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At least 1,153 records · Page 64

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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.↗

NASA NTRS1975

Spectrophotometry of Comet Kohoutek /1973f = 1973XII/

Spectrum scans have been obtained of Comet Kohoutek with a Fourier spectrometer at heliocentric distances from 0.55 to 1.05 AU. The useful spectral range is roughly 3800-8400 A and the resolution is about 10-15 A at 5000 A. Large variations occurred in the gas-to-dust ratio (pre- versus postperihelion) and in the relative strength of the NH2 bands (with heliocentric distance). There is a smaller change in the ratio of CN to C2 emission. The Swan bands of C2 show the customary anomaly in strengths implying highly non-Boltzmann distributions among vibrational levels. The red and violet CN system fluxes are in slightly better agreement with laboratory data on oscillator strengths than results on previous comets. Other species measured include Na, (O I), CH, and C3.

Ahearn, M. F.↗

NASA NTRS1976

Dust evolution from comets

The studies of the evolution of cometary debris are reviewed. The subject is divided into three major sections: (1) the developments in the immediate vicinity of the cometary nucleus, which is the source of the dust; (2) the formation of the dust tail; and (3) the blending of the debris with the dust component of interplanetary matter. The importance of the physical theory of comets is emphasized for the understanding of the early phase of evolution. A physico-dynamical model designed to analyze the particle-emission mechanism from the distribution of light in the dust tail is described and the results are presented. Increased attention is paid to large particles because of their importance for the evolution of the zodiacal cloud. Finally, implications are discussed for the future in situ investigations of comets.

Sekanina, Z.↗

NASA NTRS1976

Microwave line transitions in the 3-mm wavelength range in comet Kohoutek /1973f/

A search program for microwave line transitions in comet Kohoutek is reviewed. Observations in the 3-mm wavelength region were carried out before perihelion from December 15 to 20, 1973, and after perihelion from January 3 to 7, 1974. During these times, the comet was between 0.3 and 0.5 AU from the sun. Gas production rates of HCN are several times 10 to the 27th power to about 10 to the 28th power molecules/sec at approximately 0.4 AU. Upper limits for CH3CN and several other molecules are given.

Huebner, W. F.↗

NASA NTRS1976

Carbon production in comet West 1975n

Rocket-ultraviolet spectra of comet West 1975n are reported which were obtained with scanning spectrophotometers in the wavelength range from 1200 to 1300 A. The principal emission features observed include lines of C, O, and C(+), as well as bands of OH, CO, CO(+). The estimated fluxes of the C, O, H, CO, and OH features are found to be consistent with a photodissociation source in which the CO evaporation rate is about one-third that of water. The presence of a large number of carbon atoms in the metastable 1D state is indicated and taken to suggest that dissociative recombination is important near the cometary nucleus, that CO must originate in the nucleus, and that CO is the primary carbon constituent of the comet.

Feldman, P. D.↗

NASA NTRS1976

Physical processes in Comet Kohoutek

Observational results are brought together to establish the mass loss of Comet Kohoutek (1973 XII) in H2O, solid particles, and other molecules. The production rate of molecules is seen to have flattened prior to peaking near perihelion passage, and then fell below the rate for the incoming branch of the orbit. The total mass loss was about 10 to the 14th power gm. All abundances are consistent with a clathrate-dominant form of water. Comparison of the sizes of the nucleus as calculated from photometry and the rate of water loss requires that either an unrealistically high albedo be assumed or that only a portion of the nucleus was covered by water, in contrast with previous expectations for a new and probable first-passage comet. Moreover, if the preperihelion estimates of the nuclear brightness are rejected as being contaminated, the resulting values indicate an ice-covered nucleus with a radius of 2.1 km.

Odell, C. R.↗

NASA NTRS1976

Comet West and the scattering function of cometary dust

Observations of Comet West (1975n) at wavelengths from 0.5 to 18 microns and at a variety of scattering angles are used to infer the scattering phase function for the cometary dust. This function is strongly peaked in the forward direction. The form of the function indicates that the particles are dielectric grains with radii of approximately 1 micron. Abrupt increases in the intrinsic brightness of the coma (both in scattered sunlight and in thermal emission) are consistent with the projected times of comet fragmentation.

Ney, E. P.↗

NASA NTRS1977

The structure of cometary ionospheres. II - CO-rich comets

The structure of the ionosphere of a CO-rich comet is computed using two different models. The first one, the photochemical model, assumes that the dissociation and ionization of cometary neutrals and ions are due to photoionization, and photodissociation by solar UV radiation together with dissociative recombinations and ion-neutral reactions. The second one, the internal source model, also incorporates the ionization and dissociation effects of an electric current discharging through the inner coma. The generation of this current has been discussed in earlier papers. It is concluded that the internal source model can explain qualitatively the basic morphology of the ionospheres of CO-rich comets such as Humason (1962, VIII) and Morehouse (1908, III), whereas the photochemical model cannot. The main aim of this paper is not so much to provide accurate numerical estimates as to draw attention to a process which may very well dominate the structures of cometary ionospheres.

Ip, W.-H.↗

NASA NTRS1977

The composition of comets

Questions concerning the origin of comets are considered. It is pointed out that the molecular composition of a dense interstellar cloud appears to have exactly the composition needed to produce the observed features of the visible cometary spectrum, which consists mainly of emission bands of unstable free radicals. A working model of cometary structure is discussed together with the classical observations which have led to it. The discussed model was originally described by Whipple (1951). It is emphasized that the model can serve only as a rough guide in efforts to interpret the evolution of cometary behavior. A survey is provided of new techniques which have only recently been applied to cometary observations, taking into account the spectrum of a comet, radio observations, and ultraviolet observations.

Feldman, P. D.↗

NASA NTRS1977

Comet Halley - The orbital motion

The orbital motion of Comet Halley is investigated over the interval from A.D. 837 to 2061. Using the observations from 1607 through 1911, least-squares differential orbit corrections were successfully computed using the existing model for the nongravitational forces. The nongravitational-force model was found to be consistent with the outgassing-rocket effect of a water-ice cometary nucleus and, prior to the 1910 return, these forces are time-independent for nearly a millennium. For the 1986 return, viewing conditions are outlined for the comet and the related Orionid and Eta Aquarid meteor showers.

Yeomans, D. K.↗

NASA NTRS1977

Microwave continuum radiation from comet West 1975n

Continuum emission at wavelength 3.71 cm was observed from the nuclear region of comet West 1975n on March 5, 1976. The flux density was 0.040 Jy, which is uncertain by 25% due to calibration. Assuming that the source was a uniformly illuminated disk, the diameter was no more than about 1100 km and the brightness temperature was at least 330 + or - 85 K. On March 4, 1976, similar observations yielded only an upper limit (two standard deviations) to the flux density of 0.010 Jy. Thus it appears that the source turned on with a time scale of 1 day or less, at about the time that the short-lived cometary daughter nucleus 'C' split from the main nucleus 'A'. Similar emission was observed from comet Kohoutek 1973f. In each case, it appears that the microwave emission can be interpreted as thermal radiation from a temporarily enhanced icy-grain halo (IGH). If this interpretation is correct, then the actual temperature (which is assumed to be approximately equal to the nuclear surface temperature) must be in the range 200-250 K, roughly compatible with the observations, in order to satisfy the IGH models of Delsemme (1973).

Hobbs, R. W.↗