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At least 739 records · Page 41
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DOE OSTI2020

New Insights into the Chemical Composition of Five Oort Cloud Comets after Re-analysis of Their Infrared Spectra

We present revised results for the main molecular species in five Oort Cloud comets observed with near-infrared echelle grating spectrometer (NIRSPEC) at the Keck Observatory between 1999 and 2012 (C/1999 S4 (LINEAR), C/2001 A2 (LINEAR), C/2007 W1 (Boattini), C/2012 F6 (Lemmon), and C/2012 S1 (ISON)). The re-evaluation of these data shows the improvement of results in some of the data sets, in particular for comets observed and analyzed before the advent of new and revised fluorescence models and terrestrial retrieval methods introduced since 2011. We observe significant improvements in the resulting rotational temperatures and the production rates for all species, and in mixing ratios of minor species (relative to water). The re-analysis also allowed us to quantify species not analyzed previously, mostly due to the lack of molecular models (e.g., ammonia and formaldehyde). We note, however, that the improvement of these revised values is less substantial for comets observed (and/or analyzed) since 2010.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1972

Study of a comet rendezvous mission, volume 1

The feasibility, scientific objectives, modes of exploration and implementation alternatives of a rendezvous mission to Encke's comet in 1984 are considered. Principal emphasis is placed on developing the scientific rationale for such a mission, based on available knowledge and best estimates of this comet's physical characteristics, including current theories of its origin, evolution and composition. Studied are mission profile alternatives, performance tradeoffs, preferred exploration strategy, and a spacecraft design concept capable of performing this mission. The study showed that the major scientific objectives can be met by a Titan IIID/Centaur-launched 17.5 kw solar electric propulsion spacecraft which carries 60 kg of scientific instruments and is capable of extensive maneuvering within the comet envelope to explore the coma, tail and nucleus.

Source record↗
NASA NTRS1972

Two early missions to the comets.

Early missions to the comets Grigg-Skjellerup in 1977 and Encke in 1980 are suggested as precursors to more ambitious missions to Encke rendezvous in 1984 and Halley intercept in 1986. The spacecraft structures, power systems, telemetry, configurations, and launch vehicles considered appropriate for these two missions are actual equipments ready for applications. The mission objectives would be exploration of the nature of the solar wind interaction with comets, the composition of the gaseous cometary atmosphere, and possibly the details of the trailing portion of a comet (ion and dust tails).

Farquhar, R. W.↗
NASA NTRS1972

Reality of Comet Nucleus

The structure of the cometary nucleus is discussed. The view that the comet is a vast swarm of tiny particles separated by large distances is defended by the argument that during the meteor showers of 1966, the earth intercepted thousands of millions of meteoric particles; however, no reports of a single meteorite reaching the ground were recorded. Another feature that indicates that a comet consists of a swarm of particles is that the comets contract as they approach the sun, and expand again as they recede.

Lyttleton, R. A.↗
NASA NTRS1972

L Alpha Photometry of Comet Bennett

Results of scanning Comet Bennett in L alpha from OGO-5 are reported. A plot of the comet isophotes in L alpha is presented. The outer envelope of the comet is approximately the 50-Rayleigh contour.

Dubin, M.↗
NASA NTRS1973

The isotope ratio C-12/C-13 in Comet Tago-Sato-Kosaka 1969g.

A spectrogram of Comet 1969g obtained with the 200-inch (508-cm) Hale telescope shows a well-developed emission feature at the position of the A-X(1-0) band head of C-12C-13 (4745 A). Using new laboratory data, it is possible to remove the effect of blending with NH2 lines at this wavelength. The corrected value of the intensity of the isotopic band head is used to derive the abundance ratio C-12/C-13 = 100 plus or minus 20 for the comet. The implications of this result for models of comet formation and the early solar system are discussed.

Owen, T.↗
NASA NTRS1973

Development of an H2O atmosphere around Comet Kohoutek /1973f/ and its possible detection.

