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NASA NTRS1993

Deviations from the straight line: Bumps (and grinds) in the collisionally evolved size distribution of asteroids

Dohnanyi established that the size distribution of a collisionally relaxed population is a power-law with a -3.5 slope (incremental diameter), provided that: (1) collisional parameters are independent of size and (2) there is no lower limit on the population size for shattering impacts. In the course of studying collisional effects over a wide range of asteroid sizes (cm up to hundreds of km range), we have investigated the consequences of relaxing the second condition, while maintaining the first one.

Davis, D. R.↗

NASA NTRS1989

Collisional dynamics of perturbed particle disks in the solar system

Theoretical and computational studies were carried out on galactic and planetary disks. With the goal of addressing important open questions centered on galactic structure, the cloudy interstellar medium, giant molecular clouds, and star formation in galactic disks and the collisional dynamics of perturbed particulate matter in planetary disks, focus was largely on the self-gravitational effects, dissipative effects, and collisional dynamics of cloud-particle disks. N-body, 'cloud-particle' computational algorithms were developed for the purpose of isolating the role of gaseous self gravity from the roles of other dominant physical mechanisms and dynamical processes, e.g. the collisional dynamics and dissipative processes. The efforts focused largely on galactic disks show that observational constraints provide stringent tests of the numerical simulation techniques developed. Self gravitational effects of the galactic interstellar medium's gas clouds are included by means of Fourier Transform technique.

Roberts, William W., Jr.↗

NASA NTRS1994

Collisional records in LL-chondrites

One third of all the LL-chondrites have exposure ages of approx. 15 Ma and were exposed to cosmic rays following a collisional break-up. The probability that the 15-Ma peak represents a random signal is calculated to be less than 2%. Considerably lower probabilities are obtained if only LL5s or subgroups of high Ar-40 retention are used. Furthermore, we show that the peak shape agrees with statistical constraints obtained from multiple analyses of samples from the St. Severin LL6-chondrites. The frequency in and out of the 15-Ma peak varies significantly for different petrographic LL-types. The radiogenic Ar-40 retention systematics (most LL-chondrites retained Ar-40(sub rad) shows that no substantial heat pulse resulted in the 15-Ma collisional event. Interestingly, smaller exposure age clusters at approx. 28 Ma and approx. 40 Ma match up well with clusters in the histogram of L-chondrites. The distribution of LL-exposure ages is not consistent with that expected for a quasi-continuous injection of LL material into a resonance zone of the asteroid belt. The near absence of exposure ages shorter than 8 Ma either indicates a lack of recent collisional events or considerably longer transfer times than inferred from dynamical considerations.

Graf, Thomas↗

NASA NTRS2004

Aeolian Sand Transport with Collisional Suspension

Aeolian transport is an important mechanism for the transport of sand on Earth and on Mars. Dust and sand storms are common occurrences on Mars and windblown sand is responsible for many of the observed surface features, such as dune fields. A better understanding of Aeolian transport could also lead to improvements in pneumatic conveying of materials to be mined for life support on the surface of the Moon and Mars. The usual view of aeolian sand transport is that for mild winds, saltation is the dominant mechanism, with particles in the bed being dislodged by the impact of other saltating particles, but without in-flight collisions. As the wind becomes stronger, turbulent suspension keeps the particles in the air, allowing much longer trajectories, with the corresponding increase in transport rate. We show here that an important regime exists between these two extremes: for strong winds, but before turbulent suspension becomes dominant, there is a regime in which in-flight collisions dominate over turbulence as a suspension mechanism, yielding transport rates much higher than those for saltation. The theory presented is based on granular kinetic theory, and includes both turbulent suspension and particle-particle collisions. The wind strengths for which the calculated transport rates are relevant are beyond the published strengths of current wind tunnel experiments, so these theoretical results are an invitation to do experiments in the strong-wind regime. In order to make a connection between the regime of saltation and the regime of collisional suspension, it is necessary to better understand the interaction between the bed and the particles that collide with it. This interaction depends on the agitation of the particles of the bed. In mild winds, collisions with the bed are relatively infrequent and the local disturbance associated with a collision can relax before the next nearby collision. However, as the wind speed increases, collision become more frequent and the agitation need not decay completely. In the regime of collisional suspension, the particles near the surface of the bed are assumed to be in a state of constant agitation. We indicate the conditions at the bed corresponding to the limits of saltation and collisional suspension and outline experiments, simulations, and modeling that have been undertaken to bridge these limits.

