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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
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NASA NTRS1986

Radiative-convective equilibrium models of Uranus and Neptune

The present study of Uranus and Neptune radiative-convective equilibrium models gives emphasis to such aspects of the stratospheric energy balance as the influence of aerosol heating and convective penetration. The results obtained for Uranus imply that a 'continuum absorber' may be a significant factor in the stratosphere despite the great distance from the sun. The results obtained for Neptune show that such a continuum absorber could significantly contribute to the energy balance within a localized stratospheric region, although it probably cannot furnish sufficient power to account for the observed IR spectrum irrespective of its vertical distribution. Attention is accordingly given to the 'convective penetration' that could arise under such rapid vertical mixing that CH4's condensation cannot occur before the gas is carried above the condensation region.

Appleby, J. F.↗
NASA NTRS1986

Radiative-convective equilibrium models of Uranus and Neptune

The present study of Uranus and Neptune radiative-convective equilibrium models gives emphasis to such aspects of the stratospheric energy balance as the influence of aerosol heating and convective penetration. The results obtained for Uranus imply that a continuum absorber may be a significant factor in the stratosphere despite the great distance from the sun. The results obtained for Neptune show that such a continuum absorber could significantly contribute to the energy balance within a localized stratospheric region, although it probably cannot furnish sufficient power to account for the observed IR spectrum irrespective of its vertical distribution. Attention is accordingly given to the convective penetration that could arise under such rapid vertical mixing that CH4's condensation cannot occur before the gas is carried above the condensation region.

Appleby, J. F.↗
NASA NTRS1986

Are the aerosols on Uranus and Neptune composed of methane photopolymers?

The measured optical properties of photochemically produced aerosols in an adding-doubling radiative transfer code were used to match various points in the spectra of Uranus and Neptune. How well these points are fit are shown by different assumptions regarding the size and distribution of these aerosols in the Uranus and Neptune atmospheres. The consistency of these derived distributions with those expected from computations of the sedimentation rate of such aerosols is discussed.

Podolak, M.↗
NASA NTRS1986

Satellites of Uranus and Neptune, and the Pluto-Charon system

The orbital properties, surface compositions, opposition surges, masses, radii, and densities of the satellites of Uranus are presented. It is noted that the Uranian satellites are comparable in size to the largest of Saturn's icy satellites while density measurements suggest that the bulk compositions of Ariel and Umbriel might be different from those of Titania and Oberon. Consideration is given to the two satellites of Neptune and the question of a third satellite is addressed. The elements of Charon, determined from astrometric observations by photographic and speckle interferometric techniques, and then from eclipse observations, are given. The diurnal period of Pluto and its photometric lightcurve are discussed. The similarities and differences existing between the satellites of Uranus and Neptune and the Pluto-Charon pair are mentioned briefly.

Cruikshank, Dale P.↗
NASA NTRS1987

Infrared radiometry of Uranus and Neptune at 21 and 32 microns

Mauna Kea's NASA IRTF has been used to obtain 21- and 32-cm radiometric measurements of Uranus and Neptune; brightness temperatures of 54.1 + or - 0.3 K for Uranus and 58.1 + or 0.3 K for Neptune were obtained by calibrating the 21-cm data against Alpha Boo. A calibration of the 32-cm data against Callisto and Ganymede yielded respective temperatures of 51.8 + or - 1.5 K and 55.6 + or - 1.2 K. The general decrease of brightness temperatures with wavelength from 20 to 30 microns is confirmed. The two planets are noted to appear as bodies sufficiently different to depart from the hypothesis of smooth planetary bulk property variation as a function of heliocentric distance.

Orton, Glenn S.↗
NASA NTRS1987

The spectra of Uranus and Neptune at 8-14 and 17-23 microns

The 3-m NASA Infrared Telescope Facility was used to observe the disks of Uranus and Neptune between May 30 and June 1, 1985 in the 7-14 and 17-23 micron spectral regions. Maximum stratospheric mixing ratios of 9 x 10 to the -9th for C2H2, and of 2 x 10 to the -8th for C2H6, are found for Uranus, and the spectrum is otherwise smooth, consistent with the opacity provided by H2 collision-induced absorption and spectrally continuous stratospheric emission. Strong emission features of CH4 and C2H6 are found in the short-wavelength spectrum of Neptune, and the spectrum near 13.5 microns is consistent with C2H2 emission in local saturation equilibrium with a maximum mixing ratio of 9 x 10 to the -7th.

