions: The Army Ordnance Missile Command, the California Institute of Technology, the Goddard Space Flight Center, Harvard College Observatory, the Jet Propulsion Laboratory, the Massachusetts Institute of Technology, the State University of Iowa, the State University of Nevada, and the University of California at Berkeley.
The two planetary experiments are a microwave radiometer and an infrared radiometer. They will operate during a period of about 30 minutes from a distance of approximately 16,000 miles as Mariner approaches Venus. The closest approach of Mariner to Venus will be about 10,000 miles. These radiometers will obtain information about the planet's temperature and atmosphere.
The other experiments will make scientific measurements during the cruise through interplanetary space and in the near vicinity of Venus. They are a magnetometer, energetic-particle detectors, including an ionization chamber and several GeigerMueller counters; a cosmic dust detector; and a solar plasma detector.
One of the important considerations in choosing these experiments was the compromise between what scientists would like to measure during the mission, and what was technologically possible. For example, of the 446 pounds that could be placed in a trajectory to intercept Venus, only about 40 pounds could be allocated to scientific experiments.
Another restricting factor is time. Venus is in a favorable position for investigation by a Mariner-type spacecraft only during a few weeks period every 19 months.
In addition, scientists will ask Mariner to convert electrical power from the sunlight, report its findings from as far as 36 million miles, and, though sensitive and unattended, remain in precise working order for three to five months in the void of space.
Although Venus is our closest planetary neighbor there are many things about it that remain a mystery. Its surface is continually hidden under a mask of dense clouds impenetrable in the small region of the electromagnetic spectrum visible to the eye. Spectrographic observations (identification of materials according to the manner in which they absorb and emit light) suggest that the atmosphere of Venus contains carbon dioxide, but has probably little free oxygen or water vapor.
Earth-based temperature measurements have been made of Venus in the microwave and infrared regions of the electromagnetic spectrum. The former indicates near surface temperatures of about 615 degrees Fahrenheit, while the latter shows readings of minus 38 degrees Fahrenheit in the upper atmosphere. Because of the tremendous distances over which these measurements were made scientists cannot be sure of the exact altitude in the atmosphere where these temperature readings apply.
As a result of the fragmentary information about Venus, several theories have been proposed that attempt to explain the nature of the atmosphere and the reason for the wide range of temperatures measured.
Some scientists believe that because of the carbon dioxide in the atmosphere a "greenhouse" effect is created that holds most of the heat absorbed from the sun beneath the thick blanket of clouds. This theory relies on the assumption that water vapor is present in the atmosphere of Venus.
Other scientists say that Venus has an ionosphere with an electron density thousands of times that of the earth. If this is the case this layer of electrons could easily mislead scientists measuring temperatures of Venus from earth.
Another theory states that Venus is heated by friction produced by high winds and dust clouds.
There are still other theories that describe Venus as a swamp, a desert covered with oil and smog, and containing carbonated water.
One of the missions of the Mariner spacecraft will be to make several scientific measurements of the planet which may substantiate one of these theories, or call for the formulation of a new one.
During the cruise and encounter of Venus, the Mariner will be telemetering information to earth. As the sensors of the six experiments receive information they feed it to a data conditioning system (DCS), which is located in one of the modules in the hexagonal base of the spacecraft. The DSC prepares information from the experiments for transmission to earth in the form of a digital code.
Since all of the data collected by Mariner cannot be transmitted at the same time, an electronic clock has been built into the DCS. This clock controls the equipment so that the receiver "listens" to one experiment at a time for about one second. After 20.16 seconds the DCS switches off the scientific telemetry and starts to send spacecraft engineering data for 16.8 seconds. This cycle is continued during the cruise in interplanetary space.
Beginning at ten hours before it passes Venus, however, the spacecraft devotes its telemetry system to the full-time transmission of scientific information from its six experiments.
The integration of the scientific experiments and the generation of a number of the experiments was carried out at JPL under the direction of Dr. M. Eimer. JPL project scientist was R. C. Wycoff and J. S. Martin was responsible for the engineering of scientific experiments.
This experiment should help to resolve two vital questions about Venus: what is the atmosphere like, and what is the temperature of the surface.
