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At least 667 records · Page 37
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NASA NTRS1991

Ultraviolet and optical spectroscopy of the R Aquarii symmetrical jet

The first ultraviolet spectrum of the southwest (SW) component of the symmetrical jet in the R Aquarii binary system has been obtained in the range 1200-2000 A with the IUE. These results are compared to more encompassing spectra of the central H II region taken at the same time and also similar spectra of the northeast (NE) jet component obtained six months earlier. Moreover, optical spectra of both the NE and SW jet components in the range 3400-9800 A were obtained within about 6 months and about 1 month, respectively, of the ultraviolet spectra. These highly complementary observations argue that excitation of the symmetrical jet may be due to shock excitation as the jet components overtake and impact the previously ionized material associated with the expanding inner nebulosity. The problems with this shock model as well as problems with competing photoionization models are discussed. It is suggested that the jet components were ejected less than 90 years ago.

Hollis, J. M.↗
NASA NTRS1991

Terrestrial mantle siderophiles and the lunar impact record

Results of an analytical fit to the lunar cratering record are shown to be in good agreement with geochemical estimates of the meteoritic component mixed into the lunar crust. Upon a gravitational scaling to the earth consistent with the statistical probability that earth impactors were more massive than lunar ones, an estimate is obtained of 1.5 x 10 to the 22nd kg of material accumulated by the earth begining 4.4 Gyr ago. This is in excellent agreement with estimates of postcore-formation meteoritic input derived from geochemical considerations of the abundances of highly siderophile elements in the terrestrial mantle. Approximate agreement is therefore established between the lunar cratering record scaling and both lunar and terrestrial geochemical constraints.

Chyba, Christopher F.↗
NASA NTRS1991

An historical perspective of the NERVA nuclear rocket engine technology program

Nuclear rocket research and development was initiated in the United States in 1955 and is still being pursued to a limited extent. The major technology emphasis occurred in the decade of the 1960s and was primarily associated with the Rover/NERVA Program where the technology for a nuclear rocket engine system for space application was developed and demonstrated. The NERVA (Nuclear Engine for Rocket Vehicle Application) technology developed twenty years ago provides a comprehensive and viable propulsion technology base that can be applied and will prove to be valuable for application to the NASA Space Exploration Initiative (SEI). This paper, which is historical in scope, provides an overview of the conduct of the NERVA Engine Program, its organization and management, development philosophy, the engine configuration, and significant accomplishments.

Robbins, W. H.↗
NASA NTRS1991

Studies in planetary rings

Progress made in research devoted to examining Voyager imaging data for the purpose of understanding the kinematics and dynamics of the Neptune ring arcs is reported. It was found that a radial distortion of amplitude 30 km is traveling through the arcs with a perturbation by the nearby satellite 1989N4. Two new and smaller arcs within the outer Neptunian ring were also discovered. The longitudinal spacing of all arcs is roughly that expected for corotational arc shepherding by 1989N4 provided that not all corotation sites in the arc region are filled. The semimajor axis of the arcs' orbit inferred from their observed mean motion and the latest Neptune gravitational parameters is, within uncertainties, identical to the location of the 42:43 corotation-inclination resonance of 1989N4: an observation which supports the notion of corotational arc-shepherding. This hypothesis also explains that the 15 km radial width of the arcs are due to a spread in forced orbital eccentricities. Clumps within the arcs were observed at a greater frequency than previously reported. These features may be the manifestation of big bodies within the arcs providing a source of ring dust, or they may be a kinematical effect associated with the crossing of eccentric orbits at quadrature. The inclusion of ground based observations of the arcs taken up to five years ago will lengthen the baseline over which arc dynamics may be studied and should lead to a refinement in the position of the Neptune spin and invariable plane poles.

Porco, Carolyn C.↗
NASA NTRS1991

Ar-39-Ar-40 of achondrites: Evidence for a lunar-like cataclysm

The observation that the K-Ar, Pb-Pb, and Rb-Sr ages of a significant number of lunar highland rocks were reset in the interval of 4.1-3.8 Ga ago led to the concept of a cataclysmic bombardment of the moon during this period. An important consideration for understanding the early bombardment history of the solar system, including the moon, is whether evidence also exists in meteorites for resetting of radiometric ages by cataclysmic bombardment of their parent bodies. A comparison of Ar-39-Ar-40 ages of achondritic meteorites with ages of lunar highland rocks should help elucidate the nature of the early bombardment of the solar system. We are participating in various consortia studies of primarily Antarctic eucrites and howardites for which we measured Ar-39-Ar-40 ages of various clasts and matrix samples. The results of these studies are presented.

