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Reconfigured MGS ready for mission based on new orbit
By DIANE AINSWORTH
1997 saw the arrival of two spacecraft at Mars and the
beginning of an extended program of Mars exploration. Two months
after Pathfinder's landing, NASA's Mars Global Surveyor was
captured in orbit on Sept. 12, after a 10-month journey through
deep space.
Global Surveyor was designed to replace Mars Observer, which
was lost in August 1993. Ingenuity, teamwork and an
exceptionally dedicated group of engineers and scientists
quickly went to work to develop and launch the spacecraft within
a short amount of time and on a tight budget. The time and cost
of the mission broke all the records--26 months to build the
spacecraft at a cost of only $148 million, which was well under
the cost cap and a fraction of what it cost to build previous
spacecraft destined for Mars.
Mars Global Surveyor carried six scientific instruments to
study Mars' climate, surface topography and subsurface
resources. Its primary scientific objective, though, was to map
the entire surface of the red planet.
The journey to Mars wasn't as smooth as the team had hoped
for, but each problem that cropped up was remedied in a creative
and swift manner. In mid-November, as the spacecraft began to
aerobrake into the upper fringes of the Martian atmosphere,
structural damage to the yoke hinge of one of the solar panels,
incurred during initial deployment of the panels shortly after
launch, caused the unlatched panel to begin flexing during each
dip lower into the Martian atmosphere.
Mechanical stress analysis tests suggested that the solar
panel yoke--a triangular, aluminum honeycomb material sandwiched
between two sheets of graphite epoxy--had probably fractured on
one surface during initial deployment. The analysis further
suggested that the fractured surface, with increased pressure on
the panel during aerobraking, began to pull away from the
aluminum honeycomb beneath it.
The flight team at Lockheed Martin Astronautics in Denver, in
collaboration with atmospheric specialists at JPL, decided upon
a more gradual aerobraking strategy in which to lower the
spacecraft. Aerobraking was reinitiated at 0.2 newtons per
square meter (3/100,000 of 1 pound per square inch), about one-
third of the original aerobraking level. That level was thought
to be safe, but could be adjusted in the event of additional
trouble with the panel.
Science teams then came up with a new aerobraking strategy
and a new mapping orbit.
The new mapping orbit would be a mirror image of the original
mapping orbit, but it would take an additional year to set up.
The spacecraft would have to take a six-month hiatus in the
spring of 1998 to allow Mars to move into the proper alignment
for mapping. The spacecraft's orbit would take Global Surveyor
across Mars' equator at 2 a.m. rather than at 2 p.m., and the
side of Mars that would have been dark would now be illuminated
by the Sun.
"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 a mid-November press
briefing at JPL. Rather than reaching its final mapping orbit in
mid-January 1998, and beginning the science mission in mid-March
1998, Mars Global Surveyor would achieve its final orbital
position in mid-January 1999, and mapping was to begin in mid-
March 1999. Apart from the year's delay in beginning mapping,
the new mapping orbit would preserve all of the science
objectives of the mission.
During this 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 bonus passes 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 spectacular."
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.
With renewed vigor that the science mission had not been
compromised, the flight team resumed aerobraking on Nov. 7.
Since then, the spacecraft's scientific instruments have
performed flawlessly, continuing to return new information about
Martian magnetic properties, its atmosphere, surface features,
temperatures and mineralogy.
Among the most intriguing science discoveries was
confirmation that Mars had a weak, non-uniform, planet-wide
magnetic field. The discovery continues to baffle scientists,
but it was the first time that Mars' magnetic field had, in
fact, been studied.
The spacecraft's magnetometer, which began making
measurements of Mars' magnetic field after its capture in orbit
on Sept. 11, detected the magnetic field just four days after
the beginning of its orbit around Mars. The existence of a
planetary magnetic field has important implications for the
geological history of Mars and for the possible development and
continued existence of life on Mars.
"Preliminary evidence of a stronger than expected magnetic
field of planetary origin was collected and is now under
detailed study," said Dr. Mario Acuna, principal investigator of
the magnetometer/electron reflectometer instrument at NASA's
Goddard Space Flight Center, Greenbelt, Md. "This was the first
opportunity in the mission to collect close-in magnetic field
data. Much additional data will be collected in upcoming orbits
during the aerobraking phase of the mission to further
characterize the strength and geometry of the field.
"The current observations suggest a field with a polarity
similar to that of Earth's and opposite that of Jupiter, with a
maximum strength not exceeding 1/800 of the magnetic field at
the Earth's surface.
"This result is the first conclusive evidence of a magnetic
field at Mars," Acuna continued. "More distant observations
obtained previously by the Russian missions Mars 2,3 and 5 and
Phobos 1 and 2 were inconclusive regarding the presence or
absence of a magnetic field of internal origin."
The magnetic field holds important clues to the evolution of
Mars. Planets like Earth, Jupiter and Saturn generate their
magnetic fields by means of a dynamo made up of moving molten
metal at the core. This metal is a very good conductor of
electricity, and the rotation of the planet creates electrical
currents deep within the planet, which give rise to the magnetic
field. A molten interior suggests the existence of internal heat
sources that could give rise to volcanoes and a flowing crust
responsible for moving continents over geologic time periods.
The latter phenomenon is called plate tectonics.
"A magnetic field shields a planet from fast-moving,
electrically charged particles from the Sun, which may affect
its atmosphere, as well as cosmic rays, which are an impediment
to life," Acuna said. "If Mars had a more active dynamo in its
past, as we suspected from the existence of ancient volcanoes
there, then it may have had a thicker atmosphere and liquid
water on its surface."
It is not known whether the current weaker field now results
from a less active dynamo, or if the dynamo is now extinct and
what the scientists are observing is really a remnant of an
ancient magnetic field still detectable in the Martian crust.
"Whether this weak magnetic field implies that we are
observing a fossil crustal magnetic field associated with a now
extinct dynamo -- or merely a weak but active dynamo similar to
that of Earth, Jupiter, Saturn, Uranus and Neptune -- remains to
be seen," Acuna said.
Mars Global Surveyor is the first in a sustained program of
robotic exploration of Mars. In December 1998, a second pair of
spacecraft will be launched toward the red planet, carrying
instruments that will augment this new global portrait of Mars.
As those spacecraft arrive at Mars, Global Surveyor will be
generating a global map of the planet that will aid in the
selection of future landing sites.
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