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At least 811 records · Page 45

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NASA NTRS1979

Optical spectrophotometry of comet Tempel 2 far from the sun

With the advent of mission planning for spacecraft encounters with comets in the 1980s, the lack of basic data on cometary nuclei has become quite apparent and a real problem. More effort is needed on all comets, with special attention given to probable space flight targets such as Tempel 2. The paper presents medium-spectral-resolution spectrophotometry of P/Tempel 2, made at heliocentric distances of 2.66 and 3.02 AU on two dates in 1978. It is shown that optical spectrophotometry of comet Tempel 2 near 3 AU heliocentric distance places strict limits on its activity, the modeled CN production rate having an upper limit of 6 times 10 to the 21st molecules per sec. The observations of P/Tempel 2 are compatible with a bare nucleus of 1.6 km radius (geometric albedo = 0.15) and somewhat redder than the sun.

Spinrad, H.↗

NASA NTRS1980

Thermal history, chemical composition and relationship of comets to the origin of life

The role of thermal processes in determining the chemical composition of comets is considered, and implications of possible cometary constituents for the origin and evolution of life on earth are discussed. It is shown that the inclusion of short-lived Al-26 from a nearby supernova explosion into cometary nuclei could lead to comets with surfaces cool enough to retain H2O and interiors warm enough for thermal processing to occur, with the production of complex organic molecules such as amino acids and nucleic acid bases. It is thus suggested that comets may have played a part in seeding the primitive earth with biological polymers capable of self-replication or of evolving towards that capability, and may even be responsible for the subsequent introduction of organic material capable of infecting already existing cells.

Irvine, W. M.↗

NASA NTRS1980

Neutral cometary atmospheres. II - The production of CN in comets

Brightness profiles of the CN (0-0) band at 3883 A have been constructed from spectrograms of comets Bennett (1979 II) and West (1976 VI). The subsequent analysis of these profiles shows that the parent molecule of CN had a radial scale length of (2.19 plus or minus 0.07) x 10 to the 4th r (H-squared), in kilometers. This, combined with current theories as well as photochemistry, is consistent with production by simple photodissociation of HCN. The rather random variation of the radial scale length for the apparent decay of CN is indicative not of a true decay scale length, but of the fact that no steady state in the CN parent production rate is achieved for the time scale necessary to build up the observed profile. A revision of the average CN production law with heliocentric distance from published photometry indicates an r(H to the -2nd) law for comet West out to at least 2.555 AU. This implies that the vaporization of this comet was controlled by some species more volatile than water. Based on this and other evidence, CO2 is suggested, but even more volatile molecules like CO cannot be ruled out.

Combi, M. R.↗

NASA NTRS1981

A photometric investigation of Comet P/Encke

A photometric investigation of Comet P/Encke was undertaken using the 2.2 m telescope and the 0.6 m telescope. The two primary scientific objectives were: (1) measurement of the production rates of OH, CN, C3, and C2 as a function of heliocentric distance; and (2) determination of the comet's rotational period by searching for periodic brightness variations in the inner coma. Synoptic observation in the Canary Islands; Cambridge, Massachusetts; Flagstaff, Arizona; and Mauna Kea were planned for the study of the comet's rotation.

Millis, R. L.↗

NASA NTRS1980

A very rapid turning of the plasma-tail axis of comet Bradfield 19791 on 1980 February 6

Schmidt camera photographs of comet Bradfield 19791 obtained at the Joint Observatory for Cometary Research (JOCR) indicate that a rapid change took place in the comet's plasma tail on 1980 February 6. On that date, a sequence of photographs spanning 27.5 minutes shows a 10 deg shift occurring in the plasma-tail axis between the first and last exposures. The speed of this tail-turning event greatly exceeds that of any other known event and even exceeds turning rates for individual tail streamers. An interpretation based on the windsock theory of plasma tails is that the comet entered a region of rapidly changing solar-wind flow direction. While the search for an associated solar-wind event from near-earth spacecraft observations is a future activity, the present analysis shows that a 50 km/s change in the polar component of the solar-wind velocity, from about 30 km/s northward to about 20 km/s southward, would have produced the 10 deg shift in the tail axis.

