Engineering topics

Greenberg, R.

Publications and source records attributed to Greenberg, R..

At least 55 records · Page 3

NASA NTRS1984

Ring particles - Collisional interactions and physical nature

Attention is given to the properties of, and dynamical processes affecting individual particles of Saturn's rings. Because particles tend to be gravitationally bound when located on the surfaces of larger particles, and since net tidal stresses within the particles are small, particle collisions should produce accretion in Saturn's rings. Rapid accretionary processes within the rings are counterbalanced by tidal disruption of the larger accreted aggregates, which are presently designated 'dynamic ephemeral bodies'. The coefficient of restitution is probably very low, implying that the large particles containing most of the rings' mass are in a monolayer, although the small particles responsible for most of the rings' visible cross section form a layer many particles thick. Kinematic viscosity and interparticle erosive process models should incorporate these properties.

Weidenschilling, S. J.↗

NASA NTRS1983

Asteroids and meteorites - Parent bodies and delivered samples

Asteroid physical strengths, which critically affect the quantity of ejecta that can be placed in orbits evolving so as to cross that of the earth, vary widely due to initial composition and size, and subsequent geophysical and collisional evolutions. The meteorite yield on earth additionally depends on meteorite strength, which affects longevity in space and survival through the atmosphere. It is presently shown that meteorites may be primarily derived by cratering rather than disruptive fragmentation, and from large, main-belt asteroids rather than from small, earth-approaching bodies. The model presented combines a variety of evidence from various disciplines to yield results that are consistent with meteorite statistics, although no claim is made for the uniqueness of the model and many of its elements remain uncertain.

Greenberg, R.↗

NASA NTRS1983

The role of dissipation in shepherding of ring particles

A discussion is presented on the gravitational interaction between ring systems and nearby satellites. A shepherd satellite lacking damping mechanisms will force oscillations in the motion of a ring particle that are symmetrical with respect to the encounter geometry. If such damping mechanisms as density wave propagation or a dissipative medium are present, a lag in particle response provides the asymmetry that exerts a net torque on the rings. While the torque on a given particle depends on the degree of damping, that dependence disappears when the torque is averaged over a range of orbits spanning resonances if the degree of damping is within a certain range. A torque that is much lower than the standard formula results from excessively weak or strong damping.

Greenberg, R.↗

NASA NTRS1982

Orbital resonances and planetary formation sites

A cascaded resonance structure where planetesimal growth was accelerated at 2:1 interior and 1:2 exterior resonances, with an early-formed Jupiter producing runaway growth of planetary embryos, is hypothesized in a solar system formation model. The planetary embryos produce their own resonances, and these in turn lead to additional embryos in a process that successively propagates inwardly and outwardly to generate a resonant configuration of embryos. The early presence of Jupiter would in this way have imposed a harmonic structure on the accumulating planetesimal swarm. The positions of the planetary embryos can be moved into a degree of agreement with most of the present planetary positions which is comparable to that given by the Titius-Bode law, for the case of an accretion disk whose surface density obeys a power law of index -1.2.

Torbett, M.↗

NASA NTRS1982

Orbital interactions - A new geometrical formalism

The geometry of encounters between two bodies on independent Keplerian orbits around a third body is considered by a novel analysis, which avoids approximations made in previous studies. For the case of most of the applications considered, the formulas for collision frequencies and orbital element rates of change due to close approaches which comprise the method agree with past results. It is shown that the method can be extended to such other applications as the consideration of oscillations in orbital elements due to secular perturbations, and the computation of probabilities of escape from the system.

Greenberg, R.↗

NASA NTRS1982

Planetary geology: Impact processes on asteroids

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.

