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At least 613 records · Page 34

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NASA NTRS1972

Offset guiding through large space telescopes.

Discussion of the merits of various space telescope system designs, both of the Ritchey-Chretien and Gregorian kinds. In particular design data for an f/2.2 to f/12.0 Ritchey-Chretien system with a five-element telecentric field corrector and for another f/3.3 to f/15.0 similar system with a two-cylindrical-element telecentric field corrector are reviewed, along with data for an f/2.5 to f/15.0 Ritchey-Chretien system with a fold mirror corrector and for an f/2.2 to f/12.0 coma-corrected Gregorian system with a five-element telecentric field corrector. The transverse aberrations, optical path differences, and other characteristics of these system design varieties are examined.-

Schulte, D. H.↗

NASA NTRS1971

Cometary Science Working Group, Meeting, Williams Bay, Wis., June 9-11, 1971, Proceedings.

The scope of cometary research is delineated in papers covering the present state of knowledge, detailed objectives of further cometary investigations, the need for ground-based observations, and the types of space missions which would be most suited in the period from 1975 to the mid-1980's. Major contributions of space missions are (1) to identify parent molecules released by the nucleus before they dissociate in the coma, (2) to measure the composition of tail constituents and the electromagnetic environment in the tail, and (3) to define the nature of the cometary nucleus. Experimental payloads and mission parameters are identified for fast flyby, slow flyby, and rendezvous missions to particular comets.

Roberts, D. L.↗

NASA NTRS1973

Comet exploration - Scientific objectives and mission strategy for a rendezvous with comet Encke.

A rendezvous mission which includes an extended stay of 80 days or longer in the vicinity of the comet is considered. Such a mission with exploration maneuvers through the coma and tail and circumnavigation of the nucleus is made feasible by the use of solar-electric propulsion. The physical characteristics of comet Encke are examined, giving attention to the icy conglomerate model of the nucleus, solar wind interaction, and chemical composition. Scientific objections are discussed together with mission profile options.

Meissinger, H. F.↗

NASA NTRS1973

Galaxy formation associated with active galaxies and the binding of rich clusters.

A mechanism is proposed for recent formation of dwarf galaxies in the vicinity of active radio galaxies. In the cores of rich clusters these dwarf galaxies are stripped of gas and eventually suffer tidal disruption. This leads us to suggest that the Coma cluster of galaxies may be bound by intergalactic halo-type stars.

Silk, J.↗

NASA NTRS1973

Kohoutek - A great comet coming.

Passing inside the earth's orbit in late November, the comet Kohoutek will travel through the inner solar system during a unique period in the history of the space program, when Skylab and Mariner Venus-Mercury are in operation and the new C-141 Airborne Infrared Observatory is ready for flight. It is planned to investigate comprehensively the nature and evolution of the coma and tails of the comet. The detailed goals of the investigation include the identification of the parent molecules of the gases observed in comets, the determination of the processes that break down the parent molecules, the study of the physical nature of transient events in the comet, and the measurement of the solar-wind velocity in the inner solar system.

Maran, S. P.↗

NASA NTRS1973

A new model for cometary nuclei.

A new model for the nucleus of comets is presented, hypothesizing formation at large heliocentric distances from many independent solid bodies. It is shown that such a configuration would collapse to a single assemblage if it is to survive into the inner solar system. Prior to collapse, the bodies would be subject to coating by interstellar gas and particles, which would form the material lost into the coma at subsequent inner solar system perihelia. Quantitative estimates place an upper limit to the body sizes of 2.3 m and a lower limit of the number as 3 x 10 to the 10th power with sizes of a few tenths of a micron and numbers of about 10 to the 33rd power most probable. The major structural and evolutionary features of such comet nuclei are consistent with the Whipple icy-conglomerate model.

O'Dell, C. R.↗

NASA NTRS1973

Study of 1980 Comet Encke-asteroid missions using a spin-stabilized spacecraft. Volume 2: Technical Report

The data and analyses that support the conclusions and recommendations of a space mission to intercept comets and asteroids are presented. The scientific basis for the Encke missions is discussed and includes a summary of the physical characteristics of Encke, a statement of scientific objectives and priorities, a definition of the spacecraft payload, and a rationale for utilization and instrumentation of coma probes. Data are included for the reference mission selected as the baseline for science analysis and design studies. Alternate spacecraft configurations are compared in terms of their capability in meeting the science and mission requirements.

