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At least 73 records · Page 4

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NASA NTRS2017

Improving Estimation of Ground Casualty Risk from Reentering Space Objects

A recent improvement to the long-term estimation of ground casualties from reentering space debris is the further refinement and update to the human population distribution. Previous human population distributions were based on global totals with simple scaling factors for future years, or a coarse grid of population counts in a subset of the world's countries, each cell having its own projected growth rate. The newest population model includes a 5-fold refinement in both latitude and longitude resolution. All areas along a single latitude are combined to form a global population distribution as a function of latitude, creating a more accurate population estimation based on non-uniform growth at the country and area levels. Previous risk probability calculations used simplifying assumptions that did not account for the ellipsoidal nature of the earth. The new method uses first, a simple analytical method to estimate the amount of time spent above each latitude band for a debris object with a given orbit inclination, and second, a more complex numerical method that incorporates the effects of a non-spherical Earth. These new results are compared with the prior models to assess the magnitude of the effects on reentry casualty risk.

Ostrom, C.↗

NASA NTRS1983

On the nature of the bonding in Cu2 - An ab initio viewpoint

The bonding in Cu2 has been calculatively investigated. A CASSCF calculation is performed where the two 4s electrons are distributed in all possible ways among the 4s and 4p orbitals. The CAS results have a 3d population of 9.99. The corresponding orbitals between the CAS core orbitals at r(e) and the atomic SCF core orbitals show an overlap of 13.9992 out of a possible 14, with all of the change in sigma space. Thus, the CAS results show virtually no change in the d orbitals relative to the atom and can be considered as showing only a single bond. The reduction in 3d population, the decrease in r(e), and the increase in D(e) upon correlating the 3d shell indicates that the 3d electrons are slightly involved in the bonding, but there is no indication of the formation of a triple bond.

Bauschlicher, C. W., Jr.↗

NASA NTRS2004

Invariant Theory for Dispersed Transverse Isotropy: An Efficient Means for Modeling Fiber Splay

Most soft tissues possess an oriented architecture of collagen fiber bundles, conferring both anisotropy and nonlinearity to their elastic behavior. Transverse isotropy has often been assumed for a subset of these tissues that have a single macroscopically-identifiable preferred fiber direction. Micro-structural studies, however, suggest that, in some tissues, collagen fibers are approximately normally distributed about a mean preferred fiber direction. Structural constitutive equations that account for this dispersion of fibers have been shown to capture the mechanical complexity of these tissues quite well. Such descriptions, however, are computationally cumbersome for two-dimensional (2D) fiber distributions, let alone for fully three-dimensional (3D) fiber populations. In this paper, we develop a new constitutive law for such tissues, based on a novel invariant theory for dispersed transverse isotropy. The invariant theory is based on a novel closed-form splay invariant that can easily handle 3D fiber populations, and that only requires a single parameter in the 2D case. The model is polyconvex and fits biaxial data for aortic valve tissue as accurately as the standard structural model. Modification of the fiber stress-strain law requires no re-formulation of the constitutive tangent matrix, making the model flexible for different types of soft tissues. Most importantly, the model is computationally expedient in a finite-element analysis.

Freed, alan D.↗

NASA NTRS1988

The terrestrial ring current - From in situ measurements to global images using energetic neutral atoms

Electrical currents flowing in the equatorial magnetosphere, first inferred from ground-based magnetic disturbances, are carried by trapped energetic ions. Spacecraft measurements have determined the spectrum and composition of those currents, and the newly developed technique of energetic-neutral-atom imaging allows the global dynamics of that entire ion population to be viewed from a single spacecraft.

Roelof, Edmond C.↗

NASA NTRS2015

Microfluidic Device

Described herein are particular embodiments relating to a microfluidic device that may be utilized for cell sensing, counting, and/or sorting. Particular aspects relate to a microfabricated device that is capable of differentiating single cell types from dense cell populations. One particular embodiment relates a device and methods of using the same for sensing, counting, and/or sorting leukocytes from whole, undiluted blood samples.

