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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1975

Collisional breakup of particles in a planetary ring

Jeffreys (1947) estimated the size of fragments resulting from breakup of a satellite inside the Roche limit, obtaining a result of about 100 km. This result does not allow for the further breakup of the fragments due to collisions among themselves, which should reduce the maximum size to less than about 3 km for rock, or less than about 1 km for ice. This result affects not only Jeffreys' speculations as to the origin of Saturn's rings, but also recent speculations on the origin of the moon by capture and the possible tidal destruction of satellites of Mercury or Venus.

Harris, A. W.↗
NASA NTRS1975

Asteroid collisional evolution - Evidence for a much larger early population

The present population of asteroids is a remnant of a vastly larger one that contained perhaps a planetary mass, dominantly distributed in planetesimals approximately 500 kilometers or less in diameter. It constituted a large reservoir of objects that plausibly were responsible for cratering the moon, Mars, and Mercury. Much asteroidal dust may have accumulated on Mars and other planets.

Chapman, C. R.↗
NASA NTRS1975

On the alleged collisional origin of the Kirkwood Gaps

This paper examines two proposed mechanisms whereby asteroidal collisions and close approaches may have given rise to the Kirkwood Gaps. The first hypothesis is that asteroids in near-resonant orbits have markedly increased collision probabilities and so are preferentially destroyed, or suffer decay in population density, within the resonance zones. A simple order-of-magnitude analysis shows that this hypothesis is untenable since it leads to conclusions which are either unrealistic or not in accord with present understanding of asteroidal physics. The second hypothesis is the Brouwer-Jefferys theory that collisions would smooth an asteroidal distribution function, as a function of Jacobi constant, thus forming resonance gaps. This hypothesis is examined by direct numerical integration of 50 asteroid orbits near the 2:1 resonance, with collisions simulated by random variables. No tendency to form a gap was observed.

Heppenheimer, T. A.↗
NASA NTRS1978

Collisional excitation of interstellar molecules - H2

Cross sections for the important rotational transitions in molecular hydrogen collisions are presented for rotational levels to j = 11 and energies to 20,000 K. These have been obtained from extensive theoretical calculations. The resulting cross sections are fitted to a simple polynomial in the energy, and the coefficients are tabulated for easy utilization.

Green, S.↗
NASA NTRS1978

Collisional excitation of interstellar molecules - Linear molecules CO, CS, OCS, and HC3N

Rates for excitation of CO, CS, OCS, and HC3N by collisions with He atoms and H2 molecules are presented. These have been obtained from extensive theoretical calculations using methods of known reliability. In general, rates necessary for modeling interstellar molecular clouds with kinetic temperatures to 100 K have been included. Methods for fitting these rates in more compact form are discussed.

Green, S.↗
NASA NTRS1978

Collisional excitation of interstellar formaldehyde

Previous calculations for rates of excitation of ortho-H2CO by collisions with He have been extended to higher rotational levels and kinetic temperatures to 80 K. Rates for para-H2CO have also been computed. Pressure-broadening widths for several spectral lines have been obtained from these calculations and are found to agree with recent data within the experimental uncertainty of 10%. Excitation of formaldehyde by collisions with H2 molecules is also discussed.

Green, S.↗
NASA NTRS1978

The collisional interaction of a beam of charged particles with a hydrogen target of arbitrary ionization level

The classical theory of scattering under the Coulomb potential of both charged and neutral particles is used to derive formulae for the energy deposition rate and mean scattering of a beam of charged particles interacting with a cold hydrogen target of arbitrary ionization level as a function of the column density traversed by the beam. These general results hold for any form of stable injection energy spectrum, and their relevance to the existing literature on chromospheric heating during solar flares is discussed.

