Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT/JPLNEWS1/0190.PR

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OFFICE OF PUBLIC EDUCATION AND INFORMATION
CALIFORNIA INSTITUTE OF TECHNOLOGY JET PROPULSION LABORATORY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIFORNIA.  TELEPHONE MURRAY 1-3661, EXTENSION 3111

FOR RELEASE:  A.M.'s of July 19, 1962


                  MARINER SCIENTIFIC EXPERIMENTS
         The Mariner spacecraft contains six scientific experi-
ments 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 Geiger-
Mueller 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

SCIENTIFIC EXPERIMENTS         -2-

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 electromag-
netic 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,

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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 condi-
tioning system (DCS), which is located in one of the modules in
the hexagonal base of the spacecraft.  The DSC prepares informa-
tion from the experiments for transmission to earth in the form

SCIENTIFIC EXPERIMENTS         -4-

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.

THE EXPERIMENTS:
                       MICROWAVE RADIOMETER
         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 radia-
tion at two wave lengths, 13.5 and 19 millimeters.  In the elec-
tromagnetic spectrum 13.5mm is the location of a microwave water

SCIENTIFIC EXPERIMENTS         -5-

absorption band.  If there is water vapor above certain minimal
concentration in the atmosphere it will be possible to detect it.
         The 19mm wave length, 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 wave lengths.
         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 origi-
nate from the surface.  In this case a limb-to-limb scan would
show a gradual increase and decrease of temperature readings.

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         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

SCIENTIFIC EXPERIMENTS         -6-

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 para-
bolic 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

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opposite the face of the antenna on a quadripod.  The energy from
both wave lengths 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 calibra-
tion.  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.

                       INFRARED RADIOMETER
         This is a companion experiment to the microwave
radiometer.  As the Mariner spacecraft flies past Venus
simultaneous measurements from the two experiments will enable

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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 wave length regions of the electromagetic
spectrum.
         Measurements from earth in these two wave lengths
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 eminates 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

SCIENTIFIC EXPERIMENTS         -9-

wave lengths 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 compari-
son 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

SCIENTIFIC EXPERIMENTS         -10-

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.

                           MAGNETOMETER
         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

SCIENTIFIC EXPERIMENTS         -11-

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

SCIENTIFIC EXPERIMENTS         -12-

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

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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 inter-
planetary 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

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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

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(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.

SCIENTIFIC EXPERIMENTS         -16-

                      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 interractions 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

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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.

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         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.

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         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.













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