Space & Astronomy/Space and Astronomy (October 1993).iso/pc/text/jplnews2/1418.pr

Text File | 1993-05-03 | 4KB | 79 lines

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

Contact: Jim Doyle, JPL
FOR IMMEDIATE RELEASE                       January 28, 1992

     The Magellan spacecraft began its third radar mapping cycle
of the surface of Venus Friday, Jan. 24, and controllers at
NASA's Jet Propulsion Laboratory said today the mapping was
successful despite earlier transmitter problems.
    The second cycle ended Jan. 15 and controllers then
successfully performed a planned week of battery recharging.
    The spacecraft experienced difficulty with the primary
transmitter earlier this month. Mission Director Doug Griffith
reported that the project decided to use the faulty backup
transmitter on a reduced basis. The backup transmitter had been
turned off last year because of a spurious signal that degraded
data transmission back to Earth.
    Mapping is now continuing, Griffith said, after a new
operational mode was devised for the backup transmitter which
required the transmission data rate to be reduced from 268 to 115
thousand bits per second (kbps).
    Also mission controllers had to contend with interference on
the spacecraft's main radio signal caused by the spurious signal.
That problem is being managed by choosing one of the two primary
radio frequencies as necessary to avoid the spurious signal
frequency.
    For the first five orbits starting on January 24,
controllers tested various configurations of the radio
frequencies to analyze the spurious signal. Finally the spurious
signal settled at a frequency which allowed satisfactory radar
data to be transmitted.
    The transmitter problem is, however, still under
investigation by a special radio anomaly team. The team will try
to determine the most probable failure mechanism in both
transmitters and devise additional methods to optimize use of the
backup transmitter.
    Now that mapping is continuing, stereo mapping of selected
targets is underway as well as mapping of areas not previously
imaged. The first stereo imaging target is Maxwell Montes, the
highest mountain on Venus.
    Additionally, Project Scientist Steve Saunders said the
science team has conducted a global survey based on the data
acquired during the first two cycles and found that about 85
percent of the planet is covered by volcanic rocks, mostly lava
flows that form the great plains.
    Much of the remaining 15 percent, he said, is high standing,
chaotic material that he said was faulted and fractured. "Little
can be determined about the composition or nature of the rocks
that form these highlands," he said.
    One possibility, however, is that the highland rock
represents crustal material that formed soon after the final
accretion of the planet 4.6 billion years ago, he said.

    The high, ridged material is seen in the continent-like
terrains such as Aphrodite Terra and Maxwell Montes. "These
regions often appear as islands surrounded by the more recent
lavas that form the volcanic plains," Saunders said.
    All of the terrestrial planets appear to have formed
relatively low density crusts, Saunders said. "Little of the
ancient crust remains on Earth because of the constant churning
of plate tectonics."
    While Venus does not have Earth-like tectonic plates, it has
a long, complex history of deformation with many episodes of
tectonics and faulting.
    "The Magellan data will provide the key to understanding
that complex history and perhaps to the understanding of many of
Earth's geologic puzzles," Saunders said.
    Magellan has already mapped more than 95 percent of Venus.
The primary objective for the Magellan mission called for one
mapping cycle to obtain images of 70 percent of the planet.
    The Jet Propulsion Laboratory manages the Magellan Project
for NASA's Office of Space Science and Applications.
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