Bursnall, W. J.↗

NASA NTRS1974

A study of X-ray emission from galactic and extragalactic sources with emphasis on soft and ultra-soft wavelengths

A Black Brant VC rocket was used to scan the Coma and Virgo clusters in order to measure structure in the X-ray sources. The rocket also made measurements of soft X-ray spectra, soft X-ray background flux during a 50 deg scan of the sky, soft X-rays from De Voucoulers 50, set limits to the energy dependence of soft X-ray background spectra, and the flux of solar 584 A radiation resonantly scattered by interstellar He flowing through the solar system.

Bowyer, C. S.↗

NASA NTRS1974

Particle sizes in Comet Bennett /1970 II/

The particle size distribution in the coma and tail of Comet Bennett has been determined by several methods, each sensitive to a particular size range. It is confirmed that a minimum value of the particle density, size, and radiation pressure efficiency function exists at about .00003 to .00010 g/sq cm. The existence of such a cutoff is probably due to the decreasing radiation pressure efficiency for particles smaller than the wavelength of the light being scattered. An exact determination of this cutoff may allow identification of the particle type.

O'Dell, C. R.↗

NASA NTRS1974

Magnetisation of comets

Construction of a model explaining the means by which the interplanetary magnetic field could mix with the plasma of a comet tail. It is suggested that the magnetic fields in comet tails derive from the hydromagnetic conversion of kinetic energy into magnetic energy in the nuclear region of the comet. In particular, it is shown that, if the nuclear region consists of a supplementary distributed source for the coma gases, a turbulent flow with maximum velocities of the order of thermal velocity may be expected in this region, with the necessary ordering in the turbulent velocity field provided by the rotation of the nuclear region. It is concluded that a comet, when sufficiently close to the sun, may be able to generate an appreciable magnetic field and that the magnetic fields observed in the tail may then result from processes analogous to those producing the earth's magnetotail.

Mendis, A.↗

NASA NTRS1974

Behaviour of comet Kohoutek /1973f/

The spectral identification of CH3CN and HCN provides the first support for the hypothesis first proposed by Wurm (1943), of chemically stable 'parent molecules' for the less stable radicals and ions seen in the coma and the tail. These two molecules were among the earliest discovered in dense interstellar clouds. Circumstantial evidence for the presence of water as the dominant volatile component in the nucleus has been growing for some time. However, a much more volatile species is required to explain the observed behavior of the comet. CO, formaldehyde, or methane would quickly evaporate as the comet approached the sun.

Mendis, D. A.↗

NASA NTRS1974

Two well-corrected four-mirror telescopes

A study has been conducted of a configuration resembling that of a Cassegrain telescope with two nested mirrors in the plane of the primary and two nested mirrors in the plane of the secondary. The focal plane was located in the plane of the first and the third mirror. The four surfaces available in the device for corrections made it possible to correct for third-order spherical aberration, coma, astigmatism, and distortion.

Korsch, D.↗

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

Cometary Physics

A history of comets and cometary observations is presented. Emphasis is placed on the observations and characteristics of the Comet Kohoutek. A schematic drawing of a comet is included to show the structure of the nucleus, coma, dust tail, ion tail, and natural hydrogen cloud. A joint observatory for cometary research, located in New Mexico, is described. Observations of magnetic fields in comets are analyzed. Infrared spectral observations of comet structure in the 10 micron atmospheric window are reported.

Source record↗

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 NTRS1975

Early Mariner comet flyby

An inexpensive first mission to a faint short period comet utilizing the spare Mariner Venus/Mercury spacecraft is studied with regard to science rationale and instrumentation and engineering feasibility and design. The spare Mariner spacecraft, suitably modified, is shown to be compatible with science requirements of a first comet flyby. Important aspects of this early mission would be penetration of the comet's gaseous coma and the approach of the nucleus to within hundreds of km in order to obtain in situ measurements and imaging that will help to establish comet science models and determine the hazards of a comet environment to future comet missions.

Nock, K. T.↗

NASA NTRS1975

The light curve of Comet Kohoutek

Visual estimates of total coma brightness define the light curve of Comet Kohoutek between Nov. 24, 1974 and Feb. 6, 1974. These data are well fitted by straight lines on the M-log r diagram. The preperihelion value of n is 2.2, and the post perihelion value is 3.8 up to January 16. A standstill in the decline is suspected between January 16 and 19. If the standstill is ignored the postperihelion data can be fitted less precisely by a single line with n equals 3.3. From photoelectric measures on four nights between January 5 and 13 after perihelion, n is found to be 4.1. The average B-V with a 32.4 arc sec diaphragm for those four nights is about 0.79; no clear systematic variation with time is noted.

Angione, R. J.↗

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.↗