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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 1,027 records · Page 57
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Crystal growth of compound semiconductors in a low-gravity environment (InGaAs crystals) (M-22)

Compound semiconductor crystals, such as gallium arsenide and indium phosphide crystals, have many interesting properties that silicon crystals lack, and they are expected to be used as materials for optic and/or electro-optic integrated devices. Generally speaking, alloy semiconductors, which consist of more than three elements, demonstrate new functions. For example, values of important parameters, such as lattice constant and emission wavelength, can be chosen independently. However, as it is easy for macroscopic and/or microscopic fluctuations of composition to occur in alloy semiconductor crystals, it is difficult to obtain crystals having homogeneous properties. Macroscopic change of composition in a crystal is caused by the segregation phenomenon. This phenomenon is due to a continuous change in the concentration of constituent elements at the solid-liquid interfacing during solidification. On Earth, attempts were made to obtain a crystal with homogeneous composition by maintaining a constant melt composition near the solid-liquid interface, through suppression of the convection flow of the melt by applying a magnetic field. However, the attempt was not completely successful. Convective flow does not occur in microgravity because the gravity in space is from four to six orders of magnitude less than that on Earth. In such a case, mass transfer in the melt is dominated by the diffusion phenomenon. So, if crystal growth is carried out at a rate that is higher than the rate of mass transfer due to this phenomenon, it is expected that crystals having a homogeneous composition will be obtained. In addition, it is also possible that microscopic composition fluctuations (striation) may disappear because microscopic fluctuations diminish in the absence of convection. We are going to grow a bulk-indium gallium arsenide (InGaAs) crystal using the gradient heating furnace (GHF) in the first material processing test (FMPT). The structure of the sample is shown where InGaAs polycrystals in a crucible are doubly sealed in two quartz tubes for safety. The GHF consists of two zones, namely, high temperature and low temperature zones, which results in a large temperature gradient at the interface. Crystal growth is performed by moving the furnace (i.e. the temperature profile) from the left to right at a definite rate. Thus, we will grow crystals both on Earth and in space under the same conditions. As previously described, it is possible to obtain good quality crystals which are homogeneous in composition both macroscopically and microscopically due to the lack of convection in space. We are planning to study the effects of convection on crystal growth from a melt by comparing and characterizing the properties of crystals grown on Earth with those grown in space.

Tatsumi, Masami↗
NASA NTRS1971

Instabilities in uranium plasma.

The nonlinear evolution of unstable sound waves in a uranium plasma has been calculated using a multiple time-scale asymptotic expansion scheme. The fluid equations used include the fission power density, radiation diffusion, and the effects of the changing degree of ionization of the uranium atoms. The nonlinear growth of unstable waves is shown to be limited by mode coupling to shorter wavelength waves which are damped by radiation diffusion. This mechanism limits the wave pressure fluctuations to values of order delta P/P equal to about .00001 in the plasma of a typical gas-core nuclear rocket engine. The instability is thus not expected to present a control problem for this engine.

Tidman, D. A.↗
NASA NTRS1972

Influence of oxygen-vacancy complex /A center/ on piezoresistance of n-type silicon.

Changes in both magnitude and temperature dependence of the piezoresistance of electron-irradiated n-type silicon, induced by the latter's oxygen-vacancy complex (A center), are shown to be due to the fact that the presence of the A center causes the total conduction-band electron concentration to change with an applied stress. This change in electron concentration leads to an additional piezoresistance contribution that is expected to be important in certain many-valley semiconductors. This offers the possibility of tailoring the thermal variations of semiconductor mechanical sensors to more desirable values over limited temperature ranges.

Littlejohn, M. A.↗
NASA NTRS1972

Voltages induced in superconductors in the absence of transport currents.

Experimental observations of the production of dc and harmonic voltages are described in superconductors subjected to superimposed dc and oscillatory magnetic fields. The attempt is made to account for these observations in a qualitative manner by relating them to the asymmetric flux flow induced by the magnetic fields in the superconductor samples. The observed effects are quite complex and depend on the values of the many parameters in the experiment. Regarding some of the observed phenomena, for which no satisfactory explanation is as yet available, further understanding is expected to come from detailed work on samples with asymmetric surfaces produced in a controlled fashion.