As the Mariner spacecraft flies past Venus, the microwave radiometer will scan its surface to detect electromagnetic radiation at two wave lengths, 13.5 and 19 millimeters. In the electromagnetic spectrum 13.5mm is the location of a microwave water absorption band. If there is water vapor above certain minimal concentration in the atmosphere it will be possible to detect it.
The 19mm wavelength, however, is not affected by water vapor, and should be capable of "seeing" through the atmosphere to the surface.
Scientists studying the results of this experiment will be able to determine whether water vapor exists in the Venusian atmosphere by noting the difference in temperatures obtained from measurements at the two wavelengths.
The 19mm wave length, in addition to measuring the surface temperature may be able to test two of the theories about the atmosphere of Venus by detecting one of two conditions called "limb brightening" or "limb darkening."
The former effect may be detected if the apparent high temperatures are due to a dense ionosphere. As the microwave radiometer scans the planet it would detect larger concentrations of electrons around the limb, or edge, of the planetary disk. This is somewhat analogous to looking at the earth from thousands of miles out in space on a day when it was completely covered with a fine mist. The mist would be more evident at the limbs than in the center, since the observer would be looking through a thicker layer concentration of mist at the limbs. In much the same way, the microwave radiometer would detect effects of greater intensity around the limb of Venus. On the other hand, limb darkening would indicate that the high temperatures originate from the surface. In this case a limb-to-limb scan would show a gradual increase and decrease of temperature readings.
The microwave radiometer is mounted on the hexagonal base of the Mariner. Both wave lengths are detected by a parabolic antenna that is 20 inches in diameter and three inches deep.
At ten hours prior to Venus encounter the radiometer is turned on. Driven by an electric motor it stha the high temperatures originate surface. In this case a limb-to-limb scan would show a gradual increase and decrease of temperature readings.
The microwave radiometer is mounted on the hexagonal base of the Mariner. Both wave lengths are detected by a parabolic antenna that is 20 inches in diameter and three inches deep.
At ten hours prior to Venus encounter the radiometer is turned on. Driven by an electric motor it starts a scanning or nodding motion of 120 degrees at any rate of one degree per second. When its signals determine that it has acquired the planet the DCS sends a command to slow the scan rate to 1/10 of a degree per second.
In order to confine its attention to the planet's disk, a special command system has been built into the DCS. Whenever the radiometer indicates that it has reached the limb and is about to look out into space, the DCS reverses the direction of the scan.
In this mode it scans Venus for about 30 minutes. Since the spacecraft will be going roughly in the direction of the sun, the radiometer will first scan part of the dark side of Venus and then part of the sunlit side.
The microwave antenna is only capable of moving in a nodding motion. Lateral movement is provided by the motion of the spacecraft across the face of the planet.
As the radiated microwave energy is collected by the parabolic antenna it is focused onto a receiving horn located opposite the face of the antenna on a quadripod. The energy from both wavelengths travel down two hollow legs of the quadripod called wave guides.
Located on top of the antenna are two reference horns that are matched to receive the same two microwave bands as the parabolic antenna. These horns point at an angle of 60 degrees away from the axis of the dish antenna, and consequently are always looking at empty space.
The signals from the dish antenna and the reference horns are alternated or chopped electronically. Then they are sent to a crystal video type receiver located behind the dish antenna. Thus, this receiver measures the difference between the signals from Venus and the reference signals from space.
This information is then telemetered to earth.
The microwave radiometer weighs 23.8 pounds and requires 3.5 watts of power when operating, and 8.9 watts during calibration. The calibration sequences are automatically initiated by the DCS a number of times during the mission.
Experimenters on the microwave radiometer are Dr. A. H. Barrett, Massachusetts Institute of Technology, Dr. J. Copeland, Army Ordnance Missile Command, D. E. Jones, Jet Propulsion Laboratory, and Dr. A. E. Lilley, Harvard College Observatory.
This is a companion experiment to the microwave radiometer. As the Mariner spacecraft flies past Venus simultaneous measurements from the two experiments will enable scientists to get a better idea of the temperature and atmospheric conditions of the planet.
The infrared radiometer is rigidly attached to the microwave antenna. In this way both scan the same surface areas of Venus.
The infrared experiment operates in the 8 to 9 and the 10 to 10.8 micron wavelength regions of the electromagetic spectrum.
