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Mars Global Surveyor Status From The Project Manager

MARS GLOBAL SURVEYOR STATUS
10-16-97

Mars Global Surveyor arrived at Mars on the evening of September 11th and
achieved a very precise 45 hour long elliptical orbit around the planet.
During the spacecraft's third closest approach to the planet, the MGS
science instruments had the opportunity acquire a significant amount of new
information relative to the surface, atmosphere and interior of Mars.
Among these new findings is the observation that the northern hemisphere
area called Elysium is extremely flat, and that there are strong remnant
magnetic fields near the surface rather than a global magnetic field like
the Earth's.  (See http://mars.jpl.nasa.gov)

You may have heard that MGS has encountered some difficulty with
aerobraking and that's true.  After twelve aerobraking drag passes we've
pull up out of the Mars atmosphere to assess the unexplained motions of one
of the solar panels that we've observed and to be sure that the spacecraft
is safe and that we preserve our ability to acquire more great science data
at Mars.  We've decided to take a two week hiatus from aerobraking to do
this.  The big effect from the time lost due to the hiatus is that we will
use a different mapping orbit later on, and we are working on defining what
that orbit will be.

In the mean time, we have taken opportunity afforded us by the hiatus in
aerobraking to take a lot more significant science data at each periapsis
pass.  The Laser Altimeter has been turned back on and the Mars Orbiter
Camera, and Thermal Emission Spectrometer will have great opportunities for
science acquisition. The current periapsis altitude is two times closer to
the surface of Mars than the altitude would be during the originally
planned mapping orbit so the surface resolution is much greater.

The spacecraft, except for the uncertainty concerning the -Y solar panel,
and all its instruments are working exceptionally well, and we continue to
have great expectations for the discoveries that this mission will return.

MGS....the adventure continues.

Read on if you want  more details on the situation:

The Mars Surveyor Operations Project began, on September 16th, moving the
lowest point of the MGS's orbit from 250 km (155 miles) down into the
Martian atmosphere in order to begin aerobraking.  Aerobraking is the
process that uses the drag on the spacecraft caused by skimming through the
top of the atmosphere to reduce the period of the orbit.  Aerobraking
worked just fine!  After twelve passes through the upper atmosphere, we
reduced the orbital period from its initial 45 hours down to 35 hours on
the way to the target of 2 hours.

You may recall that when MGS was launched, the -Y solar panel did not
completely deploy and latch properly.  It ended up about 20 degrees away
from being folded out completely flat.  After a lot of analysis and test,
we determined the best way to do aerobraking, which uses the solar panels
to provide most of the drag surface area, was the turn the -Y panel around.
Thus,  the atmospheric drag pressure would push on it during aerobraking so
that we were compressing the small piece of metal that we believe is
trapped in the solar panel's hinge joint.  Then, as the drag pressures on
the panel increase as we got deeper into the atmosphere, the panel might
move further toward the fully folded out position and even latch.

On Tuesday, October 1st, at periapsis or closest approach to Mars number 12
where the altitude was 110 km (68 miles), we observed the panel move about
14 degrees closer to fully unfolded - good!  Then, on Monday, October 6th,
we had an aerobraking drag pass (periapsis 15) where the Martian atmosphere
became more dense than we had expected for that orbit, although well within
the capabilities of the spacecraft,  and we observed the panel move past
where we thought it should latch and stay past that position by about one
degree - not so good!

Atmospheric density variations like observed in periapsis 15 are expected
as the seasons change on Mars, and it is probably not the result of a great
dust storm.  Our flight operational process is designed to accommodate
these kinds of variations, and so, to reduce the dynamic pressure that the
atmosphere puts on the solar panels, we moved the spacecraft's orbit up 11
km (7 miles) to 121 km (75 miles), to where we thought the pressure would
be fairly light on the panel.   That would also give us a few days to
understand why the panel might have moved past its latch point.   As it
turned out, the pressure was small as we expected, but the -Y solar panel
continued to show us some unexpected motion during the next two drag
passes.  There has never be any concern about the electrical power that the
panel produces.

Thus, on Saturday evening, October 11th, we decided to take the spacecraft
completely out of Martian atmosphere because we are concerned that we don't
know why the panel moves as it does and we were concerned that we might
have some kind of life limiting mechanism involved.  The closest point to
Mars is now about 172 km (107 miles) and the orbital period is about 35
hours.

We are taking a two week hiatus from aerobraking, and in the process we
have forfeited our ability to reach the orbital conditions that would
result in the sun synchronous circular mapping orbit with an equator
crossing at 2 pm local solar mean time.  There are, however, other orbital
conditions that we can reach that will yield exceptional science return
from Mars.

During the hiatus, the Mars Surveyor Operations Project team, at JPL and
Lockheed Martin Astronautics, is studying the observed motion of the solar
panel, and will understand how the original launch failure model must be
modified to accommodate the new motion.  With that information, we will
then plan how to start aerobraking again in a configuration that will not
stress the panel, and given the intensity of aerobraking that is possible,
we, with the MGS science investigators, will determine the best mapping
mission to target for.   The 2 pm orbit was a compromise among the science
investigators.  We will work for another compromise orbit that yields the
best results for whatever orbital capabilities we can achieve.   In the
best case, it will be a circular orbit at some other local time.  In the
worst case, it will be an elliptical orbit with a period of about 16 hours.

We are planning on a Monday, October 27th, a decision milestone on the
plans for and safety of returning to aerobraking.

Glenn E. Cunningham
Project Manager
Mars Global Surveyor
Mars Surveyor Operations