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Mars Global Surveyor less than 7 weeks from Mars
By MARK WHALEN

     As Pathfinder continues its mission on the surface of Mars, 
another JPL spacecraft is on course for its rendezvous with the 
red planet.
     Mars Global Surveyor -- an orbiter that will study the 
planet's early history, geology and climate -- is now less than 
seven weeks from Mars orbit insertion, and mission planners have 
been busy rehearsing the spacecraft's aerobraking maneuvers and 
other key mission activities.
     During aerobraking the spacecraft skims the Martian 
atmosphere in order to reduce its velocity. Using this technique, 
MGS is able to reach a near-circular mapping orbit with a minimum 
fuel budget.
     The four-month aerobraking activities will begin with the 
spacecraft traveling in a 45-hour orbit of Mars and will 
gradually scale down to a two-hour orbit to prepare for mapping 
operations starting next March. At that point, MGS will be at an 
average of 378 kilometers (235 miles) above the planet's surface.
     A July 9-10 operations readiness test (ORT), one in a series 
of recent practice runs designed to prepare the project for the 
beginning of its operations at Mars on Sept. 11, covered the time 
when the spacecraft's orbital period will be 24 hours on Oct. 27. 
This readiness test included the flight team at JPL, support 
staff from the Telecommunications and Mission Operations 
Directorate and the MGS Lockheed Martin spacecraft team in 
Denver, according to Joe Beerer, MGS flight operations manager.
     Also this month, a live simulation of telecommunications 
procedures was conducted with the spacecraft and the Deep Space 
Network's tracking stations in Madrid, Spain and Goldstone, 
Calif.
     "With the practice we've gotten from the ORTs, and the fact 
that we will start aerobraking in 45-hour orbits -- and will 
gradually work our way down to short orbits -- I think we have a 
system that will minimize the risks to the spacecraft and the 
mission," Beerer said.
     "The key event for aerobraking is the drag pass at 
periapsis, when the spacecraft is in the atmosphere for 6 to 16 
minutes," he added. Periapsis is the point in the orbit closest 
to the planet. 
     Due to an anomaly after launch that left one of the 
spacecraft's two solar panels tilted about 20 degrees out of 
alignment, mission planners have reconfigured the panels to 
ensure that they stay fixed in a stable aerodynamic configuration 
during the drag pass.
     "An onboard sequence turns the spacecraft to the drag 
attitude," Beerer noted. "When we are in the aerobraking phase of 
the mission, the spacecraft is really ping-ponging back and forth 
between different attitudes: drag attitude, maneuver attitude and 
Earth communications attitude."
     When maneuvers are performed at apoapsis -- the point at 
which the spacecraft is farthest from Mars -- controllers must 
point the engine either in a velocity or anti-velocity direction, 
depending on the need to increase or decrease velocity. "The 
apoapsis velocity change is what determiness the altitude at 
which we go through periapsis," Beerer said.
     One of the things that makes aerobraking so difficult is 
that the atmosphere can change, from orbit to orbit, due to Mars' 
weather. To help assess atmospheric conditions, an MGS 
atmospheric advisory group will use data collected from the MGS 
spacecraft -- as well as some to be provided by Mars Pathfinder -
- to help the project manage the aerobraking events.
     Beerer also noted that the MGS thermal emission 
spectrometer, onboard for the mapping mission, will be used 
during aerobraking to measure Mars' atmospheric temperatures, 
which are indicators of density variations.
     "We expect dust storms to be brewing about a month into our 
aerobraking activities, in October," he said, referring to 
localized dust storms imaged by the Hubble Space Telescope in 
June.
     "These storms probably won't affect us that much, but 
sometimes local storms expand and develop into global dust 
storms. Global storms produce increased atmospheric densities at 
aerobraking altitudes, at 90 to 110 kilometers."
     Atmospheric models have shown mission controllers that the 
density of the Martian atmosphere can double in a 24-hour period. 
The MGS mission design can accommodate doubling of the 
atmosphere, "but we need to be able to respond to that," Beerer 
said. "If it doubles the first day, then doubles again the 
second day, we're in trouble. We have to be ready to move up to 
lighter density regions if the atmospheric advisory group tells 
us that a dust storm is growing."
     The overheating associated with dust storms could be caused 
by flying in a region where the density is too high, causing too 
much friction. Though MGS instruments could be susceptible to 
overheating, the solar panels -- which provide the spacecraft's 
primary drag surface -- are even more sensitive to it.
     Beerer underscored the importance of preparation for the MGS 
team, noting that "We're on a fine line between being too deep in 
the atmosphere -- where we have the threat of overheating-- and 
being too high, where we're not getting enough drag."
     In that case, he said, the orbit won't decrease rapidly 
enough. The spacecraft needs to be down to about a two-hour orbit 
period around the middle of January, with a science requirement 
to have the spacecraft crossing the Martian equator at 2 p.m. 
local solar time every orbit.
     As Universe went to press July 24, the project was scheduled 
to conduct another ORT for the 2 1/2-hour aerobraking orbit, 
which will occur in late December.
     "The MGS flight team will be ready for aerobraking 
operations when the spacecraft arrives at Mars in September," 
Beerer said. "We look forward to continuing the adventure that 
Pathfinder has so successfully begun." 

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