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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 307 records · Page 17
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NASA NTRS1979

Interplanetary gas. XXIV - Are cometary plasma tail disconnections caused by sector boundary crossings or by encounters with high-speed streams

The paper discusses and compares the original sector boundary model (Niedner and Brandt) and the alternative high-speed stream model (Ip and Mendis) suggested for explaining cometary plasma tail disconnection events (DE) within a largely observational framework not dependent on detailed plasma physics. Among the findings are: (1) the strong solar cycle phase dependence of the amplitude of the Rosenberg-Coleman effect (Svalgaard and Wilcox) yields inferred maximum latitudes of all of the DEs in the original survey to a one-time Rosenberg-Coleman effect measurement made by Pioneer 11, which indicated a disappearance of sectors above 16 deg latitude, and (2) approximately 70% of the post 1926 DEs in the expanded survey show a close association with corotated shorter-term polarity reversals. This result is difficult to reconcile with the predictions of the stream model, in which DEs should correlate more strongly with streams than with sector boundaries. It is concluded that the sector boundary model better describes the disconnection phenomenon and that the use of DEs as unique sector boundary markers is presently justified.

Niedner, M. B., Jr.↗
NASA NTRS1980

The contribution of cometary volatiles to the primitive earth

It has been estimated that during its early history the earth captured a mass of cometary material of the order of 10 to the 23rd grams. Since carbon is supposed to be at least three times more abundant in comets than in carbonaceous chondrites (3.5 percent C in C 1 chondrites), it can be deduced that about 1 x 10 to the 22nd grams of carbon (as carbon compounds), were added by comets to the surface of the prebiotic earth. This carbon value is of the same order of magnitude as the value of the organic carbon buried in the earth's sedimentary shell, but approximately one order of magnitude lower than the earth's surface total carbon (7 x 10 to the 22nd gm). The capture of comets by the earth would also have contributed to generating the appropriate aqueous and reducing environmental conditions necessary for organic synthesis.

Oro, J.↗
NASA NTRS1976

Simulation of a cometary conglomerate of frozen gases and dust in a zero-gravity environment: A critical evaluation of possible space experiments

It is proposed to use the Shuttle (OFT experiments) or eventually Spacelab to study, in the absence of gravity the low velocity accretion and the bulk properties of icy conglomerates simulating cometary material and their sedimentation and bulk properties in very small acceleration fields. Their behavior when exposed to the direct solar flux is also under consideration.

Delsemme, A. H.↗
NASA NTRS1981

Cometary particulate analyzer design definition study

A concept for remotely determining the relative abundance of elements contained in cometary particulates collected by a spacecraft was conducted with very encouraging results. The technique utilizes a short high intensity burst of laser radiation to vaporize and ionize collected particulate material. Ions extracted from this laser-produced plasma are analyzed in a time of flight mass spectrometer to yield an atomic mass spectrum representative of the relative abundance of elements in the particulates. A prototype analyzer system was designed, constructed, and tested. Results show that: (1) energy-time focus performs as predicted in improving resolution; (2) power densities sufficient to produce usable ionization efficiencies can be obtained; (3) complex alloys such as stainless steel can be analyzed; and (4) a tiny, simple and reliable laser used in the demonstration easily meets spacecraft power and mass limitations. A mass resolution of 150 was experimentally demonstrated at mass 108, and an analytical extrapolation predicts a resolution sufficient to separate masses 250 and 251.

Utterback, N. G.↗
NASA NTRS1980

Interplanetary gas. XXV - A solar wind and interplanetary magnetic field interpretation of cometary light outbursts

Possible relationships of cometary brightness outbursts with the solar wind and interplanetary magnetic field are examined. Two types of outburst are distinguished: those which involve a significant brightening of both the head and the tail in a comet with a conspicuous plasma tail (Class I), and those involving the brightening of the central condensation of a previously faint comet with no detectable plasma tail (Class II). Class I bursts, as exemplified by Comet Morehouse 1908c, are attributed to the generation in the head of enhanced amounts of ions and their injection into the tail shortly before it disconnects, with ionization provided by sector boundary crossings. Class II events, as exhibited by Comet P/Tuttle-Giacobini-Kresak 1973b, are interpreted as the result of the bombardment of the nucleus by disturbed solar wind near corotated high-speed streams and sector boundaries, leading to highly exothermic chemical reactions.

