ALBERT

Description: The 3.0-micrometers water of hydration absorption feature observed in the IR photometry of many low-albedo and some medium-albedo asteroids strongly correlates with the 0.7-micrometers Fe(+2) to Fe(+3) oxidized iron absorption feature observed in narrowband spectrophotometry of these asteroids. Using this relationship, an empirical algorithm for predicting the presence of water of hydration in the surface material of a Solar System body using photometry obtained through the Eight-Color Asteroid Survey nu (0.550 micrometers), w (0.701 micrometers), and x (0.853 micrometers) filters was developed and applied to the ECAS photometry of asteroids and outer planet satellites. The percentage of objects in low-albedo, outer main-belt asteroid classes that test positively for water of hydration increases from P to B to C to G class and correlates linearly with the increasing mean albedos of those objects testing positively. The medium-albedo M-class asteroids do not test positively in large number using this algorithm. Aqueously altered asteroids dominate the Solar System population between heliocentric distances of 2.6 to 3.5 AU, bracketing the Solar System region where the aqueous alteration mechanism operated most strongly. One jovian satellite, J VI Himalia, and one saturnian satellite. Phoebe, tested positively for water of hydration, supporting the hypothesis that these may be captured C-class asteroids from a postaccretional dispersion. The proposed testing technique could be applied to an Earth-based survey of asteroids or a space-probe study of an asteroid's surface characteristic in order to identify a potential water source.

Description: The infrared transmission spectra and photochemical behavior of various organic compounds isolated in solid N2 ices, appropriate for applications to Triton ad Pluto, are presented. It is shown that excess absorption in the surface spectra of Triton and Pluto, i.e., absorption not explained by present models incorporating molecules already identified on these bodies (N2, CH4, CO, and CO2), that starts near 4450/cm (2.25 microns) and extends to lower frequencies, may be due to alkanes (C(n)H(2n+2)) and related molecules frozen in the nitrogen. Branched and linear alkanes may be responsible. Experiments in which the photochemstry of N2: CH4 and N2: CH4: CO ices was explored demonsrtrate that the surface ices of Triton and Pluto may contain a wide variety of additional species containing H, C, O, and N. Of these, the reactive molecule diazomethane, CH2N2, is particularly important since it may be largely responsible for the synthesis of larger alkanes from CH4 and other small alkanes. Diazomethane would also be expected to drive chemical reactions involving organics in the surface ices of Triton and Pluto toward saturation, i.e., to reduce multiple CC bonds. The positions and intrinsic strengths (A values) of many of the infrared absorption bands of N2 matrix-isolated molecules of relevance to Triton and Pluto have also been determined. These can be used to aid in their search and to place constraints on their abundances.

Description: This paper addresses the design considerations and strategies for astrophysical observations as key elements of an international solar system exploration program. Emphasis is placed on the technical and programmatic challenges and opportunities associated with an evolving program of lunar-based astronomy. Both robotic and human tended facilities are discussed ranging from relatively small meter-class transit telescopes to large interferometer and filled-aperture systems.

Description: A method is presented by which measured modes and frequencies from a modal test can be used to determine the location and magnitude of damage in a space struss structure. The damage is located by computing the Euclidean distances between the measured mode shapes and the best achievable eigenvectors. The best achievable eigenvectors are the projection of the measured mode shapes onto the subspace defined by the refined analytical model of the structure and the measured frequencies. Loss of both stiffness and mass properties can be located and quantified. To examine the performance of the method when experimentally measured modes are employed, various damage detection studies using a laboratory eight-bay truss structure were conducted. The method performs well even though the measurement errors inevitably make the damage location more difficult.

Description: This report presents the most recent spherical harmonic topography model of Venus developed at Jet Propulsion Laboratory. It was produced by a spherical harmonic analysis of the most complete set of Magellan altimetry data, augmented by Pioneer Venus and Venera data. The harmonic coefficients of the topography were computed to degree and order 360. Compared to previous topography models, this one has the highest correlation with the gravity field of Venus.

Description: The 500-Myr average crater retention age for Venus has raised questions about the present-day level of tectonic activity. In this study we examine the relationship between the gravity and topography of four large volcanic swells, Beta, Atla, Bell, and Western Eistla Regiones, for clues about their stage evolution. The Magellan line-of-sight gravity data are inverted using a point mass model of the anomalous mass to solve for the local vertical gravity field. Spectral admittance calculated from both the local gravity inversions and a spherical harmonic model is compared to three models of compensation: local compensation, a 'flexural' model with local and regional compensation of surface and subsurface loads, and a 'hotspot' model of compensation that includes top loading by volcanoes and subsurface loading due to a deep, low density mass anomaly. The coherence is also calculated in each region, but yields an elastic thickness estimate only at Bell Regio. In all models, the long wavelengths are compensated locally. Our results may indicate a relatively old, possibly inactive plume.

Description: Following the project's first major design review, some unresolved technical issues, mainly centered on details of how to integrate Russian hardware into the U.S./international space station, remain. No 'show stoppers' were found in the review. Specific open technical issues are discussed in this article.

Description: Recent mapping studies west of Elysium Mons, Mars, have pinpointed subice features that suggest the existence of a frozen paleolake in Utopia Planitia as recently as 1.8 billion years ago. The subice features are interpreted to be hyaloclastic ridges and hills, table moutains, associated joekulhalaup deposits, and fluvial channels. Photoclinometric studies of these features and of a basal scarp around the northwest flank of Elysium Mons interpreted to have been an ice-sheet boundary indicate that the maximum thickness of ice within the basin may have been about 180 m. This thickness of ice during a relatively late stage of Martian geologic history would have important implications concerning the atmospheric, the climatic, and possibly the exobiologic history of the planet.

Description: During the final, low solar activity phase of the Pioneer Venus (PV) mission, the Orbiter Ion Mass Spectrometer (OIMS) measurements found all ion species, in the midnight-dusk sector, reduced in concentration relative to that observed at solar maximum. Molecular ion species comprised a greater part of the total ion concentration as O(+) and H(+) had the greatest depletions. The nightside ionospheric states were strikingly similar to the isolated solar maximum 'disappearing' ionospheres. Both are very dynamic states characterized by a rapidly drifting plasma and 30-100 eV superthermal O(+) ions.

Description: In October, 1992 the Pioneer Venus Orbiter entered the atmosphere of Venus, ending nearly 14 years of observations at Venus. Prior to the entry into the atmosphere and subsequent loss of the spacecraft careful management of spacecraft resources had allowed the acquisition of much low altitude data over the nightside of the planet. The long duration of the Pioneer Venus mission has enabled us to study the ionosphere and atmosphere of Venus under different levels of solar activity.

Description: Atmospheric drag measurements from the orbital decay of the Pioneer Venus Orbiter and Magellan spacecraft have recently been obtained of the Venus dayside and nightside atmosphere between 130 and 210 km during a period of low solar activity. These new measurements, combined with the earlier Pioneer Venus drag measurements (1978-80) obtained near the maximum of the 11-year solar cycle, have allowed the detection of the detailed response of temperature, atomic oxygen and carbon dioxide to solar variations. We have found a weak but detectable temperature response on the dayside which is in accord with the response predicted by Keating and Bougher when they assumed very strong CO2 radiative cooling resulting from atomic oxygen exciting CO2 into 15 micron emission. This same radiative process may cause strong cooling in the Earth's upper atmosphere with the doubling of CO2 in the future. With decreasing solar activity, the O/CO2 ratio in the lower thermosphere is found to decrease, apparently due to decreased photodissociation of CO2 and lower temperatures. The percent decrease in atomic oxygen with decreasing solar activity on the dayside is found to be approximately the same as the percent decreases of atomic oxygen transported to the nightside. A very weak response of nightside temperatures to solar activity variations has also been detected.

Description: A coupled, trajectory-based flowfield and material thermal-response analysis is presented for the European Space Agency proposed Rosetta comet nucleus sample return vehicle. The probe returns to earth along a hyperbolic trajectory with an entry velocity of 16.5 km/s and requires an ablative heat shield on the forebody. Combined radiative and convective ablating flowfield analyses were performed for the significant heating portion of the shallow ballistic entry trajectory. Both quasisteady ablation and fully transient analyses were performed for a heat shield composed of carbon-phenolic ablative material. Quasisteady analysis was performed using the two-dimensional axisymmetric codes RASLE and BLIMPK. Transient computational results were obtained from the one-dimensional ablation/conduction code CMA. Results are presented for heating, temperature, and ablation rate distributions over the probe forebody for various trajectory points. Comparison of transient and quasisteady results indicates that, for the heating pulse encountered by this probe, the quasisteady approach is conservative from the standpoint of predicted surface recession.

Description: The ion and electron momentum equations, along with Ampere's law, are solved for the ion and electron drift velocities and the electric field in the subsolar Venus ionosphere, assuming a partially ionized gas and a single ion species having the ion mean mass. All collision terms among the ions, electrons and neutral particles are retained in the equations. A general expression for the evolution of the magnetic field is derived and compared with earlier expressions. Subsolar region data in the altitude range 150-300 km from the Pioneer Venus Orbiter are used to calculate altitude profiles of the components of the current due to the electric field, gradients of pressure, and gravity. Altitude profiles of the ion and electron velocities as well as the electric field, electrodynamic heating, and the energy density are determined. Only orbits having a complete set of measured plasma temperatures and densities, neutral densities, and magnetic field were considered for analysis; the results are shown only for orbit 202. The vertical velocity at altitudes above 220 km is upgoing for orbit 202. This result is consistent with observations of molecular ions at high altitudes and of plasma flow to the nightside, both of which require upward velocity of ions from the dayside ionosphere. Above about 230 km the momentum equations are extremely sensitive to the altitude profiles of density, temperature, and magnetic field.

