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At least 1,243 records · Page 69

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NASA NTRS1972

Shielding of manned space vehicles against protons and alpha particles

The available information on the shielding of manned space vehicles against protons and alpha particles is summarized. The emphasis is placed on shielding against Van Allen belt protons and against solar-flare protons and alpha particles, but information on shielding against galactic cosmic rays is also presented. The approximation methods for use by nonexperts in the space shielding field are those that are standard in the space shielding literature.

Alsmiller, R. G., Jr.↗

NASA NTRS1976

Results of the asteroid-meteoroid particle experiment on Pioneer 11

The Asteroid-Meteoroid Detector, an electro-optical instrument that detects and measures particles in space by sensing the sunlight reflected from them, is part of the payload of both the Pioneer 10 and 11 spacecraft. The Pioneer 11 instrument is essentially identical to that on Pioneer 10, with some minor modifications. Reduction of the Pioneer 11 data was complicated by the failure of one of the four sensor channels at about 1.1 AU from the sun. Detailed analysis yielded 51 events between 1.0 and 3.5 AU, which were used to determine the particle concentration distribution in that region of space. Weighted curve-fitting of the results shows agreement with the concentrations derived from Pioneer 10 to within about a factor of two. As with the Pioneer 10 results, the exponent of the size dependency varies from about -1.7 for 100-micron radius particles to about -3.2 for 10-cm bodies. The instrument, its operation, and the method of data analysis are reviewed.

Soberman, R. K.↗

NASA NTRS1981

Electromagnetic backscattering from a sparse distribution of lossy dielectric scatterers

Electromagnetic backscattering from a sparse distribution of lossy dielectric particles having random orientation and position is studied. The paper begins by using the Foldy approximation to find an equation for the mean field. From this equation, an effective permittivity for the scattering medium is obtained. The correlation of the scattered field is found by employing the distorted Born approximation, i.e., particles embedded in the effective medium are assumed to be single scatterers. The above method is then used to find the backscattering coefficients from a leaf canopy. The leaf canopy is modeled by a half space of dielectric discs that are small in comparison to a wavelength. Numerical results show that the depolarized cross section is a sensitive function of leaf inclination angle statistics.

Lang, R. H.↗

NASA NTRS1981

Studies on the behavior of ammonia and ammonium salts in the atmosphere (1) - Fractional collection of ammonia gas and particulate ammonium

Methods for the fractional collection of trace amounts of atmospheric ammonia gas and ammonium particles on a two staged glass fiber filter are summarized. A standard glass fiber filter washed with distilled water and dried at 120 to 130 C was used. A second filter was impregnated with a mixture of 3% boric acid and 25% glycerin solution. The blank of glass fiber filters impregnated with a mixture of the above solution was very low for ammonia, i.e. 0.06 micrograms in a filter of 47 mm in diameter. The mean concentrations of ammonia and ammonium in air at Kawasaki, a polluted area, were 7.6 and 2.3 micrograms cu m, and those at Sanriku, an unpolluted area 0.9 and 0.2 micrograms cu m, respectively. Ratios of concentration levels of ammonium to total ammonia in the atmosphere were 0.3 and 0.2 for the polluted and unpolluted areas, respectively. Ammonium salts in air at both areas were not correlated with relative humidity. Variations in time of ammonia concentrations and sources in surrounding areas are also considered.

Kiin, K.↗

NASA NTRS1983

Onion-shell model of cosmic ray acceleration in supernova remnants

A method is devised to approximate the spatially averaged momentum distribution function for the accelerated particles at the end of the active lifetime of a supernova remnant. The analysis is confined to the test particle approximation and adiabatic losses are oversimplified, but unsteady shock motion, evolving shock strength, and non-uniform gas flow effects on the accelerated particle spectrum are included. Monoenergetic protons are injected at the shock front. It is found that the dominant effect on the resultant accelerated particle spectrum is a changing spectral index with shock strength. High energy particles are produced in early phases, and the resultant distribution function is a slowly varying power law over several orders of magnitude, independent of the specific details of the supernova remnant.

Bogdan, T. J.↗

NASA NTRS1985

Grid-free simulation of diffusion using random walk methods

The simulation of the diffusion of a continuum field by the random walk (RW) displacement of a set of particles is considered. Elements of the gradients of the diffusive concentration are transported by computational particles. It is demonstrated that, by the use of concentration gradients in the RW process, statistical errors are reduced and each realization of the numerical solution is a representation of the exact solution. The algorithm is grid-free, and the computational elements move to follow the gradients; hence, the algorithm is self-adaptive, and uniform resolution is achieved for all times.

