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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.

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At least 1,333 records · Page 74
All sourcesNASA NTRSDOE OSTI
NASA NTRS1974

Six-color photometry of Iapetus, Titan, Rhea, Dione and Tethys

Six-color photometric observations made during Saturn's 1972/73 opposition enable us to separate the solar phase and orbital phase contributions to the observed light variations of Iapetus, Titan, Rhea, Dione and Tethys. Titan shows no orbital variations, but has phase coefficients which range from negligible values in the infrared to 0.014 mag/deg in the ultraviolet. Rhea has a bright leading side, a light curve amplitude of about 0.2 mag, and surprisingly large phase coefficients. Combined with other available information, the observations suggest a very porous, texturally complex surface layer. Dione has a leading side which is a few tenths of a magnitude brighter than the trailing side, but the light curve amplitude has little wavelength dependence and the phase coefficients are significantly smaller than those of Rhea, suggesting a less intricate surface texture. The leading side of Tethys is probably a few tenths of a magnitude brighter than the trailing side. Our Iapetus observations generally supplement the earlier work by Millis.

Noland, M.↗
NASA NTRS1974

Statistical modeling of scintillation effects

Scintillation produces fluctuation of the complex envelope of a modulated signal. A useful way to characterize scintillation effects is to describe the signal statistics that result when a CW wave is transmitted through a random medium. Many theoretical treatments describe the signal statistics in a manner identical with the noise theory of Rice. These theories, however, predict Rice statistics only at a very great distance from the perturbing medium, and it has been suspected that generalization to permit the quadrature components of the scattered signal to be partially correlated Gaussian variates might better match the observed signal statistics. Recent tests of signals observed through three types of structured plasma have consistently confirmed this speculation and have revealed a surprising consistency in parameters describing the first-order signal statistics.

Fremouw, E. J.↗
NASA NTRS1975

Thermodynamic properties of small aggregates of rare-gas atoms

The present work reports on the equilibrium thermodynamic properties of small clusters of xenon, krypton, and argon atoms, determined from a biased random-walk Monte Carlo procedure. Cluster sizes ranged from 3 to 13 atoms. Each cluster was found to have an abrupt liquid-gas phase transition at a temperature much less than for the bulk material. An abrupt solid-liquid transition is observed for thirteen- and eleven-particle clusters. For cluster sizes smaller than 11, a gradual transition from solid to liquid occurred over a fairly broad range of temperatures. Distribution of number of bond lengths as a function of bond length was calculated for several systems at various temperatures. The effects of box boundary conditions are discussed. Results show the importance of a correct description of boundary conditions. A surprising result is the slow rate at which system properties approach bulk behavior as cluster size is increased.

Etters, R. D.↗
NASA NTRS1975

Fused silica reflecting heat shields for outer planet entry probes

The development of slip-cast fused silica is discussed as a heat shield designed to meet the needs of outer-planet entry probes. The distinguishing feature of silica is its ability to reflect the radiation imposed by planetary-entry environments. This reflectivity is particularly sensitive to degradation by the presence of trace amounts of contaminants introduced by the starting materials or by processing. The microstructure of a silica configuration also significantly influences the reflectivity and other thermomechanical properties. The processing techniques attendant on controlling microstructure while maintaining purity are discussed. The selection of a starting material of essential purity precludes the use of purified natural quartz and requires the use of synthetic fused silica. The silica is characterized in a limited combined heating test environment. The surface mass loss is controlled by liquid runoff from a relatively low-temperature melt layer; the reflectance is basically maintained and the material achieves a surprisingly high heat of ablation.

Congdon, W. M.↗
NASA NTRS1975

Do comets play a role in galactic chemistry and gamma-ray bursts

This paper explores the plausibility of the assumption that enough material from interstellar space is locked up in comets to reduce significantly the apparent growth rate of 'heavy' elements (mass greater than He) and, therefore, the present abundance of heavy elements in the interstellar medium and in the disk stars. A related suggestion concerns the influx of comets on neutron stars as a source of gamma-ray bursts. Although no interstellar comets have been observed, reasonable upper limits to the observed numbers and masses do not rule out the first suggestion. The gamma-ray-burst suggestion appears unlikely. A surprisingly large total mass of comets could be gravitationally bound to the sun and, of course, to other stars, but remain undetected. The nature of the 1908 Tunguska explosion is discussed briefly. It was probably not an encounter with an active comet.