Measurements from earth in these two wavelengths indicate temperatures below zero. It is not clear to scientists whether all of this radiation comes from the cloud tops, or whether some of it eliminates from the atmosphere or planetary surface.
The close approach of Mariner to Venus may enable scientists to measure some of the finer details of the atmosphere. This will primarily involve finding out if there are any "breaks" in the cloud cover of Venus, and if so, the amount of heat that escapes through them into space. For many years some astronomers have been able to see occasionally some kind of markings on Venus' cloud cover that change with no apparent regularity. The lack of regularity in these markings has left their nature in doubt.
If these markings are indeed cloud breaks, they will stand out with greater contrast in the infrared than if observed in the visible part of the spectrum. If the radiant energy detected by this experiment comes from the cloud top, and there are no breaks, then the temperatures obtained at both infrared wavelengths will follow a similar pattern.
If there are appreciable breaks in the clouds a substantial difference will be detected between measurements at the two wave lengths.
The reason for this is that in the 8 to 9 micron region the atmosphere is transparent, (except for clouds). In the 10 to 10.8 micron region, the low atmosphere is hidden by the presence of carbon dioxide. Through a cloud break the former would penetrate to a much lower point in the atmosphere. By a comparison of temperatures from both regions, combined with microwave data, scientists will have a more detailed picture of conditions of Venus.
The infrared radiometer is six inches long and two inches wide. It weighs 2.7 pounds and consumes two watts of power.
It contains two optical sensors, one of which scans the surface of Venus while the other obtains reference readings from space. The latter is aimed at an angle of 45 degrees away from the planetary scanner.
Radiation from Venus is collected by two f/2.4 optical systems with three inch focal lengths. As the infrared energy enters the optical system it first passes through a rotating disk with two apertures. These are positioned so that the two sensing devices can alternately see Venus and empty space. The infrared beam is chopped in this way at the rate of 20 cycles per second.
After the beam passes the disk, it is split by a dichroic filter into the two wave length regions. A second pair of filters further refines these wave lengths before they reach the radiometers sensing devices. The sensing devices are two thermistor bolometers, which are sensitive to infrared energy. The electrical output from these detectors is amplified and sent to the Mariner's DCS for processing and transmission to earth.
Experimenters on the infrared radiometer are Dr. L. D. Kaplan, and Dr. G. Neugebauer, of the Jet Propulsion Laboratory, and Dr. C. Sagan, of the University of California at Berkeley.
The magnetometer aboard Mariner is designed to measure the strength and direction of interplanetary and Venusian magnetic fields.
Many scientists believe that the magnetic field of a planet is due to a fluid motion in its interior. If such a Venusian field exists then it could be detected as Mariner approached the planet. This would depend, of course, on the strength of the field and the distance of Mariner at encounter. Also the trajectory of Mariner will permit the measurement of interplanetary magnetic fields and any variation with respect to time and distance from the sun.
Present-day theories of magnetohydrodynamics--the study of the relation between the motion of charged particles and the magnetic field which surrounds them--say that the plasma which flows away from the sun should drag with it the local solar magnetic field, since the motion of charged particles not only responds to but also creates magnetic fields. The mathematical description of this interaction between the stream of charged particles leaving the sun and the magnetic field which surrounds the sun is extremely complicated. The theories which have been used to describe these phenomena are incomplete and often contradictory.
The measurement of interplanetary magnetic fields by Mariner will be combined with simultaneous measurements from earth to help scientists understand something about the inter-relationships of these fields.
Moreover, by investigating the magnitude of any Venusian field it may be possible to draw some conclusion about the interior of the planet, as well as about planetary radiation belts, magnetic storms, and aurorae.
The magnetometer is a three axis fluxgate type. The sensors of the experiment are housed in a metal cylinder six inches long and three inches in diameter. It is located just below the Mariner's omnidirectional antenna. In this way the sensors are as far away as possible from any spacecraft components that may have magnetic fields associated with them.
Inside the cylinder are three magnetic cores, each aligned along a different axis. Each core has two windings of copper wire around it, much the same as some transformers. The primary winding leads from a frequency oscillator which produces a current. The secondary winding leads to an amplifier.
