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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 1,135 records · Page 63
All sourcesNASA NTRSDOE OSTI
NASA NTRS1975

Terminal guidance and navigation for comet and asteroid rendezvous

A terminal guidance and navigation scheme developed in earlier work was modified and evaluated for a solar electric propulsion rendezvous mission to comet Encke. The scheme is intended for autonomous, on-board use. The guidance algorithm is based on optimal control theory and minimizes the time integrated square of thrust acceleration. The navigation algorithm employs a modified Kalman filter set in measurement variables. Random sequences were generated to simulate measurement errors, and the evaluation was conducted with detailed numerical computations which include actual motions of spacecraft and comet. The evaluations showed that the scheme attains rendezvous and maintains station after rendezvous within less than 10 km for estimated best measurements and within less than 100 km for estimated worst measurements. The measurements required are angles, range, and range rate. Angles and range appear to be absolutely necessary; range rate is not as strong a measurement type, and further modifications of the filter will allow a scheme that does not require the rate measurements.

Bennett, A. G.↗
NASA NTRS1974

High resolution spectrophotometry of selected features in the 1.1 micron spectrum of Comet Kohoutek /1973f/

Fabry-Perot interferometry of Comet Kohoutek (1973f) at 1.1 microns with a resolution of 1.2 A showed emission features identified as OH and CN lines in addition to a strong Fraunhofer continuum. Central intensities have been derived for three cases (uniform, Gaussian, and Gaussian plus inverse-rho law) of brightness profiles in the comet coma. Limits for CH4, H2O, HeI, SiI and CrI are also derived.

Meisel, D. D.↗
NASA NTRS1974

Possible influence of comets on the chemical evolution of the galaxy

It has been suggested by Cameron (1973) that a cloud of comets containing a mass of condensable elements, comparable to the mass of such elements in the sun, formed on the outskirts of the solar system. If the formation of such comet clouds is a general feature of star formation, they constitute a significant sink of elements heavier than helium. It is shown here that this process provides a possible explanation for the very slow rate at which the mean metal abundance of disk stars has increased during the lifetime of the Galaxy.

Tinsley, B. M.↗
NASA NTRS1975

Imaging on ballistic missions to comet Encke

Imaging for navigation and science has been studied for a 1980 Encke flyby at 0.4 to 0.8 AU from the sun with spinning and three-axis spacecraft. Trajectory errors, maneuvers, encounter geometry, imaging performance, and data transmission were considered. Onboard comet sightings are needed for navigation. Recommended for a three-axis spacecraft are two vidicon cameras and a nucleus sensor for closed-loop pointing control. The cameras are essentially the Mariner 9 and Mariner 10 instruments; the nucleus sensor is an updated version of a sensor flown on Mariners 6 and 7. For a spinning spacecraft, a framing camera using a charge-coupled device and a spin-scan photometer are proposed. The framing camera would be a new design; it would not be despun, but the spin axis should point along the comet-centered velocity vector. The photometer is essentially that flown on Pioneer 10 and 11.-

Jaffe, L. D.↗
NASA NTRS1975

Shadow of the head of Comet Kohoutek

An earlier reported shadow of the head of Comet Kohoutek (1973f) was not found in later photographs of the comet. However, linear dark areas were seen and are described as the space between the plasma and the dust tails. The discrepancy in earlier observations is attributed to low resolution in them. Observation of a shadow is also discounted in terms of cometary physics, due to low dust content and the weakness and curvature of the dust tail.

Brandt, J. C.↗
NASA NTRS1974

Preliminary results on Comet Kohoutek - Interactions with the solar wind

A preliminary summary is presented for the types of data obtained on possible interactions between Comet Kohoutek and the solar wind. The data include Pioneer 8 solar wind density measurements along a line of sight through or near the comet's position; Pioneer 6 plasma measurements near the projected position of the ion tail 100 million km from the nucleus; Lyman-alpha observations of the cometary hydrogen cloud from Skylab and Mariner 10; and ground-based radio, spectroscopic, and photographic observations. Preliminary analysis of some data is provided.

Brandt, J. C.↗
NASA NTRS1975

Development of a comet as it pursues its orbit

The properties of cometary dust-swarms in almost parabolic long-period orbits are examined. The effect of passage of the solar system through interstellar gas clouds is shown to be capable of affecting substantially the angular momentum of a comet about the sun, thus accounting for the existence of comets with high values of perihelion distance. The same process would enable cometary particles to adsorb interstellar gases and regenerate their gas content.

Lyttleton, R. A.↗
NASA NTRS1975

A study of the icy tails of the distant comets

Properties are studied of the icy grain model for the tails of comets Baade 1955 VI and Haro-Chavira 1956 I, which have large perihelion distances. The general profiles and orientations of the tails are compared with a set of synchrones, or theoretical trajectories of particles ejected from a cometary nucleus at given instants of time. Several features observed in these comets are shown to be explained by the icy grain model, including jet activity in the coma and no appreciable broadening of the tail with increasing distance from the nucleus. It is demonstrated that the transition region between 2 and 3 AU, where water snow begins to experience rapid evaporation, has a significant effect on the dynamics of icy tails. It is suggested that the icy grains constituting these cometary tails may carry fine particles of meteoric dust which are released when the grains begin to disintegrate by evaporation.

