THE RADAR PROFILE: MEASUREMENTS FROM EARTH
In 1961, the Jet Propulsion Laboratory conducted a series of experiments from its Goldstone, California, DSIF Station, successfully bouncing radar signals off the planet Venus and receiving the return signal after it had travelled 70 million miles in 6½ minutes.
In order to complement the Mariner mission to Venus, the radar experiments were repeated from October to December, 1962 (during the Mariner mission), using improved equipment and refined techniques. As in 1961, the experiments were directed by W. K. Victor and R. Stevens.
The 1961 experiments used two 85-foot antennas, one transmitting 13 kilowatts of power at 2,388 megacycles, the other receiving the return signal after the round trip to and from Venus. The most important result was the refinement of the astronomical unit—the mean distance from the Earth to the Sun—to a value of 92,956,200 ±300 miles.
Around 1910, the astronomical unit, plotted by classical optical methods, was uncertain to 250,000 miles. Before the introduction of radar astronomy techniques such as those used at Goldstone, scientists believed that the astronomical unit was known to within 60,000 miles, but even this factor of uncertainty would be intolerable for planetary exploration.
In radar astronomy, the transit time of a radio signal moving at the speed of light (186,000 miles per second) is measured as it travels to a planet and back. In conjunction with the angular measurement techniques used by earlier investigators, this method permits a more precise calculation of the astronomical unit.
Optical and radar measurements of the astronomical unit differ by 50,000 miles. Further refinement of both techniques should lessen the discrepancy between the two values.
The 1961 tests also established that Venus rotates at a very slow rate, possibly keeping the same face toward the Sun at all times. The reflection coefficient of the planet was estimated at 12%, a bright value similar to that of the Earth and contrasted with the Moon’s 2%. The average dielectric constant (conductivity factor) of the surface material seemed to be close to that of sand or dust, and the surface was reported to be rough at a wavelength of 6 inches.
The surface roughness was confirmed in 1962. Since it is known that a rough surface will scatter a signal, the radar tests were observed for such indications. Venus had a scattering effect on the radar waves similar to the Moon’s, probably establishing the roughness of the Venusian surface as more or less similar to the lunar terrain.
Some of the most interesting work was done in reference to the rotation rate of Venus. A radar signal will spread in frequency on return from a target planet that is rotating and rough enough to reflect from a considerable area of its surface. The spread of 5 to 10 cycles per second noted on the Venus echo would suggest a very slow rotation rate, perhaps keeping the same face toward the Sun, or possibly even in a retrograde direction, opposite to the Earth’s.
In the Goldstone 1962 experiments, Venus was in effect divided into observation zones and the doppler effect or change in the returned signal from these zones was studied. The rate of rotation was derived from three months of sampling with this radar mapping technique. Also, the clear, sharp tone characteristic of the transmitted radar signal was altered on return from Venus into a fuzzy, indistinct sound. This effect seemed to confirm the slow retrograde rotation (as compared with the Earth) indicated by the radar mapping and frequency change method.
In addition to these methods of deducing the slow rotation rate, two other phenomena seemed to verify it: a slowly fluctuating signal strength, and the apparent progression of a bright radar spot across from the center of Venus toward the outside edge.
As a result, JPL scientists revised their 1961 estimate of an equal Venusian day and year consisting of 225 Earth days. The new value for Venus’ rotation rate around its axis is 230 Earth days plus or minus 40 to 50 days, and in a retrograde direction (opposite to synchronous or Sun-facing), assuming that Venus rotates on an axis perpendicular to the plane of its orbit.
Thus, on Venus the Sun would appear to rise in the west and cross to the east about once each Venusian year. If the period were exactly 225 days retrograde, the stars would remain stationary in the sky and Venus would always face a given star rather than the Sun.
A space traveller hovering several million miles directly above the Sun would thus see Venus as almost stopped in its rotation and possibly turning very slowly clockwise. All the other planets of our system including the Earth, rotate counterclockwise, except Uranus, whose axis is almost parallel to the plane of its orbit, making it seem to roll around the Sun on its side. The rotation direction of distant Pluto is unknown.
The Goldstone experiments also studied what is known as the Faraday rotation of the plane of polarization of a radio wave. The results indicated that the ionization and magnetic field around Venus are very low. These data tend to confirm those gathered by Mariner’s experiments close to the planet.
