Engineering topics

Herbert, F.

Publications and source records attributed to Herbert, F..

At least 19 records

NASA NTRS1993

Temperature, N2, and N density profiles of Triton's atmosphere - Observations and model

Improved analysis of the Voyager Ultraviolet Spectrometer observations of the solar occultation by Triton yields the isothermal temperature and N2 number densities in the altitude range 475-675 km. The signature of atomic nitrogen in the occultation spectra is identified, its density profile is derived, and an experimental value of the escape rate of N atoms is given. The one-dimensional thermal conductivity equation for a spherical atmosphere is solved, taking into account CO heating and cooling and heating by precipitating electrons, solar radiation, and chemical effects. Finally, profiles of number densities of N, H2, and H are calculated.

Krasnopolsky, V. A.↗

NASA NTRS1992

Properties of haze in the atmosphere of Triton

Voyager UV spectrometer measurements of CH4 and haze of the Triton atmosphere combined with the haze brightness profile determined by the narrow angle camera are used to infer a haze optical thickness of 0.024 at 1500 A and 0.0078 in the spectral range of the narrow angle camera centered at 4700A, rho/gamma = 0.36 +/- 0.1 g/cu cm (gamma is the quantum yield of condensate), and values of r(c) varying from 0.1 +/- 0.02 micron at 30 km to 0.15 +/- 0.03 micron near the surface. Other auxiliary properties of the haze are also determined. The value found for rho/gamma corresponds to a packing coefficient of 0.6 gamma if C2H4 is the main condensible species.

Krasnopolsky, V. A.↗

NASA NTRS1991

Interpretation of Voyager UVS observations of occultations by the atmosphere of Neptune

We propose to continue our investigation of the upper atmosphere of Neptune based primarily on the solar occultation measurements made by the Voyager Ultraviolet Spectrometer (UVS). The ultimate goal of this research is to understand the structure and composition of Neptune's upper atmosphere as encoded in the data base obtained by the UVS during the Voyager 2 encounter with Neptune. The UVS occultations provide information essential to studies of atmospheric composition and structure, energy balance, photochemistry, global transport, ionospheric structure, and airglow production mechanisms. In conjunction with other research in progress at the University of Arizona and by our colleagues at other institutions, we not only analyze and interpret individual measurements or observations by the UVS, but also construct models of the upper atmosphere which are consistent with all available data obtained by the UVS. We derive constraints from measurements of the atmospheric emissions and we combine these with constraints derived from the occultation measurements. Principal sources for the atmospheric emissions include dayglow, resonance scattering and, at long wavelengths, the reflected solar spectrum. On the dark side of the planet, a weak H Ly alpha emission is due to resonance scattering of the sky background and perhaps another source. Thus a wide range of physical processes are at work, and each provides its own constraints on the atmosphere.

Sandel, Bill R.↗

NASA NTRS1991

Development of an electrostatic propulsion engine using sub-micron powders as the reaction mass

Asteroid sample return missions would benefit from development of an improved rocket engine. Chemical rockets achieve their large thrust with high mass consumption rate (dm/dt) but low exhaust velocity; therefore, a large fraction of their total mass is fuel. Present day ion thrusters are characterized by high exhaust velocity, but low dm/dt; thus, they are inherently low thrust devices. However, their high exhausy velocity is poorly matched to typical mission requirements and therefore, wastes energy. A better match would be intermediate between the two forms of propulsion. This could be achieved by electrostatically accelerating solid powder grains, raising the possibility that interplanetary material could be processed to use as reaction mass. An experiment to study the charging properties of sub-micron sized powder grains is described. If a suitable material can be identified, then it could be used as the reaction mass in an electrostatic propulsion engine. The experiment employs a time of flight measurement to determine the exhaust velocity (v) of various negatively charged powder grains that were charged and accelerated in a simple device. The purpose is to determine the charge to mass ratio that can be sustained for various substances. In order to be competitive with present day ion thrusters, a specific impulse (v/g) of 3000 to 5000 seconds is required. Preliminary results are presented. More speculatively, there are some mission profiles that would benefit from collection of reaction mass at the remote asteroid site. Experiments that examine the generation of sub-micron clusters by electrostatic self-disruption of geologically derived material are planned.

Herbert, F.↗

NASA NTRS1990

Aurora and airglow on the night side of Neptune

The latitude-longitude distribution of emissions detected by the Voyager ultraviolet spectrometer on the dark hemisphere of Neptune have been examined. The emissions have two significant geographic features: (1) a broad peak near longitude 60 deg W that extends rather uniformly over the entire range of observed latitudes (55 deg S to 50 deg N); and (2) a brighter, narrower peak near the south pole and 240 deg W. The first peak is interpreted as due to excitation of the night side atmosphere by photoelectrons from the magnetically conjugate, sunlit atmosphere. The second peak can plausibly be attributed to a southern aurora; the field geometry would then seem to require a conjugate (and probably brighter) northern aurora that escaped detection poleward of the latitude range sampled by the UVS data. The power for such an aurora could be extracted from Neptune's rotation by the injection of plasma at Triton's orbit at a rate dm/dt of about 1 kg/s.

