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Mars Global Surveyor Update - September 27, 1997

Mars Global Surveyor
Flight Status Report
Saturday, 27 September 1997

Aerobraking operations on the Mars Global Surveyor mission continue to
proceed smoothly. As of today, the spacecraft has completed nine revolutions
around the red planet, including six aerobraking passes through the upper
Martian atmosphere. Each of these atmospheric passes took place at the start
and low point of the orbit as Surveyor zipped across the sky at altitudes
scarcely greater than 72.1 miles (116 km).

At the start of aerobraking slightly more than one week ago, the high point
of Surveyor's orbit occurred at an altitude of 33,570 miles (54,025 km).
Since then, air resistance from the atmospheric passes has slowed the
spacecraft by an average of 2.2 m.p.h. (1 meter per second) on each orbit.
The result is that the orbit's high point has dropped by 1,050 miles (1,690
km). Over the next four months, aerobraking will reduce the high-point
altitude all the way down to 280 miles (450 km).

Dr. Richard Zurek of the Surveyor science team reports that the thickness of
the Martian atmosphere continues to run slightly higher than predicted by
current models. Because a thicker atmosphere will result in more stress on
the spacecraft during aerobraking, both the navigation and atmospheric
science teams are currently studying possible changes to the baseline plan.
However, no major changes are expected because Surveyor was designed to
tolerate up to a 70% increase in atmospheric thickness.

In other news this week, minor wobbling on the spacecraft's solar panels
caused the pointing control system to off-point Surveyor's high-gain
antenna by slightly more than two degrees from the direct line to the Earth.
The flight team fixed the wobbling by commanding the solar panels into a
stable position. Then, the pointing discrepancy was corrected by allowing
Surveyor to scan and lock-up on reference stars in deep space. These distant
stars serve as fixed reference points that allow the spacecraft to determine
its proper pointing orientation relative to the Earth and Sun.

After a mission elapsed time of 324 days from launch, Surveyor is 165.28
million miles (265.99 million kilometers) from the Earth and in an orbit
around Mars with a period of 42.75 hours. The spacecraft is currently
executing the P10 command sequence, and all systems continue to be in
excellent condition.

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Status report prepared by:

Office of the Flight Operations Manager
Mars Surveyor Operations Project
NASA Jet Propulsion Laboratory
California Institute of Technology
Pasadena, CA 91109
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