The mass of Venus was another value that had never been precisely established. The mass of planetary bodies is determined by their gravitational effect on other bodies, such as satellites. Since Venus has no known natural satellite or moon, Mariner, approaching closely enough to “feel” its gravity, would provide the first opportunity for close measurement.
The distortion caused by Venus on Mariner’s trajectory as the spacecraft passed the planet enabled scientists to calculate the mass with an error probability of 0.015%. The value arrived at is 0.81485 of the mass of the Earth, which is known to be approximately 13.173 septillion (13,173,000,000,000,000,000,000,000) pounds. Thus, the mass of Venus is approximately 10.729 septillion (10,729,408,500,000,000,000,000,000) pounds.
In addition to these measurements, the extremely precise tracking system used on Mariner proved the feasibility of long-range tracking in space, particularly in radial velocity, which was measured to within 1/10 of an inch per second at a distance of about 54 million miles.
As the mission progressed, the trajectory was corrected with respect to Venus to within 10 miles at encounter. An interesting result was the very precise location of the Goldstone and overseas tracking stations of the DSIF. Before Mariner II, these locations were known to within 100 yards. After all the data have been analyzed, these locations will be redefined or “relocated” to within an error of only 20 yards.
Mariner not only made the first successful journey to Venus—it also helped pinpoint spots in the Californian and Australian deserts and the South African veldt with an accuracy never before achieved.
CHAPTER 10
THE NEW LOOK OF VENUS
The historic mission of Mariner II to the near-vicinity of Venus and beyond has enabled scientists to revise many of their concepts of interplanetary space and the planet Venus.
The composite picture, taken from the six experiments aboard the spacecraft and the data from the DSIF radar experiments of 1961 and 1962 revealed the following:
- Interplanetary space between the Earth and Venus, at least as it was during the four months of Mariner’s mission, had a cosmic dust density some ten-thousand times lower than the region immediately surrounding the Earth.
- During this period, the extremely tenuous, widely fluctuating solar winds streamed continually out from the Sun, at velocities ranging from 200 to 500 miles per second.
- An astronaut travelling through these regions in the last quarter of 1962 would not have been seriously affected by the cosmic and high-energy radiation from space and the Sun. He could easily have survived many times the amount of radiation detected by Mariner’s instruments.
- The astronomical unit, as determined by radar, the yardstick of our solar system, stands at 92,956,200, plus or minus 300 miles.
- The mass of Venus in relation to the Earth’s is 0.81485, with an error probability of 0.015%.
- The rotation rate of Venus is quite slow and is now estimated as equal to 230 Earth days, plus or minus 40 to 50 days. The rotation might be retrograde, clockwise with respect to a Sun-facing reference, with the Sun rising in the west and setting in the east approximately one Venusian year later. The planet seems to remain nearly star-fixed rather than permanently oriented with one face to the Sun.
- Venus has no magnetic field discernible at the 21,598-mile approach of Mariner II and at that altitude there were no regions of trapped high-energy particles or radiation belts, as there are near the Earth.
- The clouds of Venus are about 15 miles thick, extending from a base 45 miles above the surface to a top altitude of about 60 miles.
- At the resolution of the Mariner II infrared radiometer, there were no apparent breaks in the cloud cover. Cloud-top temperature readings are about minus 30 degrees F near the center (along the terminator), and ranging down to minus 60 degrees to minus 70 degrees F at the limbs, showing an apparent limb-darkening effect, which would indicate a hot surface and the absence of a supercharged ionosphere.
- A spot 20 degrees F colder than the surrounding area exists along the terminator in the southern hemisphere: a high mountain could exist in this region, but such an hypothesis is purely conjectural. A bright radar reflection is also found on the Equator in the same general region. Causes of these phenomena are not established.
- At their base, the clouds are about 200 degrees F and probably are comprised of condensed hydrocarbons held in oily suspension. Below the clouds, the atmosphere must be heavily charged with carbon dioxide, may contain slight traces of oxygen, and probably has a strong concentration of nitrogen.
- As determined by the microwave radiometer, Venus’ surface temperature averages approximately 800 degrees F on both light and dark sides of the planet. Some roughness is indicated and the surface reflectivity is equivalent to that of dust and sand. No water could be present at the surface but there is some possibility of small lakes of molten metal of one type or another.