Whipple, F. L.↗
NASA NTRS1975

Ionospheric and tropospheric scintillation as a form of noise

Recent tests of signals observed through the ionosphere, the solar wind, and a laboratory plasma have revealed a surprising consistency in parameters describing the first order statistics of a signal caused to scintillate by a randomly structured plasma. This paper describes a means for exploiting these new findings in a transionospheric communication channel model.

Fremouw, E. J.↗
NASA NTRS1975

Electrophysiological measurement of human auditory function

Contingent negative variations in the presence and amplitudes of brain potentials evoked by sound are considered. Evidence is produced that the evoked brain stem response to auditory stimuli is clearly related to brain events associated with cognitive processing of acoustic signals since their properties depend upon where the listener directs his attention, whether the signal is an expected event or a surprise, and when sound that is listened-for is heard at last.

Galambos, R.↗
NASA NTRS1976

On the role of dimensionality and sample size for unstructured and structured covariance matrix estimation

The experimental observation that a surprisingly small sample size vis-a-vis dimension is needed to achieve good signal-to-interference ratio (SIR) performance with an adaptive predetection filter is explained. The adaptive filter requires estimates as obtained by a recursive stochastic algorithm of the inverse of the filter input data covariance matrix. The SIR performance with sample size is compared for the situations where the covariance matrix estimates are of unstructured (generalized) form and of structured (finite Toeplitz) form; the latter case is consistent with weak stationarity of the input data stochastic process.

Morgera, S. D.↗
NASA NTRS1976

Coronal propagation: Variations with solar longitude and latitude

Observational results on the East-West effect are summarized and discussed in the context of existing models of coronal propagation. The variation of the number of events with solar longitude is shown to be surprisingly similar for particles covering a large interval of rigidities. Also, over large longitudinal distances, time delays to the event onset and maximum intensity are independent of energy and velocity. This has important implications and will require probably a transport process which is determined by fundamental properties of solar magnetic fields, e.g. reconnection processes between open and closed field configurations. The relative role of open and closed field configurations is extensively discussed. Some evidence is presented that the acceleration of protons to higher (approximately 10 MeV) energies is related with a shock wave traveling in the solar atmosphere. The importance of measurements performed from spacecraft out-of-the-ecliptic plane is stressed.

Wibberenz, G.↗
NASA NTRS1976

Inelastic scattering of 61 MeV protons by pb-207

Differential cross sections for the excitation of the first four neutron-hole states and the doublet at 2.61 MeV by 61.2 MeV protons were measured. The data are analyzed in terms of both a purely collective model description and a microscopic model supplemented by macroscopic core polarization. A realistic two-body interaction is used and knock-on amplitudes are included. Core polarization is found to be important but represents a relatively smaller contribution than in most nuclei previously studied. A parallel analysis of similar data at lower proton bombarding energies reveals a surprisingly strong energy dependence of the reaction mechanisms.

Owais, M.↗
NASA NTRS1975

SEC Vidicon spectra of Geminid meteors, 1972

The SEC Vidicon, a low light level closed circuit television system, was used to obtain 137 spectrographic records of meteors at Mt. Hopkins, Arizona, during the Geminid meteor shower in December 1972. Seven of the best Geminid meteor spectra are studied here in detail. The near infrared, out to wavelengths near 9000 A, is recorded for the first time for Geminids. The spectra, in general, exhibit the elements previously found in photographic records of this shower but show a surprising frequency of occurrence of the forbidden green line of O I at 5577 A. This line is normally absent from meteors moving as slowly as the Geminids (36 km/sec) and its presence in these records may be due to the added sensitivity available with the SEC Vidicon. The average green line duration in Geminid meteors with a luminosity near zero absolute visual magnitude is 0.73 sec at a mean height of 95 km, 11 km lower than the green line peak in Perseid meteors of the same luminosity.