Ghoniem, A. F.↗

NASA NTRS1988

Observations of comets and asteroids

All available ground-based observational techniques are used to study the chemical and physical properties of the small bodies of the solar system, primarily comets and secondarily asteroids. The ultimate goal is to use these bodies to understand the formation and evolution of the solar system. The analysis of charge coupled device (CCD) images of comet P/Halley continued. This included a mojor revision of a paper on the periodicities of CN jets in Halley showing that their periodicity is 7.3 days, not 2.2 days. It also included an analysis of the continuum images which showed that the dust in jets is much redder than in the ambient coma. Calculations with Mie theory suggest that the particles that show the strongest effects of radiation pressure. Much effort was expended devising methods to estimate the level of sky background of the many images in which comet extends to the edge of the chip. Examined images of comet Wilson were taken over many months in 1987. Wilson, a dynamically new comet, shows none of the short-term variability or jet structure (either dusty jets or jets of radicals seen in Halley. This is presumably due to the lack of a mantle or the nucleus. Median-imaging of sky was as a technique for removing residual flat-fielding errors in CCD images. A Monte Carlo model and a convolution model were developed for studying temporal variability of gaseous daughter products.

Ahearn, Michael F.↗

NASA NTRS1990

Direct simulation of reentry flows with ionization

The Direct Simulation Monte Carlo (DSMC) method is applied in this paper to the study of rarefied hypersonic reentry flows. The assumptions and simplifications involved with the treatment of ionization, free electrons and the electric field are investigated. A new method is presented for the calculation of the electric field and handling of charged particles with DSMC. In addition, a two-step model for electron impact ionization is implemented. The flowfield representing a 10 km/sec shock at an altitude of 65 km is calculated. The effects of the new modeling techniques on the calculation results are presented and discussed.

Carlson, Ann B.↗

NASA NTRS1989

Optical properties of irregular interstellar grains

In order to study the interaction of light with interstellar grains, the authors represent an irregular particle by a network of interacting dipoles whose polarizability is determined in a first approach by the Clausius-Mossoti relationship. Typically, 10,000 dipoles are considered. In the case of spherical particles, the results from Mie theory are fully recovered. The main interest of this method is to study with good accuracy the implications of surface roughness and/or inhomogeneities on optical properties in the infrared spectral range, particularly of the silicate emission features.

Perrin, J. M.↗

NASA NTRS1991

Hypervelocity impact shield

A hypervelocity impact shield and method for protecting a wall structure, such as a spacecraft wall, from impact with particles of debris having densities of about 2.7 g/cu cm and impact velocities up to 16 km/s are disclosed. The shield comprises a stack of ultra thin sheets of impactor disrupting material supported and arranged by support means in spaced relationship to one another and mounted to cover the wall in a position for intercepting the particles. The sheets are of a number and spacing such that the impacting particle and the resulting particulates of the impacting particle and sheet material are successively impact-shocked to a thermal state of total melt and/or vaporization to a degree as precludes perforation of the wall. The ratio of individual sheet thickness to the theoretical diameter of particles of debris which may be of spherical form is in the range of 0.03 to 0.05. The spacing between adjacent sheets is such that the debris cloud plume of liquid and vapor resulting from an impacting particle penetrating a sheet does not puncture the next adjacent sheet prior to the arrival thereat of fragment particulates of sheet material and the debris particle produced by a previous impact.

