Satellite collision probability estimation using polynomial chaos expansions

This paper presents the application of polynomial chaos (PC) to estimating the probability of collision between two spacecraft. Common methods of quantifying this probability for conjunction assessment use either Monte Carlo analyses or include simplifying assumptions to improve tractability. A PC expansion, or PCE, provides a means for approximating the solution to a large set of stochastic ordinary differential equations, which includes orbit propagation. When compared to Monte Carlo methods, non-intrusive, i.e., sampling-based, PCE generation techniques may greatly reduce the number of orbit propagations required to approximate the possibly non-Gaussian, a posteriori probability density function. The presented PC-based method of computing collision probability requires no fundamental simplifying assumptions, and reduces the computation time compared to Monte Carlo. This paper considers two cases where the common conjunction assessment assumptions are no longer valid. The results indeed demonstrate a reduction in computation time when compared to Monte Carlo, and improved accuracy when compared to simplified techniques.     

An extensive phase space for the potential martian biosphere

We present a comprehensive model of martian pressure-temperature (P-T) phase space and compare it with that of Earth. Martian P-T conditions compatible with liquid water extend to a depth of ～310?km. We use our phase space model of Mars and of terrestrial life to estimate the depths and extent of the water on Mars that is habitable for terrestrial life. We find an extensive overlap between inhabited terrestrial phase space and martian phase space. The lower martian surface temperatures and shallower martian geotherm suggest that, if there is a hot deep biosphere on Mars, it could extend 7 times deeper than the ～5?km depth of the hot deep terrestrial biosphere in the crust inhabited by hyperthermophilic chemolithotrophs. This corresponds to ～3.2% of the volume of present-day Mars being potentially habitable for terrestrial-like life.     

A gas management system for an ultra long duration Titan blimp

This paper presents analyses, designs and experimental results for the gas management system of a hydrogen-filled blimp capable of flying in the lower atmosphere of Titan for a period of 1 year or more. The engineering strategy has two basic elements: first, to minimize leakage rates from the blimp envelope and ballonets; and second, to provide auxiliary subsystems to mitigate the life-limiting effects resulting from those leaks. Leak minimization is achieved through use of cryogenically compatible balloon materials and adhesives, and selection of ballonet geometries that minimize pinhole generation via folding and material fatigue. Hydrogen loss to the environment through leaks in the blimp envelope is compensated by producing new hydrogen through chemical processing of atmospheric methane. Nitrogen leaked into the blimp from the ballonets is removed by a carbon absorption system and periodically vented to the atmosphere. Data is presented on the measured leak rate from a full scale (13 m long) prototype blimp envelope and on the performance of a low mass, low power prototype device that generates hydrogen from methane. These results are factored in to an overall system design that quantifies the mass and power requirements for a minimum 1 year operational lifetime.     

Banded electron cyclotron harmonic instability-a first comparison of theory and experiment

A study, which is the first of its kind, uses information derived from simultaneously measured wave spectra and particle distributions as the input to a theoretical linear instability model of an electrostatic cyclotron harmonic wave event recorded on GEOS-1. The presence of a hot loss cone component of the particle distribution is established experimentally, and the model accounts reasonably for the observed frequencies and relative strengths of the (n+1/2)f _c and upper hybrid emission features.     

Measurements of quasi-static electric fields between 3 and 7 Earth radii on GEOS-1

Quasi-static electric fields have been measured with two spherical probes supported by cable booms providing a baseline of 42 m for the measurement. The performance of the experiment is outlined to demonstrate that electric fields can be measured with accuracies of ±0.7 mV m^-1 and ±1.0 mV m^-1 in the dawn-dusk and satellite-sun directions respectively. These uncertainties can be considerably reduced under favourable plasma conditions. Examples of typical observations are described.