In the absence of a magnetic field the current induced in the secondary winding has a special symmetrical wave shape. The presence of a magnetic field changes the symmetry of this wave and produces a component with amplitude in proportion to the field strength. A third winding around the rods prevents magnetic interference from the spacecraft. This renders the three axes of the instrument sensitive to 1/2 gamma, or a field strength roughly 100,000 times weaker than that of the earth.
The magnetometer weighs 4.7 pounds and consumes six watts of power.
Experimenters are P. J. Coleman and Dr. C. P. Sonett of the National Aeronautics and Space Administration, and Dr. L. Davis and Dr. E. J. Smith of JPL.
HIGH ENERGY RADIATION EXPERIMENT
This experiment consists of an ionization chamber and a group of three Geiger-Mueller tubes. Together they will measure the number and intensity of energetic particles in interplanetary space and near Venus.
These particles are primarily cosmic rays, which are made up of protons (the nuclei of hydrogen atoms), alpha particles (the nuclei of helium atoms), the nuclei of heavier atoms, and electrons.
The measurement of these particles may contribute significantly to the knowledge of hazards to manned space flight.
Scientists have theorized that the sun has a pronounced effect on cosmic rays. During solar activity (sun spots or flares), for example, huge quantities of plasma race outward from the sun. These plasma clouds, or solar wind, carry along magnetic fields. In a rather complicated manner, not fully understood by scientists, the plasma's magnetic fields interact with those of the sun and planets. Scientists have noticed that following this solar activity, there is a considerable change in the character of the radiation that reaches the earth.
Unfortunately, because of atmosphere and magnetic field, we cannot measure all of these complicated inter-relationships from earth. We must take measurements from spacecraft traveling far from the earth. In this way we may learn something about the sun's influence on radiation.
A decrease in the number and intensity of cosmic radiation detected as we go closer to the sun would indicate that the sun's magnetic field is deflecting cosmic rays away from the solar system.
Thus by comparing the intensity of magnetic fields with the amount of cosmic radiation at earth, Venus, and in interplanetary space, some insight may be gained to these complicated inter-relationships.
The ionization chamber is of the Neher type. It consists of a five-inch-in-diameter stainless steel shell with a wall thickness of 1/100 of an inch. The sphere is filled with argon gas and is located on the superstructure of Mariner. Inside the sphere a quartz fibre is placed next to a quartz rod. Initially, both fibre and rod have the same electric potential.
As charged particles penetrate the wall of the sphere they leave behind a wake of ions in the argon gas. Negative ions accumulate on the rod, giving it a static electric charge. This causes the fibre to be attracted to the rod in proportion to the amount of the charge. Eventually, as the charge increases, the two touch. This produces an electric pulse which is amplified and sent to earth. The rod is recharged, and the fibre returns to its starting position.
In order to penetrate the wall of the ionization chamber, particles must have an energy greater than 10 million electron volts (Mev) for protons, 1/2 Mev for electrons, and 40 Mev for alpha particles.
This instrument measures the rate of ionization of cosmic rays.
Two of the GM tubes are considered companion instruments to the ionization chamber. They can be directly penetrated by particles above the same energy levels as the chamber, and can count these particles.
Both tubes consist of an enclosed volume of gas with two electrodes, at a different electrical potential. The wall of the tubes serve as the negative electrode and a thin central wire is the positive electrode. The tubes generate a current pulse each time a charged particle enters.
One of the GM tubes is shielded by a sleeve of glass and an 8/1000 of an inch thickness of stainless steel.
The second tube has a beryllium shield 24/1000 of an inch thick. Both tubes are 2.3 inches long and .6 of an inch in diameter. Because of the difference in shields, it will be possible for scientists to infer the ratio of electrons to other particles. These two GM counters along with the ionization chamber make it possible for scientists to measure the flux (velocity times the density) and the average amount of ionization of particles.
A third GM sensor is of the end window type. It measures the flux of particles not capable of penetrating the other detectors. The window is made of mica and admits protons with energies greater than one Mev, electrons over 40 thousand electronvolts.
A magnesium shield around the rest of the GM tube permits passage of protons over 20 Mev and electrons over 1 Mev. This gives the counter the ability to determine the approximate direction of particles which penetrate only the window.
The GM detectors are mounted on the superstructure of the spacecraft where they will be as far as possible from large masses that tend to produce secondary particles when struck by cosmic rays.
