THE SCIENTIFIC EXPERIMENTS
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.[2]
[Illustration: _Cosmic dust detector._]
[Illustration: _Solar plasma spectrometer._]
COLLECTOR CUP
PROGRAMMER
ELECTROMETER
DEFLECTION PLATES
[Illustration: _Magnetometer._]
[Illustration: _High-energy particle detector._]
COLLECTOR
SHIELD CAN
QUARTZ FIBER
[Illustration: _Microwave and infrared radiometers._]
REFERENCE HORNS
MICROWAVE RADIOMETER
INFRARED RADIOMETER
_Table 2. Mariner Experiments_
_Experiment_ _Description_ _Experimenters_
Microwave radiometer Determine the Dr. A. H. Barrett,
temperature of the Massachusetts
planet surface and Institute of
details concerning Technology; D. E.
its atmosphere Jones, JPL; Dr. J.
Copeland, Army
Ordnance Missile
Command and
Ewen-Knight Corp.;
Dr. A. E. Lilley,
Harvard College
Observatory
Infrared radiometer Determine the Dr. L. D. Kaplan, JPL
structure of the and University of
cloud layer and Nevada; Dr. G.
temperature Neugebauer, JPL; Dr.
distributions at C. Sagan, University
cloud altitudes of California,
Berkeley, and Harvard
College Observatory
Magnetometer Measure planetary and P. J. Coleman, NASA;
interplanetary Dr. L. Davis,
magnetic fields Caltech; Dr. E. J.
Smith, JPL; Dr. C. P.
Sonett, NASA
Ion chamber and Measure high-energy Dr. H. R. Anderson,
matched cosmic radiation JPL; Dr. H. V. Neher,
Geiger-Mueller tubes Caltech
Anton special-purpose Measure lower Dr. J. Van Allen and
tube radiation (especially L. Frank, State
near Venus) University of Iowa
Cosmic dust detector Measure the flux of W. M. Alexander,
cosmic dust Goddard Space Flight
Center, NASA
Solar plasma Measure the intensity M. Neugebauer and Dr.
spectrometer of low-energy C. W. Snyder, JPL
positively charged
particles from the Sun
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 SYSTEM
Mariner’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 no
engineering data transmission. In this mode, the science data were
sampled during 20-second intervals.
COSMIC DUST DETECTOR
The 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 EXPERIMENT
In 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.
Pointed toward the Sun, the gold-plated magnesium deflector plates
gathered particles from space. Since the walls of the tunnel each
carried different electrical charges, only particles with just the
correct energy and speed could pass through and be detected by the
collector cup without striking the charged walls. A sensitive
electrometer circuit then measured the current generated by the flow of
the charged particles reaching the cup.
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 EXPERIMENT
Mariner 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 an electrical pulse which was amplified and transmitted to the
Earth. The rod was then recharged and the fibre returned to its original
position.
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 MAGNETOMETER
Mariner 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 RADIOMETER
A 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 RADIOMETER
The 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 OBJECTIVES
Equipped 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:
1. The investigation of interplanetary space between the Earth and
Venus, measuring such phenomena as the cosmic dust, the
mysterious plasma or solar winds, high-energy cosmic rays from
space outside our solar system, charged particles from the
Sun, and the magnetic fields of space.
2. The experiments to be performed near Venus (at about 21,150 miles
out from the surface) in an effort to understand its magnetic
fields, radiation belts, the temperature and composition of
its clouds, and the temperature and conditions on the surface
of the planet.