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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 325 records · Page 18
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NASA NTRS1978

The rings of Saturn: State of current knowledge and some suggestions for future studies

The state of our current knowledge of the properties of the ring system as a whole, and of the particles individually, is assessed. Attention is primarily devoted to recent results and possibilities for exploration of the ring system by a Saturn orbiter. In particular, the infrared and microwave properties of the ring system are discussed. The behavior of the ring brightness is not well understood in the critical transition spectral region from approximately 100 micrometers to approximately 1 cm. Also, the dynamical behavior of the ring system is discussed. Recent theoretical studies show that ongoing dynamical effects continually affect the ring structure in azimuth (possibly producing the A ring brightness asymmetry) and in the vertical direction. Orbital spacecraft-based studies of the rings will offer several unique advantages and impact important cosmogonical questions. Bistatic radar studies and millimeter-wavelength spectrometer/radiometry will give particle sizes and composition limits needed to resolve the question of the density of the rings, and provide important boundary conditions on the state of Saturn's protoplanetary nebula near the time of planetary formation.

Cuzzi, J. N.↗
NASA NTRS1979

Dynamics of the accumulation process of the Earth group of planets: Formation of the reverse rotation of Venus

A numerical simulation of the process of formation of the terrestrial planets is carried within the framework of a new theory for the accumulation of planetary and satellite systems. The numerical simulation permitted determining the parameters of the protoplanetary disk from which Mercury, Venus and the Earth were formed as result of the evolution. The acquisition of a slow retrograde rotation for Venus was discovered during the course of the investigation, whereas Mercury and the Earth acquired direct rotation about their axes. Deviations of the semimajor axes of these three planets as well as the masses of the Earth and Venus from the true values are small as a rule (l 10%). It is shown that during the accumulation of the terrestrial planets, there existed a profound relationship between the process of formation of the orbits and masses of the planet and the process of formation of their rotation about their axes. Estimates are presented for the radii of the initial effective bodies and the time of evolution for the terrestrial accumulation zone.

Koslov, N. N.↗
NASA NTRS1979

Planetary rings

Two models of planetary ring formation are discussed: the break-up of a large body coming nearer to the planet than the Roche limit for the particular mechanical strength of the body would allow; and condensation of the protoplanetary nebula within a suitable Roche limit without formation of a large satellite. The location of the newly discovered Uranian rings has been interpreted in terms of three-body orbital resonance with satellites. A possible relationship between the origin and characteristics of the Uranian rings and the unusual tilt of the planetary axis is also discussed. In addition, attention is given to the polarization of light characteristic of planetary rings.

Smoluchowski, R.↗
NASA NTRS1979

The ring systems of Jupiter, Saturn and Uranus

The ring systems of Jupiter, Saturn and Uranus are compared, and their origins are discussed. The locations and widths of the rings are considered in relation to various theoretical limits, noting that all the rings lie outside the lower accretion limit and the limits at which incident and orbiting bodies will break up, and inside the upper accretion limit. The ring densities at which the rings lie within their respective Roche limits, however, are found to vary greatly among the various ring systems. The above considerations lead to the conclusions that the rings of Saturn are composed primarily of ice and are the products of condensation from the protoplanetary nebula, while those of Jupiter and Uranus are the products of the breakup of nonspherical bodies.

Smoluchowski, R.↗
NASA NTRS1980

Calculations of the evolution of the giant planets

Evolutionary calculations are presented for spherically symmetric protoplanetary configurations with a homogeneous solar composition and with masses of 1000, 1500, 28,500 and 42,000 solar masses. Recent improvements in equation-of-state and opacity calculations are incorporated. Sequences start as subcondensations in the solar nebula with densities of 10 to the -10th to 10 to the -11th g/cu cm, evolve through a hydrostatic phase lasting 100 thousand to 10 million years, undergo dynamic collapse due to dissociation of molecular hydrogen, and regain hydrostatic equilibrium with densities of about 1 g/cu cm. The nature of the objects at the onset of the final phase of cooling and contraction is discussed and compared with previous calculations.

Bodenheimer, P.↗
NASA NTRS1980

Disk-satellite interactions

The rate at which angular momentum and energy are transferred between a disk and a satellite which orbit a central mass is calculated. It is shown that the angular momentum and energy transfer at Lindblad resonances tends to increase the satellite's orbit to lowest order in eccentricity, whereas the transfer at corotation resonances tends to decrease it. The results are applied to the interaction between Jupiter and the protoplanetary disk. The angular momentum transfer is shown to be so rapid that substantial changes in both the structure of the disk and the orbit of Jupiter must have taken place on a time scale of a few thousand years.

