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At least 883 records · Page 49

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NASA NTRS1990

Requirements for a near-earth space tug vehicle

The requirement for a small but powerful space tug, which will be capable of autonomous orbital rendezvous, docking and translating cargos between near-earth orbits by the end of this decade to support the growing national and international space infrastructure focused near the Space Station Freedom, is described. An aggregate of missions drives the need for a space tug including reboosting decaying satellites back to their operational altitudes, retrieving failed or exhausted satellites to Shuttle or SSF for on-orbit refueling or repair, and transporting a satellite servicer system with an FTS to ailing satellites for supervised in-place repair. It is shown that the development and operation of a space tug to perform such numerous missions is more cost effective than separate module and satellite systems to perform the same tasks.

Gunn, Charles R.↗

NASA NTRS1990

Fractal dimension of bioconvection patterns

Shallow cultures of the motile algal strain, Euglena gracilis, were concentrated to 2 x 10 to the 6th organisms per ml and placed in constant temperature water baths at 24 and 38 C. Bioconvective patterns formed an open two-dimensional structure with random branches, similar to clusters encountered in the diffusion-limited aggregation (DLA) model. When averaged over several example cultures, the pattern was found to have no natural length scale, self-similar branching, and a fractal dimension (d about 1.7). These agree well with the two-dimensional DLA.

Noever, David A.↗

NASA NTRS1990

Presolar isotopic and chemical signatures in hibonite-bearing refractory inclusions from the Murchison carbonaceous chondrite

The mineralogy and the major-element composition of phases within individual hibonite-bearing inclusions were determined, using SEM with EDAX analysis, for 37 hibonite-bearing inclusions which comprised 26 spinel-hibonite-perovskite inclusions, ten platy hibonite crystal fragments, and one blue hibonite aggregate. Results from all inclusions studied indicated the presence of correlations in hibonite isotopic systematics and chemistry. Namely, no inclusion had both a large Ti-50 anomaly and radiogenic Mg-26, and inclusions that were depleted in the ultrarefractory elements had radiogenic Mg-26 with the exception of those which a had a large Ti-50 anomaly. These features establish a relationship between chemical and isotopic characteristics of the inclusions which must predate their formation. It is suggested that depletions in ultrarefractory elements observed in these inclusions may be a memory of stellar condensation and are not necessarily related to a condensation event in the solar nebula.

Ireland, Trevor R.↗

NASA NTRS1986

Behavior of centrosomes during fertilization and cell division in mouse oocytes and in sea urchin eggs

The behavior of centrosomes during the stages of fertilization and cell division in mouse oocytes and in sea urchin eggs was monitored in an immunofluorescence microscope, using autoimmune centrosomal antiserum derived from a patient with scleroderma to label the centrosomal material. These observations showed that centrosomes reproduce during the interphase and aggregate and separate during cell mitosis. Results supported the hypothesis of Mazia (1984), who proposed that centrosomes are 'flexible bodies'. It was also found that, while the sea urchin centrosomes are paternally inherited as was initially proposed by Bovery (1904), the mouse centrosomes are of maternal origin.

Schatten, Heide↗

NASA NTRS1990

Scattering properties of natural snow and frost - Comparison with icy satellite photometry

The Hapke (1986) equation is presently fit to ascertain the single-scattering albedo of the icy satellites of Uranus and Neptune and the one-term Henyey-Greenstein particle-phase function g for each of the Middleton and Mungall (1952) goniophotometric data samples. There emerge both very high single-scattering albedos and strongly forward-scattering particle phase functions; while these are in keeping with Mie theory-based theoretical considerations, they contrast with the observed backscattering behavior of icy satellites. It is suggested the icy satellite frost grains are aggregated into particles of complex texture, which produce the unusual backscattering behavior.

Verbiscer, Anne J.↗

NASA NTRS1990

Use of moments of momentum to predict the crystal habit in potassium hydrogen phthalate

A relatively simple calculation of the moments of momentum predicts the morphological order of crystal faces for potassium hydrogen phthalate. The effects on the habit caused by the addition of monomeric, dimeric, and larger aggregates during crystal growth are considered. The first six of the seven observed crystal faces are predicted with this method.