Ohmer, M. C.↗
NASA NTRS1973

ERTS-1 imagery use in reconnaissance prospecting: Evaluation of commercial utility of ERTS-1 imagery in structural reconnaissance for minerals and petroleum

The author has identified the following significant results. Five areas in North America (North Slope-Alaska, Superior Province-Canada, Williston Basin-Montana, Colorado, and New Mexico-West Texas) are being studied for discernibility of geological evidence on ERTS-1 imagery. Evidence mapped is compared with known mineral/hydrocarbon accumulations to determine the value of the imagery in commercial exploration programs. Evaluation has proceeded in the New Mexico-Texas area, and to date, results have been better than expected. Clearly discernible structural lineaments in this area are evident on the photographs. Comparison of this evidence with known major mining localities in New Mexico indicates a clear pattern of coincidence between the lineaments and mining localities. In West Texas, lineament and geomorphological evidence obtainable from the photographs define the petroleum-productive Central Basin Platform. Based on evaluation of results in the New Mexico-West Texas area and on cursory results in the other four areas of North America, it is concluded that ERTS-1 imagery will be extremely valuable in defining the regional and local structure in any commercial exploration program.

Saunders, D. F.↗
NASA NTRS1976

Canonical analysis for increased classification speed and channel selection

The quadratic form can be expressed as a monotonically increasing sum of squares when the inverse covariance matrix is represented in canonical form. This formulation has the advantage that, in testing a particular class hypothesis, computations can be discontinued when the partial sum exceeds the smallest value obtained for other classes already tested. A method for channel selection is presented which arranges the original input measurements in that order which minimizes the expected number of computations. The classification algorithm was tested on data from LARS Flight Line C1 and found to reduce the sum-of-products operations by a factor of 6.7 in comparison with the conventional approach. In effect, the accuracy of a twelve-channel classification was achieved using only that CPU time required for a conventional four-channel classification.

Eppler, W.↗
NASA NTRS1976

Physical processes in Comet Kohoutek

Observational results are brought together to establish the mass loss of Comet Kohoutek (1973 XII) in H2O, solid particles, and other molecules. The production rate of molecules is seen to have flattened prior to peaking near perihelion passage, and then fell below the rate for the incoming branch of the orbit. The total mass loss was about 10 to the 14th power gm. All abundances are consistent with a clathrate-dominant form of water. Comparison of the sizes of the nucleus as calculated from photometry and the rate of water loss requires that either an unrealistically high albedo be assumed or that only a portion of the nucleus was covered by water, in contrast with previous expectations for a new and probable first-passage comet. Moreover, if the preperihelion estimates of the nuclear brightness are rejected as being contaminated, the resulting values indicate an ice-covered nucleus with a radius of 2.1 km.

Odell, C. R.↗
NASA NTRS1982

Plasma characteristics of the Io torus

Simultaneous, low spectral resolution measurements of the following Jovian plasma torus forbidden emission lines show the occurrence of significant changes in characteristics on a time scale of days: S II 4069, 4076 A; S II 6716, 6731 A; S III 3722 A; and O II 3726, 3729 A. The O II emission is more extensive in magnetic latitude than the S II emission, which is the expected result of the mass difference between the ions if they have a common temperature. The S II and O II lines yield independent and consistent values for the average electron density, and variations about this average are larger than the measurement uncertainties. Unusual spectra in which the blue, but not the red S II forbidden lines are present indicate transient conditions of high plasma density and low electron temperature.

Morgan, J. S.↗
NASA NTRS1983

Possible evidence for the driving of the winds of hot stars by Alfven waves

Ultraviolet spectra of the supergiants Alpha Cam (09.5 Ia), HD 105056 (ON9.7 Iae), and 15 Sgr (O9.7 Iab) are compared, and it is shown that the terminal outflow velocity, TOV, of HD 105056 is one-half that of the other two stars even though HD 105056 has the highest effective temperature of the three stars. This anomaly, together with the fact that the observed TOV values for early-type stars scatter about an empirical correlation between TOV and log T(eff) by an amount which is larger than the amount expected according to the observational errors in determining TOV and log T(eff), leads to the conclusion that an agent in addition to radiation, Alfven waves, is driving the winds of early-type stars.

Underhill, A. B.↗
NASA NTRS1984

Comet 1983d - A contrast between data from IRAS and data from the ground

Optical photometry of comet IRAS-Araki-Alcock 1983d is presented. The data provide values for the product of albedo and average optical depth in the coma. These turn out to be at least a factor of 20 lower than expected from the IRAS measurements reported by Walker et al. (1984).