Sekanina, Z.↗
NASA NTRS1976

Photoelectric photometry of comet Kohoutek (1973f)

Comet Kohoutek was observed with the 50 cm (f/15) reflecting telescope of the European Southern Observatory, La Silla, Chile, on fourteen nights between January 16 and 30, when the heliocentric and geocentric distances of the comet were r=0.66 - 1.00 A.U. and delta=0.81 - 0.96 A.U., respectively. The 40 inch and 80 inch diaphragms were used for the photometry of the cometary head in the UBV system and with six interference filters. The atmospheric conditions were good but the accuracy of observations was low due to large extinction and twilight. The mean error of one measurement of log F in all but Na 5893 A filters can be estimated at plus or minus 0.02, whereas the accuracy through the Na filter was substantially lower.

Kohoutek, L.↗
NASA NTRS1976

Narrow band photometry of comet Kohoutek

Photometric observations of the coma of comet Kohoutek were made at the Cassegrain focus of a 36-inch telescope. The observations consisted of one wide (visual, 5454 A) and six narrow (CN, 3879 A; C3, 4057 A; C2, 4732 A, 5165 A, 5634 A; continuum, 5200 A) band interference filters. In addition each filter was used with six diaphragms. Good quality data were obtained on 13 days between November 1973 and February 1974. A small flare was observed on 1 December for all filters, a CN flare on 13 January, and a visual flare on 28 January. The data were reduced to absolute narrow band magnitudes of the comet for the 13 days. The radial dependence of the surface brightness was derived from the set of diaphragms and future work will be directed toward using these results for modeling density distributions for the coma.

Brown, L. W.↗
NASA NTRS1976

Spectrophotometry of comet Kohoutek (1973f) during pre-perihelion period

Scans of the head of comet Kohoutek obtained during the pre-perihelion period with a photoelectric spectrum scanner are discussed. The mean relative flux distributions and the continuum energy distribution of the head of comet Kohoutek are given and normalized to 479 nm. The emission features of CN, C3, Ch, and the principal Swan band sequences of C2 and Na are identified and discussed. The adopted monochromatic values relative to 479 nm are given and plotted along with the energy distribution of the sun.

Babu, G. S. D.↗
NASA NTRS1976

Nongravitational forces on comets

Methods are presented and discussed for determining the effects of nongravitational forces on the orbits of comets. These methods are applied to short-period and long-period comets. Results are briefly described.

Marsden, B. G.↗
NASA NTRS1976

Review of investigations performed in the USSR on close approaches of comets to Jupiter and the evolution of cometary orbits

Methods are reviewed for calculating the evolution of cometary orbits with emphasis on the orbital changes that take place when comets pass within the spheres of action of giant planets. Topics discussed include: differences and difficulties in methods used for the calculation of large perturbations by Jupiter; the construction of numerical theories of motion covering the whole period of observations of each comet, allowing for planetary perturbations and the effects of nongravitational forces; and investigations of the evolution of cometary orbits over the 400 year interval 1660-2060. The classical theory of cometary capture is briefly discussed.

Kazimirchak-Polonskaya, E. I.↗
NASA NTRS1976

The study of comets, part 2

Flyby missions and systematic observations of comets are projected for studying comet nuclei and cometary dust tail structures.

Donn, B.↗
NASA NTRS1976

On the origin of comets

Physico-chemical processes leading to the dynamic formation and physical evolution of comets are reviewed in relationship to the various theories that propose solar origins, protoplanetary origins, planetary origins and interstellar origins. Evidence points to the origins of comets by the growth and agglomeration of small particles from gas and dust at very low temperatures at undetermined regions in space.

Mendis, A.↗
NASA NTRS1976

Comets, interstellar clouds and star clusters

The association of comets with star formation in clusters is elaborated. This hypothesis is also used to explain origin and evaluation of the Oort cloud, the composition of comets, and relationships between cometary and interstellar molecules.

Donn, B.↗
NASA NTRS1976

The wind-sock theory of comet tails

A method is reported for calculating the shapes of ionic comet tails that use the magnetic field along the tail to channel the tail plasma. The local momentum field in the solar wind determines magnetic field line locations along the tail. Thus, the magnetic field acts as a transparent wind sock in trapping field lines in the cometary plasma around the nucleus long enough to be effectively fastened to the comet's head.

Brandt, J. C.↗
NASA NTRS1976

Particles from comet Kohoutek detected by the micrometeoroid experiment on HEOS 2

HEOS B measurements on particles ejected from comet Kohoutek reflect average particle rate as a function of particle speed and mass in relation to random distribution with known speed from the interplanetary region. The micrometeoroid experiment detector onboard the satellite passed through the orbital plane of the comet and encountered ejected particles for approximately two months.

Hoffmann, H. J.↗