Jenkins, James T.↗

NASA NTRS2008

Collisional Processing of Olivine and Pyroxene in Cometary Dust

According to the nebular theory of solar-system formation, collisions between bodies occurred frequently early in the solar system s history and continue at a lower rate even today. Collisions have reworked the surface compositions and structures of cometary nuclei, though to an unknown degree. The majority of the collisional history of a typical Jupiter-family comet takes place while it resides in the Kuiper Belt. Impacts occur on the surfaces of small bodies over a large range of velocities by impactors of all sizes, but typical encounter speeds within the Kuiper Belt are 1.5 to 2.0 km/s[1]. Durda and Stern suggest that the interiors of most cometary nuclei with diameters <5 km have been heavily damaged by collisions [2]. They estimate that over a period of 3.5 Gy, a nucleus with a diameter of 2 km and an orbit between 35-45 AU will experience 90-300 collisions with objects greater than 8 m in diameter. In this same time interval, collisions between a typical Trans-Neptunian Object (TNO) 200 km in diameter and objects with d > 8 m would rework up to one-third of that TNO s surface. In fact, it has been proposed that most short-period comets from the Kuiper Belt (90%) are collisional fragments from larger TNOs - not primordial objects themselves [3] - and that most short-period comets from the Kuiper Belt will be collisionally processed both on their surfaces as well as in their interiors.

Lederer, S. M.↗

NASA NTRS2011

A Collisional Algorithm for Modeling Circumstellar Debris Disks

Many planetary systems harbor circumstellar disks of dust and planetesimals thought to be debris left over from planet formation. These debris disks exhibit a range of morphological features which can arise from the gravitational perturbations of planets. Accurate models of these features, accounting for the interactions of the particles in a disk with each other and with whatever planets they contain, can act as signposts for planets in debris disks that otherwise could not be detected. Such models can also constrain the planet's mass and orbital parameters. Current models for many disks consider the gravitational and radiative effects of the star and planets on the disk, but neglect the morphological consequences of collisional interactions between the planetesimals. Many observed disk features are not satisfactorily explained by the current generation of models. I am developing a new kind of debris disk model that considers both the gravitational shaping of the disk by planets and the inelastic collisions between particles. I will use a hybrid N-body integrator to numerically solve the equations of motion for the particles and planets in the disk. To include the collisional effects, I begin with an algorithm that tests for collisions at each step of the orbit integration and readjusts the velocities of colliding particles. I am adapting this algorithm to the problem at hand by allowing each particle to represent a "swarm" of planetesimals with a range of masses. When the algorithm detects an encounter between swarms, two or three swarms are produced to approximate the range of possible trajectories of the daughter planetesimals. Here I present preliminary results from my collisional algorithm.

Nesvold, Erika↗

NASA NTRS2023

Anterograde Collisional Analysis of Solar Wind Ions

Owing to its low density and high temperature, the solar wind frequently exhibits strong departures from local thermodynamic equilibrium, which include distinct temperatures for its constituent ions. Prior studies have found that the ratio of the temperatures of the two most abundant ions—protons (ionized hydrogen) and α-particles (ionized helium)—is strongly correlated with the Coulomb collisional age. These previous studies, though, have been largely limited to using observations from single missions. In contrast, this present study utilizes contemporaneous, in situ observations from two different spacecraft at two different distances from the Sun: the Parker Solar Probe (PSP; r = 0.1–0.3 au) and Wind (r = 1.0 au). Collisional analysis, which incorporates the equations of collisional relaxation and large-scale expansion, was applied to each PSP datum to predict the state of the plasma farther from the Sun at r = 1.0 au. The distribution of these predicted α–proton relative temperatures agrees well with that of values observed by Wind. These results strongly suggest that, outside of the corona, relative ion temperatures are principally affected by Coulomb collisions and that the preferential heating of α-particles is largely limited to the corona.

