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At least 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

The lifetime of the OH radical in comets at 1 AU

It has been shown that the photochemical lifetime of OH in comets is a function of the comet's radial velocity. The calculated lifetime at 1 AU can vary between 69,000 and 210,000 sec for radial velocities that vary from -58 to +59 km/sec. A comparison between the scale lengths observed for three comets and those calculated based upon the theoretical lifetime has been made. This comparison shows that in two of the comets the lifetime derived from the scale lengths is a factor of 1.7 larger than the theoretical lifetime. Suggestions are made about the origin of this discrepancy.

Jackson, W. M.↗
NASA NTRS1980

Near-comet trajectory design

The Halley Flyby/Tempel 2 Rendezvous mission requires not only a new spacecraft design and a new propulsion system (the Solar Electric Propulsion System), but it also requires new approaches to the trajectory and the mission design. Because of the feebleness of Tempel 2 gravitation and because of the presence of various other competing forces, a realization and/or preservation of a spacecraft trajectory about the comet is characteristically different from that about a more massive body. This paper surveys, compares, and evaluates the significance of various forces acting on the spacecraft in the vicinity of the comet. The range of variations and uncertainties inherent in modeling of these forces are also discussed. Then, a number of different types of trajectories relevant to the design of the Tempel 2 exploration are depicted and examined. These include short-arc trajectories, near-comet short- and long-term orbits, and comet-landing trajectories.

Jones, J. B.↗
NASA NTRS1980

Rotation and outbursts of comet P/Schwassmann-Wachmann 1

The paper examines rotation and outbursts of comet P/Schwassmann-Wachmann 1. The observations of the P/Schwassmann-Wachmann 1 show this comet to flare above its quiescent level from before perihelion q=5.5 AU, to after aphelion Q of about 7AU. Cometary 'afternoon' activity suggests counterclockwise rotation of aphelion pole as seen near lambda=100 deg; outburst dates were derived from observed coma diameters, and the rotation was found to be direct with pole near lambda=280 deg. Usually one large area of the comet is active during one opposition, although two were observed concurrently in four very active oppositions; old active areas persist near the East limb of the comet and new areas resist development toward the West limb.

Whipple, F. L.↗
NASA NTRS1980

CO/+/ in comet Schwassmann-Wachmann 1 near minimum brightness

A spectrum of comet Schwassmann-Wachmann 1, taken when the comet was in a quiescent state 6.19 AU from the sun, shows the 1-0, 2-0, 3-0 and 4-0 vibrational transitions of the A2Pi-X2Sigma band system of CO(+) and a weak solar reflection continuum. It is noted that such emission has never been observed in this comet when in a quiescent state. Mechanisms for outbursts in the comet are discussed.

Larson, S. M.↗
NASA NTRS1980

Navigation accuracy analyses for two comet rendezvous missions using ion drive

Two cometary missions, making use of the solar electric propulsion system, have recently been considered for launches in the mid to late 1980's. This paper presents navigation accuracy analysis results for the rendezvous portions of these missions, the target bodies being Comet Tempel 2 and Comet Tuttle-Giacobini-Kresak. Orbit determination and guidance accuracies are presented for the baseline navigation strategies, along with the results of a number of sensitivity studies involving parameters such as data frequencies, data accuracies, ion drive thrust vector errors, comet emphemeris uncertainties, time lags associated with data processing and command sequence generation, and certain guidance law parameters. The accuracies obtained are, in some respects, significantly better than the results of previous solar electric propulsion comet rendezvous studies.

Wood, L. J.↗
NASA NTRS1980

SEPS comet rendezvous performance assessment

This paper presents an assessment of delivered payload capability for a number of selected rendezvous missions to periodic comets with launch opportunities through the years 1986-1996. These missions are chosen using a selection criteria that considers both expected scientific interest and earth-based sighting considerations. Some 22 mission opportunities are found that satisfies the selection criteria. Spacecraft performance is presented for each mission opportunity based on use of a space shuttle, a twin stage inertial-upper-stage, and a conceptual solar-electric-propulsion-system with a nominal rendezvous 50 days before comet perihelion. Additional trajectory and propulsion enhancements are investigated for several comet rendezvous missions in order to improve spacecraft performance. An indirect transfer trajectory for an Encke mission is shown to offer substantial delivered payload. In addition the use of concentrator solar arrays is considered for several of the more interesting comet missions.

