4,400 research outputs found
The control of space manipulators subject to spacecraft attitude control saturation limits
The motions of robotic manipulators mounted on spacecraft can disturb the spacecraft's positions and attitude. These disturbances can surpass the ability of the system's attitude control reaction jets to control them, for the disturbances increase as manipulator speeds increase. If the manipulator moves too quickly the resulting disturbances can exceed the saturation levels of the reaction jets, causing excessive spacecraft motions. A method for planning space manipulator's motions is presented, so that tasks can be performed as quickly as possible without saturating the system's attitude control jets
Strange hyperon and antihyperon production from quark and string-rope matter
Hyperon and antihyperon production is investigated using two microscopical
models: {\bf (1)} the fast hadronization of quark matter as given by the ALCOR
model; {\bf (2)} string formation and fragmentation as in the HIJING/B model.
We calculate the particle numbers and momentum distributions for Pb+Pb
collisions at CERN SPS energies in order to compare the two models with each
other and with the available experimental data. We show that these two
theoretical approaches give similar yields for the hyperons, but strongly
differ for antihyperons.Comment: 11 pages, Latex, 3 EPS figures, contribution to the Proceedings of
the 4th International Conference on Strangeness in Quark Matter (SQM'98),
Padova, Italy, 20-24 July 199
Self-Assembling Ice Membranes on Europa: Brinicle Properties, Field Examples, and Possible Energetic Systems in Icy Ocean Worlds
Brinicles are self-assembling tubular ice membrane structures, centimeters to
meters in length, found beneath sea ice in the polar regions of Earth. We
discuss how the properties of brinicles make them of possible importance for
chemistry in cold environments-including that of life's emergence-and we
consider their formation in icy ocean world. We argue that the non-ice
composition of the ice on Europa and Enceladus will vary spatially due to
thermodynamic and mechanical properties that serve to separate and fractionate
brines and solid materials. The specifics of the composition and dynamics of
both the ice and the ocean in these worlds remain poorly constrained. We
demonstrate through calculations using FREZCHEM that sulfate likely
fractionates out of accreting ice in Europa and Enceladus, and thus that an
exogenous origin of sulfate observed on Europa's surface need not preclude
additional endogenous sulfate in Europa's ocean. We suggest that, like
hydrothermal vents on Earth, brinicles in icy ocean worlds constitute ideal
places where ecosystems of organisms might be found
Controlled Irradiative Formation of Penitentes
Spike-shaped structures are produced by light-driven ablation in very
different contexts. Penitentes 1-4 m high are common on Andean glaciers, where
their formation changes glacier dynamics and hydrology. Laser ablation can
produce cones 10-100 microns high with a variety of proposed applications in
materials science. We report the first laboratory generation of
centimeter-scale snow and ice penitentes. Systematically varying conditions
allows identification of the essential parameters controlling the formation of
ablation structures. We demonstrate that penitente initiation and coarsening
requires cold temperatures, so that ablation leads to sublimation rather than
melting. Once penitentes have formed, further growth of height can occur by
melting. The penitentes intially appear as small structures (3 mm high) and
grow by coarsening to 1-5 cm high. Our results are an important step towards
understanding and controlling ablation morphologies.Comment: Accepted for publication in Physical Review Letter
Fire effects on temperate forest soil C and N storage
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/116995/1/eap20112141189.pd
Correspondence between geometrical and differential definitions of the sine and cosine functions and connection with kinematics
In classical physics, the familiar sine and cosine functions appear in two
forms: (1) geometrical, in the treatment of vectors such as forces and
velocities, and (2) differential, as solutions of oscillation and wave
equations. These two forms correspond to two different definitions of
trigonometric functions, one geometrical using right triangles and unit
circles, and the other employing differential equations. Although the two
definitions must be equivalent, this equivalence is not demonstrated in
textbooks. In this manuscript, the equivalence between the geometrical and the
differential definition is presented assuming no a priori knowledge of the
properties of sine and cosine functions. We start with the usual length
projections on the unit circle and use elementary geometry and elementary
calculus to arrive to harmonic differential equations. This more general and
abstract treatment not only reveals the equivalence of the two definitions but
also provides an instructive perspective on circular and harmonic motion as
studied in kinematics. This exercise can help develop an appreciation of
abstract thinking in physics.Comment: 6 pages including 1 figur
A layered approach to technology transfer of AVIRIS between Earth Search Sciences, Inc. and the Idaho National Engineering Laboratory
Since initial contact between Earth Search Sciences, Inc. (ESSI) and the Idaho National Engineering Laboratory (INEL) in February, 1994, at least seven proposals have been submitted in response to a variety of solicitations to commercialize and improve the AVIRIS instrument. These proposals, matching ESSI's unique position with respect to agreements with the National Aeronautics and Space Administration (NASA) and the Jet Propulsion Laboratory (JPL) to utilize, miniaturize, and commercialize the AVIRIS instrument and platform, are combined with the applied engineering of the INEL. Teaming ESSI, NASA/JPL, and INEL with diverse industrial partners has strengthened the respective proposals. These efforts carefully structure the overall project plans to ensure the development, demonstration, and deployment of this concept to the national and international arenas. The objectives of these efforts include: (1) developing a miniaturized commercial, real-time, cost effective version of the AVIRIS instrument; (2) identifying multiple users for AVIRIS; (3) integrating the AVIRIS technology with other technologies; (4) gaining the confidence/acceptance of other government agencies and private industry in AVIRIS; and (5) increasing the technology base of U.S. industry
Ultrasound-Assisted Peripheral Venous Access in Young Children: A Randomized Controlled Trial and Pilot Feasibility Study
OBJECTIVES: Intravenous (IV) access in children treated in the emergency department (ED) is frequently required and often difficult to obtain. While it has been shown that ultrasound can be useful in adults for both central and peripheral venous access, research regarding children has been limited. We sought to determine if the use of a static ultrasound technique could, a) allow clinicians to visualize peripheral veins and b) improve success rates of peripheral venous cannulation in young children in the ED.METHODS: We performed a randomized clinical trial of children < 7 years in an academic pediatric ED who required IV access and who had failed the first IV attempt. We randomized patients to either continued standard IV attempts or ultrasound-assisted attempts. Clinicians involved in the study received one hour of training in ultrasound localization of peripheral veins. In the ultrasound group, vein localization was performed by an ED physician who marked the skin overlying the target vessel. Intravenous cannulation attempts were then immediately performed by a pediatric ED nurse who relied on the skin mark for vessel location. We allowed for technique cross-over after two failed IV attempts. We recorded success rate and location of access attempts. We compared group success rates using differences in 95% confidence intervals (CI).RESULTS: We enrolled 44 children over a one-year period. The median age of enrollees was 9.5 months. We visualized peripheral veins in all patients in the ultrasound group (n=23) and in those who crossed over to ultrasound after failed standard technique attempts (n= 8). Venipuncture was successful on the first attempt in the ultrasound group in 13/23 (57%, CI, 35% to 77%), versus 12/21 (57%, CI, 34% to 78%) in the standard group, difference between groups 0.6% (95% CI -30% to 29%). First attempt cannulation success in the ultrasound group was 8/23 (35%, CI, 16% to 57%), versus 6/21 (29%, CI, 11% to 52%) in the standard group, difference between groups 6% (95% CI -21% to 34%).CONCLUSION: Ultrasound allows physicians to visualize peripheral veins of young children in the ED. We were unable to demonstrate, however, a clinically important benefit to a static ultrasound aided vein cannulation technique performed by clinicians with limited ultrasound training over standard technique after one failed IV attempt in an academic pediatric ED
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