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An important consideration for comet modeling is the dependence of the thermal conductivity on the comet structure. The microporosity of the comet ices changes with time as the phase of the ice and its annealing history progresses. Effects of these [modeled] changes on the overall gas production, as a function of time, are presented.
The analysis of TOPEX/POSEIDON sea level and ERS-1 zonal wind stress data gives evidence that first mode meridonal long Rossby waves fully reflected into Kelvin waves at the equatorial western Pacific boundary at different periods including the January-June 1994 period. The evolution of the conditions (zonal wind stress, sea surface temperature and sea level anomalies) in the central Pacific suggests that this reflection may have played a role in the reversal of cold to warm anomalies observed in the central Pacific in June-July 1994. To investigate the actual role of reflected Kelvin waves observed by TOPEX/POSEIDON, a simple ocean-atmosphere coupled model is designed. Starting from these initial conditions, several simulations are run for testing the role western bundary reflection evidenced in TOPEX/POSEIDON data. Coupled mechanisms are suggested to explain the low-frequency variability observed during this period.
The heterogeneous reaction of CO and O on the surface of aerosols has been proposed as an important chemical process which may contribute to the CO&sub2; stability in the Martian atmosphere.
Doppler tracking of distant spacecraft is the only method currently available for search for gravitational waves in the low-frequency band. Experiments to date and those planned for the near future all involve.
Among the low frequency (LF) gravitational wave sources that are of astronomical interest are white dwarf binaries, neutron star binaries, massive black hole binaries, and compact stars spiralling into massive black holes. A mission to detect these sources has been proposed to NASA as a possible member of its low-cost, near-term MIDEX mission series. This mission utilizes six tiny miniprobes in high Earth orbit to produce a sensitive Michelson interferometer with million kilometer arms, yielding a strain sensitivity below 10^{-21} at periods longer than a hundred seconds. At this sensitivity, known binary stars will be seen and plausible unknown massive black hole events will be searched for.
The paper will review the application of NASA developed remote sensing technology towards the monitoring and mitigation of natural hazards. The overview will be followed by recent data on three specific natural hazard applications.
JPL is responsible for deep space exploration using spacecraft and telerobotic technologies. Since all JPL's missions are one of a kind and hardware dependent, the requirements for nondestructive evaluation (NDE) of the materials and structures that are employed are significantly more stringent than the ones for conventional aerospace needs. The multidisciplinary technologies that are developed at JPL, particularily the ones for the exploration of Mars, are finding applications to a wide variety of NDE applications. Further, technology spin-offs are enabling the development of advanced actuators that are being used to drive various types of telerobotic devices. A review will be given of the recent JPL NDE and advanced actuators activity and it will include several short videos.
The Cassini RADAR is a multimode instrument built both by NASA/JPL and ASI/Alenia Spazio. The instrument operates in radiometer-only, scatterometer, altimeter, and imaging modes. This paper will describe this interesting unique radar and how it will be used to collect data at Titan.
The Alaska SAR Facility (ASF) has been receiving, processing, archiving, and distributing data for Earth scientists and operations since it began receiving data in 1991. Four radar satellites are now being handled. Recent developments have served to increase the level of services of ASF to the Earth science community considerably. These developments are discussed.
The Galileo spacecraft was launched on October 18, 1989. After a 3.7 billion kilometer journey lasting just over six years, the Galileo Orbiter and Probe arrived at Jupiter on December 7, 1995. The atmospheric Probe survived its atmospheric entry and successfully transmitted data to the Orbiter flying overhead. To date, the Orbiter has successfully completed the first three of the ten planned satellite encounters. Navigation for the Galileo Mission has proved to be a unique and challenging task. The challenges and results of navigating Galileo through the interplanetary transfer to Jupiter, delivery of the atmospheric entry Probe, Jupiter orbit insertion, and the orbital tour are presented in this paper.
The only physical observations which predate the identifications of 253 Mathilde as a potential flyby target of NEAR were measurements of its absolute magnitude in reflected light and thermal radiometry by IRAS. Mathilde has been revealed to be an unusual object, of a class not previously visited by a spacecraft.
Since 1991 a suite of international satellites have collected large amounts of high-resolution microwave radar images over Arctic and Antarctic sea ice. Together with complementary synthetic aperture radar (SAR) 100 m resolution microwave imaging, these data provide a powerful tool for addressing the characteristics of sea ice which directly influence the polar oceans and climate.
In recent years, a great deal of research has been exerted to developing NDE methods for the characterization of the material properties of composites as well as other space structural materials. The need for information about such parameters as the elastic properties, density, and thickness are critical to the safe design and operation of such structural materials. Ultrasonics using immersion methods has played an important role in these efforts due to its capability, cost effectiveness, and ease of use. The authors designed a series of ultrasonic oblique insonification experiments in order to develop a practical field applicable NDE method for space structures.
The first Danish satellite, rsted, will be launched in August of 1997. The scientific objective of sted is to perform a precision mapping of the Earth's magnetic field. Attitude data for the payload and the satellite are provided by the Advanced Stellar Compass (ASC) star tracker. The ASC consists of a CCD star camera and a capable microprocessor which operates by comparing the star image frames taken by the camera to its internal star catalogs.
NASA's SPICE information system for archiving, distributing, and accessing spacecraft navigation, orientation, and other ancillary data is described. A proposal is made for the further evolution of this concept to an internationally useful standard, to be.
The Galileo Mission is designed to study all aspects of the huge, complex Jupiter system. Its primary objectives are: 1.) Jupiter, 2.) the Galilean satellites, and 3.) the Magnetosphere. Since its arrival in the Jupiter system on 7 December 1995, Galileo has provided a nearly continuous stream of data concerning all its objectives and has already made several major discoveries. Initial results from the Probe and from the Orbiter are described in this paper.
Before the arrival of the Galileo spacecraft in the jovian system, there was little information on the interior structures of Jupiter's four Galilean satellites. Results obtained during the Galileo Mission are described.
The potential of wide-band-gap III-V nitrides as ultraviolet sensors and light emitters has prompted an increasing amount of work recently, including the fabrication of the first UV sensors from as-deposited single crystal GaN. We have used high resolution transmission electron microscopy (TEM) to study the microstructure of two novel developments of wide-band-gap III-V nitrides: the growth of ultra-short period GaN/AlN superlattices; and the incorporation of SiC layers into Al(sub x)Ga(sub 1-x)N structures. By varying the relative periods in a GaN/AlN superlattice, the band gap of the composite can be tailored to lie between the elemental values of 365 nm for GaN and 200 nm for AlN. The group IV semiconductor, SiC, has a wide band-gap and has a close lattice match (less than 3 %) to Al(sub x)Ga(sub 1-x)N for growth on the basal plane. Demonstration of epitaxial growth for Al(sub x)Ga(sub 1-x)N/SiC multilayers would introduce a wide band-gap analog to the already existing family of III-V and Si(sub 1-x)Ge(sub x) heteroepitaxial growth systems. Although good quality growth of GaN on SiC substrates has been demonstrated, Al(sub x)Ga(sub 1-x)N/SiC multilayer structures have never been grown and the interfacial structure is unknown.