Tai, Yu-Chong↗

NASA NTRS2016

Microfluidic Device

Tai, Yu-Chong↗

NASA NTRS2017

Microfluidic Device

Tai, Yu-Chong↗

NASA NTRS2017

Microfluidic Device

Tai, Yu-Chong↗

NASA NTRSDate not supplied

Deletion of Cdkn1a in Mice Rescues Reduced Osteogenesis in Microgravity and Increases Coupled Osteoclastic Bone Degenerative Activity and Bone Loss

Elevated Cdkn1a/p21 levels in mouse bone during spaceflight have led us to hypothesize that this cell cycle inhibitor could regulate osteogenesis in response to mechanical unloading in microgravity. Cdkn1a interacts with cyclins CDK1/2/4,6 during G1 and S phases, and suppression of its expression by mechanical stretch enhances proliferation and mineralization in osteoblastic cell cultures. To test this hypothesis in a whole-organism model, we used C57BL/J6 WT/Cdkn1a-null mice in the NASA RR-10, 30-day ISS microgravity experiment. To quantify active mineralization in microgravity mice were injected with Calcein seven and two days before euthanasia on-orbit, followed by high-resolution confocal imaging of optically cleared femoral endosteal surfaces. To quantify cortical and cancellous bone parameters we conducted microCT analyses of the femur and tibia. Confocal imaging revealed a population of mineralizing calcein-positive single osteoblasts, and multicellular surfaces of mineralization by osteocytes in lacunae. The Cdkn1a-null genotype shows a greater ratio of early-osteoblasts to late-osteocytes and reduced mineralization versus WT. Additionally, in microgravity WT bones have a large (80%) decrease in active mineralization, but remarkably Cdkn1a deletion fully rescued mineralized surface loss. MicroCT shows trends for moderate spaceflight bone loss in WT femurs, and significant losses in the tibia, while Cdkn1a-null femurs showed much larger significant tissue losses, including 25% thinner trabeculae (p < 0.01),40% thinner cortical bone (p < 0.005), and 10% lower percent cortical bone area (p < 0.05). These results indicate that endogenous bone Cdkn1a plays a key role in mechanosensitive downregulation of osteoprecursor proliferation and promotes osteocytic differentiation. In microgravity however, while Cdkn1a deletion rescues mineralization, bone loss quantified by microCT is greatly increased, likely due to the molecular coupling of osteogenesis and osteoclastogenesis. This molecular mechanism opens the possibility for potential pharmacological approaches to mitigate bone loss in space simultaneously targeting Cdkn1a and osteoclast inhibition.

Eduardo A.C. Almeida↗

NASA NTRS2003

How Massive Single Stars End Their Life

How massive stars die-what sort of explosion and remnant each produces-depends chiefly on the masses of their helium cores and hydrogen envelopes at death. For single stars, stellar winds are the only means of mass loss, and these are a function of the metallicity of the star. We discuss how metallicity, and a simplified prescription for its effect on mass loss, affects the evolution and final fate of massive stars. We map, as a function of mass and metallicity, where black holes and neutron stars are likely to form and where different types of supernovae are produced. Integrating over an initial mass function, we derive the relative populations as a function of metallicity. Provided that single stars rotate rapidly enough at death, we speculate on stellar populations that might produce gamma-ray bursts and jet-driven supernovae.

Heger, A.↗

NASA NTRS1984

Martian Crater Size Distributions and Terrain Age

The crater size/frequency distributions of large ( 8 km) craters on the Moon and terrestrial planets display two very different curves representing two crater populations. The heavily cratered regions of the Moon, Mercury, and Mars show the same highly structured curve which cannot be represented by a single slope distribution function. In contrast, the lunar post mare crater population has a size/frequency distribution which differs significantly from that in the highlands over the same diameter range, and can be represented by a single-slope distribution function of -2.8 differential. On areas of martian lightly cratered northern plains, the crater population is essentially identical to that of the post mare population. This indicates that the same two families of impacting objects were responsible for the cratering records on both Moon and Mars. The thickness of mantling material varies among the various plains units, and can be calculated from the depth/diameter scaling relations for martian craters.

Nadine G Barlow↗

NASA NTRS1992

Two populations of gamma-ray bursts - A solution to the BATSE conundrum

BATSE measurements suggest that the distribution of gamma-ray burst sources is spatially nonuniform, but as yet shows no discernible anisotropy. This is not consistent with the distribution expected solely from a local galactic population of neutron star sources. We suggest, however, that the inconsistency can be reconciled if there are at least two distinct classes of bursts, both of which originate on galactic neutron stars, but which differ by a factor of 10 exp 4, or more, in mean luminosity. We show, as an example, that the observed V/V(max) distributions and anisotropy constraints of gamma-ray bursts are consistent with those expected from a combination of two such classes, resulting from different burst mechanisms on a single galactic, high-velocity, neutron star population.

Lingenfelter, R. E.↗

NASA NTRS1972

Apollo 14 breccias - General characteristics and classification.

