QUESTION:
The effect of aerobreaking the MGS in the upper layers of the Martian
atmosphere depends on the density and the density depends on the altitude
above the surface. Aerobreaking means deccelaration and this should lead to
a deeper dip into the atmosphere every following turn. What is the closest
point of MGS to the surface of Mars, how big is the deccelaration and what
are the forces on the solar panels while aerobreaking? Do you use the
cameras while being so close to the surface?

ANSWER from Charles Whetsel on July 30, 1997:
We will be frequently updating ("trimming") the altitude above the Martian
surface at which we perform our aerobraking maneuvers on MGS (based on
things like the average atmoshperic density and the weather below us) but we
expect to have to descend to around 55 miles (90 km) above the surface to
get the drag level required for our aerobraking.  The actual force on the
spacecraft during an aerobraking drag pass is not that great, about 5-6
pounds (20-25 Newtons), but when considered over the length of each drag
pass and multiplied by the several hundred drag passes we will execute, the
integrated effect is quite noticable.  The camera, the Thermal Emission
Spectrometer, and several other instruments will be taking period
measurements of the Martian atmosphere during aerobraking to help us
characterize the atmosphere and watch out for dust storms or other
atmospheric weather phenomena.  These measurements will also give everyone a
"sneak-peek" at the type of science data we will be returning once we reach
the final mapping orbit, but the best will be "yet to come."

Charles Whetsel
Chief Engineer
Mars Global Surveyor