SEARCH · Search NASA
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
An aspheric grating is described which is operable to image local or distant point sources sharply in a designated wavelength, i.e., produce a perfectly stigmatic image in the given wavelength at grazing angles of incidence. The grating surface comprises a surface of revolution defined by a curve which does not have a constant radius of curvature but is defined by a nonlinear differential equation.
The paper describes the major observational features of one new component of Jupiter's radio emission spectrum, the broadband kilometer wavelength radiation, or bKOM. This study, using the Voyager Planetary Radio Astronomy (PRA) experiments, reveals that the overall occurrence morphology, dynamic spectra, and polarization character of bKOM are strong functions of the latitude and/or local time geometry of the observations. The postencounter data show a decline in the mean occurrence rates and power level of bKOM and, in particular, a depletion in the occurrence rate at those same longitudes where the detection rate is a maximum before encounter. Additionally, the polarization sense undergoes a permanent reversal in sign after encounter, whereas the time-averaged wave axial ratio and degree of polarization remain relatively unchanged. Finally, no evidence of any control by Io is found. The strong dependence of the morphology on local time suggests a source whose beam is nearly fixed relative to the Jupiter-sun line
A model is presented that accounts for several features of the dynamic spectral arcs observed at decameter wavelengths by the planetary radio astronomy experiment on Voyagers 1 and 2. It is shown that refraction of an extraordinary mode wave initially excited and nearly orthogonal to the local magnetic field is significantly influenced by the local plasma density. The source of the radiation is on the L = 6 flux, and the emission cone angle of the sheet is chosen to vary with frequency so that it is relatively small at both high and low frequencies while about 80 deg at intermediate frequencies. This functional dependence of cone angle on frequency appears to be consistent with estimates of refraction of the ordinary mode in a source region where the electron density is greater than a few hundred per cu cm.
Current understandings of the nature and probable origin of lunar magnetism are surveyed. Results of examinations of returned lunar samples are discussed which reveal the main carrier of the observed natural remanent magnetization to be iron, occasionally alloyed with nickel and cobalt, but do not distinguish between thermoremanent and shock remanent origins, and surface magnetometer data is presented, which indicates small-scale magnetic fields with a wide range of field intensities implying localized, near-surface sources. A detailed examination is presented of orbital magnetometer and charged particle data concerning the geologic nature and origin of magnetic anomaly sources and the directional properties of the magnetization, which exhibit a random distribution except for a depletion in the north-south direction. A lunar magnetization survey with global coverage provided by a polar orbiting satellite is suggested as a means of placing stronger constraints on the origin of lunar crustal magnetization.
Fluxes and exchange coefficients are derived for the transport of Sr-90, Pb-210, Bi-210, and Po-210 between the free troposphere and the marine boundary layer and between the boundary layer and the sea surface. Radionuclide concentrations previously measured near Hawaii are used in the derivations. Values obtained for the free troposphere/boundary layer exchange coefficient (expressed as a piston velocity) were 185, 228 and 203 m/d for Pb-210, Bi-210, and Sr-90, respectively. The magnitude of the local sea-surface source of Po-210 is also determined.
Data from the cosmic ray experiment on Voyagers 1 and 2 was used to examine anomalous oxygen in the time period from launch in 1977 to the end of 1981. Several time periods were found where large periodic (typically 26 day) temporal variations of the oxygen intensity between approximately 5 - 15 MeV/nuc are present. Variations in intensity by up to a factor of 10 are observed during these periods. Several characteristics of these variations indicate that they are not higher energy extensions of the low energy particle (approximately 1 MeV/nuc) increases found in many corotating interaction regions (CIR's). Many of these periodic temporal variations are correlated with similar, but much smaller, recurrent variations in the 75 MeV proton rate. Voyager 1 and Voyager 2 counting rates were compared to estimate the local radial gradient for both the protons and the oxygen. The proton gradients during periods of both maximum and minumum fluxes are consistent with the overall positive radial gradients reported by others from Pioneer and near-Earth observations, supporting the view that these variations are due to local modulation of a source outside the radial range of project measurements. In contrast, the oxygen gradients during periods of maximum proton flux differ in sign from those during minimum proton fluxes, suggesting that the origin of the oxygen variations is different from that of the protons.