1. The average electric field is always characterized by an irregular structure with time scales 0.5–5 min and with amplitudes of a few mV m^-1.
2. During substorms dawn-dusk electric fields up to 20–30 mV m^-1 have been observed over intervals of 30–60 s.
3. Oscillating electric fields with peak-to-peak amplitudes up to 10 mV m^-1 and periods of 3–10 min have been observed following magnetospheric disturbances.

The observations are discussed in terms of plasma motions and possible spatial scale sizes of the phenomena, standing magnetospheric wave modes and electrostatic potentials.     

Development of the Primary Sorption Pump for the SEIS Seismometer of the InSight Mission to Mars

We report on the development of a passive sorption pump, capable of maintaining high-vacuum conditions in the InSight seismometer throughout the duration of any extended mission. The adsorber material is a novel zeolite-loaded aerogel (ZLA) composite, which consists of fine zeolite particles homogeneously dispersed throughout a porous silica network. The outgassing species within the SEIS evacuated container were analyzed and the outgassing rate was estimated by different methods. The results were used to optimize the ZLA composition to adsorb the outgassing constituents, dominated by water, while minimizing the SEIS bakeout constraints. The InSight ZLA composite additionally facilitated substantial CO₂ adsorption capabilities for risk mitigation against external leaks in Mars atmosphere. To comply with the stringent particle requirements, the ZLA getters were packaged in sealed containers, open to the SEIS interior through \(1~\upmu\mbox{m}\)-size pore filters. Results from experimental validation and verification tests of the packaged getters are presented. The pressure forecast based on these data, corroborated by rudimentary in situ pressure measurements, infer SEIS operational pressures not exceeding \(10^{-5}~\mbox{mbar}\) throughout the mission.     

VLF electromagnetic waves observed onboard GEOS-1

This paper is concerned mainly with the information which can be extracted from frequency-time spectra in the VLF range. The instrument used is the correlator which has a good frequency resolution (50 Hz) and time resolution (30 ms) in one magnetic and one electric component simultaneously. By suitable computer analysis, it is possible for instance to distinguish between the two dominant electromagnetic emissions, hiss and chorus, as well as to display the complete spectra. This treatment is applied to the Survey periods, which are a fixed sequence of modes, repeated every hour on the hour in order to have reference data from GEOS analogous to many ground-based observatories. One result of this treatment obtained already is that hiss and chorus normally appear together, although one or the other may be dominating in intensity. The occurrence rate of these emissions in local time is also given.For continuous surveillance the filterbank data are used. There are 16 frequency filters supplying magnetic and electric amplitude at few different frequencies. Using these data, a storm sudden commencement can be followed with good time resolution (1 s), and an interesting correlation has been found in a few cases between the VLF signal amplitude and the cold plasma density (as measured by the active part of the S-300 experiment).     

Preliminary evaluation of the landsat-4 thematic mapper data for mineral exploration

Landsat-4 Thematic Mapper (TM) data recorded over an arid terrain were analyzed to determine the applicability of using of TM data for identifying and mapping hydrothermally altered, potentially mineralized rocks. Clays, micas, and other minerals bearing the OH anion in specific crystal lattice positions have absorption bands in the 2.2-μm region (TM channel 7, TM7) and commonly lack features in the 1.6-μm region (TM5). Channel ratios TM5/TM7, TM5/TM4, and TM3/TM1 were combined into a color-ratio-composite (CRC) image and used to distinguish hydrothermally altered rocks, unaltered rocks, and vegetation. These distinctions are made possible by using the TM5 and TM7, channels which are not available in the Landsat multispectral scanner (MSS). Digital masking was used to eliminate ambiguities due to water and shadows. However, some ambiguities in identification resulted between altered volcanic rocks and unaltered sedimentary deposits that contained clays, carbonates, and gypsum, and between altered volcanic rocks and volcanic tuffs diagenetically altered to zeolites. However, compared to MSS data, TM data should greatly improve the ability to map hydrothermally altered rocks in arid terrains.