Niedner, M. B., Jr.↗
NASA NTRS1980

On the origin of striae in cometary dust tails

It has been argued by Sekanina that the so-called striae or pseudo-synchronic bands observed in the dust tails of several comets are a consequence of the sudden disruption of elongated or chainlike parent grains in the tail. The present paper offers a specific mechanism for this disruption process, that is, electrostatic disruption following sudden charging to high electrostatic potentials. Assuming the grains to be prolate spheroids, it is shown that they can attain potentials in the range -60 V to -300 V when subject to energetic electron beams (1-10 keV). The energy source for these electrons is the interaction of the solar wind with the cometary ionosphere producing currents and associated electrostatic double layers.

Hill, J. R.↗
NASA NTRS1981

On the development and global oscillations of cometary ionospheres

Representing the cometary ionosphere by a single fluid model characterized by an average ionization time scale, both the ionosphere's development as a comet approaches the sun and its response to sudden changes in solar wind conditions are investigated. Three different nuclear sizes (small, average, very large) and three different modes of energy addition to the atmosphere (adiabatic, isothermal, suprathermal) are considered. It is found that the crucial parameter determining both the nature and the size of the ionosphere is the average ionization time scale within the ionosphere. Two different scales are identified. It is noted that the ionosphere can also be characterized by the relative sizes of three different scale lengths: the neutral standoff distance from the nucleus, the ion standoff distance from the nucleus, and the nuclear distance at which the ions and the neutrals decouple collisionally.

Houpis, H. L. F.↗
NASA NTRS1981

Telecommunications in cometary environments

Propagation effects on telecommunications in a cometary environment include those due to dust, the inhomogeneous plasma of the coma and tail, and ionization generated by impact of neutral molecules and dust on the spacecraft. Attenuation caused by dust particles is estimated to be on the order of 10 to the minus 5th power dB for the Halley Intercept Mission. Ionization generated by impact on the spacecraft is estimated to result in an electron content of 10 to the 12th power to 10 to the 13th power el/sq meters (3 eV electrons) along the telecommunications path. An estimate of the electron content due to Comet Halley itself is 10 to the 16th power to 10 to the 17th power el/sq meters, compared to a content of 10 to the 16th power to 10 to the 18th power el/sq meters for the Earth's ionosphere and 10 to the 17th power to 10 to the 18th power el/sq meters for the interplanetary medium. The electron content of the plasma near Comet Halley will cause excess range delay, and a Doppler shift of the signal from the spacecraft will occur in propagation to the rate of change of the path electron content. It is recommended that S and X down-link frequencies by employed to monitor the path electron content and amplitude scintillation and spectral broadening of the received signals. These measurements will provide a quantitative base of knowledge that will be valuable for radio science and telecommunications system design purposes.

Flock, W. L.↗
NASA NTRS1981

Radiative transfer in cometary dust atmospheres: Critique of recent developments

The gas and dust production rates of comets near the sun are intimately linked to the fate of the solar radiation as it is scattered and partially absorbed by the cometary dust envelope or halo surrounding the comet's nucleus. The radiation of nuclear matter (mostly water ice) and the release of dust particles embedded in the ice which are entrained by the escaping gas and subsequently form a dust atmosphere or halo. On the other hand, the interaction of the dust cloud with the solar radiation is in turn responsible for the amount of radiative energy received by the nucleus and thus determines the evaporation rate. It is shown that recent attempts to formulate this problem quantitatively are in error. The correct formulation is given and it is pointed out that negative extinction (enhancement of the primary radiation received by the nucleus due to multiple scattering) may not be as large as predicted.

Vonroos, O.↗
NASA NTRS1981

Observing facilities at the European Southern Observatory (ESO) in Chile for cometary observations

The (ESO) is located on the mountain La Silla (geographical coordinates: 4h42m55s10 west, -29 deg 15' 25".8 south, 2400 m elevation. The size of the telescopes ranges from a 40 cm Astrograph to the 3.6 m Richey-Chretien telescope. Future telescopes are discussed: a 2.2 m RC-Telescope which will be identical with the German 2.2 m telescope on Calor Alto in SE-Spain, and a 3.5 m telescope, the New Technology Telescope. In addition to these telescopes, a great number of auxiliary instrumentation are operational. Because ESO has to serve all requests of the visiting astronomers these instruments are designed for very different applications. The telescopes and auxiliary instruments that are especially suited for cometary observations are discussed. The dicussion is divided into three parts: photography, photometry-polarimetry and spectroscopy.

