3,977 research outputs found
Mass influx obtained from low-light-level television observations of faint meteors
Low light level television systems offer the ability to observe meteors as faint as 10th magnitude which allows the extension of optical meteor data to masses as small as 0.0001 gram. The results of these observations, using image orthicons and intensified vidicons, are presented along with an interpretation in terms of mass flux. This interpretation includes the development of a relationship between peak luminosity of a meteor and mass, velocity, and zenith angle that was derived from single body meteor theory and compares favorably with results obtained from the artificial meteor program. Also included in the mass flux interpretation is an analysis of the observation response of a LLLTV system to fixed and moving point sources
Space station particulate contamination environment
The origin of particulate contamination on the Space Station will mostly be from pre-launch operations. The adherence and subsequent release of these particles during space flight are discussed. Particle size, release velocity, and release direction are important in determining particle behavior in the vicinity of the vehicle. The particulate environment at the principal science instrument locations is compared to the space shuttle bay environment. Recommendations for possibly decreasing the particulate contamination are presented
Disputed Theory and Security Policy: Responding to the Rise of China
Much has been written on the security implications of the Rise of China, yet there is little consensus, posing a problem for policymakers. I highlight the areas of disagreement, arguing that the lack of consensus is a product of different theoretical positions. Since there is not an obviously correct theoretical position, policymakers must make decisions based on significant uncertainty. I argue that policymakers ought therefore reject costly and decontextualized theories, such as offensive realism, while still maintaining openness to theoretical knowledge
A digital imaging photometry system for cometary data acquisition
This report describes a digital imaging photometry system developed in the Space Science Laboratory at the Marshall Space Flight center. The photometric system used for cometary data acquisition is based on an intensified secondary electron conduction (ISEC) vidicon coupled to a versatile data acquisition system which allows real-time interactive operation. Field tests on the Orion and Rosette nebulas indicate a limiting magnitude of approximately m sub v = 14 over the 40 arcmin field-of-view. Observations were conducted of Comet Giacobini-Zinner in August 1985. The resulting data are discussed in relation to the capabilities of the digital analysis system. The development program concluded on August 31, 1985
Observational Constraints on the Averaged Universe
Averaging in general relativity is a complicated operation, due to the
general covariance of the theory and the non-linearity of Einstein's equations.
The latter of these ensures that smoothing spacetime over cosmological scales
does not yield the same result as solving Einstein's equations with a smooth
matter distribution, and that the smooth models we fit to observations need not
be simply related to the actual geometry of spacetime. One specific consequence
of this is a decoupling of the geometrical spatial curvature term in the metric
from the dynamical spatial curvature in the Friedmann equation. Here we
investigate the consequences of this decoupling by fitting to a combination of
HST, CMB, SNIa and BAO data sets. We find that only the geometrical spatial
curvature is tightly constrained, and that our ability to constrain dark energy
dynamics will be severely impaired until we gain a thorough understanding of
the averaging problem in cosmology.Comment: 6 pages, 4 figure
Spacelab mission 2: Experimental descriptions
The second Spacelab Mission and the 12 multidisciplinary experiments selected to fly on board are described. These experiments include the following: vitamin D metabolities and bone demineralization; interaction of oxygen and gravity influenced lignification; ejectable plasma diagnostics package; plasma depletion experiments for ionospheric and radio astronomical studies; small helium cooled IR telescope; elemental composition and energy spectra of cosmic ray nuclei; hard X-ray imaging of clusters of galaxies and other extended X-ray sources; solar magnetic and velocity field measurement system; solar coronal helium abundance Spacelab experiment; solar UV high resolution telescope and spectroraph; solar UV spectral irradiance monitor; and properties of superfluid helium in zero-G
Dynamics of a lattice Universe
We find a solution to Einstein field equations for a regular toroidal lattice
of size L with equal masses M at the centre of each cell; this solution is
exact at order M/L. Such a solution is convenient to study the dynamics of an
assembly of galaxy-like objects. We find that the solution is expanding (or
contracting) in exactly the same way as the solution of a
Friedman-Lema\^itre-Robertson-Walker Universe with dust having the same average
density as our model. This points towards the absence of backreaction in a
Universe filled with an infinite number of objects, and this validates the
fluid approximation, as far as dynamics is concerned, and at the level of
approximation considered in this work.Comment: 14 pages. No figure. Accepted version for Classical and Quantum
Gravit
Volume Weighted Measures of Eternal Inflation in the Bousso-Polchinski Landscape
We consider the cosmological dynamics associated with volume weighted
measures of eternal inflation, in the Bousso-Polchinski model of the string
theory landscape. We find that this measure predicts that observers are most
likely to find themselves in low energy vacua with one flux considerably larger
than the rest. Furthermore, it allows for a satisfactory anthropic explanation
of the cosmological constant problem by producing a smooth, and approximately
constant, distribution of potentially observable values of Lambda. The low
energy vacua selected by this measure are often short lived. If we require
anthropically acceptable vacua to have a minimum life-time of 10 billion years,
then for reasonable parameters a typical observer should expect their vacuum to
have a life-time of approximately 12 billion years. This prediction is model
dependent, but may point toward a solution to the coincidence problem of
cosmology.Comment: 35 pages, 8 figure
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