Bogard, D. D.↗
NASA NTRS1991

Nd-142/Nd-144 in SNCs and early differentiation of a heterogeneous Martian mantle

Sm/Nd correlated variations in Nd-142/Nd-144 have been observed for mineral phases of achondrites from decay of live Sm-146 in the early solar system. Crystallization ages of shergottites-nakhlites-Chassigny (SNC) meteorites are less than or = 1.3 Ga, so variations of Nd-142/Nd-144 among mineral phases of the SNC's are not expected. However, if SNC's were derived from source reservoirs of differing Sm/Nd ratios, established while Sm-146 was still alive, and which remained isolated except for magma extraction, then variations in Nd-142/Nd-144 would exist among individual SNC meteorites. Rb-Sr and U-Pb isotopic data for the shergottites imply differentiation of their parent planet approximately 4.6 Ga ago. The confirmation of the conclusion that the nakhlites and shergottites were derived from different source regions, and that, consequently, the shergottite parent body (SPB) mantle was heterogeneous is presented.

Nyquist, L. E.↗
NASA NTRS1991

Chaotic inflationary universe and the anisotropy of the large-scale structure

It has been realized that the inflationary universe is in fact chaotic, that globally it is strongly inhomogeneous, and that the inflation in the universe as a whole is eternal. In such a picture the region available to modern observations is just a tiny part of the universe, in which inflation finished about 10(exp 10) years ago. In spite of the great popularity of the chaotic inflationary universe models, it is usually taken for granted that their specific features (such as strong global inhomogeneity of the universe) can hardly lead to any observable consequences. The argument is that all that is seen is just a tiny part of the universe, a region about 10(exp 28) cm, and the typical scales of considerable inhomogeneities are much greater than this size. In contrast to this opinion, an attempt is made to show that such observable consequences can really exist. The phenomenon closely connected with the origin of structure (galaxies, clusters, etc.) in the observable region is discussed. The main idea considered is the vacuum fluctuations evolution on the inhomogeneous background.

Chibisov, G. V.↗
NASA NTRS1991

Paleolakes and life on early Mars

Two distinct directions have begun to elucidate key parameters in the search for extinct life on Mars. Carbonate sediments, deposited about 10,000 years ago in association with biological activity, have been sampled from the paleolake beds of Lake Vanda and Meirs in the McMurdo Dry Valleys in Antarctica. These samples are being analyzed for simple biological signatures that remain in cold and dry paleolake sediments, namely microfossils, percent carbonate, and total organic carbon. Our second initiative is the study of Colour Lake, in the Canadian Arctic, that periodically maintains a perennial ice cover. Physical measurements started this year will be used to determine one end point for ice covered lake environments and will be compared to continuous measurements from Antarctic lakes started in November 1985. Interestingly, Colour Lake also supports benthic mat communities, but the low pH precludes carbonate deposition. This research will broaden our knowledge base for what conditions are necessary for ice covered lake formation and what biological signatures will remain in paleolake deposits.

Meyer, M. A.↗
NASA NTRS1991

Terrestrial production vs. extraterrestrial delivery of prebiotic organics to the early Earth

A comprehensive treatment of comet/asteroid interaction with the atmosphere, ensuring surface impact, and resulting organic pyrolysis is required to determine whether more than a negligible fraction of the organics in incident comets and asteroids actually survived collision with Earth. Results of such an investigation, using a smoothed particle hydrodynamic simulation of cometary and asteroidal impacts into both oceans and rock, demonstrate that organics will not survive impacts at velocities approx. greater than 10 km s(exp -1), and that even comets and asteroids as small as 100m in radius cannot be aerobraked to below this velocity in 1 bar atmospheres. However, for plausible dense (10 bar CO2) early atmospheres, there will be sufficient aerobraking during atmospheric passage for some organics to survive the ensuing impact. Combining these results with analytical fits to the lunar impact record shows that 4.5 Gyr ago Earth was accreting at least approx. 10(exp 6) kg yr(exp 1) of intact cometary organics, a flux which thereafter declined with a approx. 100 Myr half-life. The extent to which this influx was augmented by asteroid impacts, as well as the effect of more careful modelling of a variety of conservative approximations, is currently being quantified. These results may be placed in context by comparison with in situ organic production from a variety of terrestrial energy sources, as well as organic delivery by interplanetary dust. Which source dominated the early terrestrial prebiotic inventory is found to depend on the nature of the early terrestrial atmosphere. However, there is an intriguing symmetry: it is exactly those dense CO2 atmospheres where in situ atmospheric production of organic molecules should be the most difficult, in which intact cometary organics would be delivered in large amounts.