Description: Advanced Satellite for Cosmology and Astrophysics (ASCA) is a high-throughput X-ray astronomy observatory which is capable of simultaneous imaging and spectroscopic observations over a wide energy range 0.5-10 keV. The scientific capabilities of ASCA and some aspects related to its operation and observations are briefly described.

Description: We report here analyses of olivines and pyroxenes, and petrofabrics of 27 chondritic interplanetary dust particles (IDPs), comparing those from anhydrous and hydrous types. Approximately 40% of the hydrous particles contain diopside, a probable indicator of parent body thermal metamorphism, while this mineral is rarely present in the anhydrous particles. Based on this evidence, we find that hydrous and anhydrous IDPs are, in general, not directly related, and we conclude that olivine and pyroxene major-element compositions can be used to help discriminate between IDPs that are (1) predominantly nebular condensates, and lately resided in anhydrous or icy (no liquids) primitive parent bodies, and (2) those originating from more geochemically active parent bodies (probably hydrous and anhydrous asteroids).

Description: This paper reports on a magnetic field phenomenon, hereafter referred to as null fields, which were discovered during the inbound pass of the recent flyby of Jupiter by the Ulysses spacecraft. These null fields which were observed in the outer dayside magnetosphere are characterised by brief but sharp decreases of the field magnitude to values less than 1 nT. The nulls are distinguished from the current sheet signatures characteristic of the middle magnetosphere by the fact that the field does not reverse across the event. A field configuration is suggested that accounts for the observed features of the events.

Description: Geochemical profiles of surface units, impact, and volcanic features are studied in detail to determine the underlying structure in an area of extensive mare/highland interface, Sinus Amoris. This study region includes and surrounds the northeastern embayment of Mare Tranquillitatis. The concentrations of two major rock-forming elements (Mg and Al), which were derived from the Apollo 15 orbital geochemical measurements, were used in this study. Mapped units and deposits associated with craters in the northwestern part of the region tend to have correlated low Mg and Al concentrations, indicating the presence of Potassium (K)-Rare Earth Elements (REE)-Phosphorus (P) (KREEP)-enriched basalt. Found along the northeastern rim of Tranquillitatis were areas with correlated high Mg and Al concentration, indicating the presence of troctolite. Distinctive west/east and north/south trends were observed in the concentrations of Mg and Al, and, by implication, in the distribution of major rock components on the surface. Evidence for a systematic geochemical transition in highland or basin-forming units may be observed here in the form of distinctive differences in chemistry in otherwise similar units in the western and eastern portions of the study region.

Description: The aerobraking orbital activities of Magelland during the gravity mapping of Venus are discussed. The goal of aerobraking was to circularize Magellan's orbit. By aerobraking the spacecraft into a nearly circula orbit, the Magellan team was able to provide scientists with a different data set to deepen their understanding of what is going on beneath Venus' surface. Before undertaking its gravity-mapping mission, Magellan completed three cycles of radar mapping. This repeated coverage allowed the spacecraft to see some of Venus' geologic features from different viewing angles. Various aspects of the mission are discussed, and maps of Venus are presented.

Description: We have combined the most recent Pioneer Venus Orbiter (PVO) and Magellan (MGN) data with the earlier 1978-1982 PVO data set to obtain a new 60th degree and order spherical harmonic gravity model and a 120th degree and order spherical harmonic topography model. Free-air gravity maps are shown over regions where the most marked improvement has been obtained (Ishtar-Terra, Alpha, Bell and Artemis). Gravity versus topography relationships are presented as correlations per degree and axes orientation.

Description: We examine the electromagnetic (EM) bias by using retracked TOPEX altimeter data. In contrast to previous studies, we use a parameterization of the EM bias which does not make stringent assumptions about the form of the correction or its global behavior. We find that the most effective single parameter correction uses the altimeter-estimated wind speed but that other parameterizations, using a wave age related parameter of significant wave height, may also significantly reduce the repeat pass variance. The different corrections are compared, and their improvement of the TOPEX height variance is quantified.

Description: Monthly Ku band sigma(sub 0) and significant wave height (SWH) histograms from the NASA altimeter on the TOPEX/POSEIDON satellite are preseneted for January through June 1993 for three latitude bands between +/- 60 degrees. The data are compared to distributions from the Geosat mission for the same months in 1987-1989. Generally, the distributions agree quite well, although there are some seasonal/hemispherical differences. The sigma(sub 0) comparison reveals an overall bias between the two altimeters with the TOPEX sigma(sub 0) higher by about 0.7 dB, which is consistent with algorithm improvements for TOPEX. The SWH distributions show strong hemispherical/seasonal changes. The seasonal/hemispherical differences between TOPEX and Geosat are consistent for SWH and sigma(sub 0). The joint distribution of sigma(sub 0) and SWH is extremely stable friom month to month. The typical SWH is independent of sigma(sub 0) for sigma(sub 0) greater than 11.3 dB. The minimum SWH grows exponentially with wind speed. This joint distribution may be useful for understanding electromagnetic bias in altimeter measurements. Finally, altimeter data are compared to buoy values from 21 overflights of the NASA verification site near Pt. Conception, California. Wave heights agree well with an root mean square (RMS) difference of only 0.2 m. Altimeter sigma(sub 0) values are compared to buoy wind speeds. The results are consistent with the -0.7 dB sigma(sub 0) offset from the histogram comparisons.

Description: To assess the accuracy of the TOPEX altimeter data, we have reprocessed the raw altimeter waveform data using more sophisticated algorithms than those implemented in the altimeter hardware. We discuss systematic contamination of the waveform which we have observed and its effect on very long wavelength errors. We conclude that these systematic errors are responsible for a very long wavelength error whose peak-to-peak magnitude for the Ku band altimeter is of the order of 1 cm. We also examine the ability of retracked data to reduce the repeat pass variance and correct for significant wave height (SWH) and acceleration dependent errors. We find that the ground postprocessing contains SWH dependent biases which depend on the altimeter fine height correction.

Description: The routine ground processing of data from the NASA radar altimeter of TOPEX/POSEIDON includes instrument corrections for the effects of significant wave height and attitude angle changes on the altimeter's estimates of range, backscattered power, and significant wave height. This paper describes how these instrument corrections were generated and how they are applied. Detailed waveform fitting to telemetered waveform samples is use to assess the effectiveness of the corrections. There are several altimeter hardware-caused small waveform departures from the model waveforms and these departures, designated waveform 'features', are described in detailed. A consequence of the waveform features, and their positioning relationship to range rate, is that range data for ground tracks moving toward the equator may differ systematically by about a centimeter compared to range data for ground tracks moving away from the equator. The results and discussion are limited to side A of the redundant altimeter, as only side A has been operated on orbit.

Description: Results of the in-flight calibration and performance evaluation campaign for the TOPEX/POSEIDON microwave radiometer (TMR) are presented. Intercomparisons are made between TMR and various sources of ground truth, including ground-based microwave water vapor radiometers, radiosondes, global climatological models, special sensor microwave imager data over the Amazon rain forest, and models of clear, calm, subpolar ocean regions. After correction for preflight errors in the processing of thermal/vacuum data, relative channel offsets in the open ocean TMR brightness temperatures were noted at the approximately = 1 K level for the three TMR frequencies. Larger absolute offsets of 6-9 K over the rain forest indicated a approximately = 5% gain error in the three channel calibrations. This was corrected by adjusting the antenna pattern correction (APC) algorithm. AS 10% scale error in the TMR path delay estimates, relative to coincident radiosondes, was corrected in part by the APC adjustment and in part by a 5% modification to the value assumed for the 22.235 FGHz water vapor line strength in the path delay retrieval algorithm. After all in-flight corrections to the calibration, TMR global retrieval accuracy for the wet tropospheric range correction is estimated at 1.1 cm root mean square (RMS) with consistent peformance under clear, cloudy, and windy conditions.

Description: The NASA altimeter on board TOPEX/POSEIDON exploits the difference in the delays of the Ku and C band radar pulses to estimate an ionosphere correction to the range measurement. The dependence of the ionosphere correction on ocean and satellite parameters is less than 1 cm. The standard deviation of the 1-s averaged ionosphere correction depends on the height of the ocean waves and ranges from 5 to 14 mm. The accuracy of the ionosphere correction is better than 1 cm at the 1 sigma confidence level. The ionosphere correction should be averaged over 140 km (20 s) along track in order to minimize its noise without sacrificing its accuracy. Ionosphere models must achieve an independent sample spacing of 900 km or less in order to allow a single-frequency altimeter to have an ionosphere correction comparable in accuracy to that of the NASA dual-frequency altimeter.

