MEDIA RELATIONS OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
http://www.jpl.nasa.gov
Contact: Diane Ainsworth
FOR IMMEDIATE RELEASE
August 10, 1998
NASA MANAGERS CONSIDER POSTPONING DEPLOYMENT OF MARS GLOBAL
SURVEYOR ANTENNA
Concern over the deployment mechanism for the high-gain
communication antenna on the Mars Global Surveyor spacecraft has
caused NASA managers to consider postponing the antenna's
deployment in order to maximize the probability of mission
success.
The project team is studying a postponement of up to nine
months in the antenna deployment, which currently is scheduled to
take place in March 1999. The spacecraft, now in orbit around
Mars, uses the undeployed high-gain antenna to communicate with
Earth, but the entire spacecraft must be turned to point the
antenna toward Earth during each communication session.
"We have not made any decisions yet, but we want to take a
conservative approach in order to protect the mission as fully as
possible," said Glenn E. Cunningham, Mars Global Surveyor project
manager at NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA.
"A delay in the antenna deployment would reduce the flow of
imagery and science data somewhat, but we have some ideas about
how to compensate for that."
Launched in November 1996 and in Mars orbit since September
1997, Mars Global Surveyor carries a dish-shaped high-gain
antenna that is to be deployed on a 2-meter-long (6.6-foot) boom
for the global mapping portion of the mission. The antenna is
stowed during launch and the early orbital phase at Mars so that
it is not contaminated by the exhaust plume from the spacecraft's
main engine. The mission plan calls for the antenna boom to be
deployed following the final use of the main engine next spring,
at the completion of the spacecraft's orbit-shaping aerobraking
activity.
During deployment, the boom is pushed outward by a powerful
spring. A damper mechanism cushions the force of the spring and
limits the speed of the deployment, somewhat like an automobile
shock absorber or the piston-like automatic closer on a screen
door. In recent months, however, engineers have become aware of
problems with similar damper devices on deployable structures
such as solar panels on other spacecraft.
New data suggest that, in the vacuum of space, air bubbles
may develop in the viscous fluid inside the damper. This may
allow the boom to move through a considerable range of motion at
a high speed before any cushioning effect begins to occur.
"To the best of our knowledge, we could deploy the antenna
boom without any adverse effect," said Cunningham. "However, the
forces that the damper and the boom would be subjected to as a
result of the bubble formation are close enough to the maximum
force that they are designed to withstand that we want to take a
cautious approach in evaluating the deployment." In a worst-case
scenario, damage resulting from damper failure could render the
spacecraft unable to communicate with Earth.
"The advantage of deploying the high-gain antenna is that we
can then use its gimbals to point the antenna at Earth to send
data at the same time science instruments are pointed at Mars
acquiring science data," said Cunningham. "Until we deploy the
antenna, we must store data on
the spacecraft's onboard recorder and then turn the entire
spacecraft periodically to transmit data to Earth." A similar
approach was used on NASA's Magellan spacecraft, which orbited
Venus from 1990 to 1994.
The project team is considering postponing the antenna
deployment until after the landing of another spacecraft, the
Mars Polar Lander, which will reach Mars in December 1999. Mars
Polar Lander carries an experiment called the Deep Space 2
microprobes, which will penetrate
the soil of Mars in search of subsurface water. Deep Space 2
relies on Global Surveyor as its only possible communication link
with Earth.
If the high-gain antenna is not deployed when Mars Global
Surveyor begins its prime mapping mission next March, Cunningham
said that small gaps would exist in coverage of the Martian
surface by the spacecraft's camera and other instruments, due to
the periods when the spacecraft is turned to communicate with
Earth. Those gaps could be filled in later in the orbital
mission.
The project team is not yet certain how a postponed
deployment would affect the total amount of data returned by the
spacecraft. An initial estimate for the first 30 days of the
global mapping mission found that it could return approximately
40 percent of the data that could be sent
with a fully articulated antenna. However, the data return rate
could be improved by strategies such as using larger ground
antennas on Earth so that the spacecraft could transmit data more
quickly, Cunningham noted.
A final decision on the antenna deployment will not be made
until a review scheduled for February 3, 1999 is held, before the
spacecraft's prime mapping mission begins the following month.
Mars Global Surveyor is managed for NASA's Office of Space
Science by JPL, a division of the California Institute of
Technology. The spacecraft was built by Lockheed Martin
Astronautics, Denver, CO.
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8/10/98 DEA
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