Cour-Palais, Burton G.↗

NASA NTRS1994

Magnetometer-only attitude and rate determination for a gyro-less spacecraft

Attitude determination algorithms that requires only the earth's magnetic field will be useful for contingency conditions. One way to determine attitude is to use the time derivative of the magnetic field as the second vector in the attitude determination process. When no gyros are available, however, attitude determination becomes difficult because the rates must be propagated via integration of Euler's equation, which in turn requires knowledge of the initial rates. The spacecraft state to be determined must then include not only the attitude but also rates. This paper describes a magnetometer-only attitude determination scheme with no a priori knowledge of the spacecraft state, which uses a deterministic algorithm to initialize an extended Kalman filter. The deterministic algorithm uses Euler's equation to relate the time derivatives of the magnetic field in the reference and body frames and solves the resultant transcendental equations for the coarse attitude and rates. An important feature of the filter is that its state vector also includes corrections to the propagated rates, thus enabling it to generate highly accurate solutions. The method was tested using in-flight data from the Solar, Anomalous, and Magnetospheric Particles Explorer (SAMPEX), a Small Explorer spacecraft. SAMPEX data using several eclipse periods were used to simulate conditions that may exist during the failure of the on-board digital sun sensor. The combined algorithm has been found effective, yielding accuracies of 1.5 deg in attitude (within even nominal mission requirements) and 0.01 degree per second (deg/sec) in the rates.

Natanson, G. A.↗

NASA NTRS1997

CGRO Guest Investigator Program

The following are highlights from the research supported by this grant: (1) Theory of gamma-ray blazars: We studied the theory of gamma-ray blazars, being among the first investigators to propose that the GeV emission arises from Comptonization of diffuse radiation surrounding the jet, rather than from the synchrotron-self-Compton mechanism. In related work, we uncovered possible connections between the mechanisms of gamma-ray blazars and those of intraday radio variability, and have conducted a general study of the role of Compton radiation drag on the dynamics of relativistic jets. (2) A Nonlinear Monte Carlo code for gamma-ray spectrum formation: We developed, tested, and applied the first Nonlinear Monte Carlo (NLMC) code for simulating gamma-ray production and transfer under much more general (and realistic) conditions than are accessible with other techniques. The present version of the code is designed to simulate conditions thought to be present in active galactic nuclei and certain types of X-ray binaries, and includes the physics needed to model thermal and nonthermal electron-positron pair cascades. Unlike traditional Monte-Carlo techniques, our method can accurately handle highly non-linear systems in which the radiation and particle backgrounds must be determined self-consistently and in which the particle energies span many orders of magnitude. Unlike models based on kinetic equations, our code can handle arbitrary source geometries and relativistic kinematic effects In its first important application following testing, we showed that popular semi-analytic accretion disk corona models for Seyfert spectra are seriously in error, and demonstrated how the spectra can be simulated if the disk is sparsely covered by localized 'flares'.

Begelman, Mitchell C.↗

NASA NTRS1960

Ionospheric Measurements Using Environmental Sampling Techniques

Two rockets were flown to peak altitudes of 220 km in September 1959 to test various methods planned for future measurements of ionization parameters in the ionosphere, exosphere, and interplanetary plasma. The experiments used techniques which sample the ambient environment in the immediate vicinity of the research vehicle. Direct methods were chosen since indirect propagation techniques do not provide the temperatures of charged particles, are insensitive to ion densities, and cannot measure local electron densities under all conditions. Very encouraging results have been obtained from a preliminary analysis of data provided by one of the two flights. A new rf probe technique was successfully used to determine the electron density profile. This was indicated by its agreement with the results of a companion cw propagation experiment, particularly when the probe data were corrected for the effects of the ion sheath which surrounds the vehicle. The characteristics of this sheath were determined directly in flight by an electric field meter which provided the sheath field, and by a Langmuir probe which measured the total potential across the sheath. The electron temperatures deduced from the Langmuir probe data are greater than the neutral gas temperatures previously measured for the same location and season, but these measurements possibly were taken under different atmospheric conditions. Ion densities were calculated from the ion trap data for several altitudes ranging from 130 to 210 km and were found to be within 20 percent of the measured electron densities.

Bourdeau, R. E.↗

NASA NTRS1999

Multi-Wavelength Analysis of the March 26, 1991: Solar Flare

We have engaged in detailed multi-wavelength analysis of the March 26, 1991, solar flare in order to develop a method of diagnostics of the physical processes responsible for the efficient acceleration of charged particles to high energies and also for diagnostics of the photospheric response to the injection of the accelerated particles. Consideration of this particular flare is of special interest because to date it is the only flare in which the gamma-ray emission with energies of 20-1000 MeV was registered throughout the entire development of the event in the optical, radio and soft x-ray bands. individual registration by the GAMMA-1 telescope of the energy and precise time for each registered gamma-ray photon allowed the maximum detailed investigation of the energy spectrum and flux evolution during the flare.

