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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 343 records · Page 19
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NASA NTRS1983

The nature of cometary dust from remote sensing

Observational data on the thermal emission from cometary dust grains and theoretical models constructed to explain them are reviewed. Consideration is given to grain temperature, the size distribution of the grains, thermal-emission models, the silicate component, dust-grain albedo, and icy grains; model predictions and observations are compared in graphs. It is pointed out that no unambiguous detection of icy grains has been achieved, probably because these grains are vaporized within a few hundred km of the nucleus when the comet is within 2.5 AU of the sun.

Hanner, M. S.↗
NASA NTRS1985

Development of a Global 3-D magnetohydrodynamic computational model for solar wind-cometary and planetary studies

The development of a global, 3-D magnetohydrodynamic computational model is described. This model is used to quantitatively describe the detailed continuum field and plasma interaction process of the solar wind with cometary and planetary magneto/ionopause shapes. The present solar wind/terrestrial planet interaction (Level 1) model (which is based on an axisymmetric gas dynamic plus frozen field approximation to the full MHD equations) is extended to a full 3-D gas dynamic (Level 2) approximation. A mass loading capability in the Level 1 interaction model is implemented.

Stahara, S. S.↗
NASA NTRS1984

Cometary Astrometry

Modern techniques for making cometary astrometric observations, reducing these observations, using accurate reference star catalogs, and computing precise orbits and ephemerides are discussed in detail and recommendations and suggestions are given in each area.

Yeomans, D. K.↗
NASA NTRS1985

Periodic cometary showers: Real or imaginary?

Since the initial reports in 1980, a considerable body of chemical and physical evidence has been accumulated to indicate that a major impact event occurred on earth 65 million years ago. The effects of this event were global in extent and have been suggested as the cause of the sudden demise or mass extinction of a large percentage of life, including the dinosaurs, at the end of the geologic time period known as the Cretaceous. Recent statistical analyses of extinctions in the marine faunal record for the last 250 million years have suggested that mass extinctions may occur with a periodicity of every 26 to 30 million years. Following these results, other workers have attempted to demonstrate that these extinction events, like that at the end of the Cretaceous, are temporally correlated with large impact events. A recent scenario suggests that they are the result of periodic showers of comets produced by either the passage of the solar system through the galactic plane or by perturbations of the cometary cloud in the outer solar system by a, as yet unseen, solar companion. This hypothesized solar companion has been given the name Nemesis.

Grieve, R. A. F.↗
NASA NTRS1985

Cometary atmospheres: Modeling the spatial distribution of observed neutral radicals

Progress on modeling the spatial distributions of cometary radicals is described. The Monte Carlo particle-trajectory model was generalized to include the full time dependencies of initial comet expansion velocities, nucleus vaporization rates, photochemical lifetimes and photon emission rates which enter the problem through the comet's changing heliocentric distance and velocity. The effect of multiple collisions in the transition zone from collisional coupling to true free flow were also included. Currently available observations of the spatial distributions of the neutral radicals, as well as the latest available photochemical data were re-evaluated. Preliminary exploratory model results testing the effects of various processes on observable spatial distributions are also discussed.

Combi, M. R.↗
NASA NTRS1985

The International Cometary Explorer (ICE)wallsheet teacher's guide

On September 11, 1985, the veteran NASA spacecraft ISEE-3 which has been renamed the International Cometary Explorer (ICE) will make the first visit of a spacecraft to a comet. A teachers' guide to the NASA wallsheet on the ICE and its mission is presented. This circumstance of course results from the current interest in the return of Halley's Comet. This teacher's guide will be helpful in understanding scientists strong interest in sending the ICE spacecraft to investigate the tail of a much less famous object Comet Giacobin-Zinner.

Maran, S. P.↗
NASA NTRS1984

A possible observation of a cometary bow shock

In February, 1982, an unusual enhancement of the IMF was observed by the Pioneer Venus orbiter at 0.72 AU. The three discontinuities within this large disturbance, previously suggested to be due to the possible passage of a comet, are examined in view of the alternative possibility of their representing the first observation to date of a cometary shock. It is noted that number density, temperature, and velocity changes, as well as the magnetic structure of the discontinuities, are consistent with this hypothesis.

Russell, C. T.↗
NASA NTRS1984

An improved thermal model for cometary nuclei

A thermal model for cometary nuclei is presented and used in comparisons of predictions with data on Halley's comet. The model considers the surface boundary conditions and heat diffusion equations, the sublimation rate of ice, the energy balance, and the dust production rate. Projections were made for the Halley nucleus over 18 latitudes from -85 to +85 deg, from 185 d before to 110 d after perihelion, and in 20 layers of its surface in 1.3 cm increments. Sublimation is predicted to increase by a factor of 2.2 every orbit, thereby allowing 320 returns for a nucleus with a coma, 710 returns with a bare ice nucleus. The gas production during the Giotto intercept at 0.9 AU will be 4.4 x 10 to the 29th molecules/sec, close to an estimate made during the 1910 return.

Weissman, P. R.↗
NASA NTRS1984

The effects of the diffuse radiation fields due to multiple scattering and thermal reradiation by dust on the dynamics and thermodynamics of a dusty cometary atmosphere

A self-consistent model of a dirty, clathrate cometary nucleus is extended to account for diffuse radiation fields caused by multiple scattering and thermal reradiation of the solar continuum by dust. The model is configured to fit conditions expected for the various spacecraft which will encounter Halley's comet at 0.89 AU. The atmosphere is assumed a chemically reactive dust-gas mixture in quasi-steady spherically symmetric expansion. The effect of electron-neutral ion collisions is accounted for, along with rate constants of the various species of clathrate ice particles and radiative transfer of the solar input in the UV, visible and near-IR intervals. The opacity of the circumnuclear dust is projected to be an order of magnitude greater than previous estimates, which severely impacts the potential visibility of the nucleus to the cameras of the flyby satellites.