Brandt, J. C.↗

NASA NTRS1980

Pristine nature of comets as revealed by their UV spectrum

Satellite and rocket VUV observations of comets are discussed with reference to observations of OH and NH; the Lyman-alpha halo of hydrogen; and C, O, S atoms, ions, and molecules. Attention is also given to the study of production rates, and elemental and molecular abundances. It is shown that the observation of the VUV spectrum of comets has revealed new clues about comets, which could serve as a link between interstellar molecules and planetary atmospheres as well as between stellar and planetary astrophysics.

Delsemme, A. H.↗

NASA NTRS1980

Pointing control for the International Comet Mission

The design of the pointing control system for the proposed International Comet Mission, intended to fly by Comet Halley and rendezvous with Comet Tempel-2 is presented. Following a review of mission objectives and the spacecraft configuration, design constraints on the pointing control system controlling the two-axis gimballed scan platform supporting the science instruments are discussed in relation to the scientific requirements of the mission. The primary design options considered for the pointing control system design for the baseline spacecraft are summarized, and the design selected, which employs a target-referenced, inertially stabilized control system, is described in detail. The four basic modes of operation of the pointing control subsystem (target acquisition, inertial hold, target track and slew) are discussed as they relate to operations at Halley and Tempel-2. It is pointed that the pointing control system design represents a significant advance in the state of the art of pointing controls for planetary missions.

Leblanc, D. R.↗

NASA NTRS1981

A study of central cryogenic cooling system for the comet rendezvous spacecraft

Several science instruments required temperatures between 100 K and 120 K on a proposed Halley Flyby, Tempel-2 Rendezvous Mission. Significant features of the thermal environment are a large variation in heliocentric as well as comet distance, very large solar panels for a Solar Electric Propulsion stage, and the comet dust environment. The best cooling is achieved by one central radiative cooler connected by insulated cryogenic heat pipes to the instruments. The conceptual design is of a single-stage rectangular shielded radiator deployed on a boom some distance from the spacecraft bus and solar panels. Thermal modeling determined sensitivity to solar and comet distance and was used to optimize cooler geometry.

Salazar, R. P.↗

NASA NTRS1981

Narrow passband imagery of comets

During an emission-line survey of the Milky Way, Comet West was accidently imaged through four different narrow passbands with a wide-field, image-intensified camera. Three passbands recorded very similar head plus tail structure. The fourth passband shows an additional large, diffuse component around the head. It was serendipitous that such was recorded as the filters, being selected for studies of emission nebulae, are not particularly suited for studies of comets. However the imagery, plus subsequent studies, suggests that much can be learned about the structure of comets using narrow passband imagery simultaneously with long slit spectroscopy.

Gull, T. R.↗

NASA NTRS1982

Spectroscopic observations of comets

Development of a spectrograph using a microchannel plate intensifier for observing faint comets is described. The spectrograph is capable of obtaining useful spectra of objects as faint as M(2) = 18. The increased guiding efficiency achieved by the optical coupling of the ISIT vidicon of the 154 cm telescope has resulted in a better signal to noise ratio. The ability to take a direct image of the comet aids in the interpretation of the spatial profile of the emissions. Spectra of comets Schwassmann-Wachmann 1, Bradfield, Encke, Tuttle, and Stephen-Oterma are discussed.

Source record↗

NASA NTRS1981

Optical polarimetry of Comet West 1976 VI

Four narrowband filters covering the 440-850 nm wavelengths were used to measure the polarization of the continuum of Comet West 1976 VI. Postperihelion observations indicated wavelength-independent linear polarization in all of the three measurements made, in agreement with other polarization measurements of the comet from the visible to the near-infrared but in contrast with the general tendency in comets toward polarization increase with wavelength. No circular polarization was detected, and it is suggested by (1) the magnitude of polarization as a function of scattering angle, (2) wavelength independence, and (3) infrared and optical photometric properties, that dirty silicates with radii approaching 5 microns may be responsible for the phenomena observed.

