PUBLIC INFORMATION 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
November 10, 1997
SURVEYOR RESUMES AEROBRAKING, HEADS FOR NEW MAPPING ORBIT
NASA's Mars Global Surveyor spacecraft has successfully
resumed aerobraking through the upper atmosphere of Mars, heading
toward a new science mapping orbit that is the mirror image of
its original target orbit, project officials announced today.
Aerobraking resumed Nov. 7 with a brief propulsive burn that
changed Global Surveyor's flight path slightly. A second
maneuver was performed successfully on Nov. 9, with a third
maneuver planned for Nov. 11 (Nov. 12 EST). These small
adjustments at the farthest part of the spacecraft's orbit around
Mars -- known as the apoapsis -- begin the process of lowering
Global Surveyor's orbit into the Martian atmosphere more
gradually than originally planned.
The more gradual aerobraking strategy will lead to a new
science mapping orbit that preserves all the original scientific
objectives of the mission. Selected by the mission's science
team, the new orbit is essentially just the reverse of the
original orbit: the data will be taken from the south to the
north along the spacecraft's orbital track, rather than north to
south, and mapping will begin one year later than originally
planned.
In this new orbit, beginning one-half Mars year (equivalent
to one Earth year) later than planned, Mars will be at a point in
its orbit that is directly opposite where it would have been in
the original mission. From the spacecraft's point of view, the
side of Mars that would have been dark will be sunlit and vice
versa.
"From the perspective of the science instruments, the orbit
will look just like the original orbit, except that instead of
taking data from north to south on the sunny side of Mars, Global
Surveyor will be making its observations in a south to north
direction in the sunlight," said Glenn E. Cunningham, Mars Global
Surveyor project manager at NASA's Jet Propulsion Laboratory,
Pasadena, CA. "The new mapping orbit will take the spacecraft
down over the Martian equator at 2 a.m. local Mars time during
each orbit, rather than the originally planned orbit that would
have crossed the Martian equator at 2 p.m."
The new mapping orbit will take an additional year to
achieve, due to both the more gradual pace of aerobraking and a
six-month hiatus in the spring of 1998, while Mars moves into the
correct alignment with the Sun for global mapping. Rather than
reaching its final mapping orbit in mid-January 1998, and
beginning the science mission in mid-March 1998, Mars Global
Surveyor will achieve its final orbital position in mid-January
1999, and mapping will begin in mid-March 1999.
"Essentially, we will begin mapping the surface of Mars in
mid-March 1999, during summer in the northern hemisphere,"
Cunningham said. "Originally we had planned to begin mapping on
March 15, 1998, during summer in the southern hemisphere."
During next year's hiatus, Global Surveyor will remain in a
fixed, elliptical orbit in which it will pass much closer to the
surface of Mars during each periapsis -- or closest part of its
orbit around Mars -- than it will in the final mapping orbit.
These close-range passes are essentially an opportunity for bonus
science and will provide superb opportunities for data
acquisition. The spacecraft's full suite of instruments,
including the laser altimeter, will be turned on during this time
to study the planet close up.
"We expect to gain some spectacular new data during this
time," Cunningham said. "The spacecraft's orbit will still be
elliptical during this period, with a duration of between eight
to 12 hours, but at periapsis, the surface resolution will be
much greater, and the lighting angles will be excellent."
Mars Global Surveyor's first two aerobraking maneuvers have
gone well, giving the operations team confidence that the
spacecraft's unlatched panel will be able to withstand an
increasing amount of pressure as it begins to dip lower into the
Martian atmosphere. The new pressure level (an average of 0.2
newtons per square meter or 3/100,000ths of 1 pound per square
inch), is about one-third the level of pressure originally
planned for aerobraking. Aerobraking is a technique that allows
a spacecraft to lower its orbit without relying on propellant, by
using the drag produced by a planet's atmosphere. The technique
was first demonstrated in the summer of 1993 during the final
months of NASA's Magellan mission to Venus.
Although these initial maneuvers have not changed Global
Surveyor's orbital period significantly, they will soon begin to
reduce the time it takes the spacecraft to complete one
revolution around Mars.
"The spacecraft's unlatched solar panel performed as
expected during the two drag passes we've conducted so far,"
Cunningham said. "Although we observed some slight movement
during the passes, the panel returned to its initial position and
its stiffness did not change. That performance gives us
confidence that the mission can proceed without further delay."
A third, 5-second maneuver to be performed at 11:30 p.m. PST
on Nov. 11 (2:30 a.m. EST Nov 12) will lower the spacecraft's
periapsis by an additional 4 kilometers (2.5 miles). With
completion of that maneuver, Global Surveyor will begin the main
phase of aerobraking well inside the upper atmosphere of Mars.
During this two-month main phase, the spacecraft will be circling
Mars every 34.5 hours to start at a periapsis altitude of about
125 kilometers (77 miles), with the apoapsis starting at 44,400
kilometers (27,500 miles) and decreasing with each pass. The
final goal is a 400-kilometer (248-mile) circular mapping orbit.
If additional problems arise with the aerobraking process,
the new mission plan will offer the Surveyor team other
opportunities to reach an elliptical orbit that will satisfy many
of the mission's science objectives. These so-called "off-ramps"
from the aerobraking process will be detailed in a new mission
plan to be reviewed by NASA officials in February 1998.
During a press briefing today at JPL, scientists also showed
stunning new images of layered rock and sediment in the canyon
walls of Valles Marineris on Mars. Other images of an ancient
valley hint at the presence of active sand dunes and dried-up
ponds. The new images are available on the Internet at
http://www.msss.com/ or at
http://barsoom.msss.com/mars/global_surveyor/camera/images/ .
Additional information about the Mars Global Surveyor
mission is available on the World Wide Web by accessing JPL's
Mars news site at http://www.jpl.nasa.gov/marsnews/ or the Global
Surveyor project home page at 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 JPL 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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