Sandel, B. R.↗

NASA NTRS1989

Asteroid volatiles inventory

Asteroids appear in light of telescopic and meteority studies to be the most accessible repositories of early solar system history available. In the cooler regions of the outer asteroid belt, apparently unaffected by severe heating, the C, P, and D populations appear to harbor significant inventories of volatiles; the larger primordial belt population may have had an even greater percentage of volatile-rich, low-albedo asteroids, constituting a potent asteroid for veneering early terrestrial planet atmospheres. The volatile-rich asteroids contain carbon, structurally bound and adsorbed water, as well as remnants of interstellar material predating the solar system.

Lebofsky, L. A.↗

NASA NTRS1989

Chemical, thermal and impact processing of asteroids

The geological effects of impacts, heating, melting, core formation, and aqueous alteration on asteroids are reviewed. A review of possible heat sources appears to favor an important role for electrical induction heating. The effects of each geologic process acting individually and in combination with others, are considered; it is concluded that there is much evidence for impacts during alteration, metamorphism and melting. These interactions vastly increased the geologic diversity of the asteroid belt. Subsequent impacts of cool asteroids did not reduce this diversity. Instead new rock types were created by mixing, brecciation and minor melting.

Scott, E. R. D.↗

NASA NTRS1986

Ultraviolet spectrometer observations of Uranus

The Voyager 2 UV spectrometer was used to scan the Uranus atmosphere at wavelengths from 500-1700 A with a field of view of 0.1 x 0.86 deg. The temperature and composition of the upper atmosphere were determined through occultations of light from gamma Pegasi, nu Geminorum and the sun. The data indicated a substantial gas density (100 million H atoms/cu cm) at about 28,000 km from the Uranus center, suggesting that gas drag plays a significant role in ring evolution. The distributions of CH4 and C2H2 in the lower atmosphere were also estimated. An electroglow emission was detected on the sunlit side, and attributed to emissions from atomic and molecular hydrogen excited by low energy electrons. An auroral glow was also observed, and exhibited evidence of an energy input equal to that of the electroglow. Finally, estimates of the C2H2 mixing ratio and the vertical column abundance of H2 are calculated.

Broadfoot, A. L.↗

NASA NTRS1985

'Alfven wing' models of the induced electrical current system at Io - A probe of the ionosphere of Io

A novel class of models of the Alfven wing interaction between the Io plasma torus and Io's ionosphere has been developed and used to compute the magnetic field fluctuations at the position of the Voyager 1 flyby. These computations of the magnetic field signature are compared with the magnetic field measurements made during the flyby, and the differences between the model and observations are used as a probe of the structure of Io's ionosphere. The results of this model fitting indicate that a significant atmospheric neutral column density is required over a major part of the trailing hemisphere. This can be consistent with the cold-trap model for Io's neutral atmoshpere only if the major plumes near the center of the trailing hemisphere provide significant volumetric coverage. The Io plasma torus charged-mass density required for a proper match to the data is larger than earlier estimates based on the width of the Alfven wing signature.

Herbert, F.↗

NASA NTRS1985

Modelling Equilibrium and Fractional Crystallization in the System MgO-FeO-CaO-Al2O3-SiO2

A mathematical modelling technique for use in petrogenesis calculations in the system MgO-FeO-CaO-Al2O3-SiO2 is reported. Semiempirical phase boundary and elemental distribution information was combined with mass balance to compute approximate equilibrium crystallization paths for arbitrary system compositions. The calculation is applicable to a range of system compositions and fractionation calculations are possible. The goal of the calculation is the computation of the composition and quantity of each phase present as a function of the degree of solidification. The degree of solidification is parameterized by the heat released by the solidifying phases. The mathematical requirement for the solution of this problem is: (1) An equation constraining the composition of the magma for each solid phase in equilibrium with the liquidus phase, and (2) an equation for each solid phase and each component giving the distribution of that element between that phase and the magma.

Herbert, F.↗

NASA NTRS1984

High-frequency wave normals in the solar wind

High-frequency (0.01-0.04 Hz) magnetic fluctuations in 506 ten-minute intervals of contemporaneous Explorer 35 and Apollo 12 measurements made in the solar wind near the morning side of the earth's bow shock show the presence of a large population of disturbances resembling Alfven waves. Each wavefront normal is systematically aligned (median deviation = 35 deg) with the associated ten-minute average of the magnetic field. Because of variability in the direction of this ten-minute average, from one interval to another, the coupled wavefront-normal distribution is nearly isotropic in solar ecliptic coordinates, in contrast with the results of other studies of waves at much lower frequency indicating outward propagation from the sun. Presumably the high frequency waves discussed here are stirred into isotropy (in solar ecliptic coordinates) by following the low frequency fluctuations. As these waves maintain their alignment of wavefront-normal with the field ten-minute average despite the great variation of the field ten-minute average, a strong physical alignment constraint is inferred.