Description: Estimates of the effectiveness of an altimetric correction, and interpretation of sea level variability as a response to atmospheric forcing, both depend upon assuming that residual errors in altimetric corrections are uncorrelated among themselves and with residual sea level, or knowing the correlations. Not surprisingly, many corrections are highly correlated since they involve atmospheric properties and the ocean surface's response to them. The full corrections (including their geographically varying time mean values), show correlations between electromagnetic bias (mostly the height of wind waves) and either atmospheric pressure or water vapor of -40%, and between atmospheric pressure and water vapor of 28%. In the more commonly used collinear differences (after removal of the geographically varying time mean), atmospheric pressure and wave height show a -30% correlation, atmospheric pressure and water vapor a -10% correlation, both pressure and water vapor a 7% correlation with residual sea level, and a bit surprisingly, ionospheric electron content and wave height a 15% correlation. Only the ocean tide is totally uncorrelated with other corrections or residual sea level. The effectiveness of three ionospheric corrections (TOPEX dual-frequency, a smoothed version of the TOPEX dual-frequency, and Doppler orbitography and radiopositioning integrated by satellite (DORIS) is also evaluated in terms of their reduction in variance of residual sea level. Smooth (90-200 km along-track) versions of the dual-frequency altimeter ionosphere perform best both globally and within 20 deg in latitude from the equator. The noise variance in the 1/s TOPEX inospheric samples is approximately (11 mm) squared, about the same as noise in the DORIS-based correction; however, the latter has its error over scales of order 10(exp 3) km. Within 20 deg of the equator, the DORIS-based correction adds (14 mm) squared to the residual sea level variance.

Description: We report infrared heterodyne spectroscopy (lambda/delta lambda is approximately 10(exp 6)) of C2H6 emission at 11.9 microns from the northern Jovian auroral region, in observations conducted over December 2-7, 1989. Accurately measured line shapes provide information on C2H6 abundance as well as temperature and permit retrieval of the source pressure region. Enhanced emission was observed in the longitude range approximately 150-180 deg at approximately 60 deg north latitude, approximately corresponding to the CH4 7.8 micron hot spot and the region of brightest UV aurora. Significant brightness variations were observed in the hot spot emissions on a time scale of approximately 20 hours. Analysis of the brightest hot spot spectra indicates C2H6 mole fractions of approximately (6.3-6.8) x 10(exp -6) at temperatures of approximately 182-184 K at 1 mbar, compared to mole fractions of (3.8 +/- 1.4) x 10(exp -6) averaged over spectra outside the hot spot at a temperature of approximately 172 K at the same pressure. Fixing the mole fraction to the lower limit retrieved in the quiescent (non-hot spot) region allows the temperature at 1 mbar to be as high as approximately 200 K within the hot spot. These results provide upper limits to the temperature increase near the source of the C2H6 thermal infrared emission. Combined with results from similar measurements of ethylene emission probing the approximately 10-microbar region (Kostiuk et al., this issue), altitude information on the thermal structure of the Jovian auroral stratosphere can be obtained for the first time.

Description: Doppler tracking data of three orbiting spacecraft have been reanalyzed to develop a new gravitational field model for the planet Mars, Goddard Mars Model 1 (GMM-1). This model employs nearly all available data, consisting of approximately 1100 days of S band tracking data collected by NASA's Deep Space Network from the Mariner 9 and Viking 1 and Viking 2 spacecraft, in seven different orbits, between 1971 and 1979. GMM-1 is complete to spherical harmonic degree and order 50, which corresponds to a half-wavelength spatial resolution of 200-300 km where the data permit. GMM-1 represents satellite orbits with considerably better accuracy than previous Mars gravity models and shows greater resolution of identifiable geological structures. The notable improvement in GMM-1 over previous models is a consequence of several factors: improved computational capabilities, the use of otpimum weighting and least squares collocation solution techniques which stabilized the behavior of the solution at high degree and order, and the use of longer satellite arcs than employed in previous solutions that were made possible by improved force and measurement models. The inclusion of X band tracking data from the 379-km altitude, nnear-polar orbiting Mars Observer spacecraft should provide a significant improvement over GMM-1, particularly at high latitudes where current data poorly resolve the gravitational signature of the planet.

Description: A series of surface-modified clays containing nanophase (np) iron/oxyhydroxides of extremely small particle sizes, with total iron contents as high as found in Mars soil, were prepared by iron deposition on the clay surface from ferrous chloride solution. Comprehensive studies of the iron mineralogy in these 'Mars-soil analogs' were conducted using chemical extractions, solubility analyses, pH and redox, x ray and electron diffractometry, electron microscopic imaging specific surface area and particle size determinations, differential thermal analyses, magnetic properties characterization, spectral reflectance, and Viking biology simulation experiments. The clay matrix and the procedure used for synthesis produced nanophase iron oxides containing a certain proportion of divalent iron, which slowly converts to more stable, fully oxidized iron minerals. The noncrystalline nature of the iron compounds precipitated on the surface of the clay was verified by their complete extractability in oxalate. Lepidocrocite (gamma-FeOOH) was detected by selected area electron diffraction. It is formed from a double iron Fe(II)/Fe(III) hydroxyl mineral such as 'green rust', or ferrosic hydroxide. Magnetic measurements suggested that lepidocrocite converted to the more stable meaghemite (gamma-Fe203) by mild heat treatment and then to nanophase hematite (aplha-Fe203) by extensive heat treatment. Their chemical reactivity offers a plausible mechanism for the somewhat puzzling observations of the Viking biology experiments. Their unique chemical reactivities are attributed to the combined catalytic effects of the iron oxide/oxyhydroxide and silicate phase surfaces. The mode of formation of these (nanophase) iron oxides on Mars is still unknown.

Description: Spatial correlation among densely packed particles can substantially change their single-scattering properties, thus making questionable the applicability of the independent scattering approximation in calculations of light scattering by planetary regoliths. The same problem arises in geophysics in light scattering computations for snow, frosts, and bare soil. In this paper, we use a dense-medium light-scattering theory based on the introduction of the static structure factor to calculate asymmetry parameters of the phase function for densely packed particles with real refractive indices 1.31 and 1.66, approximating water ice and soil particles, respectively, and imaginary refractive indices 0, 0.01, and 0.3. For sparsely distributed, independently scattering grains, the calculated asymmetry parameters are always positive and always larger than those for densely packed particles. For densely packed grains, the asymmetry parameters may be negative but only for radius-to-wavelength ratios from about 0.1 to about 0.4. With decreasing particle size, the calculated asymmetry parameters tend to zero independently of the compaction state. In the geometrical optics regime, the asymmetry parameters for densely packed scatterers are positive and very close to those for independently scattering grains. These results may have important implications for remote sensing of the Earth and solid planetary surfaces. In particular, it is demonstrated that negative asymmetry parameters derived with some approximate multiple-scattering theories may be physically irrelevant and can be the result of using an inaccurate bidirectional reflection function combined with the ill-conditionally of the inverse scattering problem.

Description: The 3-5 micrometer thermal emission of the nightside of Venus, recorded by the Near-Infrared Mapping Spectrometer (NIMS) instrument at the time of the Galileo flyby of Venus, is analysed to infer the properties of the upper cloud boundary. From the global maps of Venus at fixed wavelengths, the limb darkening of the flux is measured at several latitudes, within each infrared channel. By using the nominal Pioneer Venus thermal profile, these data give access to two parameters: the cloud deck temperature and the cloud scale height. It is verified independently, from the NIMS spectra, that this thermal profile is consistent with all the NIMS observations, and that the thermal structure does not vary significantly in the latitude range (25 deg S, 30 deg N). Within this range, the cloud scale height is found to be constant with latitude, and is H = 5.2 km, with an accuracy of about 15%, taking into account the various sources of theoretical and observational uncertainties. At higher latitudes, the temperature profile becomes more isothermal and the presented method to retrieve H is no longer valid.

Description: A large number of i.r. spectra of Venus was obtained using the Near-Infrared Mapping Spectrometer (NIMS) on the Galileo spacecraft, during the February 1990 encounter. Preliminary results show an apparent increase in the tropospheric CO volume mixing ratio (vmr) in the northern polar region. Other possible explanations of the observations are examined and rejected and an increase of the CO abundance north of 47 deg N of (35 +/- 15)% is inferred. Some possible causes of this enhancement are suggested.

Description: The spectroscopic data of the Near-Infrared Mapping Spectrometer (NIMS), recorded during the Galileo flyby of Venus, are analysed to retrieve the water vapor abundance variations in the lower atmosphere of Venus at night. The 1.18 micrometer spectral window, which probes altitude levels below 20 km, is used for this purpose. Constraints on the CO2 continuum and far-wing opacity from existing ground-based high-resolution observations are included in the modelling of the NIMS spectra. The NIMS measurements can be fitted with a water vapor mixing ratio of 30 +/- 15 ppm, in agreement with analyses of ground-based nightside observations. The water vapor abundance shows no horizontal variations exceeding 20% over a wide latitude range (40 deg S, 50 deg N) on the nightside of Venus. Within the same selection of NIMS spectra, a large enhancement in the O2 fluorescence emission at 1.27 micrometer is observed at a latitude of 40 deg S, over a spatial area about 100 km wide.

Description: Using Venus nightside data obtained by the Galileo Near-Infrared Mapping Spectrometer (NIMS), we have studied the correlation of 1.74 and 2.30 micrometer radiation which is transmitted through the clouds. Since the scattering and absorption properties of the cloud particles are different at these two wavelengths, one can distinguish between abundance variations and variations in the properties of the cloud particles themselves. The correlation of intensities shows a clustering of data into five distinct branches. Using radiative transfer calculations, we interpret these branches as regions of distinct but different mixes of Mode 2' and 3 particles. The data and calculations indicate large differences in these modal ratios, the active cloud regions varying in content from nearly pure Mode 2' particles to almost wholly Mode 3. The spatial distribution of these branches shows large scale sizes and both hemispheric symmetries and asymmetries. High-latitude concentrations of large particles are seen in both hemispheres and there is banded structure of small particles seen in both the North and South which may be related. The mean particle size in the Northern Hemisphere is greater than found in the South. If these different branch regions are due to mixing of vertically stratified source regions (e.g. photochemical and condensation source mechanisms), then the mixing must be coherent over very large spatial scales.

