19,414 research outputs found
The aerospace technology laboratory (a perspective, then and now)
The physical changes that have taken place in aerospace facilities since the Wright brothers' accomplishment 78 years ago are highlighted. For illustrative purposes some of the technical facilities and operations of the NASA Lewis Research Center are described. These simulation facilities were designed to support research and technology studies in aerospace propulsion
Will War\u27s Nature Change in the Seventh Military Revolution?
This article examines the potential implications of the combinations of robotics, artificial intelligence, and deep learning systems on the character and nature of war. The author employs Carl von Clausewitz’s trinity concept to discuss how autonomous weapons will impact the essential elements of war. The essay argues war’s essence, as politically directed violence fraught with friction, will remain its most enduring aspect, even if more intelligent machines are involved at every level
The Revenge of Geography: What the Map Tells Us about Coming Conflicts and the Battle against Fate
Photographic Effects Produced by Cadmium and Other Elements Under Neutron Bombardment
It has been found that when a duplitized x-ray film has Cd placed next to it and is then surrounded by paraffin and exposed to a neutron source, the film shows blackening under the cadmium. Under these conditions the film also shows some general blackening which is rather weak. The neutrons used in these experiments were obtained by bombarding either Li or Be with about 10 microamperes of 1.2 Mev deuterons furnished by a cyclotron. There are, of course, also p-rays incident on the Cd and the photographic film, and it was necessary to establish the blackening under the cadmium as due to slow neutrons and not to these radiations
Interplanetary propulsion using inertial fusion
Inertial fusion can be used to power spacecraft within the solar system and beyond. Such spacecraft have the potential for short-duration manned-mission performance exceeding other technologies. We are conducting a study to assess the systems aspects of inertial fusion as applied to such missions, based on the conceptual engine design of Hyde (1983) we describe the required systems for an entirely new spacecraft design called VISTA that is based on the use of DT fuel. We give preliminary design details for the power conversion and power conditioning systems for manned missions to Mars of total duration of about 100 days. Specific mission performance results will be published elsewhere, after the study has been completed
A low-loss photonic silica nanofiber for higher-order modes
Optical nanofibers confine light to subwavelength scales, and are of interest
for the design, integration, and interconnection of nanophotonic devices. Here
we demonstrate high transmission (> 97%) of the first family of excited modes
through a 350 nm radius fiber, by appropriate choice of the fiber and precise
control of the taper geometry. We can design the nanofibers so that these modes
propagate with most of their energy outside the waist region. We also present
an optical setup for selectively launching these modes with less than 1%
fundamental mode contamination. Our experimental results are in good agreement
with simulations of the propagation. Multimode optical nanofibers expand the
photonic toolbox, and may aid in the realization of a fully integrated
nanoscale device for communication science, laser science or other sensing
applications.Comment: 12 pages, 5 figures, movies available onlin
Mergers and the outside-in formation of dwarf spheroidals
We use a cosmological simulation of the formation of the Local Group to
explore the origin of age and metallicity gradients in dwarf spheroidal
galaxies. We find that a number of simulated dwarfs form "outside-in", with an
old, metal-poor population that surrounds a younger, more concentrated
metal-rich component, reminiscent of dwarf spheroidals like Sculptor or
Sextans. We focus on a few examples where stars form in two populations
distinct in age in order to elucidate the origin of these gradients. The
spatial distributions of the two components reflect their diverse origin; the
old stellar component is assembled through mergers, but the young population
forms largely in situ. The older component results from a first episode of star
formation that begins early but is quickly shut off by the combined effects of
stellar feedback and reionization. The younger component forms when a late
accretion event adds gas and reignites star formation. The effect of mergers is
to disperse the old stellar population, increasing their radius and decreasing
their central density relative to the young population. We argue that
dwarf-dwarf mergers offer a plausible scenario for the formation of systems
with multiple distinct populations and, more generally, for the origin of age
and metallicity gradients in dwarf spheroidals.Comment: 10 pages, 8 figures, Accepted for publication in MNRA
Dwarf Galaxies and the Cosmic Web
We use a cosmological simulation of the formation of the Local Group of
Galaxies to identify a mechanism that enables the removal of baryons from
low-mass halos without appealing to feedback or reionization. As the Local
Group forms, matter bound to it develops a network of filaments and pancakes.
This moving web of gas and dark matter drifts and sweeps a large volume,
overtaking many halos in the process. The dark matter content of these halos is
unaffected but their gas can be efficiently removed by ram-pressure. The loss
of gas is especially pronounced in low-mass halos due to their lower binding
energy and has a dramatic effect on the star formation history of affected
systems. This "cosmic web stripping" may help to explain the scarcity of dwarf
galaxies compared with the numerous low-mass halos expected in \Lambda CDM and
the large diversity of star formation histories and morphologies characteristic
of faint galaxies. Although our results are based on a single high-resolution
simulation, it is likely that the hydrodynamical interaction of dwarf galaxies
with the cosmic web is a crucial ingredient so far missing from galaxy
formation models.Comment: Submitted to ApJL. 6 pages, 4 figures. A set of movies showing the
interaction between dwarf galaxies and the Cosmic Web can be found at mirror
1 http://www.astro.uvic.ca/~mario/dwarf-web/ or at mirror 2
http://www.iate.oac.uncor.edu/~alejandro/dwarf-web/ . Comments are welcome
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