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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 163 records · Page 9
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NASA NTRS1976

Three-dimensional aspects of interplanetary shock waves

The three-dimensional geometries of two classes of interplanetary shock waves are analyzed: (1) the more or less spherical shock wave originating from some short-lived solar event, such as a solar flare, and (2) the corotating shock pair formed by the interaction of long-lived solar wind streams. The systematic distortion of a spherical wave due to persistent latitudinal solar wind structure should amount to 25 deg for an equator-to-pole solar wind speed differential of 200 km/sec. A geometrical argument is given for determining the heliocentric distance of the onset of corotating shocks formed by contiguous long-lived solar streams. Estimates are obtained for the heliocentric distance to the inner edges of shock waves as a function of speed difference between the streams and as a function of heliographic latitude. Means by which the three-dimensional structures studied can be observed by out-of-the-ecliptic space missions are discussed.

Siscoe, G. L.↗
NASA NTRS1977

Developing space occupancy - Perspectives on NASA future space program planning

Potential future manned space flight missions for the time after the development of the space transportation system with the space shuttle are considered, taking into account a sequence of activities concerning a gradually increasing penetration of space by man. The activities are related to the permanent occupancy of near-earth space, the permanent occupancy of near-moon space, the full self-sufficiency of man in earth-moon space, and the permanent occupancy of heliocentric space. Attention is given to Mars orbiting stations, the exploration of the asteroids, comet exploration, planet engineering programs, heliocentric orbit installation, the evolution of the earth orbit space community, space industrialization technology, lunar base development, and evolutionary paths to far-future space endeavors.

Von Puttkamer, J.↗
NASA NTRS1977

The radial gradients and collisional properties of solar wind electrons

The plasma instrument on Mariner 10 carried out measurements of electron density and temperature in the interplanetary medium between heliocentric distances of 0.85 and 0.45 AU. Due to the stable coronal configuration and low solar activity during the period of observation, the radial variations of these quantities could be obtained. The power-law exponent of the core temperature was measured to be -0.3 + or - 0.04, and the halo temperature was found to be almost independent of heliocentric distance. The exponent of the power law for the density variation was 2.5 + or - 0.2 and the extrapolated value at 1 AU was consistent with measured values during the same period. Calculations of the core electron self-collision time, and the core-halo equipartition time were made as a function of radial distance. These measurements indicate a macroscale picture of a Coulomb-collisional core and a collisionless isothermal halo. Extrapolating back to the sun, core and halo temperatures become equal at a radial distance of approx. 2-15 radii.

Gilvie, K. W.↗
NASA NTRS1977

Monochromatic brightness variations of comets. II - Core-mantle model

A class of comets, of which Comet Kohoutek (1973f) is typical, show total as well as monochromatic brightness asymmetries about perihelion. They are fainter after perihelion than before at the same heliocentric distance. A model of the cometary nucleus consisting of a growing nonvolatile dust mantle surrounding a volatile icy core is used to discuss this phenomenon. Numerical results are obtained for Comet Kohoutek (1973f). It is found that dust mantles of thickness in the range of 10-75 cm can be grown by perihelion passage for various values of the thermal conductivity of the dust if there is no substantial dust blow-off by the sublimating volatiles. The thermal conductivity of the dust mantle is quite small and is dominated by 'radiative conductivity' for heliocentric distances less than about 2 AU. Since the radiative conductivity is larger for larger grain size, the thickest mantle corresponds to coarsest matrix. The production rate of the 'parent-molecules' as well as the monochromatic brightness of their 'daughter' products increases less steeply than in the mantleless case, as the comet approaches perihelion.