Chyba, C. F.↗
NASA NTRS1991

Archaebacterial rhodopsin sequences: Implications for evolution

It was proposed over 10 years ago that the archaebacteria represent a separate kingdom which diverged very early from the eubacteria and eukaryotes. It follows that investigations of archaebacterial characteristics might reveal features of early evolution. So far, two genes, one for bacteriorhodopsin and another for halorhodopsin, both from Halobacterium halobium, have been sequenced. We cloned and sequenced the gene coding for the polypeptide of another one of these rhodopsins, a halorhodopsin in Natronobacterium pharaonis. Peptide sequencing of cyanogen bromide fragments, and immuno-reactions of the protein and synthetic peptides derived from the C-terminal gene sequence, confirmed that the open reading frame was the structural gene for the pharaonis halorhodopsin polypeptide. The flanking DNA sequences of this gene, as well as those of other bacterial rhodopsins, were compared to previously proposed archaebacterial consensus sequences. In pairwise comparisons of the open reading frame with DNA sequences for bacterio-opsin and halo-opsin from Halobacterium halobium, silent divergences were calculated. These indicate very considerable evolutionary distance between each pair of genes, even in the dame organism. In spite of this, three protein sequences show extensive similarities, indicating strong selective pressures.

Lanyi, J. K.↗
NASA NTRS1991

Biogeochemical modeling at mass extinction boundaries

The causes of major mass extinctions is a subject of considerable interest to those concerned with the history and evolution of life on earth. The primary objectives of the proposed plan of research are: (1) to develop quantitative time-dependent biogeochemical cycle models, coupled with an ocean atmosphere in order to improve the understanding of global scale physical, chemical, and biological processes that control the distribution of elements important for life at times of mass extinctions; and (2) to develop a comprehensive data base of the best available geochemical, isotopic, and other relevant geologic data from sections across mass extinction boundaries. These data will be used to constrain and test the biogeochemical model. These modeling experiments should prove useful in: (1) determining the possible cause(s) of the environmental changes seen at bio-event boundaries; (2) identifying and quantifying little-known feedbacks among the oceans, atmosphere, and biosphere; and (3) providing additional insights into the possible responses of the earth system to perturbations of various timescales. One of the best known mass extinction events marks the Cretaceous/Tertiary (K/T) boundary (66 Myr ago). Data from the K/T boundary are used here to constrain a newly developed time-dependent biogeochemical cycle model that is designed to study transient behavior of the earth system. Model results predict significant fluctuations in ocean alkalinity, atmospheric CO2, and global temperatures caused by extinction of calcareous plankton and reduction in the sedimentation rates of pelagic carbonates and organic carbon. Oxygen-isotome and other paleoclimatic data from K/T time provide some evidence that such climatic fluctuations may have occurred, but stabilizing feedbacks may have acted to reduce the ocean alkalinity and carbon dioxide fluctuations.

Rampino, M. R.↗
NASA NTRS1991

The relative importance of prebiotic synthesis on the Earth and input from comets and meteorites

The prebiotic synthesis of hydrogen cyanide and formaldehyde was studied by the action of electric discharges on various model primitive atmospheres containing CH4, CO, and CO2. Photochemical production rates would also have been important and were calculated for HCN and H2CO. A reasonable rate of synthesis of amino acids from these sources is about 10 n moles/(sq cm yr) or 0.10 moles/sq cm in 10(exp 7) yrs. This would give a concentration of 3 x 10(exp -4) M in an ocean of the present size (300 liters/sq cm). The amino acids cannot accumulate over a longer period because the entire ocean passes through the 350 C submarine vents in 10(exp 7) yrs, which decomposes all the organic compounds. A number of workers have calculated the influx of comets and meteorites on the primitive earth, both as a destructive process for organic compounds and for any life that was present, as well as a source of organic compounds. Some of the amino acids from the meteorite proposed to have hit the earth 65 x 10(exp 6) yrs ago were detected at the Cretaceous/Tertiary boundary sediments. The problem with proposing a large scale input of organic compounds from meteorites and comets is that they must survive passage through the atmosphere and impact. There are some processes that would allow survival such as showers of centimeter to meter sized meteorites and various aerodynamic braking processes for larger objects. Even if a significant amount of the organic material survived impact, the destructive processes in the hydrothermal vents would remove these compounds on the average in 10(exp 7) yrs or less. If it is assumed that the input rate was sufficient to overcome these destructive processes, then too much carbon and water, especially from comets, would have been added to the surface of the earth. It was concluded that while some organic material was added to the earth from comets and meteorites, the amount available from these sources at a given time was only a few percent of that from earth based syntheses.