Ahearn, M. F.↗
NASA NTRS1985

Study of spectral exponent on quiet and disturbed days

The spectral exponents of the diurnal and semidiurnal anisotropies are studied on quiet and disturbed days for a period from 1964 to 75. It has been observed for the diurnal anisotropy that the spectral exponent for quiet days is 0.0 from 1964 to 70, however during 1971 and onwards its value is found different from zero and is -0.4; for disturbed days the spectral exponent is observed to be 0.0 from 1964 to 75 with an exception of the year 1966. Further, it has been observed for the semi-diurnal anisotropy that the best value for the spectral exponent on quiet days being + 0.6 from 1964 to 74; however on disturbed days the value of spectral exponent is sometimes negative or positive but different from zero. It was derived from the present investigation that superimposed on the corotational anisotropy, expected from a balance between outward radial convection and inward field aligned diffusion, there is an additional component operating during the period under consideration.

Kumar, S.↗
NASA NTRS1985

A new property of the small-scale nonthermal motions in quiescent prominences

Small-scale nonthermal motions are a characteristic feature of astrophysical atmospheres, which have been studied on the basis of an interpretation of high-dispersion spectral line observations. The present paper is concerned with a study based on the close-lying lines Mg double-lambda 5173, 5167 and Fe(+) lambda 5169, in quiescent prominences. A summary of Mg b2, b4, and Fe(+) b3 prominence measurements and derived model profile parameters is presented in a table, while another table contains a summary of derived temperature and microvelocity parameter values. The most important result of the reported study is the finding that the Fe(+) b3 profiles in quiescent prominences are systematically narrower than one would expect from the Mg b2 profile widths and the assumption that both lines arise in regions of common T and v.

Landman, D. A.↗
NASA NTRS1989

A technique for interpretation of auroral bremsstrahlung X-ray spectra

A procedure is described for determining the average energy and the energy flux of precipitating electrons from the resulting bremsstrahlung X-rays based on the assumption that the precipitating electrons have an exponential electron distribution. It is shown that, for nonexponential electron distributions, this method yields an exponential electron distribution that produces an altitude profile of ionization very similar to that expected from the actual spectrum. The Hall and Pedersen electrical conductances calculated from the inferred ionization profile were also found to be close to the true values.

Robinson, Robert M.↗
NASA NTRS1989

Pluto's atmosphere near perihelion

A recent stellar occultation has confirmed predictions that Pluto has an atmosphere which is sufficiently thick to uniformly envelope the planet and to extend far above the surface. Pluto's atmosphere consists of methane and perhaps other volatile gases at temperatures below their freezing points; it should regulate the surface temperature of its volatile ices to a globally uniform value. As Pluto approaches and passes through perihelion, a seasonal maximum in the atmospheric bulk and a corresponding minimum in the exposed volatile ice abundance is expected to occur. The lag in maximum atmospheric bulk relative to perihelion will be diagnostic of the surface thermal properties. An estimate of Pluto's atmospheric bulk may result if a global darkening (resulting from the disappearance of the seasonally deposited frosts) occurs before the time of maximum atmospheric bulk. The ice deposited shortly after perihelion may be diagnostic of the composition of Pluto's volatile reservoir.

Trafton, L. M.↗
NASA NTRS1991

A comparison of measured and calculated optical properties of atmospheric aerosols at infrared wavelengths

Measurements of 10.6-micron lidar backscatter were compared with calculated backscatter based on nearly simultaneous observations of stratospheric and tropospheric aerosol size distributions. It was found that there is better agreement in the troposphere, even though the uncertainties of the calculation are greater for this region due to the variables in both the spatial concentration and the physical makeup of the aerosol. A second comparison study was made to test the consistency of the mean tropospheric extinction values at 1.02 micron (as reported by the SAGE satellite) with the values calculated from an ensemble of 400 measured size distributions thought to be representative of midcontinental tropospheric aerosol. The two methods produce consistent results within the expected degree of uncertainty. The ensemble of 400 'proven' size distributions is then used to calculate a statistical relationship between the 1.02-micron extinction and the 10.6-micron backscatter.

Rosen, James M.↗
NASA NTRS2015

Optical Communications and Sensor Demonstration

The Optical Communications and Sensor Demonstration (OCSD) project addresses two cross-cutting capabilities of value to many future small spacecraft missions: high-speed optical transmission of data and small spacecraft proximity operations. Optical data rates demonstrated by OCSD are expected to be 200 megabits persecond (Mbs) or higher, a factor of 100 increase over current high-end CubeSat communications systems. The proximity sensors developed for this mission enable relative position measurement between two small satellites - a capability not previously demonstrated.

