13,578 research outputs found
Digital computer control of a 30-cm mercury ion thruster
The major objective was to define the exact role of an onboard spacecraft computer in the control of ion thrusters. An initial computer control system with accurate high speed capability was designed, programmed, and tested with the computer as the sole control element for an operating ion thruster. The command functions and a code format for a spacecraft digital control system were established. A second computer control system was constructed to operate with these functions and format. A throttle program sequence was established and tested. A two thruster array was tested with these computer control systems and the results reported
Electrostatic propulsion system with a direct nuclear electrogenerator Patent
Nuclear electric generator for accelerating charged propellant particles in electrostatic propulsion syste
Temperature Fluctuations driven by Magnetorotational Instability in Protoplanetary Disks
The magnetorotational instability (MRI) drives magnetized turbulence in
sufficiently ionized regions of protoplanetary disks, leading to mass
accretion. The dissipation of the potential energy associated with this
accretion determines the thermal structure of accreting regions. Until
recently, the heating from the turbulence has only been treated in an
azimuthally averaged sense, neglecting local fluctuations. However, magnetized
turbulence dissipates its energy intermittently in current sheet structures. We
study this intermittent energy dissipation using high resolution numerical
models including a treatment of radiative thermal diffusion in an optically
thick regime. Our models predict that these turbulent current sheets drive
order unity temperature variations even where the MRI is damped strongly by
Ohmic resistivity. This implies that the current sheet structures where energy
dissipation occurs must be well resolved to correctly capture the flow
structure in numerical models. Higher resolutions are required to resolve
energy dissipation than to resolve the magnetic field strength or accretion
stresses. The temperature variations are large enough to have major
consequences for mineral formation in disks, including melting chondrules,
remelting calcium-aluminum rich inclusions, and annealing silicates; and may
drive hysteresis: current sheets in MRI active regions could be significantly
more conductive than the remainder of the disk.Comment: 16 pages, 13 figures, ApJ In Press, updated to match proof
Aspects of nature in early Irish religion: an essay in the phenomenology of religion
Research into a wide range of early Irish literature has yielded a
large number of texts in which nature and the Sacred are clearly
linked in some way. By comparing and contrasting these various nature
texts with each other and with attitudes to nature in the Bible and
elsewhere, it is possible to arrive at a basic understanding of the
religious significance of nature in early Irish Christianity and, to
some extent, in pre-christian Irish religion.The primal religions of Ireland and early Israel had many
features in common. Both saw the land, water, trees, sun, fire and
other aspects of nature as potentially revealing the presence and
activity of the Divine. The term 'sacramental universe' is appropriate
to both, and neither distinguished sharply between the spiritual and
the physical. How did they come by this integrated world-view? There
are of course theories of symbolism and sacramentology which one might
p
call upon in an attempt to explain the experience but such abstract
conceptions belong to a different time and place. To say that they
used their primal imagination is also to import ideas from another
world, but at least they are ideas which have been formed in dialogue
with surviving primal religions. Exactly what primal imagination is
(as distinct from what it believes) and how it operates is another
matter.It is none the less an important question. Did the people who
approached the Divine in and through nature, address an external
reality, or a projection, or a delusion or what? We asked similar
questions in relation to the girls #10 claimed to have seen the fairywoman on Knockainy, but the answers are far from clear. What is clear
however, is that in Ireland as in Israel, the life-giving energy of
nature was seen as having a supernatural source or sources, upon which
human beings were ultimately dependent. This was a cause of wonder and
also of fear. Both cultures sometimes expressed a deep ambivalence
towards the Divine.Yahweh and the deities of pre-christian Ireland represent very
different conceptions of God and the cosmos. However, the primal
imagination which we have seen at work in parts of the Hebrew Bible,
together with the wealth of nature imagery found throughout the Bible
as a whole, may have helped the early Irish Christians to retain some
of their own traditional attitudes towards nature, as they made the
transition from a primal to a more universal faith
Corrections to deuterium hyperfine structure due to deuteron excitations
We consider the corrections to deuterium hyperfine structure originating from
the two-photon exchange between electron and deuteron, with the deuteron
excitations in the intermediate states. In particular, the motion of the two
intermediate nucleons as a whole is taken into account. The problem is solved
in the zero-range approximation. The result is in good agreement with the
experimental value of the deuterium hyperfine splitting.Comment: 7 pages, LaTe
Presymmetry beyond the Standard Model
We go beyond the Standard Model guided by presymmetry, the discrete
electroweak quark-lepton symmetry hidden by topological effects which explain
quark fractional charges as in condense matter physics. Partners of the
particles of the Standard Model and the discrete symmetry associated with this
partnership appear as manifestations of a residual presymmetry and its
extension from matter to forces. This duplication of the spectrum of the
Standard Model keeps spin and comes nondegenerated about the TeV scale.Comment: 6 pages, 11 figures. To be published in the proceedings of DPF-2009,
Detroit, MI, July 2009, eConf C09072
The Influence of Metallicity on Star Formation in Protogalaxies
In cold dark matter cosmological models, the first stars to form are believed
to do so within small protogalaxies. We wish to understand how the evolution of
these early protogalaxies changes once the gas forming them has been enriched
with small quantities of heavy elements, which are produced and dispersed into
the intergalactic medium by the first supernovae. Our initial conditions
represent protogalaxies forming within a fossil H II region, a previously
ionized region that has not yet had time to cool and recombine. We study the
influence of low levels of metal enrichment on the cooling and collapse of
ionized gas in small protogalactic halos using three-dimensional, smoothed
particle hydrodynamics (SPH) simulations that incorporate the effects of the
appropriate chemical and thermal processes. Our previous simulations
demonstrated that for metallicities Z < 0.001 Z_sun, metal line cooling alters
the density and temperature evolution of the gas by less than 1% compared to
the metal-free case at densities below 1 cm-3) and temperatures above 2000 K.