Miller, S. L.↗
NASA NTRS1991

Determination of Rotordynamic Coefficients for Labyrinth Seals and Application to Rotordynamic Design Calculations

In today's rotordynamic calculations, the input parameters for a finite element analysis (FEA) determine very much the reliability of eigenvalue and eigenmode predictions. While modeling of an elastic structure by means of beam elements etc. is relatively straightforward to perform and the input data for journal bearings are usually known exactly enough, the determination of stiffness and damping for labyrinth seals is still the subject of many investigations. Therefore, the rotordynamic influence of labyrinths is often not included in FEA for rotating machinery because of a lack of computer programs to calculate these parameters. This circumstance can give rise to severe vibration problems especially for high performance turbines or compressors, resulting in remarkable economic losses. The forces generated in labyrinths can be described for small motions around the seal center with a linearized force-motion relationship. Several years ago, we started with the development of computer codes for the determination of rotordynamic seal coefficients. Our different approaches to evaluate the dynamic fluid forces generated by turbulent, compressible seal flow are introduced.

Weiser, P.↗
NASA NTRS1991

Areas of Seal R/D at GE

About four years ago, work was completed on a 36 inch diameter gas to gas carbon ring seal used to buffer low pressure turbine air at the rim of the forward outer flowpath on the GE36 unducted fan (UDF) engine. At about the same time, we were developing a long life counter-rotating intershaft air-oil seal of approximately 7.6 inch diameter for operation at 800 fps, 800 F, and 50 psid. Although we were successful in meeting most program goals with a split ring seal of the axial bushing type, the seal with the greatest payoff in life and air leakage rates, bearing many features in common with the GE36 seal, could not be successfully tested because of the structural weakness of the primary seal ring carbon material. This was a split ring seal using a hybrid combination of orifice compensated hydrostatic and shrouded hydrodynamic gas bearings. We are presently working to develop this design in conjunction with high strength materials being developed by Pure Carbon Co. In the area of engine secondary gas flow path-sealing for performance improvement, we are currently working with carbon and all metal face seals. A 15 inch diameter all metal 'aspirating' face seal, using self-acting hydrostatic bearings, was successfully tested to 700 fps, 100 psid, and 1000 F, demonstrating long life at flow reduction of 86 percent compared to a 'best' labyrinth. This seal will be developed through 1400 F, 900 fps, and 350 psid. The seal 'aspirates' closed at about idle speed pressure during engine start and reopens at engine shutdown. A hydraulic thrust balance seal, currently using orifice compensated hydrostatics, is under development. Other aspects of these projects are briefly covered.

Pope, A. Nelson↗
NASA NTRS1992

PC-SEAPAK user's guide, version 4.0

PC-SEAPAK is designed to provide a complete and affordable capability for processing and analysis of NOAA Advanced Very High Resolution Radiometer (AVHRR) and Nimbus-7 Coastal Zone Color Scanner (CZCS) data. Since the release of version 3.0 over a year ago, significant revisions were made to the AVHRR and CZCS programs and to the statistical data analysis module, and a number of new programs were added. This new version has 114 procedures listed in its menus. The package continues to emphasize user-friendliness and interactive data analysis. Additionally, because the scientific goals of the ocean color research being conducted have shifted to larger space and time scales, batch processing capabilities were enhanced, allowing large quantities of data to be easily ingested and analyzed. The development of PC-SEAPAK was paralled by two other activities that were influential and assistive: the global CZCS processing effort at GSFC and the continued development of VAX-SEAPAK. SEAPAK incorporates the instrument calibration and support all levels of data available from the CZCS archive.

Mcclain, Charles R.↗
NASA NTRS1992

Impact contribution of prebiotic reactants to Earth

It is proposed that the AIB amino acid at the K/T boundary were synthesized during entry of a comet. However, whether they were synthesized or supplied directly from space, the concentration of amino acids in the shallow K/T sea would have been about 10(exp -7) M. It is probable that clays were the dominant sinks for the amino acids in the K/T sea and in the primordial ocean. Because clay removed amino acids from sea water quickly, the amino acid contribution must be studied from individual comets in order to evaluate the effectiveness of comets for chemical evolution. Such an evaluation shows that comets would have produced amino acid concentration higher than equilibrium concentrations of amino acid from corona discharge at all times preceding the age of the oldest fossils. The perferred sites for chemical evolution of cometary amino acids are in cloud drops and tide pools where the concentration of amino acids would have been the highest. Life could have originated at the surface even during periods of intense bombardment of the earth before 3.8 billion years ago.

Aggarwal, Hans R.↗
NASA NTRS1992

Modulation and coding used by a major satellite communications company

Hughes Communications Inc., is a major satellite communications company providing or planning to provide the full spectrum of services available on satellites. All of the current services use conventional modulation and coding techniques that were well known a decade or longer ago. However, the future mobile satellite service will use significantly more advanced techniques. JPL, under NASA sponsorship, has pioneered many of the techniques that will be used.

Renshaw, K. H.↗