Millman, P. M.↗
NASA NTRS1975

Charge-transfer energy in closed-shell ion-atom interactions

The importance of charge-transfer energy in the interactions between closed-shell ions and atoms is investigated. Ab initio calculations on H(plus)-He and Li(plus)-He are used as a guide for the construction of approximate methods for the estimation of the charge-transfer energy for more complicated systems. For many alkali ion-rate gas systems the charge-transfer energy is comparable to the induction energy in the region of the potential minimum, although for doubly charged alkaline-earth ions in rare gases the induction energy always dominates. Surprisingly, an empirical combination of repulsion energy plus asymptotic induction energy plus asymptotic dispersion energy seems to give a fair representation of the total interaction, especially if the repulsion energy is parameterized, despite the omission of any explicit charge-transfer contribution. More refined interaction models should consider the charge-transfer energy contribution.

Alvarez-Rizzatti, M.↗
NASA NTRS1975

Domestic and world trends (1980 - 2000) affecting the future of aviation

Variables affecting aviation in the United States during the last fifth of the twentieth century are studied. Estimates of relevant future developments are presented and their probable impact on the aviation industry in this country are identified. A series of key trends relating to economic, social, political, technological, ecological and environmental developments are identified and discussed with relation to their possible effects on aviation. From this analysis, a series of scenarios are developed representing an array of possibilities ranging from severe economic depression and high international tension on the one hand, to a world of detente which enjoys an unprecedented economic growth rate and relaxation of tensions on the other. A surprise free scenario is presented which represents the best judgment of the manner in which events will most probably develop and the effect on the aviation industry such developments will likely produce.

Friedman, N.↗
NASA NTRS1976

Enhancement of dielectronic recombination by plasma electric microfields

A description is presented of an application of techniques from Stark-broadening theory considered by Griem (1974) to the computation of dielectronic recombination rated for multiply charged Fe ions under the influence of plasma electric microfields. Attention is given to a dielectronic recombination process for which the effects of surrounding charged particles have been found to be important at surprisingly low densities.

Jacobs, V. L.↗
NASA NTRS1976

Composition and structure of the Martian upper atmosphere - Analysis of results from Viking

Densities for carbon dioxide measured by the upper-atmospheric mass spectrometers on Viking 1 and Viking 2 are analyzed to yield height profiles for the temperature of the Martian atmosphere between 120 and 200 kilometers. Densities for nitrogen and argon are used to derive vertical profiles for the eddy diffusion coefficient over the same height range. The upper atmosphere of Mars is surprisingly cold, with average temperatures for both Viking 1 and Viking 2 of less than 200 K, and there is significant vertical structure. Model calculations are presented and shown to be in good agreement with measured concentrations of carbon monoxide, oxygen, and nitric oxide.

Mcelroy, M. B.↗
NASA NTRS1975

Response of Doppler spacecraft tracking to gravitational radiation

A calculation is made of the effect of gravity waves on the observed Doppler shift of a sinusoidal electromagnetic signal transmitted to, and transponded from, a distant spacecraft. We find that the effect of plane gravity waves on such observations is not intuitively immediate and in fact can have surprisingly different spectral signatures for different spacecraft directions and distances. We suggest the possibility of detecting such plane waves by simultaneous coherent Doppler tracking of several spacecraft.

Estabrook, F. B.↗
NASA NTRS1977

The design and implementation of the Viking mission

The Viking missions that were actually implemented were substantially different from those that were planned prior to arrival at the planet. These differences were not, however, unexpected because the fundamental tenet of the Viking mission operations strategy (MOS) was adaptability. In this paper, this unique 'design for change' strategy is explained in detail with particular emphasis on the way that the strategy allowed the missions to respond to both scientific and engineering surprises. The total result of using such an adaptive strategy is shown in a comparison of the expected mission profiles before Mars encounter and the mission profiles that were actually flown.

Lee, B. G.↗
NASA NTRS1976

Variations of the interplanetary magnetic field intensity between 1 and 0.3 AU

This is a preliminary report on the interplanetary magnetic field intensity B measured by the Rome/GSFC experiment carried on Helios 1 over the period 10 December 1974 to 3 April 1975 when the spacecraft moved from 1 AU to 0.3 AU (on 15 March 1975) and back to 0.47 AU. The large-scale radial variations between 1 and 0.3 AU are consistent with Parker's model for a spiral field. The distribution of B between 0.30 AU and 0.32 AU is comparable to that generally observed near 1 AU. Large changes are observed from day to day. Surprisingly large and abrupt changes in B (about 30 gammas in a few hours) are observed near perihelion.

Mariani, F.↗