1,337 research outputs found

    Investigation of Atmospheric Reentry for the Space Maneuver Vehicle

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    This study investigated the atmospheric reentry of the Space Maneuver Vehicle from low-earth orbit using an entry guidance concept similar to the Space Shuttle. The Space Maneuver Vehicle was modeled as a point mass with aerodynamic properties as determined using Newtonian impact theory. For the rarefied-flow transition regime bridging formulae are used to capture the effects of both hypersonic continuum flow and free molecular flow. Constraints to the reentry are developed and a reentry corridor is defined in the drag-velocity plane. Bank angle modulation is established as the primary means for controlling drag and range. The guidance concept is applied to both a high inclination orbit and a low inclination orbit with cross-range requirement. Monte Carlo error analysis validates the ability of the control algorithm to guide reentry in the presence of initial state errors, as well as atmospheric variations

    Viability of a Sustainable and Feasible National Transportation System

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    Spitzer observations of extragalactic H II regions - III. NGC 6822 and the hot star, H II region connection

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    Using the short-high module of the Infrared Spectrograph on the Spitzer Space Telescope, we have measured the [S IV] 10.51, [Ne II] 12.81, [Ne III] 15.56, and [S III] 18.71-micron emission lines in nine H II regions in the dwarf irregular galaxy NGC 6822. These lines arise from the dominant ionization states of the elements neon (Ne++^{++}, Ne+^+) and sulphur (S3+^{3+}, S++^{++}), thereby allowing an analysis of the neon to sulphur abundance ratio as well as the ionic abundance ratios Ne+^+/Ne++^{++} and S3+^{3+}/S++^{++}. By extending our studies of H II regions in M83 and M33 to the lower metallicity NGC 6822, we increase the reliability of the estimated Ne/S ratio. We find that the Ne/S ratio appears to be fairly universal, with not much variation about the ratio found for NGC 6822: the median (average) Ne/S ratio equals 11.6 (12.2±\pm0.8). This value is in contrast to Asplund et al.'s currently best estimated value for the Sun: Ne/S = 6.5. In addition, we continue to test the predicted ionizing spectral energy distributions (SEDs) from various stellar atmosphere models by comparing model nebulae computed with these SEDs as inputs to our observational data, changing just the stellar atmosphere model abundances. Here we employ a new grid of SEDs computed with different metallicities: Solar, 0.4 Solar, and 0.1 Solar. As expected, these changes to the SED show similar trends to those seen upon changing just the nebular gas metallicities in our plasma simulations: lower metallicity results in higher ionization. This trend agrees with the observations.Comment: 22 pages, 13 figures. To be published in MNRAS. reference added and typos fixed. arXiv admin note: text overlap with arXiv:0804.0828, which is paper II by Rubin et al. (2008

    A First Comparison of SLOPE and Other LIGO Burst Event Trigger Generators

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    A number of different methods have been proposed to identify unanticipated burst sources of gravitational waves in data arising from LIGO and other gravitational wave detectors. When confronted with such a wide variety of methods one is moved to ask if they are all necessary, i.e. given detector data that is assumed to have no gravitational wave signals present, do they generally identify the same events with the same efficiency, or do they each 'see' different things in the detector? Here we consider three different methods, which have been used within the LIGO Scientific Collaboration as part of its search for unanticipated gravitational wave bursts. We find that each of these three different methods developed for identifying candidate gravitational wave burst sources are, in fact, attuned to significantly different features in detector data, suggesting that they may provide largely independent lists of candidate gravitational wave burst events.Comment: 10 Pages, 5 Figures, Presented at the 10th Gravitational Wave Data Analysis Workshop (GWDAW-10), 14-17 December 2005 at the University of Texas, Brownsvill

    DNA Fingerprinting of Mycobacterium tuberculosis: Lessons Learned and Implications for the Future

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    DNA fingerprinting of Mycobacterium tuberculosis—a relatively new laboratory technique—offers promise as a powerful aid in the prevention and control of tuberculosis (TB). Established in 1996 by the Centers for Disease Control and Prevention (CDC), the National Tuberculosis Genotyping Surveillance Network was a 5-year prospective, population-based study of DNA fingerprinting conducted from 1996 to 2000. The data from this study suggest multiple molecular epidemiologic and program management uses for DNA fingerprinting in TB public health practice. From these data, we also gain a clearer understanding of the overall diversity of M. tuberculosis strains as well as the presence of endemic strains in the United States. We summarize the key findings and the impact that DNA fingerprinting may have on future approaches to TB control. Although challenges and limitations to the use of DNA fingerprinting exist, the widespread implementation of the technique into routine TB prevention and control practices appears scientifically justified
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