Description: The Radio and Plasma Wave Science (RPWS) experiment being built for the Cassini spacecraft will study a wide range of plasma and radio wave phenomena in the magnetosphere of Saturn and will also make valuable measurements during the cruise phase and at other encounters. A feature of data from wave receivers is the capability of producing vastly more data than the spacecraft telemetry link is capable of transmitting back to the Earth. Thus, techniques of on-board data compression and data reduction are important. The RPWS instrument has one processor dedicated to data compression tasks.

Description: The first comprehensive discussion of the south seasonal polar cap spectra obtained by the Mariner 7 infrared spectrometer in the short-wavelength region (2-4 microns) is presented. The infrared spectra is correlated with images acquired by the wide-angle camera. Significant spectral variation is noted in the cap interior and regions of varying water frost abundance, CO2 ice/frost cover, and CO2-ice path length can be distinguished. Many of these spectral variations correlate with heterogeneity noted in the camera images, but certain significant infrared spectral variations are not discernible in the visible. Simple reflectance models are used to classify the observed spectral variations into four regions. Region I is at the cap edge, where there is enhanced absorption beyond 3 microns inferred to be caused by an increased abundance of water frost. The increase in water abundance over that in the interior is on the level of a few parts per thousand or less. Region II is the typical cap interior characterized by spectral features of CO2 ice at grain sizes of several millimeters to centimeters. These spectra also indicate the presence of water frost at the parts per thousand level. A third, unusual region (III), is defined by three spectra in which weak CO2 absorption features are as much as twice as strong as in the average cap spectra and are assumed to be caused by an increased path length in the CO2. Such large paths are inconsistent with the high reflectance in the visible and at 2.2 microns and suggest layered structures or deposition conditions that are not accounted for in current reflectance models. The final region (IV) is an area of thinning frost coverage or transparent ice well in the interior of the seasonal cap. These spectra are a combination of CO2 and ground signatures.

Description: We report new measurements of the sodium emission intensity seen in a line of sight just above the surface of the Moon. These data show a strong dependence on lunar phase. The emission intensity decreases from a maximum around first quarter (phase angle 90 deg) to very small values near full Moon (phase angle 0 deg). This suggests that the rate of sodium vapor production from the lunar surface is largest at the subsolar point and becomes small near the terminator. However, the sodium emission near full Moon falls below that which would be expected for solar photon-driven processes. Since the solar wind flux decreases substantially when the Moon enters the Earth's magnetotail near full Moon, while the global solar photon flux is undiminished, we suggest that solar wind sputtering is the dominant process for sodium production.

Description: Infrared diffuse reflectance spectra (2.53-25 microns) of some carbonaceous (C) chondrites were measured. The integrated intensity of the absorption bands near 3 microns caused by hydrous minerals were compared with the modal content of hydrous minerals for the meteorites. The CM and CI chondrites show larger values of the intergated intensity than those of the unique C chondrites Y82162, Y86720 and B7904, suggesting that the amount of hydrous minerals in the CM and CI chondrites is larger, which supports the contention that hydrous minerals were dehydrated by thermal metamorphism in the unique chondrites. Orgueil (CI) has the largest value of the integrated intensity among the C chondrites we measured and shows a sharp absorption band at 3685/cm (2.71 microns) that is not seen in the spectra of the CM chondrites. There is an excellent correlation between the observed hydrogen content in C chondrites and the integrated intensity. The CM chondrites show a wide variation in the strength of absorption bands at 1470/cm (6.8 microns), despite the similarity in absorption features near 3 micron for all CM chondites. The 1470/cm band could be due to the presence of some hydrocarbons but may also be a result of terrestrial alteration processes.

Description: Oxygen production from a lunar rock has been experimentally demonstrated for the first time. A 10 g sample of high-Ti basalt 70035 was reduced with hydrogen in seven experiments at temperatures of 900-1050 C and pressures of 14.7-150 psia. In all experiments, water evolution began almost immediately and was essentially complete in tens of minutes. Oxygen yields ranged from 2.93 to 4.61% of the starting sample weight, and showed weak dependence on temperature and pressure. Analysis of the solid samples demonstrated total reduction of Fe(2+) in ilmenite and small degrees of reduction in olivine and pyroxene. Ti O2 was also partially reduced to one or more suboxides. Data from these experiments provide a basis for predicting the yield of oxygen from lunar basalt as well as new constraints on natural reduction in the lunar regolith.

Description: The DET/MPS programs model and simulate the Direct Energy Transfer and Multimission Spacecraft Modular Power System in order to aid both in design and in analysis of orbital energy balance. Typically, the DET power system has the solar array directly to the spacecraft bus, and the central building block of MPS is the Standard Power Regulator Unit. DET/MPS allows a minute-by-minute simulation of the power system's performance as it responds to various orbital parameters, focusing its output on solar array output and battery characteristics. While this package is limited in terms of orbital mechanics, it is sufficient to calculate eclipse and solar array data for circular or non-circular orbits. DET/MPS can be adjusted to run one or sequential orbits up to about one week, simulated time. These programs have been used on a variety of Goddard Space Flight Center spacecraft projects. DET/MPS is written in FORTRAN 77 with some VAX-type extensions. Any FORTRAN 77 compiler that includes VAX extensions should be able to compile and run the program with little or no modifications. The compiler must at least support free-form (or tab-delineated) source format and 'do do-while end-do' control structures. DET/MPS is available for three platforms: GSC-13374, for DEC VAX series computers running VMS, is available in DEC VAX Backup format on a 9-track 1600 BPI tape (standard distribution) or TK50 tape cartridge; GSC-13443, for UNIX-based computers, is available on a .25 inch streaming magnetic tape cartridge in UNIX tar format; and GSC-13444, for Macintosh computers running AU/X with either the NKR FORTRAN or AbSoft MacFORTRAN II compilers, is available on a 3.5 inch 800K Macintosh format diskette. Source code and test data are supplied. The UNIX version of DET requires 90K of main memory for execution. DET/MPS was developed in 1990. A/UX and Macintosh are registered trademarks of Apple Computer, Inc. VMS, DEC VAX and TK50 are trademarks of Digital Equipment Corporation. UNIX is a registered trademark of AT&T Bell Laboratories.

Description: The DET/MPS programs model and simulate the Direct Energy Transfer and Multimission Spacecraft Modular Power System in order to aid both in design and in analysis of orbital energy balance. Typically, the DET power system has the solar array directly to the spacecraft bus, and the central building block of MPS is the Standard Power Regulator Unit. DET/MPS allows a minute-by-minute simulation of the power system's performance as it responds to various orbital parameters, focusing its output on solar array output and battery characteristics. While this package is limited in terms of orbital mechanics, it is sufficient to calculate eclipse and solar array data for circular or non-circular orbits. DET/MPS can be adjusted to run one or sequential orbits up to about one week, simulated time. These programs have been used on a variety of Goddard Space Flight Center spacecraft projects. DET/MPS is written in FORTRAN 77 with some VAX-type extensions. Any FORTRAN 77 compiler that includes VAX extensions should be able to compile and run the program with little or no modifications. The compiler must at least support free-form (or tab-delineated) source format and 'do do-while end-do' control structures. DET/MPS is available for three platforms: GSC-13374, for DEC VAX series computers running VMS, is available in DEC VAX Backup format on a 9-track 1600 BPI tape (standard distribution) or TK50 tape cartridge; GSC-13443, for UNIX-based computers, is available on a .25 inch streaming magnetic tape cartridge in UNIX tar format; and GSC-13444, for Macintosh computers running AU/X with either the NKR FORTRAN or AbSoft MacFORTRAN II compilers, is available on a 3.5 inch 800K Macintosh format diskette. Source code and test data are supplied. The UNIX version of DET requires 90K of main memory for execution. DET/MPS was developed in 1990. A/UX and Macintosh are registered trademarks of Apple Computer, Inc. VMS, DEC VAX and TK50 are trademarks of Digital Equipment Corporation. UNIX is a registered trademark of AT&T Bell Laboratories.

Description: The Flexible Spacecraft Dynamics and Control program (FSD) was developed to aid in the simulation of a large class of flexible and rigid spacecraft. FSD is extremely versatile and can be used in attitude dynamics and control analysis as well as in-orbit support of deployment and control of spacecraft. FSD has been used to analyze the in-orbit attitude performance and antenna deployment of the RAE and IMP class satellites, and the HAWKEYE, SCATHA, EXOS-B, and Dynamics Explorer flight programs. FSD is applicable to inertially-oriented spinning, earth oriented, or gravity gradient stabilized spacecraft. The spacecraft flexibility is treated in a continuous manner (instead of finite element) by employing a series of shape functions for the flexible elements. Torsion, bending, and three flexible modes can be simulated for every flexible element. FSD can handle up to ten tubular elements in an arbitrary orientation. FSD is appropriate for studies involving the active control of pointed instruments, with options for digital PID (proportional, integral, derivative) error feedback controllers and control actuators such as thrusters and momentum wheels. The input to FSD is in four parts: 1) Orbit Construction FSD calculates a Keplerian orbit with environmental effects such as drag, magnetic torque, solar pressure, thermal effects, and thruster adjustments; or the user can supply a GTDS format orbit tape for a particular satellite/time-span; 2) Control words - for options such as gravity gradient effects, control torques, and integration ranges; 3) Mathematical descriptions of spacecraft, appendages, and control systems- including element geometry, properties, attitudes, libration damping, tip mass inertia, thermal expansion, magnetic tracking, and gimbal simulation options; and 4) Desired state variables to output, i.e., geometries, bending moments, fast Fourier transform plots, gimbal rotation, filter vectors, etc. All FSD input is of free format, namelist construction. FSD is written in FORTRAN 77, PASCAL, and MACRO assembler for batch execution and has been implemented on a DEC VAX series computer operating under VMS. The PASCAL and MACRO routines (in addition to the FORTRAN program) are supplied as both source and object code, so the PASCAL compiler is not required for implementation. This program was last updated in 1985.

