5,781 research outputs found
An earth pole-sitter using hybrid propulsion
In this paper we investigate optimal pole-sitter orbits using hybrid solar sail and solar electric propulsion (SEP). A pole-sitter is a spacecraft that is constantly above one of the Earth's poles, by means of a continuous thrust. Optimal orbits, that minimize propellant mass consumption, are found both through a shape-based approach, and solving an optimal control problem, using a direct method based on pseudo-spectral techniques. Both the pure SEP case and the hybrid case are investigated and compared. It is found that the hybrid spacecraft allows consistent savings on propellant mass fraction. Finally, is it shown that for sufficiently long missions (more than 8 years), a hybrid spacecraft, based on mid-term technology, enables a consistent reduction in the launch mass for a given payload, with respect to a pure SEP spacecraft
Generation of optimal trajectories for Earth hybrid pole sitters
A pole-sitter orbit is a closed path that is constantly above one of the Earth's poles, by means of continuous low thrust. This work proposes to hybridize solar sail propulsion and solar electric propulsion (SEP) on the same spacecraft, to enable such a pole-sitter orbit. Locally-optimal control laws are found with a semi-analytical inverse method, starting from a trajectory that satisfies the pole-sitter condition in the Sun-Earth circular restricted three-body problem. These solutions are subsequently used as first guess to find optimal orbits, using a direct method based on pseudospectral transcription. The orbital dynamics of both the pure SEP case and the hybrid case are investigated and compared. It is found that the hybrid spacecraft allows savings on propellant mass fraction. Finally, it is shown that for sufficiently long missions, a hybrid pole-sitter, based on mid-term technology, enables a consistent reduction in the launch mass for a given payload, with respect to a pure SEP spacecraft
Long Days Enhance Recognition Memory and Increase Insulin-like Growth Factor 2 in the Hippocampus
Light improves cognitive function in humans; however, the neurobiological mechanisms underlying positive effects of light remain unclear. One obstacle is that most rodent models have employed lighting conditions that cause cognitive deficits rather than improvements. Here we have developed a mouse model where light improves cognitive function, which provides insight into mechanisms underlying positive effects of light. To increase light exposure without eliminating daily rhythms, we exposed mice to either a standard photoperiod or a long day photoperiod. Long days enhanced long-term recognition memory, and this effect was abolished by loss of the photopigment melanopsin. Further, long days markedly altered hippocampal clock function and elevated transcription of Insulin-like Growth Factor2 (Igf2). Up-regulation of Igf2 occurred in tandem with suppression of its transcriptional repressor Wilm’s tumor1. Consistent with molecular de-repression of Igf2, IGF2 expression was increased in the hippocampus before and after memory training. Lastly, long days occluded IGF2-induced improvements in recognition memory. Collectively, these results suggest that light changes hippocampal clock function to alter memory, highlighting novel mechanisms that may contribute to the positive effects of light. Furthermore, this study provides insight into how the circadian clock can regulate hippocampus-dependent learning by controlling molecular processes required for memory consolidation
Interaction of quasilocal harmonic modes and boson peak in glasses
The direct proportionality relation between the boson peak maximum in
glasses, , and the Ioffe-Regel crossover frequency for phonons,
, is established. For several investigated materials . At the frequency the mean free path of the
phonons becomes equal to their wavelength because of strong resonant
scattering on quasilocal harmonic oscillators. Above this frequency phonons
cease to exist. We prove that the established correlation between
and holds in the general case and is a direct consequence of
bilinear coupling of quasilocal oscillators with the strain field.Comment: RevTex, 4 pages, 1 figur
The Apparent Fractal Conjecture: Scaling Features in Standard Cosmologies
This paper presents an analysis of the smoothness problem in cosmology by
focussing on the ambiguities originated in the simplifying hypotheses aimed at
observationally verifying if the large-scale distribution of galaxies is
homogeneous, and conjecturing that this distribution should follow a fractal
pattern in perturbed standard cosmologies. This is due to a geometrical effect,
appearing when certain types of average densities are calculated along the past
light cone. The paper starts reviewing the argument concerning the possibility
that the galaxy distribution follows such a scaling pattern, and the premises
behind the assumption that the spatial homogeneity of standard cosmology can be
observable. Next, it is argued that to discuss observable homogeneity one needs
to make a clear distinction between local and average relativistic densities,
and showing how the different distance definitions strongly affect them,
leading the various average densities to display asymptotically opposite
behaviours. Then the paper revisits Ribeiro's (1995: astro-ph/9910145) results,
showing that in a fully relativistic treatment some observational average
densities of the flat Friedmann model are not well defined at z ~ 0.1, implying
that at this range average densities behave in a fundamentally different manner
as compared to the linearity of the Hubble law, well valid for z < 1. This
conclusion brings into question the widespread assumption that relativistic
corrections can always be neglected at low z. It is also shown how some key
features of fractal cosmologies can be found in the Friedmann models. In view
of those findings, it is suggested that the so-called contradiction between the
cosmological principle, and the galaxy distribution forming an unlimited
fractal structure, may not exist.Comment: 30 pages, 2 figures, LaTeX. This paper is a follow-up to
gr-qc/9909093. Accepted for publication in "General Relativity and
Gravitation
Total versus superficial parotidectomy for stage III melanoma
BackgroundThe primary purpose of this study was to describe the parotid recurrence rates after superficial and total parotidectomy.MethodsA retrospective cohort study was performed on patients with cutaneous melanoma metastatic to the parotid gland who underwent parotidectomy from 1998 through 2014. Primary outcome was parotid bed recurrence. Secondary outcomes were facial nerve function postoperatively and at last follow‐up.ResultsOne hundred twenty‐nine patients were included in the study. Thirty‐four patients (26%) underwent a total parotidectomy and 95 patients underwent superficial parotidectomy. Twelve patients (13%) developed parotid bed recurrence after superficial parotidectomy alone versus zero after total parotidectomy (P = .035). Facial nerve function, clinically detected disease, stage, and adjuvant treatment were not statistically different between the groups (P = .32, .32, .13, and 0.99, respectively).ConclusionParotid bed melanoma recurrence was more common after superficial parotidectomy compared to total parotidectomy, and recurrence resulted in significant facial nerve functional deficit. Our results support total parotidectomy when metastatic melanoma involves the parotid nodal basin.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137735/1/hed24810_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137735/2/hed24810.pd
Feeding Nucleotides with Corn Germ Meal or Dried Corn Distillers Grains Does Not Promote Growth Performance of Receiving and Growing Calves
Corn germ meal is often used in swine and poultry diets, but very little information exists on the effects on beef cattle and on newly arrived stressed cattle. When formulating receiving and growing diets, calf health and stress are important factors to consider. Including nucleotides, an immune-boosting feed additive may aid in gastrointestinal health of an animal and furthermore improve growth performance. The objective of these experiments was to determine: 1) the effects of corn germ meal in comparison to dried corn distillers grains on growth performance and 2) the effects of nucleotides on growth performance, by receiving and growing cattle
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