63,839 research outputs found
Collision rates in the present-day Kuiper Belt and Centaur Regions: Applications to surface activation and modification on Comets, Kuiper Belt Objects, Centaurs, and Pluto-Charon
We extend previous results showing that the surfaces of Edgeworth-Kuiper Belt
objects are not primordial and have been moderately to heavily reworked by
collisions. Objects smaller than about km have collisional disruption
lifetimes less than 3.5 Gyr in the present-day collisional environment and have
been heavily damaged in their interiors by large collisions. In the 30--50 AU
region, impacts of 1 km radius comets onto individual 100 km radius objects
occur on -- yr timescales, cratering the surfaces of
the larger objects with 8--54 craters 6 km in diameter over 3.5 Gyr.
Collision time scales for impacts of 4 meter radius projectiles onto 1 km
radius comets range from 3--5 yr. The cumulative fraction of the
surface area of 1 and 100 km radius objects cratered by projectiles with radii
larger than 4 m ranges from a few to a few tens percent over 3.5 Gyr. The flux
of EKO projectiles onto Pluto and Charon is also calculated and is found to be
3--5 times that of previous estimates. Our impact model is also applied
to Centaur objects in the 5--30 AU region. We find the collisional/cratering
histories of Centaurs are dominated by the time spent in the Edgeworth-Kuiper
Belt rather than the time spent on planet-crossing orbits. Hence, the
predominant surface activity of Centaur objects like Chiron is almost certainly
not impact-induced.Comment: 17 pages, 8 figures. Icarus, 2000, in pres
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Behavioral synthesis from VHDL using structured modeling
This dissertation describes work in behavioral synthesis involving the development of a VHDL Synthesis System VSS which accepts a VHDL behavioral input specification and performs technology independent synthesis to generate a circuit netlist of generic components. The VHDL language is used for input and output descriptions. An intermediate representation which incorporates signal typing and component attributes simplifies compilation and facilitates design optimization.A Structured Modeling methodology has been developed to suggest standard VHDL modeling practices for synthesis. Structured modeling provides recommendations for the use of available VHDL description styles so that optimal designs will be synthesized.A design composed of generic components is synthesized from the input description through a process of Graph Compilation, Graph Criticism, and Design Compilation. Experiments were performed to demonstrate the effects of different modeling styles on the quality of the design produced by VSS. Several alternative VHDL models were examined for each benchmark, illustrating the improvements in design quality achieved when Structured Modeling guidelines were followed
Collisional evolution in the Vulcanoid region: Implications for present-day population constraints
We explore the effects of collisional evolution on putative Vulcanoid
ensembles in the region between 0.06 and 0.21 AU from the Sun, in order to
constrain the probable population density and population structure of this
region today. Dynamical studies have shown that the Vulcanoid Zone (VZ) could
be populated. However, we find that the frequency and energetics of collisional
evolution this close to the Sun, coupled with the efficient radiation transport
of small debris out of this region, together conspire to create an active and
highly intensive collisional environment which depletes any very significant
population of rocky bodies placed in it, unless the bodies exhibit orbits that
are circular to ~10^-3 or less, or highly lossy mechanical properties that
correspond to a fraction of impact energy significantly less than 10% being
imparted to ejecta. The most favorable locale for residual bodies to survive in
this region is in highly circular orbits near the outer edge of the dynamically
stable Vulcanoid Zone (i.e., near 0.2 AU), where collisional evolution and
radiation transport of small bodies and debris proceed most slowly. If the mean
random orbital eccentricity in this region exceeds ~10^-3, then our work
suggests it is unlikely that more than a few hundred objects with radii larger
than 1 km will be found in the entire VZ; assuming the largest objects have a
radius of 30 km, then the total mass of bodies in the VZ down to 0.1 km radii
is likely to be no more than ~10^-6Mearth, <10^-3 the mass of the asteroid
belt. Despite the dynamical stability of large objects in this region (Evans &
Tabachnik 1999), it is plausible that the entire region is virtually empty of
km-scale and larger objects.Comment: text plus 7 .ps figures, gzipped. Icarus, 2000, in pres
Swearwords Used by Gangsters in the “Alpha Dog” Movie
Many people assume that swearwords is a rude word and should be avoided. Those words are used to insult, to curse, to offend or to mock at something when the speaker has strong emotion (Hughes, 1991,05). To swear at someone or something is to insult and deprecate the object of abuse, as well as to use other kinds of dysphemism (Allan & Burridge, 2006,76). Apparently some experts and scholars have been able to prove that swearwords also has a purpose and a meaning beside the one as commonly held. Therefore, the writer took the "Alpha Dog" movie as an example of the analyzed cases. Examples have been found by the writer including the categories of epithets derived from tabooed bodily organs, epithets derived from bodily effluvia, epithets derived from sexual behaviours, dysphemistic epithets that pick on real physical characteristics that are treated as though they are abnormalities, imprecations and epithets invoking mental subnormality or derangement. Finally, the writer also managed to find a purpose or a reason for the people to use swearwords in real life
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Structured modeling for VHDL synthesis
This report will describe a proposed modeling style for the use of the VHSIC Hardware Description Language (VHDL) in design synthesis. We will describe the operations and underlying assumptions of four design models currently understood and used in practice by designers: combinational logic, functional descriptions (involving clocked components such as counters), register transfer (data path) descriptions, and behavioral (instruction set or processor) designs. We will illustrate the various uses of the VHDL description styles (structural, dataflow and behavioral) to represent characteristics of each of these design models. Emphasis is placed on how VHDL constructs should be used in order to synthesize optimal designs
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VSS : a VHDL synthesis system
This report describes a register transfer synthesis system that allows a designer to interact with the design process. The designer can modify the compiled design by changing the input description, selecting optimization and mapping strategies, or graphically changing the generated design schematic. The VHDL language is used for input and output descriptions. An intermediate representation which incorporates signal typing and component attributes simplifies compilation and facilitates design optimization. The compilation process consists of two phases. First, a design composed of generic components is synthesized from the input description. Second, this design is translated into components from a particular library by a mapper and optimized by a logic optimizer. Redesign to new technologies can be accomplished by changing only the component library
Atomistic subsemirings of the lattice of subspaces of an algebra
Let A be an associative algebra with identity over a field k. An atomistic
subsemiring R of the lattice of subspaces of A, endowed with the natural
product, is a subsemiring which is a closed atomistic sublattice. When R has no
zero divisors, the set of atoms of R is endowed with a multivalued product. We
introduce an equivalence relation on the set of atoms such that the quotient
set with the induced product is a monoid, called the condensation monoid. Under
suitable hypotheses on R, we show that this monoid is a group and the class of
k1_A is the set of atoms of a subalgebra of A called the focal subalgebra. This
construction can be iterated to obtain higher condensation groups and focal
subalgebras. We apply these results to G-algebras for G a group; in particular,
we use them to define new invariants for finite-dimensional irreducible
projective representations.Comment: 14 page
Attitude sensor alignment calibration for the solar maximum mission
An earlier heuristic study of the fine attitude sensors for the Solar Maximum Mission (SMM) revealed a temperature dependence of the alignment about the yaw axis of the pair of fixed-head star trackers relative to the fine pointing Sun sensor. Here, new sensor alignment algorithms which better quantify the dependence of the alignments on the temperature are developed and applied to the SMM data. Comparison with the results from the previous study reveals the limitations of the heuristic approach. In addition, some of the basic assumptions made in the prelaunch analysis of the alignments of the SMM are examined. The results of this work have important consequences for future missions with stringent attitude requirements and where misalignment variations due to variations in the temperature will be significant
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