Chapman, C. R.↗

NASA NTRS1982

Orbital evolution of the Galilean satellites

The orbital motions of the Galilean satellites exert dramatic control over their physical properties (most notably Io's) through tidal heating. In turn, tidal dissipation in the satellites, as well as in Jupiter, has governed the evolution of the orbits and, in particular, of the Laplace resonance. If the system started out of the resonance and evolved into it, forced eccentricities would have increased with time. Hence, the tidal melting of Io and the cracking of Europa'a surface may have occurred relatively recently. This theory requires that Jupiter's tidal dissipation factor be greater than about two million, a rather low value (high rate of tidal dissipation) compared with most models of Jovian interior processes. Alternatively, the system may have started even deeper in the resonance than it is today, a scenario which is consistent with larger values of the tidal dissipation factor. This model, with its correspondingly large initial forced eccentricities, would imply (1) that Io melted early and fast, and may have remained molten with only a thin solid skin until the present and (2) that the water mantles of both Europa and Ganymede remained largely molten for considerably longer than Callisto did, but later froze as their eccentricities and tidal heating decreased.

Greenberg, R.↗

NASA NTRS1981

The unusual dynamical environment of Phobos and Deimos

A three-dimensional numerical model is used to study the dynamical environment of Phobos and Deimos. Surface gravity, escape speeds, and ejecta impact contours are calculated both for the satellites at their present orbit distances, and for orbit distances they may have had in the past. Impact loci for Stickney ejecta are also calculated and compared with the observed groove locations in order to evaluate a possible secondary-impact origin for the grooves on Phobos. Attention is also given to the possible influence of the dynamical environment on shaping the satellites' surfaces.

Davis, D. R.↗

NASA NTRS1981

Apsidal precession of orbits about an oblate planet

It is noted that expressions for the apsidal precession rates to second order on J2 appear in the literature in at least three apparently mutually contradictory forms. The expressions are reconciled by accounting for subtle differences in the definitions of orbital elements.

Greenberg, R.↗

NASA NTRS1981

Tidal evolution of the Galilean satellites - A linearized theory

The Laplace resonance among the Galilean satellites Io, Europa, and Ganymede is traditionally reduced to a pendulum-like dynamical problem by neglecting short-period variations of several orbital elements. However, some of these variations that can now be neglected may once have had longer periods, comparable to the 'pendulum' period, if the system was formerly in deep resonance (pairs of periods even closer to the ratio 2:1 than they are now). In that case, the dynamical system cannot be reduced to fewer than nine dimensions. The nine-dimensional system is linearized here in order to study small variations about equilibrium. When tidal effects are included, the resulting evolution is substantially the same as was indicated by the pendulum approach, except that evolution out of deep resonance is found to be somewhat slower than suggested by extrapolation of the pendulum results. This slower rate helps support the hypothesis that the system may have evolved from deep resonance.

Greenberg, R.↗

NASA NTRS1980

Phobos and deimos: Analysis of surface features, ejecta dynamics and a volatile loss mechanism

The question of whether the crater population on Phobos represents a production population or an equilibrium population is considered. The absolute ages of cratered surfaces are interpreted and analyzed. A computer program was developed to study the dynamics of material ejected from Martian satellites and to investigate the hypothesis that at least some of the extensive set of linear features discovered on the surface of Phobos could be the result of secondary cratering from the Stickney impact. The possibility that Deimos was catastrophically disrupted by a large impact but subsequently reaccreted is considered as well as the probability the Phobos had an impact nearly large enough to disrupt it are also discussed.

Davis, D. R.↗

NASA NTRS1980

Collisional growth of planetesimals

Safronov's (1972) demonstration that relative velocities of planetesimals would be comparable to the dominant size bodies' escape velocities, combined with a plausible size distribution that has most mass in the largest bodies, yielded his evolution model with limited growth of the largest planetesimal with respect to its next largest neighbors. A numerical simulation of planetesimal accretion (Greenberg et al., 1978) suggests that at least over one stage of collisional accretion, velocities were much lower than the escape velocity of the largest bodies, because the bulk of the mass still resided in km-scale bodies. The low velocities at this early stage may conceivably have permitted early runaway growth, which, in turn, would have kept the velocities low and permitted continued runaway growth of the largest bodies.