NASA's Crustal Dynamics Project (CDP) has the objective to improve the understanding of geodynamics by measuring crustal deformation, tectonic motion, and polar motion and earth rotation. Three different approaches are utilized for obtaining these measurements. One is based on satellite laser ranging (SLR), while another makes use of very long baseline interferometry (VLBI) which uses reception of radio signals from quasars. The third approach involves laser ranging to the moon. An important part of the CDP is to compare baselines periodically or the straight-line distance between two points on the earth's surface as determined by either SLR or VLBI. Attention is given to the SLR analysis, the VLBI analysis, a local survey, and error sources. A table is presented with the baselines between SLR survey markers as measured by VLBI and SLR.
This Working Paper Series entry describes the PC R and D development effort initiated as part of the NASA/RECON Project at the University of Southwestern Louisiana. This effort involves the development of a PC-based environment for the prototyping and evaluation of various tools designed to enhance the interaction between scientists and engineers and remote information systems. The design of PC-based tools for the enhancement of the NASA/RECON university-level courses is described as well as the design of a multi-functional PC-based workstation to support access to and processing of information from local, distributed, and remote sources. Course preparation activities are described in a companion report entitled A Report on the USL NASA/RECON Project: Part 1, the Development of a Transportable, University-Level, IS and R Educational Program, by Suzy Gallagher and Martin Granier, USL/DBMS NASA/RECON Working Paper Series report number DBMS.NASA/RECON-7.
The major observational characteristics of the smooth, narrow bandwidth component of Uranus' radio emissions are well described by sources radiating near the local electron gyrofrequency, confined to the magnetic equatorial plane and encircling the planet at radial distances of approximately 2 to 3 R(U). The most intense emission appears to be generated in association with the epsilon ring at 2.0 R(U) radial distance. A cold electron density of less than or equal to 4/cu cm are inferred in this region.
Both the Earth's auroral kilometric radiation (AKR) and Jupiter's decametric radio S-bursts are attributed to natural radio lasing. Presumably consisting of self-excited, closed-loop wave feedback oscillations between local irregularities of the source plasma density, this radio lasing is comparable to that which occurs in man-made optical lasers, although at radio, rather than optical wavelengths. As a result, it should produce a multiple discrete emission spectrum and intense, coherent beams. Recent observations of the AKR's discreteness and coherence have clearly ruled out the previous open-loop amplifier model for such emissions, and recent observations of the Jovian S-bursts have shown the expected, regularly-spaced, longitudinal laser modes. These new observations thus confirm the proposed planetary cyclotron radio lasing at both planets.
Giotto observations of the inner coma of Comet Halley are analyzed, focusing on the dust distribution and the gas-dust interaction. Differences between the azimuthal and radial dust distributions are discussed. The profiles of dust jets suggest that gas emission is localized in the same source areas that give rise to dust. The observational results are compared with models of cometary dust and suggestions are made for improving the models to give better agreement with observations.
In preparation for the Ulysses Gravitational Wave Experiment a Doppler detector has been constructed for the Bologna VLBI antenna and tested in a differential mode with the spacecraft Voyager 2 at 25 AU. In this mode the signal was sent from the Deep Space Network station in Canberra and received at Madrid and Bologna. The correlation between the received signals allows a discrimination between local and common noise sources. The successful test was performed in August, 1988 showed that special communication procedure will have to be implemented to allow a smooth and reliable operation during the real experiment.
