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Location: UFOUpDatesList.Com > 1997 > Oct > Oct 30

Mars Global Surveyor to Resume Aerobraking

From: NASANews@hq.nasa.gov
Date: Thu, 30 Oct 1997 15:59:13 -0500 (EST)
Fwd Date: Thu, 30 Oct 1997 17:36:36 -0500
Subject: Mars Global Surveyor to Resume Aerobraking

Don Savage
Headquarters, Washington, DC                  October 30, 1997
(Phone:  202/358-1547)

Diane Ainsworth
Jet Propulsion Laboratory, Pasadena, CA
(Phone:  818/354-5011)

RELEASE: 97-249

MARS GLOBAL SURVEYOR TO RESUME AEROBRAKING

    After a two-week hiatus, NASA's Mars Global Surveyor (MGS)
flight team will resume lowering the spacecraft's orbit around
Mars beginning Nov. 7.  The effort will proceed at a more
gradual pace than before, which will extend the mission's
aerobraking phase by several months, and will change Global
Surveyor's final science mapping orbit.

    The decision to resume aerobraking came after intensive
engineering analysis, computer simulations and tests with
representative hardware to characterize the current condition
of one of the spacecraft's two solar panels, which began to
flex more than expected during the spacecraft's lowest dip into
the Martian atmosphere on Oct. 6.

    Under normal circumstances, the spacecraft's two 11-foot-
long (3.5-meter) solar panels should remain fixed and nearly
motionless during each aerobraking pass through the upper
atmosphere of Mars.  One of the panels, which did not fully
deploy and latch after launch, moved past its latched position
and has shown slight movement during the spacecraft's last
three closest approaches to the Martian surface.

    "After sufficient time to study the observed motion, we
concluded that it is possible to perform additional aerobraking
at a slower rate, without putting undue stress on the solar
panel in question," said Glenn E. Cunningham, Mars Global
Surveyor mission manager at NASA's Jet Propulsion Laboratory
(JPL), Pasadena, CA.  "This changes Mars Global Surveyor's
final mapping orbit, but it should not have a significant
impact on the ability of Global Surveyor to accomplish the
mission science objectives."

    The spacecraft's scientific instruments have performed
flawlessly and continue to return new information about Martian
magnetic properties, its atmosphere, surface features,
temperatures and mineralogy since Mars Global Surveyor entered
orbit around the red planet on Sept. 11.

    The spacecraft is currently in a 35-hour elliptical orbit
which brings it 107 miles (172 kilometers) above the surface of
Mars at its closest approach to the planet.  The operations
team at JPL and Lockheed Martin Astronautics, Denver, CO, will
begin to reduce that orbit using a more moderate level of
aerobraking that will slowly bring the spacecraft into the
desired nearly circular mapping orbit.  Aerobraking, a
technique first demonstrated in the summer of 1993 during the
final months of the Magellan mission to Venus, allows a
spacecraft to lower its orbit without relying on propellant, by
using the drag produced by a planet's atmosphere.

    "There are several types of desirable orbits for us to
consider in the next several weeks that will give us global
coverage of the planet and yield all of the science data we
expected to return," Cunningham said.  "In the meantime, the
instruments are performing marvelously, and we will continue
gathering new science data as we begin to reduce the
spacecraft's altitude and bring it down into the upper Martian
atmosphere.  Even if we wind up in an elliptical orbit, we will
have an opportunity to study Mars at closer range than we
originally planned because the spacecraft's periapsis -- or
closest passage over Mars -- will be closer than the 234-mile
(378-kilometer) circular orbit that was to be its original
mapping distance."

    The spacecraft's current orbit was raised Oct. 12 after the
flight operations team observed that the unlatched solar panel
had moved more than 20 degrees and beyond what should have been
its fully deployed and latched position.  Significant movement
was observed on periapsis 15 -- or the 15th closest pass over
Mars, which occurred on Oct. 6 -- when the Martian atmosphere
had become twice as dense as it had been during previous
passes.  The thickness of the atmosphere amounted to a 50
percent increase in pressure over what was expected on the
spacecraft's solar array.

    Although atmospheric variations like these were anticipated
as the seasons change on Mars, the spacecraft's orbit was
raised by about seven miles (11 kilometers) to adjust the
pressure level.  Subsequent motion of the panel at periapsis 16
through 18 caused the flight team to raise the orbit further on
Oct. 12, taking the spacecraft out of the atmosphere altogether.

    "The investigation of the unexpected motion of the
unlatched panel led us to identify a secondary source of damage
in the yoke, a piece of structure that connects the solar panel
to the spacecraft," Cunningham said.  "This secondary source of
damage was a result of the failure of the damper arm that
jammed in the panel's hinge joint shortly after launch, when
the solar panels were initially deployed."

    Mechanical stress analysis tests suggest that the yoke -- a
triangular, aluminum honeycomb material sandwiched between two
sheets of graphite epoxy -- probably fractured on one surface.
The analysis further suggests that the fractured surface, with
increased pressure on the panel during aerobraking, began to
pull away from the aluminum honeycomb beneath it.

    "Aerobraking will be reinitiated at 0.2 newtons per square
meter (0.00003 pounds per square inch), which is about one-
third of the original aerobraking level," Cunningham said.
"This is a pressure that we currently believe is safe but we
will continue to work with ground tests, analysis and close
monitoring of in-flight spacecraft data to assure that it is safe."

    "Aerobraking will take much longer, perhaps eight to 12
months, at this more gradual rate.  In the meantime, we will
continue collecting science data and work in the next several
weeks toward selection of the best possible orbit to fulfill
the science objectives of the mapping mission," Cunningham said.

    A new color image from the MGS Mars Orbiter Camera of the
giant volcano Olympus Mons is available on the Internet at the
following URL:

http://barsoom.msss.com/mars/global_surveyor/camera/images/index.html

    Additional information about the Mars Global Surveyor
mission is available on the World Wide Web by accessing JPL's
Mars news site at URL:

http://www.jpl.nasa.gov/marsnews

or the Global Surveyor project home page at URL:

http://mars.jpl.nasa.gov.

    Mars Global Surveyor is part of a sustained program of Mars
exploration, known as the Mars Surveyor Program.  The mission
is managed by the Jet Propulsion Laboratory for NASA's Office
of Space Science, Washington, DC.  JPL's industrial partner is
Lockheed Martin Astronautics, Denver, CO, which developed and
operates the spacecraft.  JPL is a division of the California
Institute of Technology, Pasadena, CA.

                              - end -



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