Schnur, G. F. O.↗
NASA NTRS1981

A possible technique for cometary studies with high angular and spectral resolution

The echelle spectrographs, designed for and used at the Cassegrain stations of the KPNO and CTIO 4-meter telescopes, are capable of cometary spectroscopy with seeing-limited angular resolution along the slit and with spectral resolving power (lambda/delta lambda) ranging from 10 to the 4th power to 10 to the 5th power. Various gratings, cameras and detectors can be used in combination for specific studies in the 3000A to 10,000A range.

Gull, T. R.↗
NASA NTRS1981

A semi-empirical photometric theory of cometary gas and dust production: Application to P/Halley's gas production rates

The semiempirical photometric theory of cometary gas and dust production is recalibrated using UV observations from 14 comets and uniform visual photometry from 8 comets. The calibration is not changed significantly but becomes more secure. The complete theory is presented with all approximations evaluated and explained. Numerical calibration aspects are presented in tables. The theory is applied to P/Halley using a light curve without the artifact caused by the close approach to Earth in 1910. Gas production rates predicted for the 1985/86 apparition are similar to those for 1910.

Newburn, R. L.↗
NASA NTRS1981

Effect of the size distribution on the thermal emission from cometary dust

The IR thermal emission from cometary dust is analyzed by two size distribution functions, Sekanina A and a modified Sekanina Miller distribution. The resulting thermal emission spectra are illustrated for heliocentric distances 0.15 to 1.6 AU. The Sekania A distribution does not predict the observed heliocentric variation in the flux ratio I (4.8 micron) I (3.5 micron) for bright comets. A size distribution intermediate between the two presented is a better representation of the dust in recently observed comets.

Hanner, M. S.↗
NASA NTRS1981

Heat content and evolution of cometary nuclei

The heat flux into cometary nuclei at various distances from the sun, before and after perihelion, is investigated for both the isothermal case and that of the fixed subsolar point. It is found that the heat flux may be a large fraction of the incident total heat input, so that surface temperature and the associated evaporation rate are lower than usually calculated, and that the effect depends on the porosity of the nucleus. An ideal, initially spherical and homogeneous nucleus cannot remain isothermal, so that it must develop surface nonuniformities through localized phase changes, evaporation, and breakaway. The splitting of comets as far as 9 AU from the sun may be explained in terms of the heating of a CO2-rich inclusion in the nucleus.

Smoluchowski, R.↗
NASA NTRS1981

Millimeter-wave experiments for cometary space missions

Predicted brightness temperatures, computed by means of radiative transfer techniques adapted from the modeling of terrestrial ice and snow fields, are given for cometary nucleus models consisting of homogeneous layers of water ice and refractory grain mixtures presented as functions of wavelength. The computed millimeter-wave spectra are sensitive to the values of such physically significant nucleus parameters as (1) crust thickness, (2) subsurface temperature gradient, and (3) sublimating surface boundary temperature. Although antenna beam dilution is a major obstacle for ground-based molecular spectral line radio observations of comets, a millimeter-wave radiometer in the vicinity of the comet would be immune to this effect and able to make observations of several candidate parent molecules in the gas phase.

Hobbs, R. W.↗
NASA NTRS1981

On the dust zoning of rapidly rotating cometary nuclei

The effects of nuclear rotation on the surface of a cometary nucleus (a comet at 1 AU that is H2O dominated and has a radius of 1 km) are considered. It is shown that this dust does not accumulate uniformly on the surface, which here is considered spherical. While dust particles in the two polar cap regions and an equatorial belt remain at rest on the surface, those in two midlatitude bands migrate toward the equator, stopping at the two low latitudes to form dust ridges. As the nucleus spins up, both the polar caps and the equatorial belt shrink in size, and the dust ridges move toward the equator, eventually spinning off the dust from the nucleus when the nuclear rotation period is less than about 3.3 hr. For larger particles for which the gas buoyancy is negligible, migration takes place only if the rotation period is not significantly larger than the critical value of 3.3 hr or if the surface friction is abnormally small.

Houpis, H. L. F.↗
NASA NTRS1982

Cometary NH - Ultraviolet and submillimeter emission

A description is presented of the results of computer calculations on the molecule NH, which is an important constituent of cometary comae, and a probable, but an as yet undetected, interstellar species. Attention is given to spectroscopic properties, the treatment of solar spectral data, and radiative transfer calculations. The results of calculations for the Einstein A-coefficients are given in an appendix, where the entries for the X-X and A-A transitions depend on the assumed permanent electric-dipole moment squared. Rotational-spin-Lambda-doublet energy levels used for statistical equilibrium population densities are listed in a table.

Litvak, M. M.↗