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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.

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