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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.

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

The forthcoming observations by Venus Express provide an ideal opportunity to comprehensively study the atmosphere of Venus for the first time since Pioneer Venus (1978–1992), and for the first time ever in detail at polar latitudes. This article reviews some of our current knowledge from space and ground-based observations about the upper atmosphere of Venus, such as its thermal structure, the global distribution of gases and dynamics. We discuss the processes most likely responsible for phenomena such as the cold nightside cryosphere, the cloud top superrotation and waves, and highlight outstanding scientific challenges for Venus Express measurements. In particular, we describe an experiment to measure atmospheric drag using the on-board accelerometers.

Aerobraking was an enabling technology for the Mars Odyssey mission even though it involved risk due primarily to the variability of the Mars upper atmosphere. Consequently, numerous analyses based on various data types were performed during operations to reduce these risk and among these data were measurements from spacecraft accelerometers. This paper reports on the use of accelerometer data for determining atmospheric density during Odyssey aerobraking operations. Acceleration was measured along three orthogonal axes, although only data from the component along the axis nominally into the flow was used during operations. For a one second count time, the RMS noise level varied from 0.07 to 0.5 mm/s2 permitting density recovery to between 0.15 and 1.1 kg per cu km or about 2% of the mean density at periapsis during aerobraking. Accelerometer data were analyzed in near real time to provide estimates of density at periapsis, maximum density, density scale height, latitudinal gradient, longitudinal wave variations and location of the polar vortex. Summaries are given of the aerobraking phase of the mission, the accelerometer data analysis methods and operational procedures, some applications to determining thermospheric properties, and some remaining issues on interpretation of the data. Pre-flight estimates of natural variability based on Mars Global Surveyor accelerometer measurements proved reliable in the mid-latitudes, but overestimated the variability inside the polar vortex.

International audience; DYNAMO is a small multi-instrument payload aimed at characterizing current atmospheric escape, which is still poorly constrained, and improving gravity and magnetic field representations, in order to better understand the magnetic, geologic and thermal history of Mars. The internal structure and evolution of Mars is thought to have influenced climate evolution. The collapse of the primitive magnetosphere early in Mars history could have enhanced atmospheric escape and favored transition to the present arid climate. These objectives are achieved by using a low periapsis orbit. DYNAMO has been proposed in response to the AO released in February 2002 for instruments to be flown as a complementary payload onboard the CNES Orbiter to Mars (MO-07), foreseen to be launched in 2007 in the framework of the French PREMIER Mars exploration program. MO-07 orbital phase 2b (with an elliptical orbit of periapsis 170 km), and in a lesser extent 2a, offers an unprecedented opportunity to investigate by in situ probing the chemical and dynamical properties of the deep ionosphere, thermosphere, and the interaction between the atmosphere and the solar wind, and therefore the present atmospheric escape rate. Ultraviolet remote sensing is an essential complement to characterize high, tenuous, layers of the atmosphere. One Martian year of operation, with about 5,000 low passes, should allow DYNAMO to map in great detail the residual magnetic field, together with the gravity field. Additional data on the internal structure will be obtained by mapping the electric conductivity, sinergistically with the NETLANDER magnetic data. Three options have been recommended by the International Science and Technical Review Board (ISTRB), who met on July 1st and 2nd, 2002. One of them is centered on DYNAMO. The final choice, which should be made before the end of 2002, will depend on available funding resources at CNES.

A methodology is proposed for using sparsely-distributed data to calibrate physics-based tidal functions, that can then be used to reconstruct global wind, temperature and density patterns associated with diurnal and semidiurnal tides throughout Mars’ upper atmosphere (i.e., 25–200 km). The functions, called Hough Mode Extensions, maintain self-consistent internal relationships between winds, densities and temperatures, and account for changes in vertical and latitudinal shape within the dissipative thermosphere. In the present work, total mass densities from the Mars Global Surveyor accelerometer experiment during Phases I and II of aerobraking are separated into zonal wavenumber components ( k s ) viewed from the satellite reference frame. Fits are performed to two of the wavenumber components to illustrate how the technique works, and to provide insight into its potential for analysing more comprehensive datasets anticipated for the future. Results indicate Phase-II wind and temperature amplitudes for the eastward-propagating diurnal tide with s = −1( k s = 2) to be of order 10–40 ms −1 (eastward) and 2–10 K, maximizing in the equatorial region above 120 km. Similar values are found for the westward-propagating semidiurnal tide with s = 1 ( k s = 1) during Phase I at polar latitudes, a wave that is identified for the first time in MGS aerobraking data. This is the same oscillation that appears prominently in the terrestrial lower thermosphere over south pole, and is thought to be excited by nonlinear interaction between the terrestrial migrating semidiurnal tide and the stationary planetary wave with s = 1. The above wind and temperature estimates must be viewed as preliminary, however, as more data are required to more definitively constrain the fits.

Mars Global Surveyor accelerometer observations of the martian upper atmosphere revealed large variations in density with longitude during northern hemisphere spring at altitudes of 130–160 km, all latitudes, and mid-afternoon local solar times (LSTs). This zonal structure is due to tides from the surface. The zonal structure is stable on timescales of weeks, decays with increasing altitude above 130 km, and is dominated by wave-3 (average amplitude 22% of mean density) and wave-2 (18%) harmonics. The phases of these harmonics are constant with both altitude and latitude, though their amplitudes change significantly with latitude. Near the South Pole, the phase of the wave-2 harmonic changes by 90° with a change of half a martian solar day while the wave-3 phase stays constant, suggesting diurnal and semidiurnal behaviour, respectively. We use a simple application of classical tidal theory to identify the dominant tidal modes and obtain results consistent with those of General Circulation Models. Our method is less rigorous, but simpler, than the General Circulation Models and hence complements them. Topography has a strong influence on the zonal structure.