Solar wind↗

NASA NTRS1972

Interstellar formaldehyde. I - The collisional pumping mechanism for anomalous 6-centimeter absorption.

Investigation of the quantum mechanics of the collisional pumping process which Townes and Cheung (1969) propose as the cause of 'anomalous' formaldehyde absorption in diffuse dark nebulae discussed by Palmer et al. (1969). Quantum effects are taken into account in an attempt to determine whether such nebulae are likely to provide the physical conditions required for the collisional pumping process.

Thaddeus, P.↗

NASA NTRS1978

Spectral line profiles in a planetary corona - A collisional model

An attempt is made to report on an extension of the theory of spectral line profiles for a planetary corona to include the effects of H-H(+) resonant charge exchange in which satellite particles are omitted. A computational procedure is outlined, three classes of particle orbits are distinguished (ballistic, satellite, escaping), and several complications are considered. A collisional profile is obtained, and appropriate collisionless and collisional curves are compared. It is concluded that good profiles could disclose the presence of a nonthermal hydrogen component. Effects of omitting satellite particles are briefly discussed.

Prisco, R. A.↗

NASA NTRS1978

The radial gradients and collisional properties of solar wind electrons

The plasma electron detector on Mariner 10 is used to obtain measurements of electron density and temperature in the interplanetary medium between heliocentric distances of 0.85 and 0.45 AU. The observations show quantitatively that the core of the electron distribution function can be described as collisional at least for radial distances within 1 AU, since with a very few well-marked exceptions associated with high-speed streams, the Coulomb collisional momentum relaxation length is less than the density scale height at all times and all radial distances at which data were obtained. It is found that the Coulomb energy exchange collisions between the core and the (test) halo population are negligible. The power law exponent of the core temperature is about -0.3, whereas the halo temperature is almost independent of heliocentric distance.

Ogilvie, K. W.↗

NASA NTRS1979

Asteroid rotation. I - Tabulation and analysis of rates, pole positions and shapes. II - A theory for the collisional evolution of rotation rates

Rotation properties and shape data for 182 asteroids are compiled and analyzed, and a collisional model for the evolution of the mean rotation rate of asteroids is proposed. Tabulations of asteroid rotation rates, taxonomic types, pole positions, sizes and shapes and plots of rotation frequency and light curve amplitude against size indicate that asteroid rotational frequency increases with decreasing size for all asteroids except those of the C or S classes. Light curve data also indicate that small asteroids are more irregular in shape than large asteroids. The dispersion in rotation rates observed is well represented by a three dimensional Maxwellian distribution, suggestive of collisional encounters between asteroids. In the proposed model, the rotation rate is found to tend toward an equilibrium value, at which spin-up due to infrequent, large collisions is balanced by a drag due to the larger number of small collisions. The lower mean rotation rate of C-type asteroids is attributed to a lower means density of that class, and the increase in rotation rate with decreasing size is interpreted as indicative of a substantial population of strong asteroids.

Harris, A. W.↗

NASA NTRS1980

Collisional narrowing by polyatomic buffer gases in an optically pumped CH3F laser

The gain linewidth of an optically pumped CH3F molecular laser is observed with the addition of various polyatomic buffer gases. This is interpreted as collisional (Dicke) narrowing. The measurement is the first observation of collisional narrowing by polyatomic buffer gases. It is also the first observation of the effect in a laser oscillator. The effect was observed using a heterodyne mixing technique at the laser emission frequency of 604 GHz. Collision cross sections for SF6-CH3F and CS2-CH3F are obtained.

