Gerald M. Keating

Evidence of long term global decline in the Earth's thermospheric densities apparently related to anthropogenic effects

A study was performed of the long-term orbital decay of five Earth satellites with perigee altitudes averaging near 350km. To decouple long-term trend measurements from the effects of solar variability, measurements were evaluated during the years of solar minimum (1976, 1986 and 1996). Atmospheric densities derived from these essentially global measurements showed substantial evidence of a decline averaging 9.8 ± 2.5% in thermospheric density over 20 years pointing toward a long-term cooling of the upper atmosphere. Increases in greenhouse gases induced by human activity are hypothesized to warm the Earths surface and lower atmosphere, but strongly cool the upper atmosphere. Assuming that the 10% increase in CO2 over these 20 years caused cooling resulting in the 10% decline in density, a doubling of CO2 could cause the thermospheric densities measured near 350km to decrease by a factor of 3. This decrease may shrink the altitude of a constant density surface by 40km before the end of the 21st century.

Mars Global Surveyor aerobraking: Atmospheric trends and model interpretation

Abstract Mars Global Surveyor (MGS) recently obtained coordinated lower-atmosphere (thermal and dust) measurements and simultaneous upper atmosphere accelerometer data (densities, scale heights and temperatures) for the purpose of safely aerobraking the spacecraft toward its mapping orbit (Keating et al. 1998). Much useful scientific information was also gleaned that describes the coupling of these atmospheric regions during this Phase I aerobraking period (September 1997–March 1998; Ls = 184–300). The major features of this aerobraking data are presented, and its trends elucidated in order to: (1) illustrate the aerobraking environment experienced by the spacecraft, and (2) decompose the processes responsible for the atmospheric variations observed. Coupled general circulation models of the Mars lower and upper atmospheres are exercised to investigate the solar-orbital, seasonal, wave, and dust variations observed during MGS aerobraking. The precession of the MGS periapsis position during Phase I permits longitudinal, latitudinal, local time, and vertical variations of the thermosphere to be monitored. Future aerobraking activities at Mars will benefit greatly from this MGS aerobraking data and its model interpretation.

Application of Accelerometer Data to Mars Global Surveyor Aerobraking Operations

Aerobraking was selected for the Mars Global Surveyor mission as a primary and enabling operation. The application of accelerometer data for determining atmospheric density during operations for the e rst phase of aerobraking isreported.Acceleration was measured alongthebody zaxis, which is theaxisnominally into thee ow. For a 1-s count time, thedata have a resolution of 0.332 mm /s, permitting the recovery of density to 3% at nominal aerobraking altitudes near 115 km and on many orbits, permitting the recovery of density to altitudes as high as 180 km. Accelerometer data were analyzed in near real timeto provideestimates ofdensity at periapsis, maximum density, density scale height, latitudinal gradient information, and longitudinal wave variations. Summaries are givenoftheaerobrakingphaseofthemission,theaccelerometerdata analysismethodsand operationalprocedures, some applications to determining thermospheric properties, and some remaining issues on interpretation of the data. Pree ight estimates of 70% 2 ae natural variability are shown to be realistic, and predictions that dust storms could produce rapid and large increases in thermospheric density have been verie ed.

Application of Acclerometer Data to Atmospheric Modeling During Mars Aerobraking Operations

R. H. Tolson∗ North Carolina State University, Hampton, Virginia 23666-6147 G. M. Keating George Washington University, Newport News, Virginia 23602 R. W. Zurek Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109-8099 S. W. Bougher University of Michigan, Ann Arbor, Michigan 48109-2143 C. G. Justus Morgan Research Corporation, Huntsville, Alabama 35805-1948 and D. C. Fritts∗∗ NorthWest Research Associates, Inc., Boulder, Colorado 80301

Atmospheric Modeling Using Accelerometer Data During Mars Reconnaissance Orbiter Aerobraking Operations

Aerobraking as an enabling technology for the Mars Reconnaissance Orbiter mission was used in numerous analyses based on various data types to maintain the aerobraking time line. Among these data types were measurements from spacecraft accelerometers. This paper reports on the use of accelerometer data for determining atmospheric density during Mars Reconnaissance Orbiter aerobraking operations. Acceleration was measured alongthreeorthogonalaxes,althoughonlydatafromthecomponentalongtheaxisnominallyintothe flowwereused during operations. For a 1-s count time, the root-mean-square noise level was 0:004 mm=s 2 , permitting density recovery to 0:008 kg=km 3 , or about 0.023% of the mean density at periapsis, during aerobraking. Accelerometer data were analyzed in near real time to provide estimates of density, density scale height, orbit-to-orbit variability, latitudinal-seasonalvariations,longitudinalwaves,andotherphenomenainthethermosphere.Summariesaregiven of the aerobraking phase of the mission, the accelerometer data analysis methods and operational procedures, some applications to determining thermospheric properties, correlation with the Mars Global Surveyor and Odyssey missions, and some remaining issues on interpretation of the data.

Application of MGS and ODY Aerobraking Accelerometer Data to Atmospheric Modeling

This paper reviews the use of accelerometer data for determining atmospheric density during the Mars Global Survey and Mars Odyssey missions and provides preliminary results from the Mars Reconnaissance Orbiter aerobraking operations. For MGS and ODY, accelerometer data were analyzed in both near real time and post flight to provide estimates of density, density scale height, latitudinal gradients, global longitudinal wave structure, and small scale (gravity) wave spectra. MGS (Odyssey) provided data during about 850 (300) passes at altitudes ranging from 100 to 160 km (95 to 150) and covering a latitude range of 60 o N to 90 o S (30N to 90N). A summary is given of the atmospheric phenomena encountered during the aerobraking phase of the missions and of some of the scientific results based on these data. MRO is expected to provide another 500 aerobraking passes over an altitude range from 95 to 170 km and from the south pole to the equator. These data provide powerful constraints on upper atmospheric models. Accelerometer and space craft requirements to enhance scientific return are discussed.