An Aerobraking Strategy for Determining Mars Upper Atmospheric Structure
S.W. Bougher (LPL, U. of Arizona), J.R. Murphy, R.M. Haberle (NASA Ames)
The Mars Global Surveyor (MGS) spacecraft will enter Mars orbit on
Sept. 12, 1997, and thereafter undergo aerobraking for roughly 4-months.
The final data-taking orbit to be achieved is sun-synchronous (2PM/2AM).
An aerobraking strategy has been developed that not only will provide
the walk-in capability needed to safely achieve the required Mars orbit,
but also will provide a careful monitoring of the atmospheric structure.
In particular, the linkage between the lower (0-100 km) and upper (100-
150 km) Mars atmospheres will be investigated. A suite of complementary
measurements is planned that will probe the atmosphere over 0-150 km,
including : (1) MGS Accelerometer density and inferred temperatures
(100-150 km), (2) MGS Thermal Emission Spectrometer (TES) nadir (25-30
km) and limb (up to about 55 km) temperatures, (3) MGS Electron
Reflectometer (ER) F1-peak heights (near 130 km), (4) ground-based
microwave disk-averaged temperatures (0-70 km), and (5) Mars Pathfinder
(MPF) surface meteorological data at 20 N latitude. These datasets
acquired during the aerobraking phase will enable the current state of
the atmosphere to be examined. Potential dust storm activity and its
manifestations throughout the atmosphere can be monitored over Ls =
184 to 250. A corresponding library of coupled 3-D model simulations,
based upon the NASA Ames Mars GCM and the NCAR Mars Thermospheric GCM
(MTGCM), will be used to : (1) validate the current state of the Mars
atmosphere, (2) investigate the various orbital, seasonal, LAT-LT-LON,
and potential dust storm trends, and (3) predict the structure of the
Mars atmosphere in the aerobraking corridor that is approaching in
future MGS orbits. The in-situ accelerometer and ER data will eventually
be used to construct a Mars empirical model covering 100-150 km. We will
present a few selected GCM simulations to illustrate the expected
atmospheric response to a dust storm event. In addition, we will discuss
why these upper atmosphere datasets are important to future Mars
missions.