Here, we present the results of high-resolution simulations using particle
splitting to improve resolution in regions of interest. These simulations allow
us to address the question of whether there is a critical metallicity above
which fine structure cooling from metals allows efficient fragmentation to
occur, producing an initial mass function (IMF) resembling the local Salpeter
IMF, rather than only high-mass stars.Comment: 3 pages, 2 figures, First Stars III conference proceeding
Hydrogen bonding in substituted nitroanilines : isolated nets in 1,3-diamino-4-nitrobenzene and continuously interwoven nets in 3,5-dinitroaniline
Peer reviewedPublisher PD
Multiple steady-states in the terrestrial atmosphere-biosphere system: a result of a discrete vegetation classification?
International audienceMultiple steady states in the atmosphere-biosphere system can arise as a consequence of interactions and positive feedbacks. While atmospheric conditions affect vegetation productivity in terms of available light, water, and heat, different levels of vegetation productivity can result in differing energy- and water partitioning at the land surface, thereby leading to different atmospheric conditions. Here we investigate the emergence of multiple steady states in the terrestrial atmosphere-biosphere system and focus on the role of how vegetation is represented in the model: (i) in terms of a few, discrete vegetation classes, or (ii) a continuous representation. We then conduct sensitivity simulations with respect to initial conditions and to the number of discrete vegetation classes in order to investigate the emergence of multiple steady states. We find that multiple steady states occur in our model only if vegetation is represented by a few vegetation classes. With an increased number of classes, the difference between the number of multiple steady states diminishes, and disappears completely in our model when vegetation is represented by 8 classes or more. Despite the convergence of the multiple steady states into a single one, the resulting climate-vegetation state is nevertheless less productive when compared to the emerging state associated with the continuous vegetation parameterization. We conclude from these results that the representation of vegetation in terms of a few, discrete vegetation classes can result (a) in an artificial emergence of multiple steady states and (b) in a underestimation of vegetation productivity. Both of these aspects are important limitations to be considered when global vegetation-atmosphere models are to be applied to topics of global change
Thermoregulation of the \u3ci\u3epap\u3c/i\u3e Operon: Evidence for the Involvement of RimJ, the N-terminal Acetylase of Ribosomal Protein S5
Our previous work showed that pap pilin gene transcription is subject to a thermoregulatory control mechanism under which pap pilin is not transcribed at a low temperature (23°C) (L. B. Blyn, B. A. Braaten, C. A. White-Ziegler, D. H. Rolfson, and D. A. Low, EMBO J. 8:613-620, 1989). In order to isolate genes involved in this temperature regulation of gene expression, chromosomal mini-TnlO (mTnlO) mutations that allowed transcription of the pap pilin gene at 23°C were identified, and the locus was designated tcp, for thermoregulatory control of pap (C. A. White-Ziegler, L. B. Blyn, B. A. Braaten, and D. A. Low, J. Bacteriol. 172:1775-1782, 1990). In the present study, quantitative analysis showed that the tcp mutations restore pap pilin transcription at 23°C to levels similar to those measured at 37°C. By in vivo recombination, the tcp mutations were mapped to phage E4H1OS of the Kohara library of the Escherichia coli chromosome (Y. Kohara, K. Akiyama, and K. Isono, Cell 50:495-508, 1987). The tcp locus was cloned by complementation, in which a 1.3-kb DNA fragment, derived from the Kohara phage, was shown to restore thermoregulation to the mTnlO mutants. DNA sequencing revealed two open reading frames (ORFs) encoding proteins with calculated molecular masses of 22.7 and 20.3 kDa. The sequence of the 22.7-kDa ORF was identical to that of rimJ, the N-terminal acetylase of the ribosomal protein S5. The gene encoding the 20.3-kDa ORF, designated g20.3 here, did not display significant homology to any known. DNA or protein sequence. On the basis of Northern (RNA) blot data, rimj and g20.3 are located within the same operon. Two of the mTnlO transposons in the thermoregulatory mutants were inserted within the coding region of rimi, indicating that the RimJ protein plays an important role in the temperature regulation ofpap pilin gene transcription. However, rimj itself is not thermoregulated, since rim. transcripts were detected at both 23 and 37°C. Disruption of the g20.3 gene by insertion and deletion mutagenesis did not affect thermoregulation of the pap operon, suggesting that, although g20.3 lies within the same operon as rimj, it does not play a role in thermoregulation
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