The Apollo 14 breccias are complex and are characterized by a wide range of clast types and textures. The principal fragment types in the breccias are (1) hornfelsed noritic microbreccias; (2) micronorite hornfels; (3) devitrified glass; and (4) ophitic and other kinds of basalt. Based on fragment population, nature of the matrix, grain size and porosity, metamorphic history, and bulk chemical composition, the Apollo 14 breccias can be classified chiefly into regolith microbreccias, Fra Mauro breccias, and spherule-rich microbreccias. Further subdivisions are based principally on the interpretation of metamorphic history. Twenty-six Apollo 14 breccias have been so classified. The complex fragment types contained in Apollo 14 breccias represent multiple episodes of heating and fragmentation. They probably are pre-Imbrian and were not produced by the impact effects of a single event. The variability of their fragment populations is important in the interpretation of pre-Imbrian geologic history.

Chao, E. C. T.↗

NASA NTRS2012

Rotating Radio Transients and Their Place Among Pulsars

Six years ago, the discovery of Rotating Radio Transients (RRATs) marked what appeared to be a new type of sparsely-emitting pulsar. Since 2006, more than 70 of these objects have been discovered in single-pulse searches of archival and new surveys. With a continual inflow of new information about the RRAT population in the form of new discoveries, multi-frequency follow ups, coherent timing solutions, and pulse rate statistics, a view is beginning to form of the place in the pulsar population RRATs hold. Here we review the properties of neutron stars discovered through single pulse searches. We first seek to clarify the definition of the term RRAT, emphasising that "the RRAT population" encompasses several phenomenologies. A large subset of RRATs appears to represent the tail of an extended distribution of pulsar nulling fractions and activity cycles; these objects present several key open questions remaining in this field.

stars↗

NASA NTRSDate not supplied

A Novel Bone Marrow Single Cell Atlas of Mechanotransduction in Microgravity, Normal Gravity, Exercise, and Hindlimb Unloading for WT and CDKN1A-Null Regenerative Mice

Mechanical loading of adult stem cell progenitors is a key factor in modulating their proliferation, differentiation, and tissue regenerative potential, with loading generally promoting tissue formation and unloading mediating tissue loss. To address the role of mechanotransduction in maintaining stem cell-based tissue regenerative health, we generated a single cell transcriptomic atlas mapping responses of femur bone marrow mesenchymal and hematopoietic lineages to a range of altered mechanical loading conditions, both in WT B1629SF2/J mice as well as the p21/CDKN1A-null regenerative mice. We selected the femur marrow compartment as a model because of the diversity and high numbers of stem cell progenitor stages present, and because it undergoes static and cyclic hydrostatic pressure loading associated with weight-bearing and ambulation. Our study included normally loaded mice at 1g, unloading in microgravity during spaceflight as well as tail suspension hindlimb unloading, and voluntary running wheel exercise. Basal, one-year natural aging, and habitat controls were also conducted. Overall, the atlas encompasses 18 different experimental conditions with N=3 mice per condition and includes more than 500,000 single cell expressomes. Key specific findings include: increased mature reticulocyte populations in aging and unloading mice compared to active mice; greater hematopoietic and mesenchymal differentiating progenitors cluster identification in CDKN1A-null samples, especially the exercise model; distinct pseudotime cell trajectory shifts in the hematopoietic lineage for spaceflight and unloaded mice; and shifts in monocytic cell populations toward osteoclastic bone degenerative lineages in unloading and spaceflight samples. Overall mechanical loading shows increased marrow progenitor population differentiation while unloading is associated with increased CDKN1A expression and maintenance of marrow population stemness. Deletion of CDKN1A appears to remove a negative check on progenitor lineages commitment to differentiation. Finally, the Bone Marrow Mechanotransduction Single Cell Atlas will serve as a reference tool for studying marrow regenerative responses across a range of mechanical environments as they relate to CDKN1A status.

single-cell↗

NASA NTRS1975

Signal dispersion within a hippocampal neural network

A model network is described, representing two neural populations coupled so that one population is inhibited by activity it excites in the other. Parameters and operations within the model represent EPSPs, IPSPs, neural thresholds, conduction delays, background activity and spatial and temporal dispersion of signals passing from one population to the other. Simulations of single-shock and pulse-train driving of the network are presented for various parameter values. Neuronal events from 100 to 300 msec following stimulation are given special consideration in model calculations.

