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Posted by baalke on August 28, 2006, 4:16 pm
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http://mars.jpl.nasa.gov/mro/newsroom/pressreleases/20060825a.html
Mars Reconnaissance Orbiter Nears End of Aerobraking
Jet Propulsion Laboratory
August 25,2 006
NASA's Mars Reconnaissance Orbiter has begun the final and
fastest-paced
portion of its "aerobraking" process of using friction with the top of
Mars' atmosphere to shrink the spacecraft's orbit.
After nearly 400 drag passes into the atmosphere during the
closest-to-Mars portion of each orbit, the spacecraft has reduced the
farthest point in its orbit to an altitude of 1,100 kilometers (684
miles). The spacecraft takes 2 hours, 7 minutes to complete one orbit,
as of Aug. 25. In contrast, during the weeks between Mars
Reconnaissance
Orbiter's arrival at Mars on March 10 and its start of aerobraking in
early April, the most distant point of each orbit was about 43,000
kilometers (27,000 miles) away from the planet and each orbit lasted
about 35 hours. By using the aerobraking technique, the project has
saved carrying 600 kilograms (about 1,300 pounds) of additional
propellant to the red planet.
"We are in the end-game part of aerobraking from now until August 30,
when we will fire our thrusters to move the spacecraft out of an
atmosphere-grazing orbit," said Mars Reconnaissance Orbiter Project
Manager Jim Graf, of NASA's Jet Propulsion Laboratory, Pasadena, Calif.
"Because of the variations in the atmosphere and the short orbits, this
period is the most exciting and most dangerous of the five months of
aerobraking. The team is on '24-seven' until the 30th."
The lowest-altitude part of each drag pass has been about 100
kilometers
(62 miles), varying by a few percent up or down due to gravitational
variations in the lumpy planet and as adjusted by small maneuvers that
keep the right balance between atmospheric drag and heating as the
spacecraft flies through the variable atmosphere.
On each of the aerobraking passes, the spacecraft records the density
of
the atmosphere on both the inbound and outbound legs as it flies
through
its closest approach to the planet. This information is used to
characterize atmospheric variability, which helps guide the flight team
to design the right maneuvers. The data will also be analyzed to
understand the structure and circulation of the upper atmosphere.
The desired orbit for systematic observations by the spacecraft's six
scientific instruments ranges from an altitude of 320 kilometers (199
miles) over Mars' north pole to an altitude of 255 kilometers (158
miles) over the south pole, a loop that takes one hour and 53 minutes
to
fly. The flight team plans to get the spacecraft's orbit into that size
and shape by mid-September, with two maneuvers to raise the
low-altitude
portion of the orbit following completion of the aerobraking that is
lowering the high-altitude portion. The mission's main science
observations are scheduled to begin in November, after a period of
intermittent communications while Mars passes nearly behind the sun.
As the flight team manages the end-game aerobraking, engineers are also
studying a stuck waveguide transfer switch in the orbiter's X-band
radio
communication system. This switch allows one of the spacecraft's two
X-band amplifiers to transmit through either the low-gain antenna,
which
has a broad field-of-view, or the high-gain antenna, a dish antenna 3
meters (10 feet) in diameter that is used to downlink high-rate science
data. The present spacecraft operation is stable and allows for the
full
amount of science data to be transmitted to Earth. The stuck switch
limits the flexibility of choosing which amplifier is used to transmit
data. Engineers are conducting tests to understand the root cause of
the
switch becoming stuck and to explore possibilities for restoring the
operability of the switch.
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