Orton, Glenn S.↗
NASA NTRS1988

Near-infrared stellar-occultation predictions for Uranus and Neptune - 1987-1990

Stellar-occultation predictions are presented for Uranus and Neptune covering the period 1987-1990, based on automated scans of R and I plates taken with the UK Schmidt Telescope in Australia. These scans have revealed many late-type or highly reddened stars suitable for stellar-occultation observations at 2.2 microns that were missed in previous searches of yellow-sensitive astrographic plates (Mink and Klemola, 1985). Comparisons involving several hundred stars measured by Mink and Klemola (1985) indicate that the internal random errors in the stellar positions estimated here are less than about 0.2 arcsec, comparable to the precision reached using conventional astrographic methods. In addition, JHK photometry was obtained for many of the Neptune stars, which is used to establish an empirical (I-K) vs (R-I) calibration for the remaining occultation candidates, and thus provide estimates of their K magnitudes.

Nicholson, Philip D.↗
NASA NTRS1988

Structure of scintillations in Neptune's occultation shadow

An exceptionally high-quality data set from a Neptune occultation is used here to derive a number of new results about the statistical properties of the fluctuations of the intensity distribution in various parts of Neptune's occultation shadow. An approximate numerical ray-tracing model which successfully accounts for many of the qualitative aspects of the observed intensity fluctuation distribution is introduced. Strong refractive scintillation is simulated by including the effects of 'turbulence' with projected atmospheric properties allowed to vary in both the direction perpendicular and parallel to the limb, and an explicit two-dimensional picture of a typical intensity distribution throughout an occulting planet's shadow is presented. The results confirm the existence of highly anisotropic turbulence.

Hubbard, W. B.↗
NASA NTRS1988

Theory of anisotropic refractive scintillation - Application to stellar occultations by Neptune

A theory of refractive scintillation due to a thin phase-changing screen with an anisotropic power-law spectrum of phase fluctuations is presented. Scintillation theory for an isotropic medium is discussed, and anisotropy of the mean density and anisotropy in the scattering are discussed. The theory of refractive scintillation in an anisotropic medium is developed, deriving a general expression for the cross-correlation of flux variations at two points on the 'observer screen'. From this, estimates of the coherence lengths and amplitudes of flux variations are obtained for important parameter regimes. The application of the theory to the analysis of two occultations by Neptune is addressed. The projected dimensions of the occulting stars, a normalization constant to describe the phase fluctuations, and an anisotropy parameter are determined, and the theory is shown to agree well with observation. The significance of the theory for understanding of the physics of Neptune's atmosphere is discussed.

Narayan, Ramesh↗
NASA NTRS1988

IUE observations of Neptune for H Lyman-alpha emission

Study of seven IUE observations has not resulted in the detection of H Ly-alpha emission from Neptune, with 1-sigma upper limits to the planet-averaged surface brightness as low as 180 Rayleighs. The intrinsic brightness is found to be less than the 400-1500 R that would be expected from scaling arguments, and it is noted that this upper limit excludes neither the possibility of scattered solar H Ly-alpha emission from a Jupiter-like atmosphere, nor auroral emission from an active magnetosphere. The results suggest that Neptune has a lower upper atmospheric temperature than Uranus.

Clarke, John T.↗
NASA NTRS1988

Voyager flight engineering preparations for Neptune encounter

Voyager 2 will make the first close observations of the planet Neptune, during the period from June 1 to October 1,1989. A number of flight engineering activities are being conducted in preparation for the encounter. This paper discusses the most significant of these activities: new image motion compensation techniques, attitude control system changes, new exposure capabilities, new data handling capabilities, radiation protection measures, and new navigation methods. In addition, the process of performing late sequence updates is discussed. An overview of the Neptune mission is also presented.