Emslie, A. G.↗
NASA NTRS1978

Collisional evolution of mass-distribution spectrum of planetesimals

The early growth of planetesimals by mutual direct collisions is numerically simulated with a Monte Carlo technique to show how planetesimals with specific mass and velocity distributions evolve into a full-size planet. Four types of collisions are taken into account: rebound, erosion, catastrophic break-up, and coagulation. It is shown that evolution of mass-distribution spectrum is expressed by an inverse power relation. It is also suggested that whether or not a planetesimal can survive catastrophic collision is primarily dependent on mean relative velocity and mechanical properties of planetesimals. It is necessary for the early growth of rocky (basaltic) materials that mean relative velocity be much smaller than 0.1 km/s. Otherwise it is necessary to introduce something like nucleating agents (such as iron bodies which have plastic properties at temperatures higher than 200 K) for the formation of terrestrial planets.

Matsui, T.↗
NASA NTRS1979

Collisional evolution of the mass-distribution spectrum of planetesimals. II

An investigation of conditions for the early growth of planetesimals through their mutual collisions using Monte Carlo technique is presented. Rebound, erosion, catastrophic breakup, and coagulation are considered, mean random velocity is assumed to vary with time, and growth of km-sized planetesimals is shown to occur only when they have the mechanical properties similar to iron meteorites. The formation time of several hundred-km sized planetesimals is much longer than previously reported by Greenberg (1978), and an expression was derived for the temporal variation of the mean mass.

Matsui, T.↗
NASA NTRS1980

Collisional excitation of interstellar molecules - Water

Rate constants for excitation of water molecules by collisions with He atoms have been obtained from extensive theoretical calculations. For ortho-H2O, rates are presented for all spectroscopic levels below 1000/cm and for kinetic temperatures to 500 K; for para-H2O, levels below 300/cm and kinetic temperatures to 250 K are included. It is suggested that rates of excitation by collisions with hydrogen molecules are similar to those for He atoms.

Green, S.↗
NASA NTRS1980

The excitation and collisional deactivation of metastable N/2P/ atoms in auroras

The concentration and altitude distribution of metastable N(2P) atoms was measured in a diffuse IBC II(+) auroral arc. The dominant N(2P) source is shown to be the dissociative excitation of N2 by electron impact with a minor contribution from the dissociative recombination of N2(+) ions. The possibility that an ion-molecule process involving atomic oxygen and vibrationally excited N2(+) ions is a significant N(2P) source is examined. Values for the proportional yield of N(+), N(2P), N(2D), and N(4S) atoms from electron-impact dissociation of N2 under optically thick conditions are given.

Zipf, E. C.↗
NASA NTRS1980

Collisional redistribution of radiation. II - The effects of degeneracy on the equations of motion for the density matrix. III - The equation of motion for the correlation function and the scattered spectrum

The effect of correlations between an absorber atom and perturbers in the binary-collision approximation are applied to degenerate atomic systems. A generalized absorption profile which specifies the final state of the atom after an absorption event is related to the total intensities of Rayleigh scattering and fluorescence from the atom. It is suggested that additional dynamical information to that obtainable from ordinary absorption experiments is required in order to describe redistributed atomic radiation. The scattering of monochromatic radiation by a degenerate atom is computed in a binary-collision approximation; an equation of motion is derived for the correlation function which is valid outside the quantum-regression regime. Solutions are given for the weak-field conditions in terms of generalized absorption and emission profiles that depend on the indices of the atomic multipoles.

Burnett, K.↗
NASA NTRS1980

Collisional redistribution of radiation. III - The equation of motion for the correlation function and the scattered spectrum

Computations were made of the scattering of monochromatic radiation by a degenerate atom in the binary-collision approximation for field strengths whose products of the Rabi frequency for atomic transition and the duration of a strong collision are much less than 1. An expression of motion for the correlation function is derived which does not exclude the region where thermal correlations may be neglected; the equation is valid outside the quantum-regression regime, and has a straightforward solution for practical cases. Solutions for the weak-field linear response regime are presented in terms of generalized absorption and emission profiles which depend on the indices of the atomic multipoles.

Burnett, K.↗