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  • 1
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 368 (1994), S. 721-723 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Planetesimals form after particles settle to the central plane of the gaseous solar nebula. If the particle layer attains a critical density, it can fragment into gravitationally bound condensations6. Goldreich and Ward7 suggested direct formation of planetesimals from microscopic dust grains. But ...
    Type of Medium: Electronic Resource
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  • 2
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    In:  Other Sources
    Publication Date: 2019-01-25
    Description: The lunar origin model considered involves processing of protolunar material through a circumterrestrial swarm of particles. Once such a swarm has formed, it can gain mass by capturing infalling planetesimals and ejecta from giant impacts on the Earth, although the angular momentum supply from these sources remains a problem. Examined is the first stage of formation of a geocentric swarm by capture of planetesimals from initialy heliocentric orbits. The only plausible capture mechanism that is not dependent on very low approach velocities is the mutual collision of planetesimals passing within Earth's sphere of influence. This capture scenario was tested directly by many body numerical integration of planetesimal orbits in near Earth space. Results agree that the systematic contribution of angular momentum is insufficient to maintain an orbiting swarm under heavy bombardment. Thus, a circumterrestrial swarm can be formed rather easily, but is hard to sustain because the mean net angular momentum of a many body swarm is small.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar Planetary Inst. Conf. on the Origin of the Moon; p 54
    Format: text
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  • 3
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 352 (1991), S. 190-192 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] THE most distant bodies visible in the Solar System are slowly yielding their secrets. The discovery of Pluto's large satellite, Charon, and observations of their mutual occupations led to the first meaningful measurements of their size and mass1. The encounter of Voyager II with Neptune yielded a ...
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  • 4
    Publication Date: 2013-08-31
    Description: Research on a variety of dynamical processes relevant to the formation of planets, satellites and ring systems is discussed. The main focus is on studies of accretionary formation of early protoplanets using a numerical model, structures and evolution of ring systems and individual bodies within planetary rings, and theories of lunar origin.
    Keywords: ASTROPHYSICS
    Type: NASA, Washington, Reports of Planetary Geology and Geophysics Program, 1986; p 112-114
    Format: application/pdf
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  • 5
    Publication Date: 2013-08-31
    Description: Research on the accumulation of solid bodies in the solar nebula is discussed. Studies of the earliest stage of accumulation of solid bodies in the solar system, which occured in the presence of the gaseous component of the solar nebula, are discussed. The combined effects of gas drag and gravitational perturbations of a planetary embryo on the orbital evolution of planetesimals, the effects of resonant trapping on planetesimals, and planetary mass accretion are discussed.
    Keywords: SOLAR PHYSICS
    Type: NASA, Washington, Reports of Planetary Geology and Geophysics Program, 1986; p 106-107
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  • 6
    Publication Date: 2013-08-31
    Description: An improved numerical code was constructed to model coagulation and settling of particles in disk nebula containing generic turbulence with arbitrary velocities in the gas. The turbulence is assumed to have a Kolmogorov eddy spectrum. Relative velocities of particles, which lead to collisions and possible coagulation, are computed as due all significant causes in their appropriate regimes: thermal motion, shear and inertial effects in turbulent eddies, and systematic motions due to settling and non-keplerian rotation of the gas. Significant improvements to this program were produced. One significant problem was the disparity of timescales for turbulent mixing and coagulation. To accurately compute the former, the timestep must be shorter than the smallest spatial scale (layer thickness) divided by the turbulent velocity. However, the size distribution often varies due to coagulation on much longer timescales. To minimize the computational overhead associated with collisions between particles of all sizes, a dual timestep was introduced. Collisional changes in the size distribution are computed once in every N substeps, where the substep is controlled by the turbulent diffusion velocity, and N is determined by the rate of collisions. This algorithm allowed simulations to be extended to longer times and later stages.
    Keywords: ASTROPHYSICS
    Type: NASA, Washington, Reports of Planetary Geology and Geophysics Program, 1990; p 424-425
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  • 7
    Publication Date: 2013-08-31
    Description: The reduction and publication of an extensive data set collected in experiments over several years at Ames and PSI is briefly examined. Hartmann has been assembling data sets from his experiments on catastrophic fragmentation of various materials, including basalt, other igneous rock, ice, and weak dirt clods. Weidenschilling and Davis have continued to gather and reduce data on oblique impacts. The data indicate a power law distribution of ejecta mass vs. velocity, with a slope that is independent of azimuth, and does not vary with impact angle from normal impacts to at least 75 deg from vertical. In order to improve models of coagulation of dust aggregates in the solar nebula, SJW developed an apparatus for drop tests of fragile projectiles. Davis and Weidenschilling continued to collect and analyze experimental data on collisional catastrophic disruption at the Ames Vertical Gun Range.
    Keywords: GEOPHYSICS
    Type: NASA, Washington, Reports of Planetary Geology and Geophysics Program, 1986; p 388-390
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  • 8
    Publication Date: 2013-08-31
    Description: The fundamental geological and geophysical properties of asteroids were studied by theoretical and simulation studies of their collisional evolution. Numerical simulations incorporating realistic physical models were developed to study the collisional evolution of hypothetical asteroid populations over the age of the solar system. Ideas and models are constrained by the observed distributions of sizes, shapes, and spin rates in the asteroid belt, by properties of Hirayama families, and by experimental studies of cratering and collisional phenomena. It is suggested that many asteroids are gravitationally-bound "rubble piles.' Those that rotate rapidly may have nonspherical quasi-equilibrium shapes, such as ellipsoids or binaries. Through comparison of models with astronomical data, physical properties of these asteroids (including bulk density) are determined, and physical processes that have operated in the solar system in primordial and subsequent epochs are studied.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA-CR-169709 , NAS 1.26:169709
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  • 9
    Publication Date: 2013-08-31
    Description: We use a combination of analytical and numerical methods to study dynamical processes involved in the formation of planets and smaller bodies in the solar system. Our goal was to identify and understand critical processes and to link them in a numerical model of planetesimal accretion. We study effects of these processes by applying them in the context of the standard model of solar system formation, which involves accretion of the terrestrial planets and cores of the giant planet from small planetesimals. The principal focus of our research effort is the numerical simulation of accretion of a swarm of planetesimals into bodies of planetary size. Our computer code uses a Monte Carlo method to determine collisional interactions within the swarm. These interactions are not determined simply by a relative velocity, but rather by explicit distributions of keplerian orbital elements. The planetesimal swarm is divided into a number of zones in semimajor axis, which are allowed to interact. The present version of our code has the capability of following detailed distributions of size, eccentricity, and inclination in each zone.
    Keywords: ASTROPHYSICS
    Type: NASA, Washington, Reports of Planetary Geology and Geophysics Program, 1990; p 433-435
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  • 10
    Publication Date: 2013-08-31
    Description: Asteroids can lose angular momentum due to so called splash effect, the analog to the drain effect for cratering impacts. Numerical code with the splash effect incorporated was applied to study the simultaneous evolution of asteroid sized and spins. Results are presented on the spin changes of asteroids due to various physical effects that are incorporated in the described model. The goal was to understand the interplay between the evolution of sizes and spins over a wide and plausible range of model parameters. A single starting population was used both for size distribution and the spin distribution of asteroids and the changes in the spins were calculated over solar system history for different model parameters. It is shown that there is a strong coupling between the size and spin evolution, that the observed relative spindown of asteroids approximately 100 km diameter is likely to be the result of the angular momentum splash effect.
    Keywords: ASTROPHYSICS
    Type: NASA, Washington, Reports of Planetary Geology and Geophysics Program, 1990; p 399-402
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