Miller, L. J.↗
NASA NTRS1988

Navigating Neptune

The 1989 Voyager spacecraft encounter with Neptune is analyzed from a navigation system viewpoint. Some sources of navigation challenge are mentioned, e.g. the extreme distance to Neptune, the two body geometry, the closeness of the flyby, the low light level, and the complexity of science observations to be supported. However, the emphasis is on certain techniques which have been developed by the Voyager project to meet these challenges. Each technique will be outlined and its relevance to navigation will be discussed.

Gray, Donald L.↗
NASA NTRS1988

Mission design challenges posed by the Voyager 2 Neptune encounter

A major challenge for the Voyager 2 Neptune encounter lies in the detailed design of a trajectory that achieves science objectives at the planet as well as at its large satellite, Triton. This achievement demands a close flyby of the primary, whereas the planet's great distance makes such an undertaking especially challenging. Changing estimates and uncertainties of parameters characterizing the Neptune environment, particularly ring, atmosphere and radiation models, affect the mission design. These effects are investigated and trade-offs among candidate trajectories are examined with respect to spacecraft performance, avoidance of risk and science objective achievement.

Cesarone, R. J.↗
NASA NTRS1989

D/H for Uranus and Neptune

Searches for absorption features of HD near 6050 A are reported for Uranus and Neptune. The existence of blends of the HD features with weak features due to minor species in the atmosphere of Uranus is demonstrated. These blends make the unambiguous identification of the weak HD features exceedingly difficult. The data are analyzed with inhomogeneous scattering models to ascertain a D/H upper limit for Uranus and Neptune of 0.0001, a factor of 2 smaller than the upper limit reported from CH3D measurements.

Smith, Wm. Hayden↗
NASA NTRS1989

Properties of possible polar rings around Neptune

The near-polar-position rings whose possible existence and character around Neptune are presently considered correspond to the equilibrium configuration of a test-particle orbital plane in the gravitational potential of Neptune and Triton. At these equilibria, of which two whose invariable-plane nodal line is approximately perpendicular to the nodal line of Triton on the same plane are dynamically stable, the orbit of the test particle maintains a fixed orientation with respect to the invariable plane and the node of triton on that plane. The stable equilibria are noted to correspond to a maximum of the total energy for a given semimajor axis; their stability is found to remain unperturbed despite collisions among particles.

Borderies, Nicole↗
NASA NTRS1989

Disk-integrated photometry of Neptune at methane-band and continuum wavelengths

The present disk-integrated photometry of Neptune, which was obtained in 1986 and 1987, is used to study the diurnal and short-term variability; the fact that this photometry was obtained from high-resolution CCD images allows the discrete cloud features' causing of the rotational lightcurve seen at methane-band wavelengths to be definitively demonstrated. A quiescent state of the Neptune atmosphere is suggested by the planet's 8900 A reflectivity in the absence of bright clouds, appears to have remained nearly constant over the observational period.

Hammel, H. B.↗
NASA NTRS1989

Stratospheric aerosols from CH4 photochemistry on Neptune

A combined photochemical-condensation model has been used to study hydrocarbon ices produced from CH4 photolysis in the stratosphere of Neptune. A total stratospheric haze production rate of 4.2 x 10 to the -15th g/sq cm/s. The total production rate is insensitive to within a factor of two to order of magnitude changes in the eddy diffusion coefficient and methane mixing ratio, which is within the present estimate of uncertainty for this number. The condensation temperatures are 97 K for C4H2, 71 K for C2H2, and 64 K for C2H6. Voyager 2 images of Neptune will be able to confirm the presence of stratospheric aerosols and provide constraints on their production rate and location.

Romani, Paul N.↗
NASA NTRS1989

The ionosphere of Neptune

Limited knowledge of ionospheric processes of the outer planets and practically no information on the upper atmosphere of Neptune result in a range of possible ionospheres for Neptune. Various cases are investigated in order to establish a theoretical framework of the ionospheric structure against which the radio occultation measurements can be evaluated. The peak electron densities could range from 1000/cu cm to 10 to the 6th/cu cm depending on ion loss processes and particle ionization processes. The scale height could also range from 300 km to 2000 km depending on the exospheric temperature. Although various assumptions must be made, it is predicted that an auroral ionosphere with large electron densities and a large scale height will be observed by the ingress radio occultation measurement, and that a compressed ionosphere with small electron densities will be seen during the egress measurement.

Shinagawa, H.↗