Greenberg, R.↗

NASA NTRS1980

The optimization air separation plants for combined cycle MHD-power plant applications

Some of the design approaches being employed during a current supported study directed at developing an improved air separation process for the production of oxygen enriched air for magnetohydrodynamics (MHD) combustion are outlined. The ultimate objective is to arrive at conceptual designs of air separation plants, optimized for minimum specific power consumption and capital investment costs, for integration with MHD combined cycle power plants.

Juhasz, A. J.↗

NASA NTRS1979

Asteroidal regoliths

A physical model is developed for the evolution of regoliths on small bodies and applied to the asteroid and meteorite parent bodies. The model considers global deposition of that fraction of cratering ejecta that is not lost to space. In addition, it is applied to cases of both strong, cohesive bodies and to bodies of weak, unconsolidated materials. It is found that large, strong asteroids generate surficial regoliths of a few kilometers depth while strong asteroids smaller than 10-km diameter generate negligible regoliths. In conclusion, it is noted that the theory that substantial regoliths are produced predominantly by blanketing differs from earlier hypotheses that asteroidal regoliths might be thin or absent and that short surface exposure of asteroidal materials is due chiefly to erosion rather than blanketing.

Housen, K. R.↗

NASA NTRS1979

Growth of large, late-stage planetesimals

The paper discusses planetesimals which may have been produced without perturbations by Venus and changes that can be made in Wetherill's model (1976) of growth of large planetisimals. The late stage of terrestrial planets' growth determined their thermal properties and petrology, impact records, and possibly the existence of the moon. A critical result of late-stage models is the size of the largest planetesimals that grew near, and later impacted, those that became full-size planets. The relation between the size of planetesimals and their relative velocities has been misinterpreted, and some models neglect the possible decrease in relative velocity as control is transferred from the largest to the second-largest body in an accretion zone. The size of the second-largest planetesimal in the earth's zone may range from 300 to 2500 km, with corresponding accretion times of 7 times 10 to the 6th and 10 to the 8th power, respectively.

Greenberg, R.↗

NASA NTRS1979

Collisional evolution of asteroids - Populations, rotations, and velocities

The collisional evolution of various initial populations of asteroids is simulated numerically and compared with the present asteroid size-frequency distribution to find those populations which collisionally relax to the present belt. Both orbital and size distributions are treated, as well as the simultaneous evolution of two collisionally interacting populations with different physical properties. If the initial belt distribution was a power law, the initial belt population at the time when the present high-collision speed was established was probably only modestly larger than the present population. However, other distributions allow a more massive early belt. The rotational evolution due to collisions of asteroids with power-law distributions is also examined and compared with observations, leading to conclusions generally in agreement with those of size evolution. The high-collision speed in the present belt is likely due to Jupiter. Gravitational stirring by massive Jupiter-scattered planetesimals or secular resonances sweeping through the belt are the most probable mechanisms.

Davis, D. R.↗

NASA NTRS1978

Planetesimals to planets - Numerical simulation of collisional evolution

In a simulation of collisional and gravitational interaction in the early solar system, planets of approximately 500 km diameter are generated from an initial swarm of kilometer-sized planetesimals. Collisions are treated in accordance with experimental and theoretical impact results (such as rebound, cratering, and catastrophic fragmentation) for a variety of materials whose parameters span plausible values for early solid objects. In this model, the small planets form in approximately 10,000 yr; during this time, most of the mass of the system continues to reside in particles near the original size. It is thought that the few 500-km planets may act as 'seeds' for the subsequent gradual accretional growth into full-sized planets.

Greenberg, R.↗

NASA NTRS1978

Orbital resonance in a dissipative medium

Orbital characteristics of solar system bodies are reviewed with reference to their degree of circularity or eccentricity. Attention is given to orbital perturbations in the vicinity of Jupiter, Saturn, and the Asteroid Belt. The trapping of orbital periods is explained on the basis of relative velocity and gravitational interaction between particles. Enhancement of orbital eccentricity through resonance with other bodies is dampened by the drag-effect of small particles in the orbital path. This mechanism is seen operating in interactions between Titan and Hyperion, and between individual asteroids in the vicinity of Jupiter; with implications for the accretion dynamics of a proto-planetary nebula.

Greenberg, R.↗