Space & Astronomy/Space and Astronomy (October 1993).iso/pc/text/jplnews2/1259.pr

Text File | 1993-05-03 | 7KB | 140 lines

PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109. TELEPHONE [phone]



FOR IMMEDIATE RELEASEAugust 16, 1989



          A tracking station in Japan has been added to the
network of giant antennas trained on Voyager 2 during its
flyby of Neptune on August 24-25 to help the spacecraft
mission's radio science experiment.
          Scientists say they will be able to "see" twice as
deeply into the atmosphere of the giant gas planet, thanks to
the participation of the 64-meter (210-foot) antenna at the
Usuda Deep Space Center.
          The collaboration was arranged through an agreement
signed in 1988 between NASA and Japan's Institute of Space
and Aeronautical Studies (ISAS), which operates the Usuda
center.
          Under the agreement, physicist Dr. Nobuki Kawashima
of ISAS will join the Voyager Radio Science Team.
          "Using Usuda will allow us to extract information
on deeper parts of Neptune's atmosphere," said Dr. Len Tyler
of Stanford University, principal investigator for the
Voyager radio experiment.  "Also, the quality of the data we
will have for any given point will be twice as good."
    During these observations, scientists will listen
not to the information carried by Voyager's radio signal but
rather to the signal itself -- especially its strength and

frequency -- as the spacecraft sails over Neptune's north
pole and dips behind the planet.
    Barely perceptible changes in the radio signal
convey signatures of the structure, composition and
temperature of the seas of gases that constitute Neptune's
atmosphere, as well as of the distant planet's gravity field.
          As the spacecraft disappears behind a planet or
moon from the Earth's point of view, its radio signal is
refracted, or bent, during passage through the planet's or
moon's atmosphere.
          Tiny changes in the signal's frequency and strength
give scientists a portrait of the atmosphere's structure,
composition, temperature and location of clouds, as well as
information on small-scale atmospheric dynamics.
          Tyler likened the refraction effect to how a sunset
appears on Earth, as the Sun seems to linger at the horizon
before it disappears.
          "You would think that the Sun would appear to keep
moving.  But because its light is being refracted, you can
continue to see it as it seems to stand still for a few
moments," he explained.
          In addition to studying atmospheres of planets and
moons, scientists have used the Voyagers' radio systems
during the mission to investigate rings surrounding planets.
As the spacecraft flies behind a ring system, changes in the
radio signal provide information not only on the rings'
overall dimensions but also on the particles that constitute
the rings.

          Also, the radio experiment is able to detect minute
changes in Voyager's velocity as it curves past each planet,
offering a detailed look at the planet's gravity field.
          Some of the most significant findings from the
radio experiment during the 12-year Voyager Mission have
included revelations on the nitrogen atmosphere surrounding
and the surface pressure at Saturn's largest moon, Titan;
measurements on wave-like structures within Saturn's
sprawling rings and the sizes of particles that constitute
them; detection of a methane cloud layer in the atmosphere of
Uranus; and the nature of Uranus's coal-black rings.
          The experiment has also provided data on the
densities of moons at planets the Voyagers have visited, as
well as the planets' gravity fields.  In concert with the
Voyagers' infrared interferometer spectrometer and radiometer
(IRIS) instrument, the radio science experiment additionally
has determined the ratio of hydrogen to helium, the two
overwhelmingly dominant elements in the atmospheres of each
of the planets.
          As Voyager 2 has headed into the outer solar
system, the strength of its radio signal at Earth has become
fainter.  During the Neptune flyby, the radio's signal
strength at Earth will be 1/36th of what it was when the
Voyagers flew by Jupiter.
         The collaboration with Japan's ISAS will help offset
the diminished signal, Tyler explained.
          During Voyager 2's closest approach to Neptune,
radio science data will be recorded at the NASA/JPL Deep

Space Network Station in Canberra, Australia; at the Parkes
Radio Observatory in Australia; and at the Usuda center in
Japan.
          Each receiver is linked to an atomic clock which
gives an exceedingly precise time-stamp to the received radio
signal.  This allows the recorded signals from all the
stations to be combined later so that they mesh extremely
closely, with the peaks and troughs of the radio waves
matching virtually exactly.
          The combined signal is then analyzed to yield the
experiment's science data.  Factors such as the chemistry and
temperature of a planet's atmosphere will have minute, but
measurable, effects on the signal's frequency and strength.
          Tyler noted that his team studies frequency changes
that amount to only one one-hundredth of a cycle per second
(1/100th Hz) in a signal from Voyager 2 being transmitted at
a frequency of billions of cycles per second.  That is
similar, he said, to measuring a change in position of 1
millimeter (1/25th inch) from an observing position 5
billion kilometers (3 billion miles) away.
          "The best ear of any trained musician can detect a
change of a partial musical step, which is some number of
cycles per second," Tyler added.  "Needless to say, the
changes we study in Voyager's radio signal are much more
subtle than that."
          Opened in October 1984, the Usuda Deep Space Center
is set in the mountains of Japan's Nagano Prefecture at 1,456
meters (4,777 feet) above sea level, some 100 kilometers

(about 60 miles) northwest of Tokyo.
          In addition to performing tracking, telemetry and
commanding for Japan's solar system missions, such as the
Suisei and Sakigake spacecraft which flew by Comet Halley in
1986, the Usuda center supports experiments involving
sounding rockets and balloons.
          The Deep Space Network, which includes complexes in
the California desert and Spain in addition to the Australian
site, is managed by the Jet Propulsion Laboratory for NASA's
Office of Space Operations.
          The Parkes Radio Observatory in Australia is
operated by the Commonwealth Scientific and Industrial
Research Organization.
          JPL manages the Voyager Project for NASA's Office
of Space Science and Applications.
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