Barber, Patrick G.↗

NASA NTRS1991

Shedding light on baryonic dark matter

Halo dark matter, if it is baryonic, may plausibly consist of compact stellar remnants. Jeans mass clouds containing 10 to the 6th to 10 to the 8th solar masses could have efficiently formed stars in the early universe and could plausibly have generated, for a suitably top-heavy stellar initial mass function, a high abundance of neutron stars as well as a small admixture of long-lived low mass stars. Within the resulting clusters of dark remnants, which eventually are tidally disrupted when halos eventually form, captures of neutron stars by nondegenerate stars resulted in formation of close binaries. These evolve to produce, by the present epoch, an observable X-ray signal associated with dark matter aggregations in galaxy cluster cores.

Silk, Joseph↗

NASA NTRS1991

The interstellar connection to solar system bodies

Solar system bodies such as comets and meteorites are examined for interstellar signatures which may ultimately indicate what the protoplanetary nebula conditions were at the time the bodies formed. This involves retracing the sequence starting with a dense interstellar cloud, through the accretion disk phase of the protoplanetary nebula to the various steps in the aggregation process and finally to the physical and chemical metamorphic processes in the final body during the subsequent 4.55 billion years. Models of interstellar dust and comets which aid in such analysis are presented.

Greenberg, J. Mayo↗

NASA NTRS1991

Origin of plagioclase-olivine inclusions in carbonaceous chondrites

The origin of plagioclase-olivine inclusions (POIs) from three CV chondrites and one ungrouped chondrite was investigated by examining the chemical, mineralogical, and isotopic characteristics of a group of POIs from these chondrites. Results of these analyses demonstrate that the mixing and the partial melting processes in these inclusions were superimposed on more ancient isotopically heterogeneous material. A comparison of the essential characteristics of POIs and CAIs suggests that the major processes leading to the formation of POIs (such as condensation, dust/gas fractionation, aggregation of chemically and isotopically disparate materials, and partial melting) are common to most CAIs and chondrules. A scenario for the origin of POIs is proposed, showing that the homogeneity of the final assemblage (whether a POI, a CAI, or a chondrite) is primarily a reflection of the thermal history rather than the nature of precursor materials.

Sheng, Y. J.↗

NASA NTRS1990

Unique cell culture systems for ground based research

The horizontally rotating fluid-filled, membrane oxygenated bioreactors developed at NASA Johnson for spacecraft applications provide a powerful tool for ground-based research. Three-dimensional aggregates formed by cells cultured on microcarrier beads are useful for study of cell-cell interactions and tissue development. By comparing electron micrographs of plant seedlings germinated during Shuttle flight 61-C and in an earth-based rotating bioreactor it is shown that some effects of microgravity are mimicked. Bioreactors used in the UAH Bioreactor Laboratory will make it possible to determine some of the effects of altered gravity at the cellular level. Bioreactors can be valuable for performing critical, preliminary-to-spaceflight experiments as well as medical investigations such as in vitro tumor cell growth and chemotherapeutic drug response; the enrichment of stem cells from bone marrow; and the effect of altered gravity on bone and muscle cell growth and function and immune response depression.

Lewis, Marian L.↗

NASA NTRS1990

The role of data management in discipline-independent data visualization

The common data format (CDF) is described in terms of its support applications for the database management of visualization systems. The CDF is a self-describing data abstraction technique for the storage and manipulation of multidimensional data that are based on block structures. The discipline-independent approach is designed to manage, manipulate, archive, display, and analyze data, and can be applied to heterogeneous equipment communicating different data structures over networks. An improved CDF version incorporates a hyperplane access allowing random aggregate access to subdimensional blocks within a multidimensional variable. The visualization pipeline is also discussed, which controls the flow of data and permits the visualization of different classes of data representation techniques. The system is found to accommodate a large variety of scientific data structures and large disk-based data sets.