Description: The ORBSIM program was developed for the accurate extraction of geophysical model parameters from Doppler radio tracking data acquired from orbiting planetary spacecraft. The model of the proposed planetary structure is used in a numerical integration of the spacecraft along simulated trajectories around the primary body. Using line of sight (LOS) Doppler residuals, ORBSIM applies fast and efficient modelling and optimization procedures which avoid the traditional complex dynamic reduction of data. ORBSIM produces quantitative geophysical results such as size, depth, and mass. ORBSIM has been used extensively to investigate topographic features on the Moon, Mars, and Venus. The program has proven particulary suitable for modelling gravitational anomalies and mascons. The basic observable for spacecraft-based gravity data is the Doppler frequency shift of a transponded radio signal. The time derivative of this signal carries information regarding the gravity field acting on the spacecraft in the LOS direction (the LOS direction being the path between the spacecraft and the receiving station, either Earth or another satellite). There are many dynamic factors taken into account: earth rotation, solar radiation, acceleration from planetary bodies, tracking station time and location adjustments, etc. The actual trajectories of the spacecraft are simulated using least squares fitted to conic motion. The theoretical Doppler readings from the simulated orbits are compared to actual Doppler observations and another least squares adjustment is made. ORBSIM has three modes of operation: trajectory simulation, optimization, and gravity modelling. In all cases, an initial gravity model of curved and/or flat disks, harmonics, and/or a force table are required input. ORBSIM is written in FORTRAN 77 for batch execution and has been implemented on a DEC VAX 11/780 computer operating under VMS. This program was released in 1985.

Description: In an effort to place payloads into orbit at the lowest possible costs, the use of air-breathing space-planes, which reduces the need to carry the propulsion system oxidizer, has been examined. As this approach would require the space-plane to fly at hypersonic speeds for periods of time much greater than that required by rockets, many factors must be considered when analyzing its benefits. The Basic Hypersonic Data and Equations spreadsheet provides data gained from three analyses of a space-plane's performance. The equations used to perform the analyses are derived from Newton's second law of physics (i.e. force equals mass times acceleration); the derivation is included. The first analysis is a parametric study of some basic factors affecting the ability of a space-plane to reach orbit. This step calculates the fraction of fuel mass to the total mass of the space-plane at takeoff. The user is able to vary the altitude, the heating value of the fuel, the orbital gravity, and orbital velocity. The second analysis calculates the thickness of a spherical fuel tank, while assuming all of the mass of the vehicle went into the tank's shell. This provides a first order analysis of how much material results from a design where the fuel represents a large portion of the total vehicle mass. In this step, the user is allowed to vary the values for gross weight, material density, and fuel density. The third analysis produces a ratio of gallons of fuel per total mass for various aircraft. It shows that the volume of fuel required by the space-plane relative to the total mass is much larger for a liquid hydrogen space-plane than any other vehicle made. This program is a spreadsheet for use on Macintosh series computers running Microsoft Excel 3.0. The standard distribution medium for this package is a 3.5 inch 800K Macintosh format diskette. Documentation is included in the price of the program. Macintosh is a registered trademark of Apple Computer, Inc. Microsoft is a registered trademark of Microsoft Corporation.

Description: The DET/MPS programs model and simulate the Direct Energy Transfer and Multimission Spacecraft Modular Power System in order to aid both in design and in analysis of orbital energy balance. Typically, the DET power system has the solar array directly to the spacecraft bus, and the central building block of MPS is the Standard Power Regulator Unit. DET/MPS allows a minute-by-minute simulation of the power system's performance as it responds to various orbital parameters, focusing its output on solar array output and battery characteristics. While this package is limited in terms of orbital mechanics, it is sufficient to calculate eclipse and solar array data for circular or non-circular orbits. DET/MPS can be adjusted to run one or sequential orbits up to about one week, simulated time. These programs have been used on a variety of Goddard Space Flight Center spacecraft projects. DET/MPS is written in FORTRAN 77 with some VAX-type extensions. Any FORTRAN 77 compiler that includes VAX extensions should be able to compile and run the program with little or no modifications. The compiler must at least support free-form (or tab-delineated) source format and 'do do-while end-do' control structures. DET/MPS is available for three platforms: GSC-13374, for DEC VAX series computers running VMS, is available in DEC VAX Backup format on a 9-track 1600 BPI tape (standard distribution) or TK50 tape cartridge; GSC-13443, for UNIX-based computers, is available on a .25 inch streaming magnetic tape cartridge in UNIX tar format; and GSC-13444, for Macintosh computers running AU/X with either the NKR FORTRAN or AbSoft MacFORTRAN II compilers, is available on a 3.5 inch 800K Macintosh format diskette. Source code and test data are supplied. The UNIX version of DET requires 90K of main memory for execution. DET/MPS was developed in 1990. A/UX and Macintosh are registered trademarks of Apple Computer, Inc. VMS, DEC VAX and TK50 are trademarks of Digital Equipment Corporation. UNIX is a registered trademark of AT&T Bell Laboratories.

Description: NASA-JPL's four-frequency telecommunication system design entails the creation and integration of a frequency-selective surface (FSS) subreflector into the high-gain antenna subsystem. The FSS design, which incorporates a periodic array of conducting elements on a kevlar/polymer composite structure, will be able to multiplex S, X, Ku, and Ka frequency-band wavelengths. Accounts are presented of the FSS's development, mechanical testing, and electrical testing.

Description: A novel modeling technique is developed and applied to the long-wavelength gravity, topography, and internal density structure of Venus. The focus of this study is to employ data on the gravity and topography of Venus, which are obtained from the Pioneer Venus Orbiter mission, to enhance understanding some key aspects of the internal structure of the planet. A modeling strategy utilizing 'stokeslets' or basic units of slow viscous flow (governed by the Stokes equations) to duplicate the observed gravity and topography is implemented.

Description: Technology is being developed to process signals from distributed sensors using distributed computations. These distributed sensors provide a new feedback capability for vibration control that has not been exploited. Additionally, the sensors proposed are of an optical and distributed nature and could be employed with known techniques of distributed optical computation (Fourier optics, etc.) to accomplish the control system functions of filtering and regulation in a distributed computer. This paper reviews a procedure for the analytic design of control systems for this application. For illustration, the procedure is applied to the problem of suppressing the vibrations of a simply supported beam. A simulator has been developed to study the effects of sensor and processing errors. An extensive study of the effects of these errors on estimation and regulation performance is presented.

Description: The Task Committee on Methods for Identification of Large Structures in Space was founded in Jul. 1984. The charter of the committee was to prepare a state-of-the-art report on methods of system identification applicable to large space structures (LSS). Funding to support preparation of the report was received in Aug. 1985 from the Air Force Rocket Propulsion Laboratory (now the Air Force Astronautics Laboratory), in the form of a contract to the ASCE. The report was completed, and published by AFRPL in Sep. 1986. The Task Committee consisted of ten members, including ASCE and AFRPL representatives. The membership represented Government, Industry, and Universities, and consisted of electrical, mechanical, and civil engineers, with backgrounds in Structural Dynamics, Optimization, and Controls. An effort was made to use consistent terminology and notation throughout the report which would be compatible with the terminology used in both the structures and controls communities.

Description: This paper describes an autonomous control concept for pointing and articulation of science instruments on the Eos (Earth observing system) NASA/NOAA platforms intended to be operational by the late 1990s. Key features of this concept include advanced control adaptation and tuning strategies which provide performance robustness over a wide range of system uncertainties and mission time criticality. System identification-control modification paradigms are synthesized to form an adaptation continuum over this extended regime of autonomous operations.

Description: Large space structures are characterized by a large number of modes, grouped frequencies, and small inherent damping. Model reduction techniques in time domain may not be effective due to small damping. The model truncation method is generally used. This method can not solve the problem of grouped frequencies, and will lose all the information about the higher order modes. A new method developed in this paper, which tries to minimize the error of interested transfer functions, makes use of all the information of the original system, and achieves improvement not only from a smaller error of transfer functions but also from better frequency distribution.

Description: A space flight experiment being developed by NASA's Office of Aeronautics and Space Technology (OAST) that uses the Space Station as a testbed to study techniques for determining the dynamic characteristics of large space structures (LSS) is described. The experiment is separate from the Space Station Program itself with research objectives outside the domain of Space Station Program objectives. A brief description of the experiment, in terms of the general objective and approach, is given along with a statement of the potential benefits to NASA and others. The bulk of material to follow deals with the experiment definition activity that is underway. The scope of an 'initial' definition study and preliminary results from supporting Space Station dynamics analyses is presented. The term initial is used to indicate that the study currently being conducted has limited objectives and is not expected to complete the required experiment definition. A follow on study is planned and is mentioned in the summary.

