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Now showing items 17 - 32 of 1702

  • Star catalog position and proper motion corrections in asteroid astrometry II:The Gaia era

    Eggl, Siegfried   Farnocchia, Davide   Chamberlin, Alan B.   Chesley, Steven R.  

    Astrometric positions of moving objects in the Solar System have been measured using a variety of star catalogs in the past. Previous work has shown that systematic errors in star catalogs can affect the accuracy of astrometric observations. That, in turn, can influence the resulting orbit fits for minor planets. In order to quantify these systematic errors, we compare the positions and proper motion of stellar sources in the most utilized star catalogs to the second release of the Gaia star catalog. The accuracy of Gaia astrometry allows us to unambiguously identify local biases and derive a scheme that can be used to correct past astrometric observations of solar system objects. Here, we provide substantially improved debiasing tables for 26 astrometric catalogs that were extensively used in minor planet astrometry. Revised corrections near the galactic center eliminate artifacts that could be traced back to reference catalogs used in previous debiasing schemes. Median differences in stellar positions between catalogs now tend to be on the order of several tens of milliarcseconds (mas) but can be as large as 175 mas. Median stellar proper motion corrections scatter around 0.3 mas/yr and range from 1 to 4 mas/yr for star catalogs with and without proper motion, respectively. The tables presented in this work contain a posteriori corrections meant to improve orbit fits based on optical observations that were measured against astrometric catalogs other than Gaia. However, astrometrists are strongly encouraged to make use of the most recent Gaia astrometric catalog when submitting new observations. Since previous debiasing schemes already reduced systematics in past observations to a large extent, corrections beyond the current work may not be needed in the foreseeable future.
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  • Nongravitational Accelerations on Comets

    Chesley, Steven R.   Yeomans, Donald K.  

    The orbital motion of comets is difficult to characterize accurately due to the rocket-like outgassing of material from the cometary nucleus. The resulting nongravitational accelerations often appear to be fundamentally stochastic in nature and thus pose severe modeling challenges in orbit determination, especially when the comet has been observed for many revolutions. Even so, new techniques have arisen in recent years that give new insight, not only into the motion of the comets, but also into their physical characteristics and spin states. These approaches include modeling of spin axis precession over many decades and the consideration of the seasonal variation in the thrust from discrete jets acting on a rotating nucleus. Such advances have been enabled, in part, by the increasing efforts and capabilities of comet observers worldwide as more and more comets with longer and longer observing arcs become available for study. In this review we specifically consider the application of the Rotating Jet Model to several space mission targets, indicating how this model can often be used to infer the orientation of a comet's spin axis.
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  • Potential impact detection for Near-Earth asteroids: the case of 99942 Apophis (2004 MN 4 )

    Chesley, Steven R.  

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  • A Global Analysis of The Generalized Sitnikov Problem

    Chesley, Steven R.  

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  • High-fidelity Simulations of the Near-Earth Object Search Performance of the Large Synoptic Survey Telescope

    Veres, Peter   Chesley, Steven R.  

    We perform high-fidelity simulations of a wide-field telescopic survey searching for Near-Earth Objects (NEOs) larger than 140 m, focusing on the observation and detection model, as well as detection efficiency and accuracy. As a test survey, we select the Large Synoptic Survey Telescope (LSST). We use its proposed pointings for a 10-year mission, and model the detection of NEOs in the fields. We discuss individual model parameters for magnitude losses, vignetting, fading, asteroid rotation and colors, fill factor, limiting magnitude, rate of motion, field shape and rotation, and survey patterns. We assess results in terms of the cumulative completeness of the detected population as a function of size and time. Additionally, we examine the sources of modeling uncertainty, and derive the overall NEO population completeness for the baseline LSST survey to be 55 +/- 5% for NEOs with absolute magnitude brighter than 22. Including already discovered objects and ongoing surveys, the NEO completeness at the end of the LSST baseline survey should reach similar to 77%.
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  • Constraints on the perturbed mutual motion in Didymos due to impact-induced deformation of its primary after the DART impact

    Hirabayashi, Masatoshi   Schwartz, Stephen R.   Yu, Yang   Davis, Alex B.   Chesley, Steven R.   Fahnestock, Eugene G.   Michel, Patrick   Richardson, Derek C.   Naidu, Shantanu P.   Scheeres, Daniel J.   Cheng, Andrew F.   Rivkin, Andrew S.   Benner, Lance A. M.  

    Binary near-Earth asteroid (65803) Didymos is the target of the proposed NASA Double Asteroid Redirection Test (DART), part of the Asteroid Impact & Deflection Assessment (AIDA) mission concept. In this mission, the DART spacecraft is planned to impact the secondary body of Didymos, perturbing mutual dynamics of the system. The primary body is currently rotating at a spin period close to the spin barrier of asteroids, and materials ejected from the secondary due to the DART impact are likely to reach the primary. These conditions may cause the primary to reshape, due to landslides or internal deformation, changing the permanent gravity field. Here, we propose that if shape deformation of the primary occurs, the mutual orbit of the system would be perturbed due to a change in the gravity field. We use a numerical simulation technique based on the full two-body problem to investigate the shape effect on the mutual dynamics in Didymos after the DART impact. The results show that under constant volume, shape deformation induces strong perturbation in the mutual motion. We find that the deformation process always causes the orbital period of the system to become shorter. If surface layers with a thickness greater than similar to 0.4 m on the poles of the primary move down to the equatorial region due to the DART impact, a change in the orbital period of the system and in the spin period of the primary will be detected by ground-based measurement.
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  • Steven R. Garfin

    Goel, Vijay   Bono, Chris   Herkowitz, Harry   Boden, Scott  

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  • Steven R. Garfin

    Goel, Vijay; Bono, Chris; Herkowitz, Harry; Boden, Scott  

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  • Adam Usk’s Secret\r (by Steven Justice)

    Barrington   Candace  

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  • Promoting Interdisciplinary Practice:An Interview With Steven R. Forness

    Zabel, Robert H.   Kaff, Marilyn   Teagarden, James  

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  • Steven Best,\r The Politics of Total Liberation

    Garner Robert  

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  • EnvStats: An R Package for Environmental Statisticsby Steven P. Millard

    Peter F. Craigmile  

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  • EnvStats: An R Package for Environmental Statisticsby Steven P. Millard

    Peter F. Craigmile  

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  • Promoting Interdisciplinary Practice: An Interview With Steven R. Forness

    Zabel, R. H.   Kaff, M.   Teagarden, J.  

    As part of an ongoing oral history project, a conversation was held with Dr. Stephen Forness on the past, present, and possible future of the field of providing services to children with emotional-behavioral disorders. Dr. Forness stresses the increasing importance of providing an interdisciplinary approach to meeting these needs.
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  • Plotinus: Myth, Metaphor and Philosophical Practice by Steven R. L. Clark

    Mortley, Raoul  

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  • Reading Steven Friedman’s\r race, class, and power

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