Treinish, Lloyd A.↗

NASA NTRS1991

The solar wind interaction with Mars - Mariner 4, Mars 2, Mars 3, Mars 5, and Phobos 2 observations of bow shock position and shape

An aggregate Mars bow shock data set using Mariner 4, Mars 2, Mars 3, Mars 5, and Phobos 2 observations has been analyzed. The results support the earlier conclusion that the mean distance to the subsolar shock at Mars is nearly 1.5 planetary radii, from which gas dynamic models predict an obstacle altitude of 500 km. The Martian bow shock does not appear to vary significantly in shape or altitude with the phase of the solar cycle. The unusually distant dayside bow shock crossings reported by Mars 2 and 3 also appear in the Phobos 3 observations, suggesting that the dayside obstacle can on rare occasions reach altitudes over 1000 km. The Martian bow shock differs from that of Venus in that its mean altitude is greater, it lacks a strong solar cycle variation, and its location is far more variable, including the occurrence of strong bow shocks over the dayside hemisphere at distances at least as great as the orbit of Phobos 2, i.e., 2.8 Mars radii.

Slavin, J. A.↗

NASA NTRS1991

Sintering of two phase lamellar microstructures

The effect of an inert phase on neck growth rates during sintering of lamellar structures is described. Sintering diagrams are constructed for an aggregate of two-phase Cu spheres with lamellar microstructure containing an inert phase. It is shown that the regions corresponding to lattice diffusion domination shrink due to the inert phase presence. As the dispersion becomes finer, the alterations in the map become substantial.

Courtney, T. H.↗

NASA NTRS1991

Antarctic Sea ice variations and seasonal air temperature relationships

Data through 1987 are used to determine the regional and seasonal dependencies of recent trends of Antarctic temperature and sea ice. Lead-lag relationships involving regional sea ice and air temperature are systematically evaluated, with an eye toward the ice-temperature feedbacks that may influence climatic change. Over the 1958-1087 period the temperature trends are positive in all seasons. For the 15 years (l973-l987) for which ice data are available, the trends are predominantly positive only in winter and summer, and are most strongly positive over the Antarctic Peninsula. The spatially aggregated trend of temperature for this latter period is small but positive, while the corresponding trend of ice coverage is small but negative. Lag correlations between seasonal anomalies of the two variables are generally stronger with ice lagging the summer temperatures and with ice leading the winter temperatures. The implication is that summer temperatures predispose the near-surface waters to above-or below-normal ice coverage in the following fall and winter.

Weatherly, John W.↗

NASA NTRS1991

Degradation of Fe-Mg silicates in hot CO2 atmospheres: Applications to Venus

Experiments demonstrated that oxidation of ferromagnesian silicates and magnetite occurs when these minerals are heated at 800 C in 1 atmosphere of CO2, under which conditions hematite is thermodynamically stable. The 30 ppm oxygen impurity in CO2 presumably facilitates the oxidation of some of the ferrous iron initially present in the crystal structures of the minerals. Mossbauer spectral measurements reveal, however, that only CO2 degraded olivine and pigeonite is hematite formed as a magnetically ordered phase at ambient temperatures. In orthopyroxene, some of the ferric iron produced by oxidation is present as nanophase hematite which, because it remains superparamagnetic until 4.2 K, must exist as particles less than or equal to 4 nm in diameter. In the calcic pyroxenes much of the oxidized ferrous iron may still remain as structural Fe3(+) in the host silicates. Some ferric iron may also be present as unit cell sized Fe2O3 inclusions in the pyroxenes, or be segregated along cleavage planes, or be coating mineral grains. In these states of aggregation, the Fe2O3 is unidentifiable by x ray diffraction and in low temperature Mossbauer spectra. Applications of this research to the surface of Venus are discussed.

Straub, Darcy W.↗

NASA NTRS1991

Dynamic crystallization characteristics of enstatite chondrite chondrules

Although the chemical properties of enstatite and ordinary chondrites are distinctly different, they both contain chondrules with a similar array of textures. This similarity suggests like origins. Textural studies using chondrule compositions from ordinary chondrites suggest that these chondrules have an igneous origin: either by crystallization from melts or from partial melts of crystalline material. In contrast, the cathodoluminescence (CL) properties of the enstatite from enstatite chondrites were interpreted to mean that mechanical aggregation played an important part in their formation. An alternative interpretation of these CL properties, however, suggests that variations in the minor element content of the enstatite, a probable result of igneous fractionation processes, could also produce different CL colors. An attempt was made to evaluate the two models by performing dynamic crystallization experiments on an average enstatite chondrule composition and by looking at the resultant CL. The textures grown on experimentally crystallized E-chondrite melts confirm that formational processes are similar to those for the ordinary chondrites with the obvious exception of the oxidation state.