Marconi, M. L.↗
NASA NTRS1985

Models of cometary emission in the 18-cm OH transitions - The predicted behavior of Comet Halley

The return of Halley's comet to the inner solar system in 1985-1986 presents an excellent opportunity to study the behavior of a bright comet, since observations may be planned ahead of time to yield the maximum scientific return. Observations of the OH radical in the 18-cm wavelength Alpha-doublet transitions can be expected to yield significant results to cometary science in a number of areas from the measurement of the production rate of OH parent molecules (water) to the elucidation of the kinematics in the coma. Therefore, models of the OH emission from the comae of comets are presented together with predictions of brightness and spectra of Halley's Comet during the current apparition as an aid to prospective observers. The desirability of certain observational programs, including monitoring, mapping, polarimetry, and occultation experiments, are also discussed.

Schloerb, F. P.↗
NASA NTRS1985

Three spacecraft measurements of an unusual disturbance in the solar wind - Further evidence for a cometary encounter

It is noted that, while the Pioneer Venus orbiter detected an unusual IMF and solar wind plasma disturbance on Feb. 11, 1982, the Venera 13 and 14 spacecraft were conducting IMF measurements 6 million miles away which exhibit very weak disturbances several hours after the peak of the Pioneer Venus event. It is speculated that, if the event had been a solar-initiated disturbance, it should have been detected in virtually unaltered form by Venera 13 and 14; whose data therefore furnish further evidence for the cometary nature of this event.

Russell, C. T.↗
NASA NTRS1985

Solar pressure and molecular decay in cometary atmospheres

The effects of solar pressure and molecular decay on number density in cometary atmospheres are rigorously separated and scale lengths for each are determined from an analysis of observed brightness profiles in the solar and antisolar directions. It is found that the pressure scale length of CN is approximately 160,000 km and that of C2 is approximately 110,000 km. The scale length for molecular decay, heretofore incorrectly inferred from the observational data, is approximately 3 times as long as the pressure scale lengths. It is difficult to determine adequately from observations that extend no more than about 100,000 km from the comet nucleus. The scale length for molecular decay by photodissociation or whatever cause is found to be about 350,000 km for C2 and 500,000 km for CN.

Beard, D. B.↗
NASA NTRS1986

Arecibo Observatory support of the US international cometary Explorer mission encounter at comet Giacobini-Zinner

The Arecibo Observatory in Puerto Rico participated in the support of the U.S. International Cometary Explorer (ICE) mission when the ICE spacecraft passed through the tail of comet Giacobini-Zinner on September 11, 1985. The Arecibo Observatory is a research facility of the National Astronomy and Ionosphere Center (NAIC) operated by Cornell University under contract to the National Science Foundation (NSF). Coverage of the encounter involved the use of the observatory's 305-m (1000-ft) radio reflector antenna and RF and data system equipment fabricated or modified specifically for support of the ICE mission. The successful implementation, testing, and operation of this temporary receive, record, and data relay capability resulted from a cooperative effort by personnel at the Arecibo Observatory, the Goddard Space Flight Center, and the Jet Propulsion Laboratory.

Gordon, D. D.↗
NASA NTRS1986

The nature of cometary grains from remote sensing

Physical property data of cometary grains are derived primarily from thermal emission measurement and optical scattering. Results consistent with the properties of micrometeorites collected in the stratosphere are described.

Hanner, M. S.↗
NASA NTRS1986

Preliminary development of a global 3-D magnetohydrodynamic computational model for solar wind-cometary and planetary interactions

This is the final summary report by Resource Management Associates, Inc., of the first year's work under Contract No. NASW-4011 to the National Aeronautics and Space Administration. The work under this initial phase of the contract relates to the preliminary development of a global, 3-D magnetohydrodynamic computational model to quantitatively describe the detailed continuum field and plasma interaction process of the solar wind with cometary and planetary bodies throughout the solar system. The work extends a highly-successful, observationally-verified computational model previously developed by the author, and is appropriate for the global determination of supersonic, super-Alfvenic solar wind flows past planetary obstacles. This report provides a concise description of the problems studied, a summary of all the important research results, and copies of the publications.

Stahara, S. S.↗
NASA NTRS1986

A digital imaging photometry system for cometary data acquisition

This report describes a digital imaging photometry system developed in the Space Science Laboratory at the Marshall Space Flight center. The photometric system used for cometary data acquisition is based on an intensified secondary electron conduction (ISEC) vidicon coupled to a versatile data acquisition system which allows real-time interactive operation. Field tests on the Orion and Rosette nebulas indicate a limiting magnitude of approximately m sub v = 14 over the 40 arcmin field-of-view. Observations were conducted of Comet Giacobini-Zinner in August 1985. The resulting data are discussed in relation to the capabilities of the digital analysis system. The development program concluded on August 31, 1985.

Clifton, K. S.↗
NASA NTRS1986

The International Cometary Explorer (ICE) mission to Comets Giacobini-Zinner and Halley

Use of the ISEE-3 satellite (renamed ICE) to study the interaction between the solar wind and a cometary atmosphere by passing through the plasma tail by intercepting Comet Giacobini-Zinner on 11 September 1985 is described. Details of the targeting strategy are discussed. Additional scientific objectives following the tail intercept of Comet Giacobini-Zinner include the support of Comet Halley studies through the measurement of solar-wind conditions upstream of P/Halley in October 1985 and March 1986.

Brandt, J. C.↗