The important principles of direct- and heterodyne-detection optical free-space communications are reviewed. Signal-to-noise-ratio (SNR) and bit-error-rate (BER) expressions are derived for both the direct-detection and heterodyne-detection optical receivers. For the heterodyne system, performance degradation resulting from received-signal and local oscillator-beam misalignment and laser phase noise is analyzed. Determination of interfering background power from local and extended background sources is discussed. The BER performance of direct- and heterodyne-detection optical links in the presence of Rayleigh-distributed random pointing and tracking errors is described. Finally, several optical systems employing Nd:YAG, GaAs, and CO2 laser sources are evaluated and compared to assess their feasibility in providing high-data-rate (10- to 1000-Mbps) Mars-to-Earth communications. It is shown that the root mean square (rms) pointing and tracking accuracy is a critical factor in defining the system transmitting laser-power requirements and telescope size and that, for a given rms error, there is an optimum telescope aperture size that minimizes the required power. The results of the analysis conducted indicate that, barring the achievement of extremely small rms pointing and tracking errors (less than 0.2 microrad), the two most promising types of optical systems are those that use an Nd:YAG laser (lambda = 1.064 microns) and high-order pulse position modulator (PPM) and direct detection, and those that use a CO2 laser (lambda = 10.6 microns) and phase shifting keying homodyne modulation and coherent detection. For example, for a PPM order of M = 64 and an rms pointing accuracy of 0.4 microrad, an Nd:YAG system can be used to implement a 100-Mbps Mars link with a 40-cm transmitting telescope, a 20-W laser, and a 10-m receiving photon bucket. Under the same conditions, a CO2 system would require 3-m transmitting and receiving telescopes and a 32-W laser to implement such a link. Other types of optical systems, such as a semiconductor laser systems, are impractical in the presence of large rms pointing errors because of the high power requirements of the 100-Mbps Mars link, even when optimal-size telescopes are used.
The Mangala Valles channel system differs from other Mars outflow channels as they emanate from a point source determined by the local interplay of tectonism, volcanism, and volatiles that led directly to the release of groundwater. Water discharge rates through the southern reaches of Mangala Valles may have been substantially lower than estimates determined fron the canyon system as a whole, suggesting that the accumulation and release of water may have lasted longer than previously envisioned. The sequence of events determined from mapping of units along the southern reaches of Mangala Valles is closely related to the area structures.
The Ulysses X-ray and gamma-ray instrument is described, and its scientific objectives are presented. Some aspects of the history of the Ulysses mission are explained, as well as of the design considerations for this experiment. As this mission will be the most distant one from earth ever to carry a burst detector, the primary cosmic objective will be to localize gamma-burst sources by triangulation.
A meaningful interpretation of airborne flux estimates over nonuniform terrain must consider local advection from nonhomogeneous source distributions (footprint correction). An empirical procedure for footprint correction of airborne flux data obtained in the First ISLSCP Field Experiment (FIFE) 1989 is presented, based on simple analytical solutions of the diffusion equation which have been adjusted to approximate more realistic solutions. Optimized estimates of surface flux 'maps' derived from airborne observations are then constructed for maximum correlation between flux estimates and independently observed surface characteristics. This paper also includes an analysis of resolution for the given procedure, in terms of amplitude recovery of hypothetical square-wave distributions of surface flux density, and discusses its implications for the interpretation of the optimized estimates of FIFE surface flux maps. Results show that corrected spatial sink distributions are obtained by the current empirical procedure at the cost of considerable reduction in amplitude recovery of small-scale variations in surface flux density. The high spatial correlation observed between corrected flux maps and surface characteristics such as greenness or surface temperature excess is ascribed to the approximately correct positioning of the main flux gradients across the site, which can be expected to have been reproduced with amplitude recovery equal to or greater than 60 percent.
In this paper, I discuss an extremely luminous X-ray source found on the northern arm of the nearly face-on spiral galaxy NGC 6946. The source was originally discovered in an Einstein observation, but the poor spatial resolution prevented an accurate identification. Here, Roentgen Satellite (ROSAT) data provide a position to approximately 5 sec - 10 sec. The spectrum is rather soft, and moderately absorbed. The source appears to be local to NGC 6946 and a supernova remnant, based upon the positional coincidence with an optical counterpart. The luminosity, assuming a distance to NGC 6946 of 5.1 Mpc, is 2.8 x 10(exp 39) erg/s, among the highest known X-ray luminosities of a remnant. The optical counterpart is at least 100 years old.
We present a summary of gamma-ray and X-ray observations of the intriguing COS-B field, 2CG 075+00, in order to search for potential counterparts. The third EGRET (Energetic Gamma Ray Experiment Telescope) (3EG) catalog shows that the COS-B emission corresponds to at least two localized gamma-ray sources, 3EG 2016+3657 and 3EG J2021+3716. We present analyses of archival X-ray fields which overlap error boxes of both the EGRET sources.