Lofgren, Gary E.↗

NASA NTRS1991

Early Archean (approximately 3.4 Ga) prokaryotic filaments from cherts of the apex basalt, Western Australia: The oldest cellularly preserved microfossils now known

In comparison with that known from later geologic time, the Archean fossil record is miniscule: although literally hundreds of Proterozoic formations, containing more that 2800 occurrences of bona fide microfossils are now known, fewer than 30 units containing some 43 categories of putative microfossils (the vast majority of which are of questionable authenticity) have been reported from the Archean. Among the oldest known fossils are Early Archean filaments reported from cherts of the Towers Formation and the Apex Basalt of the 3.3-3.6 Ga-old Warrawoona Group of Western Australia. The paleobiologic significance of the Towers Formation microstructures is open to question: thin aggregated filaments are properly regarded as dubiomicrofossils (perhaps biogenic, but perhaps not); therefore, they cannot be regarded as firm evidence of Archean life. Although authentic, filamentous microfossiles were reported from a second Towers Formation locality, because the precise layer containing the fossiliferous cherts was not relocated, this discovery can neither be reconfirmed by the original collector nor confirmed independently by other investigators. Discovery of microfossils in bedded cherts of the Apex Basalt, the stratigraphic unit immediately overlying the Towers Formation, obviates the difficulties stored above. The cellularly preserved filaments of the Apex Basalt meet all of the criteria required of a bona fide Archean microfossils. Recent studies indicate that the Apex assemblage includes at least six morphotypes of uniseriate filaments, composed of barrel-shaped, discoidal, or quadrate cells and exhibiting rounded or conical terminal cells and medial bifurcated and paired half-cells that reflect the occurrence of prokaryotic binary cell division. Interestingly, the majority of these morphotypes are morphologically more similar to extant cyanobacteria than to modern filamentous bacteria. Prokaryotes seem clearly to have been hypobradytelic, and the evidence suggests (but does not prove) that physiologically advanced oxygen-producing photosynthesizers may have been represented in the Early Archean biota.

Schopf, J. W.↗

NASA NTRS1991

Study of geopotential error models used in orbit determination error analysis

The uncertainty in the geopotential model is currently one of the major error sources in the orbit determination of low-altitude Earth-orbiting spacecraft. The results of an investigation of different geopotential error models and modeling approaches currently used for operational orbit error analysis support at the Goddard Space Flight Center (GSFC) are presented, with emphasis placed on sequential orbit error analysis using a Kalman filtering algorithm. Several geopotential models, known as the Goddard Earth Models (GEMs), were developed and used at GSFC for orbit determination. The errors in the geopotential models arise from the truncation errors that result from the omission of higher order terms (omission errors) and the errors in the spherical harmonic coefficients themselves (commission errors). At GSFC, two error modeling approaches were operationally used to analyze the effects of geopotential uncertainties on the accuracy of spacecraft orbit determination - the lumped error modeling and uncorrelated error modeling. The lumped error modeling approach computes the orbit determination errors on the basis of either the calibrated standard deviations of a geopotential model's coefficients or the weighted difference between two independently derived geopotential models. The uncorrelated error modeling approach treats the errors in the individual spherical harmonic components as uncorrelated error sources and computes the aggregate effect using a combination of individual coefficient effects. This study assesses the reasonableness of the two error modeling approaches in terms of global error distribution characteristics and orbit error analysis results. Specifically, this study presents the global distribution of geopotential acceleration errors for several gravity error models and assesses the orbit determination errors resulting from these error models for three types of spacecraft - the Gamma Ray Observatory, the Ocean Topography Experiment, and the Cosmic Background Explorer.

Yee, C.↗