Goldreich, P.↗
NASA NTRS1981

Solar wind origin of Ar-36 on Venus

An examination is conducted concerning the circumstances under which the difference between earth and Venus (and Mars) fits naturally into theories in which the terrestrial planets formed by the gradual sweeping up of planetesimals in an essentially gas-free protoplanetary swarm. The primary purpose of the reported investigation is to use observational data to define restrictions on planetary formation theories that would be imposed if most of Venus' inert gases come from the solar wind. The observational data support the suggestion that the abundances of Ar, Kr, and Xe on Venus have been augmented by a component of solar composition. Solar wind implantation at an early stage of accumulation provides a natural way of producing the observed extreme heliocentric distribution of this component, provided that accumulation occurred after dissipation of solar gas from the solar nebula

Wetherill, G. W.↗
NASA NTRS1983

Ultraviolet observations of M1-2 (= VV 8)

First IUE spectrograms of the peculiar emission-line object M1-2 (= VV 8) are presented. In the wavelength region 1150-3200 A this object is characterized by emission lines superposed on a moderately strong continuum. Some ultraviolet and optical properties of M1-2 are discussed in the context of symbiotic stars and protoplanetary and planetary nebulae. An E(B-V) extinction of 0.37 mag is obtained from the ratio of the He II flux at 1640 and 4686 A and is compared with values derived from other parameters. The ultraviolet observations suggest that M1-2 is a young, high-density planetary nebula of excitation class 5 or 6 with characteristics similar to IC 4997.

Feibelman, W. A.↗
NASA NTRS1985

The Structure and Composition of Uranus and Neptune

Uranus and Neptune form a special class of planetary objects; intermediate in mass and composition between the giant Hydrogen-rich planets, Jupiter and Saturn, and the small, rocky terrestrial planets. Their structure and composition are not only of intrinsic importance, but also should provide information as to the nature of the protoplanetary nebula and the processes of planetary formation. A detailed set of theoretical models of these planets within the framework of two and three shell models was constructed. The ratio of ice to rock (1/r) is varied. The three shell model fits the data on the two planets best.

Reynolds, R. T.↗
NASA NTRS1985

A model for diagenesis in proto-planetary bodies

A model is proposed which eases the thermal constraints imposed by earlier models of low-temperature aqueous alteration in primitive extraterrestrial materials in which formation of an aqueous medium promoting these alterations required complete melting of the water-ice. It is argued that low-temperature alterations in a protoplanetary body such as the formation of hydrated silicates will occur because of the presence of an interfacial water layer.

Rietmeijer, F. J. M.↗
NASA NTRS1986

Debris-cloud collisions: Accretion studies in the space station

The growth of planetesimals in the Solar system reflects the success of collisional aggregation over disruption. Recent experiments performed at the NASA-Ames Vertical Gun Range are discussed using the production of debris cloud impaction to model protoplanetary accretion. The impact experiment assessed the differences between clustered and single body impacts on particulate surfaces. The preliminary results would indicate that collisions between two debris clouds might produce aggregates, thereby increasing particle sizes, whereas a single particle impacting a particle results in disruption and comminution. Such an experiment could provide new insight for early planetary growth processes and for interpreting the record of this stage. The use of the microgravity environment of the Space Station to further the research is discussed.

Schultz, P. H.↗
NASA NTRS1985

Rings and moons - Clues to understanding the solar nebula

Planetary satellites, ranging in size from ring particles to the Galilean moons of Jupiter, constitute a small but diverse compmonent of the solar system. In some respects, the satellite systems of the giant planets can be thought of as miniature solar systems. The chemical and physical properties of these systems can yield clues to the conditions under which they may have formed, and thus can constrain models of the circumplanetary nebulae and the solar nebula. Planetary rings contain a great deal of structure, most strikingly displayed in Voyager spacecraft images. Understanding the causes of this structure can yield important insights into the dynamics of the protoplanetary disk. Taken together, rings and moons offer clues as to the nature of the processes which led to the formation of the planets themselves.

Lissauer, J. J.↗
NASA NTRS1985

IRAS observations of matter around nearby stars

A systematic search of the IRAS point-source catalog has led to the identification of eight new nearby stars that are Vega-like in terms of their large 60-micron excess. These stars are distinguished by the predominance of spectral type A and the absence of double stars in the Vega-like group. Both effects are intuitively consistent with the interpretation that the 60-micron excess radiation is due to a disk of protoplanetary material, suggesting an early phase in the evolution of a planetary system; however, this distribution can also be due to luminosity and brightness selection effects.