Although chemically stable 'parent molecules' for the observed radicals in comets, have been suspected for more than thirty years none of them has yet been observed spectroscopically. The newly discovered great comet Kohoutek may provide an excellent opportunity for directly observing a 'parent molecule' (namely, H2O) for the first time. The continuous development of a H2O atmosphere around the nucleus as the comet approaches the sun is discussed together with the detection of this atmosphere, using ground-based instruments and instruments carried in aircraft, balloons, and rockets.

Ip, W.-H.↗
NASA NTRS1974

Ephemeris data and error analysis in support of a Comet Encke intercept mission

Utilizing an orbit determination based upon 65 observations over the 1961 - 1973 interval, ephemeris data were generated for the 1976-77, 1980-81 and 1983-84 apparitions of short period comet Encke. For the 1980-81 apparition, results from a statistical error analysis are outlined. All ephemeris and error analysis computations include the effects of planetary perturbations as well as the nongravitational accelerations introduced by the outgassing cometary nucleus. In 1980, excellent observing conditions and a close approach of comet Encke to the earth permit relatively small uncertainties in the cometary position errors and provide an excellent opportunity for a close flyby of a physically interesting comet.

Yeomans, D. K.↗
NASA NTRS1973

Terminal navigation analysis for the 1980 comet Encke slow flyby mission

The initial results of a terminal navigation analysis for the proposed 1980 solar electric slow flyby mission to the comet Encke are presented. The navigation technique employs onboard optical measurements with the scientific television camera, groundbased observations of the spacecraft and comet, and groundbased orbit determination and thrust vector update computation. The knowledge and delivery accuracies of the spacecraft are evaluated as a function of the important parameters affecting the terminal navigation. These include optical measurement accuracy, thruster noise level, duration of the planned terminal coast period, comet ephemeris uncertainty, guidance initiation time, guidance update frequency, and optical data rate.

Jacobson, R. A.↗
NASA NTRS1974

Photometry of Comet Kohoutek /1973f/

Data are reported which provide the first large, homogeneous set of photoelectric and infrared photometry obtained for any comet. Assuming an albedo of 0.5 (a high albedo is assumed because of the large ratio of gas to dust in Kohoutek), the diameter of the nucleus of the comet is estimated to be 10 to 15 km. Based on a comparison with the infrared spectrum of Comet Bennett, it was calculated that the latter had about 16 times as much dust as Kohoutek. The data are used to obtain an evaluation of O'Dell's (1971) expression for the albedo of cometary dust grains in terms of the integrated infrared surface brightness and the surface brightness of scattered light in the continuum.

Rieke, G. H.↗
NASA NTRS1974

Energy source for comet outbursts

Development of a mechanism explaining the internal source of energy of comet outbursts. A mechanism is proposed which automatically provides a source of particulate matter which creates a huge surface area which contains a substantial percentage of amorphous ice, so that the phase transition of the amorphous ice to a cubic structure provides a release of energy which may be responsible for the outbursts observed in many comets. In addition, the volume into which the transition can propagate is estimated for a spherical comet with a radius of 5 km.

Patashnick, H.↗
NASA NTRS1974

The scale length of OH and the production rates of H and OH in comet Bennett /1970 II/

Analysis of the OAO-2 photometry of the OH-lambda-3090 and H-lambda-1216 emission features of comet Bennett (1970 II). The results are consistent with the assumption that water is the most abundant parent molecule in comets and that water vaporization controls the production rate of gas in comets at small heliocentric distances.

Keller, H. U.↗
NASA NTRS1974

Note on ultraviolet hydroxyl observations of Comet Kohoutek

Observations of hydroxyl (OH) emission from Comet Kohoutek were made during the evening of 24 January 1974, from an altitude of 41,000 feet. The comet, of visual magnitude 5.5 at the time of observation, was photographed with an f/1.3 Maksutov slitless spectrograph of 150-mm aperture. This spectrograph used a double quartz prism as the dispersion element to isolate the OH radiation in the low-resolution spectrum. A nearly spherical coma of OH about three arc minutes diameter, as observed from an earth-comet distance of 0.88 AV, was photographed. The OH emission was the strongest feature in the spectrum and produced an irradiance of 3 billionths erg per sq cm per sec at the earth.