Kurt, V. G.↗

NASA NTRS2000

Automated Propellant Blending

An automated propellant blending apparatus and method that uses closely metered addition of countersolvent to a binder solution with propellant particles dispersed therein to precisely control binder precipitation and particle aggregation is discussed. A profile of binder precipitation versus countersolvent-solvent ratio is established empirically and used in a computer algorithm to establish countersolvent addition parameters near the cloud point for controlling the transition of properties of the binder during agglomeration and finishing of the propellant composition particles. The system is remotely operated by computer for safety, reliability and improved product properties, and also increases product output.

Hohmann, Carl W.↗

NASA NTRS2003

Vertically Resolved Aerosol Optical Properties over the ARM SGP Site

In order to meet one of its goals - to relate observations of radiative fluxes and radiances to the atmospheric composition - the Department of Energy's Atmospheric Radiation Measurement (ARM) program has pursued measurements and modeling activities that attempt to determine how aerosols impact atmospheric radiative transfer, both directly and indirectly. However, significant discrepancies between aerosol properties measured in situ or remotely remain. To this end, the ARM program will conduct an Aerosol Intensive Operational Period (IOP) in May 2003 at the ARM Southern Great Plains (SGP) site in north central Oklahoma. The IOP involves airborne measurements from two airplanes over the heavily instrumented SGP site. We will give an overview of early airborne results obtained aboard Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft. The aircraft will carry instrumentation to perform in-situ measurements of aerosol absorption, scattering, extinction and particle size including such novel techniques as the photoacoustic and cavity ring-down methods. Aerosol optical depth and extinction will be measured with the NASA Ames Airborne Tracking 14-channel sunphotometer. Furthermore up- and downwelling solar (broadband and spectral) and infrared radiation will be measured using three different instruments. The up-looking radiation instruments will be mounted on a newly developed stabilized platform, which will keep the instruments level up to aircraft pitch and roll angles of 10 degrees. Additional effort will be directed toward measurement of cloud condensation nucleus concentration as a function of supersaturation and relating CCN concentration to aerosol composition and size distribution. This relation is central to description of the aerosol indirect effect.

Schmid, B.↗

NASA NTRS1989

Direct Simulation of Reentry Flows with Ionization

The Direct Simulation Monte Carlo (DSMC) method is applied in this paper to the study of rarefied, hypersonic, reentry flows. The assumptions and simplifications involved with the treatment of ionization, free electrons and the electric field are investigated. A new method is presented for the calculation of the electric field and handling of charged particles with DSMC. In addition, a two-step model for electron impact ionization is implemented. The flow field representing a 10 km/sec shock at an altitude of 65 km is calculated. The effects of the new modeling techniques on the calculation results are presented and discussed.

Carlson, Ann B.↗

NASA NTRS1999

Effect of osteoblastic culture conditions on the structure of poly(DL-lactic-co-glycolic acid) foam scaffolds

Poly(DL-lactic-co-glycolic acid) (PLGA) foams are an osteoconductive support that holds promise for the development of bone tissue in vitro and implantation into orthopedic defects. Because it is desirable that foams maintain their shape and size, we examined a variety of foams cultured in vitro with osteoblastic cells. Foams were prepared with different porosities and pore sizes by the method of solvent casting/porogen leaching using 80, 85, and 90 wt% NaCl sieved with particle sizes of 150-300 and 300-500 microm and characterized by mercury intrusion porosimetry. Foams seeded with cells were found to have volumes after 7 days in static culture that decreased with increasing porosity: the least porous exhibited no change in volume while the most porous foams decreased by 39 +/- 10%. In addition, a correlation was observed between decreasing foam volume after 7 days in culture and decreasing internal surface area of the foams prior to seeding. Furthermore, foams prepared with the 300-500 microm porogen had lower porosities, greater mean wall thicknesses between adjacent pores, and larger volumes after 7 days in culture than those prepared with the smaller porogen. Two culture conditions for maintaining cells, static and agitated (in a rotary vessel), were found to have similar influences on foam size, cell density, and osteoblastic function for 7 and 14 days in culture. Finally, we examined unseeded foams in aqueous solutions of pH 3.0, 5.0, and 7.4 and found no significant decrease in foam size with degradation. This study demonstrates that adherent osteoblastic cells may collapse very porous PLGA foams prepared by solvent casting/particulate leaching: a potentially undesirable property for repair of orthopedic defects.

NASA Program Fundamental Space Biology↗