Herbert, F.↗

NASA NTRS1984

Models of angular momentum input to a circumterrestrial swarm from encounters with heliocentric planetesimals

Preliminary experiments show that heliocentric planetesimals passing through the Earth environment possess significant angular momentum. However it also appears that these same planetesimals impacting a circularized circumterrestrial planetesimal swarm would likely remove angular momentum (though possibly increasing mean kinetic energy), presumably promoting both swarm infall upon the Earth and escape to heliocentric space. Only a distribution of highly eccentric satellite orbits with mean tangential velocities of a few tens of percent of local circular velocity would be immune against angular momentum loss to passing heliocentric planetesimals.

Herbert, F.↗

NASA NTRS1983

An upper bound on the radius of a highly electrically conducting lunar core

Parker's (1980) nonlinear inverse theory for the electromagnetic sounding problem is converted to a form suitable for analysis of lunar day-side transfer function data by: (1) transforming the solution in plane geometry to that in spherical geometry; and (2) transforming the theoretical lunar transfer function in the dipole limit to an apparent resistivity function. The theory is applied to the revised lunar transfer function data set of Hood et al. (1982), which extends in frequency from 10 to the -5th to 10 to the -3rd Hz. On the assumption that an iron-rich lunar core, whether molten or solid, can be represented by a perfect conductor at the minimum sampled frequency, an upper bound of 435 km on the maximum radius of such a core is calculated. This bound is somewhat larger than values of 360-375 km previously estimated from the same data set via forward model calculations because the prior work did not consider all possible mantle conductivity functions.

Hobbs, B. A.↗

NASA NTRS1982

The deep lunar electrical conductivity profile - Structural and thermal inferences

Simultaneous lunar surface and orbital magnetometer records are reexamined, to ascertain intervals which may be suitable for measuring lunar inductive response in the solar wind and terrestrial magnetosheath. Power spectral estimates of the response tangent to the lunar surface, defined in terms of transfer and gain functions, are obtained for the 0.0001-0.01 Hz frequency range. The maximum consistency of estimates from different time intervals is found when the initial analysis is limited to the tangential direction of maximum incident power, or that direction in which the ratio of signal to background noise is greatest. Spherically symmetric plasma confinement theory is used in the interpretation of transfer function data, by way of forward model calculations, under the assumption of continuous electrical conductivity increase with depth. Results are presented for internal electrical conductivity profile, metallic core radius, and selenotherm limits.

Hood, L. L.↗

NASA NTRS1982

Further efforts to limit lunar internal temperatures from electrical conductivity determinations

A representative set of 18 electrical conductivity profiles consistent with lunar transfer function data in the 0.00001 to 0.001 Hz range is generated by means of a Monte Carlo procedure. Laboratory electrical conductivity vs temperature data extrapolated to lower and higher temperatures are used to convert the 18 profiles into temperature profiles for a series of radially homogeneous olivine-pyroxene mixtures ranging from 100% olivine to 100% aluminous orthopyroxene. The former yields selenotherms approaching the Ringwood-Essene solidus at depths near 500 km, while additions of the latter in concentrations greater than 15-30 vol% leads to cooler selenotherms which approach the solidus only at depths greater than about 1000 km. On the basis of independent geophysical constraints, the profile envelopes for compositions containing more than 15-30 vol% aluminous orthopyroxene are considered to be the most probable.

Hood, L. L.↗

NASA NTRS1980

Joule heating of Io's ionosphere by unipolar induction currents

Electrical induction in Io's ionosphere, due to the corotating plasma bound to the Jovian magnetosphere, is one possible source for the attainment of the high temperatures suggested by the large scale height of Io's ionosphere. Unipolar induction models are constructed to calculate ionospheric joule heating numerically, whose heating rates lie between 10 to the -9th and 10 to the -8th W/cu m. The binding and coupling of the ionosphere is due to the dense, and possibly ionized, neutral SO2 atmosphere, and there appears to be no need to postulate the existence of an intrinsic Ionian magnetic field in order to retain the observed ionnosphere.

Herbert, F.↗

NASA NTRS1980

Lunar core detection using two surface magnetometers at very low frequency

A method is described which allows the analysis of the dipole component of the lunar induction using only measurements of two surface magnetometers. This is possible because the very low frequency of the relevant signal components limits the lunar response to the dipole partial wave. Because only surface magnetometers are necessary the simultaneous measurements made by the Apollo 15 and 16 surface modules can be used. Thus, additional information concerning the lunar transfer function can be applied to the determination of the existence and size of a metallic lunar core.

Herbert, F.↗

NASA NTRS1980

Electromagnetic inductive heating of the asteroids and moon as evidence bearing on the primordial solar wind

Thermal evolutionary models of the asteroids and the moon based on the hypothesis of electrical inductive heating by a dense primordial solar wind are compared with observation. Inferences (derived from spectroscopy) of asteroidal mineralogy when compared with electrical heating models, the variation of cooling rates recorded in iron meteorites, and the occurrence of the lunar magma ocean event lend support to the hypothesis of inductive heating. A high primordial solar rotation rate and magnetic field together with the loss due to plasma outflow of a substantial fraction of the initial solar mass would follow.

Herbert, F.↗