Harvey, G. A.↗
NASA NTRS1975

Fabry-Perot observations of comet Kohoutek

Observations of H alpha, H20(+), and emission lines from comet Kohoutek were made. Analyses of H alpha line profiles and line intensities indicate that the mean outflow velocity of the hydrogen atoms was 7.8 + or - 0.2 km s(-1) and that the hydrogen atom production rate varied for comet-sun distances between 1 AU and 0.4 AU. The identification of an H20(+) emission feature in certain H alpha scans indicates that the H20(+) ions were moving in a tailward direction with a velocity of 20 to 40 km s(-1) with respect to the comet nucleus. An upper limit of 1 part in 100 was found for the D/H ratio in the cometary atomic hydrogen cloud.

Roesler, F. L.↗
NASA NTRS1975

Ballistic intercept missions to Comet Encke

The optimum ballistic intercept of a spacecraft with the comet Encke is determined. The following factors are considered in the analysis: energy requirements, encounter conditions, targeting error, comet activity, spacecraft engineering requirements and restraints, communications, and scientific return of the mission. A baseline model is formulated which includes the basic elements necessary to estimate the scientific return for the different missions considered. Tradeoffs which have major impact on the cost and/or scientific return of a ballistic mission to comet Encke are identified and discussed. Recommendations are included.

Mumma, M.↗
NASA NTRS1975

Thermal conductivity of comets

The thermal conductivity of two comet models is calculated. Both models assume the comets to be heterogeneous in composition. One model considers the comet to have a nucleus of water-ice mixed with basaltic and meteoritic material in the form of dust and agglomerated particles surrounded by a layer of water-frost. The second model assumes the frost and water-ice layers have sublimated, leaving a porous structure to some depth composed of solid basaltic and meteoritic material with residual gases. K values are calculated as a function of depth, density, temperature and porosity.

Kumar, G. N.↗
NASA NTRS1974

Visual observations of Comet Kohoutek from Skylab III

Sketches of Comet Kohoutek drawn by the author during operations on Skylab are presented. The sketches are black and white, with darker color implying a higher observed brightness. Sketches were also made of what the isophotes would have looked like if they could have been measured. In addition, color sketches made after the flight and corresponding to each of the inflight sketches are presented. The sketches show the comet at perihelion minus 10 days and at perihelion plus 1-9 days. Sketches show the comet changing appreciably in form, color, texture, and length after perihelion. The sunward spike, visible at perihelion plus one day, is no longer visible at perihelion plus 4 days.

Gibson, E. G.↗
NASA NTRS1974

Lyman-alpha imagery of Comet Kohoutek

Electrographic imagery of Comet Kohoutek in the 1100-1500 A wavelength range was obtained from a sounding rocket on Jan. 8, 1974, and from the Skylab space station on 13 occasions between Nov. 26, 1973 and Feb. 2, 1974. These images are predominantly due to Lyman-alpha (1216 A) emission from the hydrogen coma of the comet. The rocket pictures have been calibrated for absolute sensitivity and a hydrogen production rate has been determined. However, the Skylab camera suffered degradation of its sensitivity during the mission, and its absolute sensitivity for each observation can only be estimated by comparison of the comet images with those taken by the rocket camera, with imagery of the geocoronal Lyman-alpha glow, of the moon in reflected Lyman-alpha, and of ultraviolet-bright stars. The rocket and geocoronal comparisons are used to derive a preliminary, qualitative history of the development of the cometary hydrogen coma and the associated hydrogen production rate.

Carruthers, G. R.↗