Description: The interaction of the thermosphere and ionosphere is largely governed by collisions between ions and neutral particles. On Venus and the Earth, O(+) is a dominant ion, and atomic O dominates throughout much of the thermosphere; therefore an accurate O(+)-O cross section is an important prerequisite for understanding the dynamics of planetary upper atmospheres. The cross section and momentum transfer collision frequency are calculated with a quantum mechanical code which includes resonance charge exchange, polarization, and charge-quadrupole effects. Our results agree well with earlier calculations of Stubbe (1968) and Stallcop et al. (1991).

Description: Synthetic aperture radar (SAR) focusing can be achieved either based on accurate ephemeris data or on an autofocusing process. For the Magellan project, such a decision must be made in the early phase of Magellan SAR system design. The analysis of the emphemeris requirement is complicated. The analysis given by the author leads to the conclusion that empheris data obtained from the Magellan navigation system provide sufficient accuracy to meet the Magellan image resolution requirement.

Description: An overview of the history and current status of research on planetary noble gases is presented. The discovery that neon and argon are vastly more abundant on Venus than on earth points to the solar wind rather than condensation as the fundamental process for placing noble gases in the atmospheres of the terrestrial planets; however, solar wind implantation may not be able to fully reproduce the observed gradient, nor does it obviously account for similar planetary Ne/Ar ratios and dissimilar planetary Ar/Kr ratios. More recent studies have emphasized escape rather than accretion. Hydrodynamic escape, which is fractionating, readily accounts for the difference between atmospheric neon and isotopically light mantle neon. Atmospheric cratering, which is nearly nonfractionating, can account for the extreme scarcity of nonradiogenic noble gases (and other volatiles) on Mars.

Description: A reanalysis of the evolution configuration module pattern for Space Station Freedom is presented. The module pattern is to consist of one habitation module, one airlock, one cupola, two Space Shuttle Orbiter pressurized docking adapters, one assured crew return vehicle, and one pressurized logistics module. Three laboratory modules are also included. A phased plan has been developed on the basis of utilization analysis being performed and derived through actual and projected user mission requirements for the growth of pressurized elements. The evolution requirement impacts to the module pattern are outlined. Application of the growth philosophy and the physical limitations resulted in a number of core element ground rules for module pattern growth. Nodes and modules are to be grown symmetrically in order to optimize poor flight control characteristics and maximize the capability for crew dual egress between elements. Core module pattern elements are not to be grown along Y-Y axis from nodes 1 or 2 due to physical clearance problems with the thermal control system radiators operational envelope.

Description: The study points out three difficulties with the hypothesis of Hartung (1993) that the Corvid meteor system, observed only once in 1937, may be the return of the clump of ejecta from the formation of a lunar crater, specifically, an event recorded in the chronicles of Gervase of Canterbury on June 25, 1178, which Hartung (1976) previously suggested may be an eyewitness account of the formation of the lunar crater Giordano Bruno. On the basis of this, he predicts that another Corvid shower may be observed in 2003 or 2006. This hypothesis is rejected on the contention that it is implausible that a clump of ejecta could be launched into heliocentric orbit with a low enough dispersion in velocity among separate pieces that it would produce a meteor shower in just one year and not others. Subsequent perturbations by the earth on parts of the clump passing near the earth but not impacting would destroy the coherence of the clump on a time scale much shorter than the 759-yr interval proposed. There are so many orbits that could yield a shower after 759 yr that it is unlikely that a prediction of a return in a specific year would be correct.

Description: A new gravity field determination that has been produced combines both the Pioneer Venus Orbiter (PVO) and the Magellan Doppler radio data. Comparisonsbetween this estimate, a spherical harmonic model of degree and order 21, and previous models show that significant improvements have been made. Results are displayed as gravity contours overlaying a topographic map. We also calculate a new spherical harmonic model of topography based on Magellan altimetry, with PVO altimetry included where gaps exist in the Magellan data. This model is also of degree and order 21, so in conjunction with the gravity model, Bouguer and isostatic anomaly maps can be produced. These results are very consistent with previous results, but reveal more spatial resolution in the higher latitudes.

Description: Results of a comprehensive morphological analysis of the dayside superthermal ion presence in the dayside ionopause region of Venus made using the Orbiter Ion Mass Spectrometer measurements are presented. There is a strong inbound-outbound difference in both the number and intensity of the superthermal ions with most events detected on the inbound crossing of the ionopause. The ambient energy of the detected ions is comparable to the ram energy of cold ions relative to the moving spacecraft. A comparison of the locations of the superthermal O(+) ion occurrences with respect to other parameters shows that the superthermals are a characteristic of the ionopause transition separating the essentially stationary ionosphere plasma from the flowing ionosheath plasma as deduced by Taylor et al. (1980) from individual examples.

Description: Compared to most other Yamato polymict eucrites, Yamato Y792769 eucrite includes fewer and smaller eucritic clasts with homogenized pyroxenes, and its fine-grained matrix is shock-compacted and sintered. In this work, the relationships between the Antarctic eucrite Y792769, monomict eucrites, polymict eucrites, and isotopic ages are investigated, using results of Ar-39/Ar-40 method to date the time of the major thermal event on the Y792769 body and the Rb-Sr and Sm-Nd methods to determine whether relict older ages might have been preserved in some of the breccia materials. The Ar-39/Ar-40 time of the last thermal event which produced the Y792769 texture is 3.99 +/- 0.04 Ga. The complete resetting of the Ar-39/Ar-40 age is consistent with the texture of Y792769 observed in TEM, suggesting that shock compaction converted part of the matrix plagioclase to maskelynite. The Sm-Nd data give an age of 4.23 +/- 0.12 Ga, reflecting partial resetting of the Sm-Nd system during breccia formation. The 3.9 Ga Ar-39/Ar-40 age probably reflects a period of intense meteoroid bompardment which affected the entire inner solar system.

Description: In three-dimensional numerical simulations of a rapidly rotating Boussinesq fluid shell, thermally driven convection in the form of columns parallel to the rotation axis generates an alternately directed mean zonal flow with a cylindrical structure. The mean structure at the outer spherical surface consists of a broad eastward flow at the equator and alternating bands of westward and eastward flows at higher latitudes in both hemispheres. The banded structure persists even though the underlying convective motions are time-dependent. These results, although still far from the actual motions seen on Jupiter and Saturn, provide support for theoretical suggestions that thermal convection can account for the remarkable banded flow structures on these planets.

Description: A review of the planetary system is presented from a uniform perspective, taking advantage of the data obtained during the planetary space missions. A comparative approach is used, which makes it possible to consider each planet or a satellite in context, focusing both on their common origin and on the processes that have influenced their evolution. The discussion focuses on the Jovian planets, the moon and the Mercury, Venus and Earth, and small bodies in the outer solar system. Attention is also given to comets, asteroids, and cosmic impacts.

Description: An electron probe microanalyzer is used to measure the Mn, Fe, and oxygen zoning profiles of olivines in the ALHA 77257 ureilite. This is done to study the effects of reduction on the Mn-Fe value, as ureilite olivines exhibit thin reduced rims. Since the Mn content gradually increases toward the rim of ureilite olivines, while the Fa (= 100 x Fe/(Mg + Fe), mol percent) component decreases, the Mn-Fe content of olivine is likely related to redox conditions. The results of melting experiments suggest that the Mn-Fe positive correlation is related to temperature and that the negative correlation of Mn-Fe in olivine and low-Ca pyroxene is related to reduction.

Description: Voyager IRIS observations of the NEB reveal longitudinal variability of 5-micron brightness temperatures of order 100 C. An anisotropic multiple scattering radiative transfer model is used to calculate synthetic spectra for comparison with the IRIS observations. Mie theory is used to model the spectral dependence of cloud extinction from 180 to 2300/cm. Cloud base locations within the model vary with assumed gas abundances according to thermochemical equilibrium. It is found that spatial variations in the abundance profiles of the condensible species, parahydrogen profiles and cloud optical depths can be used as tracers of the local and large-scale dynamics. The variation of cloud opacity is strongly correlated with the variation of relative humidity, which suggests that dynamic depletion of water vapor above the cloud forming level is the most plausible model to explain the spatial variation in the water profile within the NEB.

Description: Io's brightness was monitored following emergence from eclipse by Jupiter on 14 occasions during the Jupiter apparitions from 1981 to 1989 and no instance is found of what has been called posteclipse brightening. If all the observations are averaged, a 2 percent effect cannot be ruled out; however, this effect is also the size of the rms errors of the summed data set. If condensation followed by sublimation of SO2 frost is hypothesized to be the mechanism which causes posteclipse brightening, then solar insolation alone may not be sufficient to remove an optically thick layer of SO2 frost in the time required to conform with the positive reports of posteclipse brightening in the literature. An additional source of energy is required, such as warming caused by the impact of magnetospheric particles as they dissipate the energy of their motion into Io's surface.

Description: For a fixed heat flow, the surface flow velocity of a convecting layer is not strongly sensitive to the variation of viscosity as a function of depth. Thus, the inferred absence of a low viscosity asthenosphere on Venus can not account for the limited surface motions there. The surface velocity is dependent on the convective geometry. Cartesian geometry convection can produce large surface velocities if the high viscosity surface layer is broken in places by weak zones. On the other hand, a high viscosity surface layer may inhibit the development of large surface velocities in axisymmetric convection.