- Some reddish sunlight, in the filterable infrared spectrum, may find its way through the 15-mile-thick cloud cover, but the surface is probably very bleak.
- The heavy, dense atmosphere creates a surface pressure of some twenty times that found on the Earth, or equal to about 600 inches of mercury.
The mission was completed and the spacecraft had gone into an endless orbit around the Sun. But before Mariner II lost its sing-song voice, it produced 13 million data words of computer space lyrics to accompany the music of the spheres.
APPENDIX
SUBCONTRACTORS
Thirty-four subcontractors to JPL provided instruments and other hardware for Mariners I and II.
The subcontractors were:
| Aeroflex Corporation Long Island City, New York | Jet vane actuators |
| American Electronics, Inc. Fullerton, California | Transformer-rectifiers for flight telecommunications |
| Ampex Corporation Instrumentation Division Redwood City, California | Tape recorders for ground telemetry and data handling equipment |
| Applied Development Corporation Monterey Park, California | Decommutators and teletype encoders for ground telemetry equipment |
| Astrodata, Inc. Anaheim, California | Time code translators, time code generators, and spacecraft signal simulators for ground telemetry equipment |
| Barnes Engineering Company Stamford, Connecticut | Infrared radiometers Planet simulator |
| Bell Aerospace Corporation Bell Aerosystems Division Cleveland, Ohio | Accelerometers and associated electronic modules |
| Computer Control Company, Inc. Framingham, Massachusetts | Data conditioning systems |
| Conax Corporation Buffalo, New York | Midcourse propulsion explosive valves Squibs |
| Consolidated Electrodynamics Corp. Pasadena, California | Oscillographs for data reduction |
| Consolidated Systems Corporation Monrovia, California | Scientific instruments Operational support equipment |
| Dynamics Instrumentation Company Monterey Park, California | Isolation amplifiers for telemetry Operational support equipment |
| Electric Storage Battery Company Missile Battery Division Raleigh, North Carolina | Spacecraft batteries |
| Electro-Optical Systems, Inc. Pasadena, California | Spacecraft power conversion equipment |
| Fargo Rubber Corporation Los Angeles, California | Midcourse propulsion fuel tank bladders |
| Glentronics, Inc. Glendora, California | Power supplies for data conditioning system |
| Groen Associates Sun Valley, California | Actuators for solar panels |
| Houston Fearless Corporation Torrance, California | Pin pullers |
| Kearfott Division General Precision, Inc. Los Angeles, California | Gyroscopes |
| Marshall Laboratories Torrance, California | Magnetometers and associated operational support equipment |
| Matrix Research and Development Corporation Nashua, New Hampshire | Power supplies for particle flux detectors |
| Menasco Manufacturing Company Burbank, California | Midcourse propulsion fuel tanks and nitrogen tanks |
| Midwestern Instruments Tulsa, Oklahoma | Oscillographs for data reduction |
| Mincom Division Minnesota Mining & Manufacturing Los Angeles, California | Tape recorders for ground telemetry and data handling equipment |
| Motorola, Inc. Military Electronics Division Scottsdale, Arizona | Spacecraft command subsystems, transponders, and associated operational support equipment |
| Nortronics Division of Northrop Corporation Palos Verdes Estates, California | Attitude control gyro electronic, autopilot electronic, and antenna servo electronic modules, long-range Earth sensors and Sun sensors |
| Ransom Research Division of Wyle Laboratories San Pedro, California | Verification and ground command modulation equipment |
| Rantec Corporation Calabasas, California | Transponder circulators and monitors |
| Ryan Aeronautical Company Aerospace Division San Diego, California | Solar panel structures |
| Spectrolab Division of Textron Electronics, Inc. North Hollywood, California | Solar cells and their installation and electrical connection on solar panels |
| State University of Iowa Iowa City, Iowa | Calibrated Geiger counters |
| Sterer Engineering & Manufacturing Company North Hollywood, California | Valves and regulators for midcourse propulsion and attitude control systems |
| Texas Instruments, Inc. Apparatus Division Dallas, Texas | Spacecraft data encoders and associated operational support equipment, ground telemetry demodulators |
| Trans-Sonic, Inc. Burlington, Massachusetts | Transducers |
In addition to these subcontractors, over 1,000 other industrial firms contributed to the Mariner Project.