Michalsky, J. J.↗

NASA NTRS1981

Interaction of comets with the interplanetary medium

The interaction of solar radiation and the solar wind with a comet as it moves around the sun is discussed. The overall structure of the comet-solar wind interaction, which is believed to give rise to a tangential discontinuity surface excluding the solar wind from the cometary ionosphere, a collisionless bow shock serving to decelerate and divert the solar wind, and a weak outer shock, is considered. The variable nature of the interaction as the comet moves around the sun and as solar wind conditions vary is then examined. It is shown that the cometary neutrals play a dominant role in standing off the solar wind, particularly under quiet conditions, making the cometary ionosphere in general highly incompressible. Conditions are found under which cometary neutrals cannot penetrate the ionopause to interact with the solar wind, and the inner shock becomes a hybrid ion-neutral shock. Finally, consideration of the direct interaction of the solar wind and solar ultraviolet radiation with the cometary nucleus indicates that electrostatic charging of the surface is capable of causing transport and blow-off of cometary dust at large heliocentric distances.

Mendis, D. A.↗

NASA NTRS1981

Comets and the origin of life

The possible role of comets as sites for chemical and/or biological evolution is considered from both the positive and the negative viewpoints. On the negative side, it is pointed out that direct evidence for the chemical composition and size of cometary nuclei is lacking and the thermal history of comets is unknown, so that the length of time that liquid water could have been available ranges from zero to over a billion years. On the positive side, there remains the possibility that there may have been comets which retained liquid water in their interior for a sufficient time and that have more complex molecules than the amino acids and nucleic acid bases found in carbonaceous chondrities, and that the cometary environment may have been more favorable to the origin of life than the terrestrial. It has also been demonstrated that cometary material falls to earth at a significant rate. Problems in understanding the course of chemical evolution on earth may thus have a solution in cometary processes.

Irvine, W. M.↗

NASA NTRS1982

The ultraviolet bands of the CO2/plus/ ion in comets

Eight comets are studied with the International Ultraviolet Explorer spectrographs. The existence of the CO2(plus) ion in a comet is confirmed through the presence of the 2890 A doublet in at least three of these objects. Spatial and spectral resolution obtained in comets Bradfield (1979 X) and Seargent (1978 XV) permit a discussion of the production mechanisms of this ion. The spectra reveal new ionic features in the 3100-3400 A range, which are attributed to resonance fluorescence of the Fox-Duffendack-Barker system of the CO2(plus) ion and, near 3350 A, to the OH(plus) ion.

Festou, M. C.↗

NASA NTRS1982

Comets - Chemistry and chemical evolution

Research on the chemical composition and conditions in comets and their possible role in the origin of life on earth is surveyed. The inorganic and organic compounds and ions indicated in the ultraviolet and visible spectra of comets are noted, and evidence for the existence of at least a small proportion of complex organic molecules in comets is presented. It is then pointed out that while cometary material could have reached the earth and provided volatile elements from which biochemical compounds could have formed, it is unlikely that a cometary nucleus could have withstood the temperatures and pressures necessary to sustain an environment in which life could have originated.

Donn, B.↗

NASA NTRS1982

Halley's comet and plans for its observation during its return in 1986

The general characteristics and properties of Halley's comet, as they can be derived from earlier apparitions, are briefly discussed. Consideration is given to historical data on the comet, orbital characteristics, and physical characteristics, including the estimated diameter and density of nucleus, rotation period, observed spectra, tails, and associated meteor streams. The coming 1985/86 apparition is then discussed together with planned space missions for in situ observations. Finally, the major goals of the International Halley Watch, a comprehensive program to prepare for the next apparition of Halley's comet, are reviewed.

Rahe, J.↗

NASA NTRS1982

The spectrum of comets as derived from IUE observations

Eight comets were observed with the IUE at various-heliocentric and geocentric distances. Their UV spectra are remarkably similar despite the large differences in the dust to gas ratios. Since all the dominant atomic species (except N) radicals and ions of the coma are detected in this spectral region, the total gaseous output of the nucleus can be estimated. The abundance of the carbon atom-bearing species is still not very well known and there are indications that the CO content of the coma could vary from comet to comet.

Festou, M. C.↗

NASA NTRS1982

A CO2-rich coma model applied to the neutral coma of Comet West

Models of the cometary coma in which the dominant volatile is CO2 have been constructed for a range of heliocentric distances. Model coma abundances of C2, C3, and CN are compared with the abundances observed in Comet West and are found to be in good agreement. Furthermore, the variation with heliocentric distance of C2, C3, and CN model abundances agree well with the observed variation in Comet West. The present work lends detailed support to a previous suggestion that a substance more volatile than water, such as CO2, controls the evaporation of the nucleus of Comet West. The implications for cometary formation are briefly discussed.

Mitchell, G. F.↗