Sauer, C. G., Jr.↗
NASA NTRS1980

Dynamical and photometric investigation of comets

The findings of dynamic and photometric investigations of comets are summarized, and include discussions of the comets Bennett 1970 II, Kohoutek 1973f, West 1976 VI, and periodic comets d'Arrest, Encke, and Swift Tuttle. The phenomena examined include striated and anomalous tails, tail composition and the dynamics of vaporizing dust particles, the evolution of dust jets, and split and dissipating comets.

Sekanina, Z.↗
NASA NTRS1976

Ultraviolet spectroscopy of comets

The advantages of the space shuttle being used as an observing platform with advanced instrumentation for the study of comets are discussed. The ground-based observations of the comet West (1976) were used to illustrate the many problems caused by the Earth's atmosphere. The spectrum of comet Bennett (1970 II) was compared to that the of comet West in order to correct instrument errors.

Lillie, C. F.↗
NASA NTRS1980

Abundance correlations among comets

Results of narrowband filter photometry are presented for comets 1977g, 1978b, 1978f, 1978j, 1979c, and 1979i. There appears to be a remarkable homogeneity among comets with regard to the relative production rates for those species which are readily measured, specifically C2, CN, C3, and OH. Most comets appear to show identical relative abundances, except for a rather well-defined variation with heliocentric distance. Molecular production rates appear to be unrelated to either the emission-to-continuum (gas-to-dust) ratio or the dynamical age of the comet. The correlation with visual magnitudes is excellent, though the slope is not what could be intuitively expected.

Ahearn, M. F.↗
NASA NTRS1981

Hydrogen production rates from ground-based Fabry-Perot observations of comet Kohoutek

The only ground-based observations of a cometary hydrogen corona that have been obtained up to the present were carried out during the appearance of comet Kohoutek (1973 XII). Hydrogen Balmer alpha (H-alpha) emission from the gas cloud surrounding the comet was detected using a Fabry-Perot spectrometer at Kitt Peak National Observatory. These observations have been reexamined using (1) recently obtained solar full-disk Lyman beta emission line profiles, (2) a new calibration of the absolute sensitivity of the Fabry-Perot spectrometer based on comparison of NGC 7000 with standard stars and the planetary nebula NGC 7662, and (3) corrections for atmospheric extinction instead of the geocoronal H-alpha comparison method used previously to obtain comet H-alpha intensities. The new values for hydrogen production rates are in good agreement with results obtained from Lyman alpha observations of comet Kohoutek.

Scherb, F.↗
NASA NTRS1981

Observing chemical abundances in comets

The atomic resonance lines of the major elements were observed in the atmospheres of a few comets, by using vacuum ultraviolet spectrographs on board rockets or orbiting observatories. Dust-to-gas ratios were also deduced for two comets through a Finson-Probstein's analysis of their dust-tail isophotes. The geometric albedo of the dust for the phase angle alpha of the observations is not accurately known but, the dust-to-gas ratio is not overly sensitive to the actual value of this albedo. Infrared observations of the dust head of some comets show that the bulk of cometary dust must be silicates, although a minor component (5-10 percent) of carbon compounds is rather likely, because of poor dielectric properties of the grains. This interpretation is confirmed by the fact that interplanetary dust probably of cometary origin, that was collected in the stratosphere by NASA-U2 Spacecraft, is chondritic in nature. Metal abundances in the head of a sungrazing comet support the chondritic hypothesis.

Delsemme, A. H.↗
NASA NTRS1981

Ground-based photometry of comets in the spectral interval 3000 to 3500 angstrom

Abundances and production rates of CN, C3, and C2 for fifteen comets were determined and the variation of these parameters with helicentric distance for two comets were monitored. While these measurements provide much information about the similarities and differences among comets, only a small fraction of the total material in any of the comets observed was sampled.