Horowitz, J. M.↗

NASA NTRS1995

Optical characterization of the oceanic unicellular cyanobacterium Synechococcus grown under a day-night cycle in natural irradiance

The optical properties of the ocenanic cyanobacterium Synechococcus (clone WH8103) were examined in a nutrient-replete laboratory culture grown under a day-night cycle in natural irradiance. Measurements of the spectral absorption and beam attenuation coefficients, the size distribution of cells in suspension, and microscopic analysis of samples were made at intervals of 2-4 hours for 2 days. These measurements were used to calculate the optical properties at the level of a single 'mean' cell representative of the acutal population, specifically, the optical cross sections for spectral absorption bar-(sigma(sub a)), scattering bar-sigma(sub b))(lambda), and attentuation bar-(sigma(sub c))(lambda). In addition, concurrent determinations of chlorophyll a and particulate organic carbon allowed calculation of the Chl a- and C-specific optical coefficients. The refractive index of cells was derived from the observed data using a theory of light absorption and scattering by homogeneous spheres. Low irradiance because of cloudy skies resulted in slow division rates of cells in the culture. The percentage of dividing cells was unusually high (greater than 30%) throughout the experiment. The optical cross sections varied greatly over a day-night cycle, with a minimum near dawn or midmorning and maximum near dusk. During daylight hours, bar-(sigma(sub b)) and bar-(sigma(sub c)) can increase more than twofold and bar-(sigma(sub a) by as much as 45%. The real part of the refractive index n increaed during the day; changes in n had equal or greater effect than the varying size distribution on changes in bar-(sigma(sub c)) and bar-(sigma(sub b)). The contribution of changes in n to the increase of bar-(sigma(sub c))(660) during daylight hours was 65.7% and 45.1% on day 1 and 2, respectively. During the dark period, when bar-(sigma(sub c))(660) decreased by a factor of 2.9, the effect of decreasing n was dominant (86.3%). With the exception of a few hours during the second light period, the imaginary part of the refractive index n' showed little variation over a day-night cycle, and bar-(sigma(sub a)) was largely controlled by variations in cell size. The real part of the refractive index at lambda = 660 nm was correlated with the intracellular C concentration and the imaginary part at lambda = 678 nm with the intracellular Chl a concentration. The C-specfic attenuation coefficient showed significant diel variability, which has implications for the estimation of oceanic primary production from measurements of diel variability in beam attenuation. This study provides strong evidence that diel variability is an important component of the optical characterization of marine phytoplankton.

Stramski, Dariusz↗

NASA NTRS2001

Distribution and Spectroscopy of Green Fluorescent Protein and Acyl-CoA: Cholesterol Acytransferase in Sf21 Insect Cells

Acyl-CoA: cholesterol acyltransferase (ACAT) is thought to significantly participate in the pathway of cholesterol esterification that underlies the pathology of artherosclerosis. This enzyme is a membrane protein known to be preferentially bound within the endoplasmic reticulum of mammalian cells, from which location it esterifies cholesterol derived from low density lipoprotein. Cultures of insect cells were separately infected with baculovirus containing the gene for green fluroescent protein (GFP) and with baculovirus containing tandem genes for GFP and ACAT. These infected cultures expressed GFP and the fusion protein GCAT, respectively, with maximum expression occurring on the fourth day after infection. Extraction of GFP- and of GCAT-expressing cells with urea and detergent resulted in recovery of fluorescent protein in aqueous solution. Fluorescence spectra at neutral pH were identical for both GFP and GCAT extracts in aqueous solution, indicating unperturbed tertiary structure for the GFP moiety within GCAT. In a cholesterol esterification assay, GCAT demonstrated ACAT activity, but with less efficiency compared to native ACAT. It was hypothesized that the membrane protein ACAT would lead to differences in localization of GCAT compared to GFP within the respective expressing insect cells. The GFP marker directly and also within the fusion protein GCAT was accordingly used as the intracellular probe that was fluorescently analyzed by the new biophotonics technique of hyperspectral imaging. In that technique, fluorescence imaging was obtained from two dimensional arrays of cells, and regions of interest from within those images were then retrospectively analyzed for the emission spectra that comprises the image. Results of hyperspectral imaging of insect cells on day 4 postinfection showed that GCAT was preferentially localized to the cytoplasm of these cells compared to GFP. Furthermore, the emission spectra obtained for the localized GCAT displayed a peak blue shift from 518nm obtained in neutral aqueous solution to 505nm obtained in localized regions within the cells. This blue shift indicates change in the fluorescence coupling of the GFP moiety of GCAT. It is hypothesized that change in tertiary environment of GCAT, coincident with intracellular deposition of GCAT, follows from intracellular trafficking of GCAT leading to membrane interactions with the ACAT moiety, and/or self-assembly of GCAT, that alters the chromophore environment of the GFP moiety of GCAT. These findings introduce a new technique of biophotonic imaging to studies of intracellular protein trafficking and interactions. This technique of hyperspectral imaging could contribute to advancing the emergent field of proteomics. Because of the noninvasive nature of this technique, kinetic processes associated with intracellular protein trafficking, and interactions of proteins within cellular domains, can be considered for investigation within a single cell as well as a cell population.

Richmond, R. C.↗