Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT/JPLNEWS1/0225.PR

Text File | 1993-04-21 | 9KB | 164 lines

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:  P.M.'s of Friday, December 28, 1962
                  MARINER RADIATION EXPERIMENTS

          Mariner II carried two experiments designed to measure
the charged-particle radiation in space, including galactic
cosmic rays and streams of high-energy particles which are
released intermittently from the sun.  Virtually continuous
measurements of the particle fluxes in space were made by the
instruments throughout the 109-day journey to Venus and during
the passage near the planet on December 14, and additional data
have been received for approximately 10 hours per day since that
time.
          One experiment, for observing the higher-energy
particles (protons above 10 million electron volts (Mev) and
electrons above 0.5 Mev in energy) was designed by Dr. H. R.
Anderson of the Jet Propulsion Laboratory and Dr. H. V. Neher of
the California Institute of Technology.  Somewhat lower-energy
particles (protons above 0.5 Mev or electrons above 0.04 Mev) are
detected by the experiment of L. A. Frank and Dr. J. A. Van Allen
of the State University of Iowa.  Preliminary results of the two
experiments were reported at the Stanford meeting by Dr. Anderson
and Frank, respectively.
          The instrumentation for the high-energy experiment
consisted of a large spherical ionization chamber and two matched
Geiger counters.  The ionization chamber, which was invented by

MARINER RADIATION EXPERIMENTS    -2-

Dr. Neher, has been widely used by him and by other investigators
for several years as a standard instrument for surveying the
absolute intensity of the cosmic rays.
          In addition to its use in almost countless balloon
flights, airplane flights, and ground-based experiments, this
type of chamber was also carried on the earth satellite Explorer
VI and on this country's only previous successful interplanetary
probe, Pioneer V.  The two Geiger counters are matched to count
the same kind of particles which are registered by the ionization
chamber.
          The detector for the lower-energy particles is a
cigarette-sized Geiger counter, the Anton 213, which was used in
several of the early Explorer and Pioneer satellites for
investigating the Van Allen radiation belts around the earth and
also in numerous more recent satellites.
          These experiments have three principal scientific
objectives, all of which were reported on at the Stanford meeting.
          Objective 1:  To detect, if possible, the presence of
magnetically-trapped particle belts about Venus.  For this
purpose, the Anton 213 counter was the most sensitive indicator.
At 20,000 miles from the earth it is known to have a counting
rate of several thousand per second, but during the closest
approach to Venus it detected an average count of only one
particle per second, in agreement with the rate observed during
most of the month of November.

MARINER RADIATION EXPERIMENTS    -3-

          The absence of additional particles near the planet was
confirmed also by the other radiation detectors.  Near the earth,
the number of trapped particles observed decreases very sharply
with distance near the boundary between the earth's magnetic
field and the interplanetary field.
          Thus the absence of particles near Venus indicates that
the planet's magnetic field does not extend as far out as the
trajectory of Mariner.  This fact was confirmed by the magneto-
meter on board.  The small intensity and extent of the field is
believed to be explained by the very slow rate of rotation of the
planet.
          Objective 2:  To measure the intensity of the galactic
cosmic rays far away from the perturbing effect of any planet,
and to look for variations in this intensity in different parts
of the solar system.  Years of earth-based research have shown
that the flux of relatively low-energy galactic cosmic rays (5000
Mev and below) have a systematic variation with a period of about
eleven years which is somehow connected with the solar activity
cycle (sunspot cycle).
          It is hoped that cosmic-ray measurements made simultane-
ously in widely separated parts of the solar system will elucidate
the nature of the mechanism responsible for this variation.  For
this purpose, the ionization chamber is best suited.  It measured
a rate of ionization near 670 ion pairs per cubic centimeter per
atmosphere of air.  The value did not change significantly during

MARINER RADIATION EXPERIMENTS    -4-

the flight, and furthermore is in agreement with measurements in
high-altitude balloons made last summer at Thule, Greenland, by
Dr. Neher.
          The Geiger counter on Mariner indicated a cosmic-ray
flux of approximately 3.0 particles per square centimeter per
second throughout the flight.  The constancy of the cosmic-ray
intensity over the very great distance traveled by Mariner is a
new and significant piece of information, but its real meaning
will not become clear until we have repeated the experiment
several times on space vehicles going out away from the sun as
well as in toward it.
          Objective 3:  To study the number and the nature of the
high-energy changed particles emitted by the sun.  (Another
Mariner experiment investigated the very low-energy solar
particles also.)
          The presence of these particles is indicated by sudden
increases above the cosmic-ray background reading of the various
particle detectors.  Some idea of their composition can be
obtained from a comparison of the response of different detectors.
The Mariner results were that high-energy solar particles, such
as could be detected by the JPL-Caltech experiment, were generally
absent except for a single event which began on October 23.  The
Iowa counter, on the other hand, detected not only this event but
at least eight others, which must therefore have been produced by
radiation or particles of very low penetrating power.  Its exact
nature is still in doubt at this time.

MARINER RADIATION EXPERIMENTS    -5-

          The nature of the solar-particle event of October 23
was described in detail by Dr. Anderson.  A solar flare of a type
which has frequently produced streams of charged particles was
observed between 9:42 A.M. and 10:45 A.M., and the reading of the
ionization chamber began to increase even before the flare had
disappeared.  Its reading rose rapidly from a background of 670
to a peak of above 18,000, underwent several oscillations, and
remained above 10,000 for about six hours before declining
gradually over the next few days.  The flux of particles detected
by the Geiger counters rose from a background of 3 to a peak of
16 particles per square centimeter per second.  The fact that the
ionization increased much more than did the number of particles
indicates that the solar particles had much lower average energies
than the galactic cosmic rays, and it is calculated that a typical
energy in this event was about 25 Mev.  The details of the time
and energy variations will be further studied in the hope of
learning more about how the particles were produced in the photo-
sphere of the sun and how they may have been trapped in the
magnetic fields around the sun before being released to the region
where Mariner was waiting to detect them.
          The problem of solar flares and their production of
high-energy charged particles is a particularly important one for
interplanetary space research because the very largest solar
particle streams may contain particles in such numbers and of such
high energies as to constitute a significant hazard to manned
space missions.  No such events have been observed by Mariner,
however.

MARINER RADIATION EXPERIMENTS    -6-

          The total radiation dose seen by the ionization chamber
in the October 23 event was only about 0.24 roentgen inside its
0.01-inch thick steel wall, and the radiation was so non-penetrat-
ing that a moderate increase in the wall thickness would have
excluded the particles almost entirely. For comparison, the radia-
tion dose recorded during the entire flight to Venus was about 3
roentgens, and much of this radiation was etremely penetrating.







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