Description: The paper characterizes the mineralogy and spectral properties of three slightly palagonitized basaltic tephra samples collected near the summit of Mauna Kea in order to contribute to the basis for inferring mineralogy and processes for Martian surface materials. The mineralogy of size fractions of these samples is examined by diffuse reflectance and FIR spectroscopy, optical microscopy, X-ray diffraction, Moessbauer spectroscopy, magnetic analysis, EMPA, TEM, and SEM. For the 20-1000 micron size fraction, sample HWMK11 (red) is essentially completely oxidized and has a hematite (Ti-hematite) pigment dispersed throughout the silicate matrix. Sample HWMK12 (black) has the lowest proportion of ferric-bearing phases and is thus least weathered. For HWMK11, the amount of hematite is essentially constant, and mica is present only in the coarse clay-sized fraction; smectites are low in structural Fe.

Description: Results are presented of a thermal design optimization study of the segmented GFRP primary reflector of the earth-orbiting Submillimeter Imager and Line Survey telescope. The paper examines the thermal requirements of the primary reflector and the thermal environment of the telescope and describes the thermal design of the primary reflector. Particular attention is given to the geometric math model and the thermal math model of the telescope. A summary for the steady-state thermal performance of the optimized design is presented, showing that the optimized design has reduced, by an order of magnitude, structural spatial temperature gradients, which were earlier shown to be the most significant obstacle in maintaining the required telescope figure accuracy.

Description: The research experience of SCOLE developed by the NASA Langley Research Center is reviewed. Particular attention is given to the advances made in control and modeling techniques, the experimental facilities that are now available, and the continuing needs to advance and to validate the technologies in controlling flexible structures.

Description: An analytic redundancy management approach for on-line component failure detection is described and illustrated using a simulation of the NASA Langley Spacecaft Control Laboratory Experiment (SCOLE) research facility. The SCOLE experimental apparatus simulated is a functional model of the Space Shuttle with a large, flexible, offset-feed antenna cantilevered from the payload bay. This approach uses a single, active, Kalman filter selected from a bank of filters, each element of which was previously designed to accommodate a specific failure condition. The residuals of this active filter are processed through a sequential probability ratio test (SPRT) filter to identify the failure state of the system and, hence, closing the failure accommodation loop, to select the active filter. Results are presented that illustrate the ability of the system to detect and recover from failures introduced in the angular rate and linear acceleration sensors on the SCOLE facility.

Description: Recent laboratory polarimetric measurements of light scattering by layers of alumina grains of different size are used to examine the relevance of the Wolff & Dollfus (1990) theory of negative polarization, which has been developed to describe light scattering by regolithic grains of arbitrary size. It is demonstrated that, while reproducing satisfactorily the main polarization characteristics of light scattering by grains much greater than the wavelength, this theory does not describe the negative polarization of light measured by Geake & Geake (1990) for subwavelength-sized grains. The smallest grains exhibit a polarization feature that may be called the 'polarization opposition effect'. This feature requires an alternative explanation and may be due to the so-called weak localization of photons in discrete random media.

Description: It is shown that N2 may be present in the troposphere of Neptune in an amount difficult to evaluate but which could easily be as high as 0.003, while there is no evidence that it is present in the atmosphere of Uranus. The estimate of the helium abundance depends on the assumed value for N2. If there is no N2 in the observed region of the atmosphere of Uranus and an N2 mole fraction of 0.003 on Neptune, the central value of the estimates of the helium abundance are equal to 0.26 by mass in both planets, which is close to the protosolar value of 0.28. This would imply that the He/H2 ratios measured in the outer atmospheres of Uranus and Neptune are representative of the ratio in the primitive solar nebula and thus were not modified during planetary formation.

Description: The GISS GCM is used here to examine the hypothesis that equatorial superrotation on slowly rotating planets is sensitive to the nature of the vertical radiative heating profile and can exist in the absence of diurnally varying forcing. The general circulation, diabatic heating, and thermal structure produced in the experiments are described and the heat and angular momentum budgets and energy cycles are analyzed to understand the factors conducive to equatorial superrotation. The implications of the results for future planetary missions are addressed.

Description: The spins of the terrestrial planets likely arose as the planets formed by the accretion of planetesimals. Depending on the masses of the impactors, the planet's final spin can either be imparted by many small bodies (ordered accretion), in which case the spin is determined by the mean angular momentum of the impactors, or by a few large bodies (stochastic accretion), in which case the spin is a random variable whose distribution is determined by the rms angular momentum of the impactors. In the case of ordered accretion, the planet's obliquity is expected to be near 0 or 180 deg, whereas, if accretion is stochastic, there should be a wide range of obliquities. Analytic arguments and extensive orbital integrations are used to calculate the expected distributions of spin rate and obliquity as a function of the planetesinal mass and velocity distributions. The results imply that the spins of the terrestrial planets are determined by stochastic accretion.

Description: The history and construction of Martian place names are examined. The 24 specific descriptor terms in use for Mars are defined. Informal names of individual rocks are discussed: the human fondness for informality is evident in the names attached to individual rocks at the Viking Lander sites.

Description: One of the basic requirements in engineering analysis is the development of a mathematical model describing the system. Frequently comparisons with test data are used as a measurement of the adequacy of the model. An attempt is typically made to update or improve the model to provide a test verified analysis tool. System identification provides a systematic procedure for accomplishing this task. The terms system identification, parameter estimation, and model correlation all refer to techniques that use test information to update or verify mathematical models. The goal of system identification is to improve the correlation of model predictions with measured test data, and produce accurate, predictive models. For nonmetallic structures the modeling task is often difficult due to uncertainties in the elastic constants. A finite element model of the shell was created, which included uncertain orthotropic elastic constants. A modal survey test was then performed on the shell. The resulting modal data, along with the finite element model of the shell, were used in a Bayes estimation algorithm. This permitted the use of covariance matrices to weight the confidence in the initial parameter values as well as confidence in the measured test data. The estimation procedure also employed the concept of successive linearization to obtain an approximate solution to the original nonlinear estimation problem.

Description: Reliable structural dynamic models will be required as a basis for deriving the reduced-order plant models used in control systems for large space structures. Ground vibration testing and model verification will play an important role in the development of these models; however, fundamental differences between the space environment and earth environment, as well as variations in structural properties due to as-built conditions, will make on-orbit identification essential. The efficiency, and perhaps even the success, of on-orbit identification will depend on having a valid model of the structure. It is envisioned that the identification process will primarily involve parametric methods. Given a correct model, a variety of estimation algorithms may be used to estimate parameter values. This paper explores the effects of modeling errors and model deficiencies on parameter estimation by reviewing previous case histories. The effects depend at least to some extent on the estimation algorithm being used. Bayesian estimation was used in the case histories presented here. It is therefore conceivable that the behavior of an estimation algorithm might be useful in detecting and possibly even diagnosing deficiencies. In practice, the task is complicated by the presence of systematic errors in experimental procedures and data processing and in the use of the estimation procedures themselves.

Description: On-orbit system identification (ID) of large space systems is essential for various reasons. For example, the complex composite structure of such systems cannot be ground-tested; their structural dynamic characteristics must be known accurately in order to accomplish active control. Furthermore, such capability can be used to characterize/identify various disturbances. The identification process is consisted of four principal elements: (1) modeling, (2) the estimation algorithm, (3) input system, and (4) measurement system. These elements are highly correlated and all togerher determine the success of the identification problem. Accurate modeling of large space systems is the most important element of the identification process. Large flexible structures are non-linear and infinite dimensional systems with highly coupled parameters and low frequency packed modes. In addition, these systems are subject to stochastic and time-varying disturbances, they have structural parameters which can vary due to on-orbit assembly deployment, and operations. These systems are generally; however, represented by constant coefficient, finite order differential equations. The non-linearities, coupling and noise effects are also often neglected. Moreover, identification experiment designs which lead to highly complex optimization problems usually require the simultaneous choice of ID algorithm, sensor, and actuator type and placement. On-orbit bandwidth and power restrictions on excitation, limited data window, and restrictions on sensor/actuator type, placement and number, has led to practical questions of implementations.

Description: This paper examines the use of on-orbit identification based on Maximum Likelihood Estimation (MLE) to provide these high-order, high-accuracy control design models for large space structures (LSS's). First, it outlines a general MLE identification algorithm, together with a covariance-analysis procedure to assess algorithm performance in terms of systematic and stochastic errors. Next, it examines various simplifications appropriate for the LSS identification application. Simplified analytical performance results are presented, as are numerical results to support these analyses. Finally, a graphical interpretation of these results is given.

Description: This talk focuses on the determination of state-space models for large space systems using only the output data. The output data could be generated by the unknown or deliberate initial conditions of the space structure in question. We shall review some relevant fundamental work on the state-space modeling of sequential output data that is potentially applicable to large space structures. If formulated in terms of some generalized Markov parameters, this approach is in some sense similar to, but much simpler than, the Juang-Pappa Eigensystem Realization Algorithm (ERA) and the Ho-Kalman construction procedure.