High-speed optical transmission of data↗
NASA NTRS2019

Validation, Comparison, and Integration of GOCI, AHI, MODIS, MISR, and VIIRS Aerosol Optical Depth Over East Asia During the 2016 KORUS-AQ Campaign

Recently launched multichannel geostationary Earth orbit (GEO) satellite sensors, such as the Geostationary Ocean Color Imager (GOCI) and the Advanced Himawari Imager (AHI), provide aerosol products over East Asia with high accuracy, which enables the monitoring of rapid diurnal variations and the transboundary transport of aerosols. Most aerosol studies to date have used low Earth orbit (LEO) satellite sensors, such as the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Multi-angle Imaging Spectroradiometer (MISR), with a maximum of one or two overpass daylight times per day from midlatitudes to low latitudes. Thus, the demand for new GEO observations with high temporal resolution and improved accuracy has been significant. In this study the latest versions of aerosol optical depth (AOD) products from three LEO sensors – MODIS (Dark Target, Deep Blue, and MAIAC), MISR, and the Visible/Infrared Imager Radiometer Suite (VIIRS), along with two GEO sensors (GOCI and AHI), are validated, compared, and integrated for a period during the Korea–United States Air Quality Study (KORUS-AQ) field campaign from 1 May to 12 June 2016 over East Asia. The AOD products analyzed here generally have high accuracy with high R (0.84–0.93) and low RMSE (0.12–0.17), but their error characteristics differ according to the use of several different surface-reflectance estimation methods. High-accuracy near-real-time GOCI and AHI measurements facilitate the detection of rapid AOD changes, such as smoke aerosol transport from Russia to Japan on 18–21 May 2016, heavy pollution transport from China to the Korean Peninsula on 25 May 2016, and local emission transport from the Seoul Metropolitan Area to the Yellow Sea in South Korea on 5 June 2016. These high-temporal-resolution GEO measurements result in more representative daily AOD values and make a greater contribution to a combined daily AOD product assembled by median value selection with a 0.5∘×0.5∘ grid resolution. The combined AOD is spatially continuous and has a greater number of pixels with high accuracy (fraction within expected error range of 0.61) than individual products. This study characterizes aerosol measurements from LEO and GEO satellites currently in operation over East Asia, and the results presented here can be used to evaluate satellite measurement bias and air quality models.

Myungje Choi↗
NASA NTRS2021

Fit to be Tied: Embracing Tethered Robots for Exploring Extreme Planetary Environments

Tethers are supportive wires that could provide power, communication, and even science instrument capabilities to planetary rovers, landers, and future human settlements. Whether it be power distribution and optical communication connecting robots exploring extreme, undersea and deep subsurface environments, to your home office on Earth, we live, and will continue to live, in a predominantly ‘wired’ world. Accordingly, as humanity continues to push beyond Earth to exploring extreme planetary terrains like caves, cliffs, craters, and crevasses, and moves towards establishing human settlements on the Moon and Mars, we can expect tethers to play a major role. For exploration purposes, tethers are critical to enabling next-generation, science-focused missions to access high-value, resource-limited targets, where exploring rovers or astronauts lack direct communication or access to viable sunlight for solar power. NASA’s Jet Propulsion Laboratory (JPL) is formulating new mission concepts that could deploy robotic assets into some of the Solar System’s harshest and steepest terrain and, in many cases, tethers are the enabling technology. Examples include i) a rappelling rover to explore vertically along lunar pit walls to determine how the Moon formed, ii) tethered rovers to reach ice deposits on Mars and the Moon, iii) large, tether-deployed antenna structures to probe into the subsurface of a planet and/or look skyward to image the highly red-shifted, early universe, iv) probes that descend into the icy layers of Europa and Enceladus in search of subsurface oceans and, potentially, extant life, and v) science instruments that could be suspended on supportive tethers from balloons on Venus to directly image the surface and sample its clouds. This paper will present a survey of the state-of-the-art for tether related exploration of the solar system and chronicle ongoing work at JPL, which is leading to robust tether designs, high-power and communication transmission over multi-km distances, and highly capable, tether-based rover systems.

McGarey, Patrick↗