Aumann, H. H.↗
NASA NTRS1986

Modeling of infrared flux spectra from disk-shaped interstellar dust clouds

Although there is growing observational and theoretical evidence for many disk-shaped objects of astrophysical interest, spherical geometry is assumed in most radiative transfer models. Recently we generalized the quasi-diffusion method developed by Leung (1975, 1976) for spherical geometry to solve the problem of scattering, absorption, and reemission by dust grains in a medium of disk geometry. The method is applicable to a variety of astronomical sources whose dynamics are angular-momentum dominated and hence not accurately treated by spherical geometry, e.g., protoplanetary nebulae, circumstellar disks, bipolar-flow molecular clouds, accretion disks and disk galaxies. Using this technique and realistic grain opacities we construct theoretical models to determine self-consistently the dust temperature distribution and infrared emission from disk-shaped, quiescent dark globules heated externally by the intersteller radiation field. The effects of the following parameters on the temperature structure and the emergent spectrum are studied: grain type (graphite and silicate), optical depth, density inhomogeneity, and degree of disk flattening. The disk models are characterized by an aspect ratio and an optical depth at 0.55 microns. For inhomogeneous models, a gaussian density distribution is assumed such that the ratio of central to surface density is 100. To study the effects of source geometry, we also compare results for models with spherical and disk geometry. In this case both models have the same radius and central optical depth, the disk models having a 1:1 aspect ratio. While the dust temperature distributions in the two cases are very similar, the emergent flux for the disk model depends sensitively on the viewing angle. For clouds which are unresolved, one would expect, since the thermal emission is isotropic in the neighborhood of an emitting grain, and since the emission (in the far infrared) is optically thin, that the emergent flux spectrum should be characteristic only of the dust temperature, and independent of viewing angle. However, because of the lack of complete symmetry and the resulting radiation anisotropy, this is found not to be the case for the disk models. Angle is that it implies a large uncertainty in estimating the radiation dust mass.

Spagna, G. F., Jr.↗
NASA NTRS1987

Uranus and Neptune: Questions and possible answers

Uranus and Neptune form a special class of planetary objects; intermediate in mass and composition between the giant hydrogen-rich planets of Jupiter and Saturn, and the small, rocky terrestrial planets, their structure and composition are not only of intrinsic importance, but also should provide information regarding the nature of the protoplanetary nebula and the processes of planetary formation. A detailed set of theoretical models of these planets within the framework of two and three shell models was costructed. The ratio of ice to rock was varied. The three shell model fits the data on the two planets best.

Reynolds, R. T.↗
NASA NTRS1987

Rare gas analysis of size fractions from the Fayetteville meteorite

Eight size separates of grains from the Fayetteville meteorite ranging from less than 20 microns to greater than 1 millimeter are being analyzed for their rare gas elemental and isotopic composition. Measurements on five samples were performed. All five reveal a mixture of solar, planetary, cosmic ray produced and radiogenic gases. The solar component is of particular interest since it suggest that the meteorite may represent a fragment of an ancient protoplanetary regolith which was exposed to the solar wind. Solar wind elements are implanted in the outer few hundred Angstroms of exposed grains and are therefore expected to be surface correlated. At present the data do not suggest that such a correlation exists, but the final conclusion must await further analyses and data reduction.

Jordan, Jim L.↗
NASA NTRS1987

The relationship of active comets, 'extinct' comets, and dark asteroids

The possibility of distinguishing between outer solar system asteroids and dark cometary objects is discussed. Planetesimals that could be comets in various evolutionary phases have been detected at distances greater than about 3 AU. Comparisons are made among observed planetesimal and cometary albedos and spectra. It is noted that carbonaceous material, assumed to originate from comets, has been captured in the stratosphere. The data indicate that extinct or dormant comets have loose aggregate surface structures consisting of reddish organic compounds. A protoplanetary nebular condensation scenario is discussed which would have produced the dark cometary bodies.

Hartmann, William K.↗
NASA NTRS1987

Numerical solution of the radiation transport equation in disk geometry

An efficient numerical method for solving the problem of radiation transport in a dusty medium with two dimensional (2-D) disk geometry is described. It is a generalization of the one-dimensional quasi-diffusion method in which the transport equation is cast in diffusion form and then solved as a boundary value problem. The method should be applicable to a variety of astronomical sources, the dynamics of which are angular-momentum dominated and hence not accurately treated by spherical geometry, e.g., protoplanetary nebulae, circumstellar disks, interstellar molecular clouds, accretion disks, and disk galaxies. The computational procedure and practical considerations for implementing the method are described in detail. To illustrate the effects of 2-D radiation transport, some model results (dust temperature distributions and IR flux spectra) for externally heated, interstellar dust clouds with spherically symmetric and disk geometry are compared.

Spagna, George F., Jr.↗