Lawandy, N. M.↗

NASA NTRS1981

Collisional radiative recombination in high-pressure noble gas mixtures

Rate coefficients are calculated for neutral-assisted collisional radiative recombination in dense, weakly ionized rare-gas mixtures using the quasiequilibrium model developed by Bates. It is shown that the recombination rate can be significantly altered by the presence of a buffer gas. In particular, the presence of helium can enhance the collisional radiative recombination rate coefficient of heavy rare gases by a factor of 5.

Whitten, B. L.↗

NASA NTRS1982

Collisional redistribution of radiation in strong fields - Modification of the collision dynamics

The theory of light scattering from an atom undergoing collisions is extended to the case of strong laser fields. In particular, the conditions necessary to relate the observable quantities to intense-field collisional rate constants in a dressed-state basis are considered. It is shown that by studying the spectrum emitted by the atoms in the presence of a strong field one may measure the collisional rates for transfer between the dressed states of atom plus radiation field.

Burnett, K.↗

NASA NTRS1982

Dissipation of ionospheric irregularities by wave-particle and collisional interactions

The nonlinear dissipation of plasma irregularities aligned parallel to an ambient magnetic field is studied numerically using a model which employs both wave-particle and collisional diffusion. A wave-particle diffusion coefficient derived from a local theory of the universal drift instability is used. This coefficient is effective in regions of nonzero plasma gradients and produces triangular-shaped irregularities with spectra which vary as f to the -4th, where f is the spatial frequency. Collisional diffusion acts rapidly on the vertices of the irregularities to reduce their amplitude. The simultaneous action of the two dissipative processes is more efficient than collisions acting alone. In this model, wave-particle diffusion mimics the forward cascade process of wave-wave coupling.

Bernhardt, P. A.↗

NASA NTRS1983

Collisional rates for vibrational-rotational transitions in circumstellar SiO masers

Calculations for rotational population inversion in circumstellar SiO masers require collisional rates for vibrational-rotational transitions. This paper reports quantum mechanical, state-to-state collisional rate coefficients for pure rotational and vibrational-rotational transitions among the first three vibrational states of SiO. These are expressed in terms of simple two-parameter power-law fits accurate in the temperature range 1000-3000 K.

Bieniek, R. J.↗

NASA NTRS1984

Collisional narrowing effects in the Raman Q-branch spectral profiles of N2, CO, and NO

A description is presented of coherent Raman spectroscopic studies of N2, CO, and NO molecules. It is pointed out that the Q-branch Raman spectra of these simple diatomics are characterized by lines which overlap with increasing pressure as a result of collisional line broadening. Attention is given to Q-branch vibrational spectra and their determination, the spectral line shape of isolated lines, the line overlap regime, fitting laws, and the isotropic Raman spectrum of NO. It is found that the room temperature spectra of the considered molecules at low pressure (approximately 0.2 atm) may be treated as a sum of isolated lines. It is possible to extract J-dependent pressure-broadening coefficients. With increasing pressure, collisional narrowing occurs and the description of the spectrum must include the effects of interference among neighboring lines.

Hurst, W. S.↗

NASA NTRS1985

Asteroid Collisional Evolution Studies

Understanding asteroid collisional evolution is important for characterizing the physical state of asteroids today and for learning about the processes that acted in this region of the solar system early in its history. The collisional outcome algorithm in the numerical simulation of asteroid evolution was revised to reflect pressure-strengthening. Asteroid collisions are now treated as a distribution of oblique impacts rather than as only head-on collisions. The initial and evolved size distribution of a plausible asteroid population is compared with the observed size distribution. Asteroid accretion times and reconstruction of the primordial solar nebula suggest that there was significantly more mass in this part of the solar system when the asteroids were accreting.

Davis, D. R.↗