Willis, R. L.↗
NASA NTRS1981

A search for millimeter-wave emission from HCN and other molecules in Comet Bradfield /1979l/

Limits on column densities and production rates are derived in light of negative results from a search for HCN, CS, and CH3OH millimeter-wave emissions, along with two unidentified lines observed in Comet Kohoutek (1973 XII), from Comet Bradfield (1979l) after perihelion passage. In the case of HCN, the production rate is found to be lower than that reported for Comet Kohoutek even upon scaling of measurements relative to the observed OH emission from each comet.

Ekelund, L.↗
NASA NTRS1981

Comets and the origin of life; Proceedings of the Fifth College Park Colloquium on Chemical Evolution, University of Maryland, College Park, MD, October 29-31, 1980

Papers are presented concerning the characteristics of comets and their possible role in the origin of life. Specific topics include the characteristics, origin and structure of the cometary nucleus, cometary chemical abundances, the nature of interplanetary dust and its entry into terrestrial planet atmospheres, and the mechanism of ray closure in comet tails. Attention is also given to chemically evolved interstellar dust as a source of prebiotic material, the relation of comets to paleoatmospheric photochemistry, comets as a vehicle for panspermia, limits to life posed by extreme environments, and the status of cometary space missions as of 1980.

Ponnamperuma, C.↗
NASA NTRS1982

Comet Bowell /1980b/ - An active-looking dormant object

The absence of molecular emissions in the spectrum of Comet Bowell, a nearly constant amount of dust in its coma, a slow expansion rate of the coma and implied low particle velocities, a tail width that does not increase with increasing distance from the nucleus, the absence of tail particles less than 0.5 mm in diameter, and the development of an elongated coma, lead to the conclusion that the solid particles in the coma and tail are not recent ejecta, and that the comet has probably been dormant. It is speculated that 10 to the 13th g of observed coma and tail particles is either a leftover pristine material that has never contacted the nucleus surface, or a product of erratic activity associated with chemical instability, at temperatures below 40 K, stimulated by cosmic ray and/or UV irradiation of the surface layer during the comet's Oort cloud stay. It remains possible that the comet may temporarily become active near perihelion.

Sekanina, Z.↗
NASA NTRS1982

Halley's Comet

Since 240 B.C., Chinese observers have documented a nearly unbroken record of scientifically useful observations of Periodic Comet Halley (P/Halley). Investigations of the comet's motion by Western astronomers are discussed, taking into account the first successful prediction of a cometary return by Halley (1705), computations conducted by Rosenberger (1830), and studies performed by Cowell and Crommelin (1910). Comet Halley's motion and nongravitational forces are considered along with meteor showers associated with P/Halley. The physical properties of P/Halley are examined, giving attention to the visual observations, the light curve of P/Halley, the coma, the tails, direct photographs, spectrograms, and the emission spectrum of P/Halley. Other subjects explored are related to the cometary nucleus, the mass of P/Halley, the rotation period and axial inclination, the composition, a nominal model of P/Halley's coma, and plans for investigations in connection with the coming apparition of Comet Halley.

Newburn, R. L., Jr.↗
NASA NTRS1982

Narrowband photometry of Comet P/Stephan-Oterma and the backscattering properties of cometary grains

The results of narrowband filter photometry of Comet P/Stephan-Oterma are presented. For heliocentric distances between 1.95 and 1.58 AU prior to perihelion, OH, CN, and C2 production rates varied approximately as r to the -5th power. Following perihelion, an even steeper dependence of production rate on heliocentric distance was observed for these species. A pronounced brightening of the comet's continuum near opposition is interpreted in terms of preferential backscattering by the cometary dust. While Comet P/Stephan-Oterma is one of the dustier comets observed, it shows no indication of compositional anomalies in either the gas or the dust.

Millis, R. L.↗
NASA NTRS1982

The rotation of comet nuclei

Spin-vector research on cometary nuclei is reviewed with emphasis on the actual determination of rotation period and spin-axis orientation. The rotation periods of 47 comets are compared with those of 41 asteroids with diameters of not more than 40 km. It is shown that the median periods for the comets is 15.0 hr as compared with 6.8 hr for the asteroids and that the preliminary distribution curve for the logarithms of the comet periods is not Gaussian and is flatter than the corresponding curve for the asteroids. Slow accumulation at low relative velocities is suggested as the cause of the longer comet rotation periods.

Whipple, F. L.↗