The three GM tubes protrude from a box that houses their electronic circuitry. The box is six inches wide, six inches long and two inches thick. The end window GM tube is inclined at an angle fo 20?o from the other two tubes.
The total weight of both experiments is 2.78 pounds and they consume 4/10 of a watt.
Experimenters are Dr. H. R. Anderson of JPL, Dr. H. V. Neher of Caltech, and Dr. James Van Allen of the State University of Iowa.
SOLAR PLASMA DETECTOR
The purpose of this experiment is to determine the flow and density of solar plasma and the energy of its particles.
Solar plasma is frequently called "solar wind" and consists of charged particles that are continually streaming outward from the sun. Since direct measurements such as the one on Mariner have been infrequent, scientists know very little about the solar plasma. Some feel that it is merely an extension of the sun's atmosphere, or corona. Although there are many theories, some conflicting, we do know that during solar activity (sun spots or flares) the flux of plasma increases.
One of the most complicated and interesting areas of space science is the study of how solar plasma interacts with the magnetic fields in space. Since the plasma carries an electrical charge, it not only is affected by magnetic fields, but also creates one of its own.
If a field is strong enough it may control and divert the solar winds, and, conversely, if the electrical energy in the plasma is great enough, the planetary magnetic fields may be trapped in the cloud and move with it through space.
Therefore, to study the complex interactions between solar wind and magnetic fields, space probes that carry plasma experiments generally carry magnetometers.
Most particle detectors are designed to operate inside a sealed tube and the tube walls keep out very low energy particles. The solar plasma detector on Mariner, however, is open to space and can collect and measure positively-charged particles of very low energy.
The sensor for this experiment is mounted on the outside of one of the electronic boxes in the base of the Mariner. The aperture of the analyzer is pointed along the roll axis of the spacecraft, and during most of the mission will be facing the sun.
As a charged particle enters the analyzer it finds itself in a curving tunnel. The two sides of this tunnel are metal plates carrying static electric charges--one negative, and the other positive. The charged particle is attracted by one plate and repelled by the other, and so follows a curved path down the curved tunnel. If it is moving too slowly or too rapidly, it runs into one wall or the other, but if it is moving at just the right speed, it passes to the end and is detected by a charge collecting cup. The electric current produced by the flow of charged particles is measured by a very sensitive electrometer circuit.
Thus, all the particles moving in the right direction to enter the tunnel and moving with the right speed to get all the way through will be detected.
Periodically the amount of voltage on the plates is changed and a different energy is required by the particles to get through to the collector cup. The voltage is automatically changed ten times. In this way it is possible to measure a spectrum of particle energies of 240 to 8400 electron volts.
The plasma detector has a total weight of 4.8 pounds a power requirement of 1 watt. Experiments are Dr. C. W. Snyder and M. Neugebauer of JPL.
COSMIC DUST DETECTOR
This experiment is designed to measure the flux and momentum of cosmic dust in interplanetary space and around Venus. It may contribute to an understanding of the hazards of manned flight through space.
This information will help scientists in understanding the history and evolution of the solar system.
There are many theories about these dust particles. One is that when the solar system was formed billions of years ago by the condensation of a huge cloud of gas and dust, these cosmic particles were debris left over, or they could be remnants of comets that rush through the solar system leaving a trail of dust behind. Some scientists believe cosmic dust has its origin in galactic space and is somehow trapped by the interaction of magnetic fields from the sun and planets.
Scientists have been trying to study cosmic dust with earth satellites and sounding rockets, but Mariner may provide the first data on its distribution in interplanetary space.
The experiment is located on the top of Mariner's hexagonal bus. It consists of a rectangular magnesium "sounding board" five inches wide and 10 inches long. A crystal microphone is located in the center of this plate. This acoustical device measures the impact of particles of cosmic dust.
As a particle hits the acoustical plate it is recorded by the microphone whose output excites a voltage-sensitive amplifier. The number of dust particles striking the plate is recorded on two counters, one for particles with high momentum and one for particles with low momentum.
During the cruise part of the trajectory the data conditioning system will read out the counters every 37 seconds and telemeter this to the ground. During planetary encounter the counting rate will be reduced to 20-second intervals.
The cosmic dust detector weighs 1.85 pounds and consumes Goddard Space Flight Center, Greenbelt, Maryland, under the direction of W. M. Alexander.