Caspofungin is the first in a new class of antifungal agents, the glucan synthesis inhibitors, that interfere with fungal cell wall synthesis. Caspofungin exhibited in vitro and in vivo efficacy against a wide range of fungi and yeasts including Aspergillus and Candida species. A complete or partial response to caspofungin therapy was seen in 40.7% of immunocompromised adults with invasive aspergillosis who did not respond to, or did not tolerate, other antifungal agents in a noncomparative multicentre study. Caspofungin was effective in patients with oropharyngeal or oesophageal candidiasis, according to the preliminary results of 2 randomised double-blind trials. Caspofungin was generally well tolerated in a multicentre noncomparative trial involving patients with invasive aspergillosis. One or more drug-related clinical adverse effects were experienced by 13.8% of caspofungin recipients (the most common were fever, nausea, vomiting and complications associated with the vein into which caspofungin was infused). The tolerability of caspofungin appeared to be better than that of amphotericin B and similar to that of fluconazole in double-blind, randomised trials involving patients with mucosal candidiasis.

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 Earth's 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 (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.

The Mars Global Surveyor (MGS) z -axis accelerometer has obtained over 200 vertical structures of thermospheric density, temperature, and pressure, ranging from 110 to 170 kilometers, compared to only three previous such vertical structures. In November 1997, a regional dust storm in the Southern Hemisphere triggered an unexpectedly large thermospheric response at mid-northern latitudes, increasing the altitude of thermospheric pressure surfaces there by as much as 8 kilometers and indicating a strong global thermospheric response to a regional dust storm. Throughout the MGS mission, thermospheric density bulges have been detected on opposite sides of the planet near 90°E and 90°W, in the vicinity of maximum terrain heights. This wave 2 pattern may be caused by topographically-forced planetary waves propagating up from the lower atmosphere.

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.

International audience; DYNAMO is a small Mars orbiter planned to be launched in 2005 or 2007, in the frame of the NASA/ CNES Mars exploration program. It is aimed at improving gravity and magnetic field resolution, in order to better understand the magnetic, geologic and thermal history of Mars, and at characterizing current atmospheric escape, which is still poorly constrained. These objectives are achieved by using a low periapsis orbit, similar to the one used by the Mars Global Surveyor spacecraft during its aerobraking phases. The proposed periapsis altitude for DYNAMO of 120–130 km, coupled with the global distribution of periapses to be obtained during one Martian year of operation, through about 5000 low passes, will produce a magnetic/gravity field data set with approximately five times the spatial resolution of MGS. Additional data on the internal structure will be obtained by mapping the electric conductivity. Low periapsis provides a unique opportunity to investigate the chemical and dynamical properties of the deep ionosphere, thermosphere, and the interaction between the atmosphere and the solar wind, therefore atmospheric escape, which may have played a crucial role in removing atmosphere and water from the planet.

Measurements of satellite drag obtained from the orbital decay of the Pioneer Venus orbiter on the nightside of Venus indicate an atomic oxygen atmosphere near 155 kilometers (an order of magnitude less dense than expected) with nighttime inferred exospheric temperatures averaging as low as 110 K. Densities at these altitudes decrease sharply from day to night, contrary to the predicted nighttime oxygen bulge. This decrease may be indicative of an unexpectedly weak transport across the evening terminator or a very strong heat sink at night that is possibly related to vertical eddy heat transport. Large periodic oscillations in density and inferred exospheric temperature are detected with a period of 5 to 6 days. We have subsequently discovered temperature variations of the same period in the stratosphere, which are tentatively interpreted as planetary-scale waves that may propagate upward producing the periodic variations in the thermosphere and exosphere. The peak-to-peak amplitude of the temperature oscillations associated with these waves apparently increases with altitude approximately as follows: 1 K (70 kilometers), 3 K (90 kilometers), 40 K (155 kilometers). Inferred nighttime exospheric temperatures are found to be asymmetric relative to midnight, minimizing on the morning side. The possibility of superrotation of the thermosphere, and exosphere is discussed.

Atmospheric drag measurements obtained from the study of the orbital decay of Pioneer Venus 1 indicate that atomic oxygen predominates in the Venus atmosphere above 160 kilometers. Drag measurements give evidence that conditions characteristic of a planetary thermosphere disappear near sundown, with inferred exospheric temperatures sharply dropping from approximately 300 K to less than 150 K. Observed denisities are generally lower than given by theoretical models.

Neutral upper atmosphere response to solar activity variations, discussing temperature, density and helium/oxygen composition

Monthly averaged variations of global ozone determined from infrared interferometer spectrometer measurements aboard Nimbus 4 are shown to be consistently in agreement with monthly averaged variations of solar activity. The relationship between monthly changes of ozone and solar activity are found to be generally consistent with the longer-term variations of ozone over the last solar cycle and can be accounted for by assuming monthly solar flux variations near 0.2 micron of about 2%.