Description: For the future space systems, on-orbit identification (ID) capability will be required to complement on-orbit control, due to the fact that the dynamics of large space structures, spacecrafts, and antennas will not be known sufficiently from ground modeling and testing. The computational requirements for ID of flexible structures such as the space station (SS) or the large deployable reflectors (LDR) are however, extensive due to the large number of modes, sensors, and actuators. For these systems the ID algorithm operations need not be computed in real-time, only in near real-time, or an appropriate mission time. Consequently the space systems will need advanced processors and efficient parallel processing algorithm design and architectures to implement the identification algorithms in near real-time. The MAX computer currently being developed may handle such computational requirements. The purpose is to specify the on-board computational requirements for dynamic and static identification for large space structures. The computational requirements for six ID algorithms are presented in the context of three examples: the JPL/AFAL ground antenna facility, the space station (SS), and the large deployable reflector (LDR).

Description: This proposal discusses a new nonlinear, nonparametric method for off-line modeling and on-line estimation of the deformation of a flexible structure undergoing rapid retargeting maneuvers. In these circumstances, the structural stiffness and damping coefficients depend on the angular acceleration omega(dot), the angular rate omega, and the square of the angular rate omega. In the single axis case, the excitation of the structure is represented by the vector u(exp T) = (omega(dot), omega(exp 2), 2(omega)), to which the structural dynamics responds as a 'bilinear' (i.e., parametrically excited) system. A similar technique for multiaxial rotations yields a bilinear model with respect to matrix valued excitations. Three methods of estimation and modeling are described in this proposal to achieve deformation state determination: (1) a method based on a feedback linearized procedure which gives an estimate of the state by means of observers installed in the deformable body; (2) off-line modeling of the deformation state of the structure by means of topological interpolators; and (3) an on-line structural state estimation method based on a combination of the two previous techniques.

Description: Viewgraphs on current analytical gaps for future needs for large space systems are presented. Topics covered include: future spacecraft; common control objectives; accuracy requirements; increasing complexity; promising approaches; and analytical gaps.

Description: The initial results of a NIR spectral imaging study conducted from Mauna Kea Observatory during the 1990 opposition are presented. The study's main goals were to determine whether it is possible to map and monitor subtle variations in surface and atmospheric components on Mars from ground-based observations and to constrain Mars surface and airborne dust composition. Small variations near 2.3 microns include both atmospheric (CO) and surface/dust mineral absorption, although the data cannot yet accurately discriminate these components or assess their relative contributions. The present data set, even after preliminary calibration steps, demonstrates the potential of the Mars imaging spectroscopic observations in the near-IR. Focused ground-based telescopic studies in selected wavelength regions accompanied by detailed high spectral resolution atmospheric modeling will allow the mapping and monitoring of surface minerals, atmospheric aerosols, and atmospheric trace gases.

Description: The Magellan prime mission involves mapping the planet of Venus once around its rotational axis. The Magellan synthetic aperture radar (SAR) data processing facility processes all SAR data collected by the Magellan spacecraft into image data on an orbit-by-orbit basis. About 1700 million bits of radar data were collected. A complete description of the Magellan SAR Data Processing Facility is provided with emphasis on key design features of the data processors that satisfied the project data processing requirements. A summary of the attained data processing performance is included, as well as a brief discussion of some of the constraints and considerations regarding the applicability of the processors to meeting the data processing goals anticipated for the follow-on mission phases (i.e., cycles II, III, and beyond).

Description: The evolution of solid particles in the solar nebula (or other circumstellar disk) is described. Motions of bodies less than about 1 km in size were dominated by gas drag rather than gravity. An original population of microscopic grains had to produce greater than km-sized planetesimals before gravitational accretion of planets could begin. Planetesimals probably formed by coagulation of grain aggregates that collided due to differential settling, turbulence, and drag-induced orbital decay. Growth of such aggregates depended on sticking mechanisms and their mechanical properties, which are poorly understood. Their growth was aided by concentration of larger bodies toward the central plane of the disk. The nebula could remain optically thick during this process. It is unlikely that a particle layer formed by settling would undergo gravitational instability, as a small amount of turbulence would keep the particle layer from reaching the critical density. This conclusion is independent of the particle size, as even large bodies do not effectively decouple from the gas. Even in a laminar disk, shear in the particle layer would generate enough turbulence to keep it stirred up.

Description: This chapter presents a review of the structure and composition of the giant planets and the theory of their formation and growth. All of the giant planets have heavy-element cores, and have envelopes which contain large amounts of high-Z material in addition to hydrogen and helium. The planets most probably formed through the core instability mechanism. This is a much more complex mechanism than was previously thought, depending, as it does, on several time-dependent parameters. We present the results of new, more detailed, simulations. Towards the end of accretion, the transfer of angular momentum to the outer layers of the contracting protoplanet should lead to the formation of a disk. This disk may be the site of satellite formation. Some recent simulation results are shown.

Description: Structural sizing and performance data are presented for two different aerobrake hexagonal heatshield panel structural concepts. One concept features a sandwich construction with an aluminum honeycomb core and thin quasi-isotropic graphite-epoxy face sheets. The other concept features a skin-rib isogrid construction with thin quasi-isotropic graphite-epoxy skins, and graphite-epoxy ribs oriented at 0, +60, and -60 deg along the panel. Linear static, linear bifurcation buckling, and nonlinear static analyses were performed to compare the structural performance of the two panel concepts and assess their feasibility for a Lunar Transfer Vehicle aerobrake application.

Description: The nature of the unusually narrow photometric and polarization opposition effects exhibited by Saturn's A and B rings is examined using a theory and results of laboratory measurements. It is pointed out that the small angular width of both phenomena makes it difficult to explain them on the basis of the commonly used shadowing models. On the other hand, it is known from laboratory experiments and theoretical studies, that a strong and very narrow opposition peak in the reflected intensity can be produced by coherent backscattering of light from powderlike layers of small regolithic grains. Using Ozrin's (1992) rigorous vector theory of coherent backscattering, it is shown that, for subwavelength-sized regolithic particles, the photometric opposition effect is accompanied by a polarization opposition effect of the same angular width. This suggests that the polarization opposition effect of the Saturn's ring has the same origin as the photometric opposition effect and is due to coherent backscattering of light from the regolithic layer composed of the submicrometer-sized ice grains.

Description: Polar projections of 50 images of Saturn at 889 nanometers and 25 images at 718 nanometers taken by the HST in November 1990, as well as three images at each wavelength taken in June 1991, have been examined. Among them, 31 show the north polar spot, which is associated with Saturn's polar hexagon, in locations suitable for measurement. The movement of the spot with respect to Saturn's system III rotation rate was studied. During the period of observation, the polar spot had first a short-term westward movement and then a long-term eastward drift. The rate of the long-term drift was -0.060 +/- 0.008 deg/day with respect to system III, approximately 50 percent greater than previously determined from Voyager. The original 1980 and 1981 Voyager data were combined with the new Hubble images to form an 11-yr baseline. The eastward drift over the longer period was -0.0569 degrees per day. The long-term drift could be due to uncertainty in the standard value of the internal rotation period. The short-term movement in November 1990 has a rate that is greater in magnitude but opposite in sign and probably represents a real, transient motion of the spot relative to the internal rotation system.

Description: An effort is currently being carried out by the Jet Propulsion Laboratory (JPL) to study mission feasibility and to define functional requirements for various subsystems of the Space Infrared Telescope Facility (SIRTF). As a major part of this effort, structural design requirements have been derived based on the stated mission objectives. Design concerns addressed by these requirements include the limits on mass and location of the center of gravity, launch stiffness and dynamic characteristics, design loads and analysis criteria, survivability of the TITAN IV/Centaur launch environment, thermal control for maintaining a near absolute-zero operating temperature, and helium cryogen volume and storage for a five-year mission. To illustrate how the structural design requirements can be met, a point design of the SIRTF flight hardware system was developed, modeled, and analyzed. A description of the key features of this point design, along with pertinent modeling and analysis results, are discussed in this Paper.

Description: The progress in spacecraft charging is reviewed with particular attention given to the interactions of plasma and penetrating radiation with dielectrics. Topics discussed include the charging environments, elementary charging theory, the anomalies attributed to charging or discharging phenomena, and spacecraft engineering.

Description: Recently, Harris et al. (1989) have observed a strong and unusually narrow (HWHM approximately 0.8 deg in the yellow) opposition effect for high-albedo asteroids 44 Nysa and 64 Angelina. In this paper, we apply the theory of coherent backscattering of light from discrete random media to interpret this remarkable opposition brightening. It is shown that coherent backscattering of sunlight from a regolithic layer composed of submicrometer-sized grains with an index of refraction close to that of the mineral enstatite can be a reasonable explanation of the observed opposition effect.

Description: The objectives of the Solar Probe mission and the current status of the Solar Probe thermal shield subsystem development are described. In particular, the discussion includes a brief description of the mission concepts, spacecraft configuration and shield concept, material selection criteria, and the required material testing to provide a database to support the development of the shield system.

Description: A spherical harmonic model of the gravitational field of Venus complete to degree and order 50 has been developed using the S-band Doppler tracking data of the Pioneer Venus Orbiter (PVO) collected between 1979 and 1982. The short wavelengths of this model could only be resolved near the PVO periapse location (about 14 deg N latitude), therefore a priori constraints were applied to the model to bias poorly observed coefficients towards zero. The resulting model has a half-wavelength resolution of 400 km near the PVO periapse location, but the resolution degrades to greater than 1000 km near the poles. This gravity model correlates well with a degree 50 spherical harmonic expansion of the Venus topography derived from a combination of Magellan and PVO data. New tracking data from Magellan's gravity mission should provide some improvement to this model, although a complete model of the Venusian gravity field will depend on tracking of Magellan after the circularization of its orbit using aerobraking.