Mendis, D. A.↗
NASA NTRS1978

On the monochromatic brightness variations of comets

The nonsteady development of the multispecies atmosphere of a long-period comet approaching the sun is modeled, and the associated monochromatic brightness variations are discussed in terms of the varying abundances of the atmospheric species. A simple homogeneous clathrate structure, 85% of which is H2O, provides the representation for the cometary nucleus. Monochromatic brightness variations of the emission bands due to OH, CN and C2 are found to follow the law proposed by Levin (1943) for small heliocentric distance where a quasi-steady approximation is valid. However, the large time-scales of residence for emitting species leads to a breakdown of Levin's law at large heliocentric distances.

Mendis, D. A.↗
NASA NTRS1978

The radial gradients and collisional properties of solar wind electrons

The plasma electron detector on Mariner 10 is used to obtain measurements of electron density and temperature in the interplanetary medium between heliocentric distances of 0.85 and 0.45 AU. The observations show quantitatively that the core of the electron distribution function can be described as collisional at least for radial distances within 1 AU, since with a very few well-marked exceptions associated with high-speed streams, the Coulomb collisional momentum relaxation length is less than the density scale height at all times and all radial distances at which data were obtained. It is found that the Coulomb energy exchange collisions between the core and the (test) halo population are negligible. The power law exponent of the core temperature is about -0.3, whereas the halo temperature is almost independent of heliocentric distance.

Ogilvie, K. W.↗
NASA NTRS1978

Postperihelion interference filter photometry of the 'annual' Comet P/Encke

Interference filter photometry was taken of Comet Encke on June 14, 1974 (1.07 AU heliocentric distance, postperihelion) at the CTIO (Cerro Tololo Interamerican Observatory) 150-cm reflector. Production rates were calculated as 4.1 x 10 to the 23nd power mol/sec for CN, 5.3 x 10 to 23nd power mol/sec for C3, and 4.3 x 10 to the 24th power mol/sec for C2. These are about three times smaller than at comparable heliocentric distance preperihelion, assuming a value of 100 for the ratio H2O/(C2 + C3 + CN). An upper limit was placed on the production of nonvolatiles at about one-third that of volatiles in mass by assuming a bulk density of 1 g/cu cm, a particle geometric albedo of 0.1, and a phase function of 0.2.

Newburn, R. L., Jr.↗
NASA NTRS1978

The radial evolution of the bulk properties of the solar wind

Simultaneous solar-wind proton data obtained at several heliocentric distances during radial alignments are compared. The radial variations associated with two high-speed streams in the solar wind are studied as examples of the radial evolution of the solar-wind speed distribution and of high-speed streams in the solar wind as observed in the ecliptic plane in 1973. Pioneer 11 data on high-speed streams in the solar wind observed at about 1.5 and 3.7 AU are compared with the corresponding high-speed-stream data obtained at earth. These analyses indicate that as these high-speed streams propagated to these extended heliocentric distances, there was an erosion of the highest speeds and a general narrowing of the speed distribution. These observations are consistent with the exchange of momentum in the solar wind between high-speed streams and low-speed streams as they propagate outward from the sun.

Intriligator, D. S.↗
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 NTRS1979

Spacecraft radio scattering observations of the power spectrum of electron density fluctuations in the solar wind

Solar wind electron density power spectra in the solar equatorial region are inferred from observations of phase scintillations and spectral broadening made with the Viking, Helios, and Pioneer spacecraft. The heliocentric distance range covered is 2-215 solar radii and for some observations close to the sun the spectra extend to fluctuation frequencies as high as 100 Hz. For heliocentric distances of about 20 solar radii the equivalent spacecraft-measured one-dimensional density spectrum is well modeled by a single power law in the frequency range 0.0001-0.05 Hz. The flattening of the density spectrum within 20 solar radii is presumably associated with energy deposition in the near-sun region and acceleration of the solar wind.

Woo, R.↗
NASA NTRS1980

Brightness profiles of CO/+/ in the ionosphere of Comet West /1976 VI/

The observed sunward and tailward brightness profiles of the (2-0) band of the (A2Pi-X2Sigma) system of CO(+) (4260 A) in Comet West (1976 VI) are established over a range of heliocentric distance from 0.440 to 0.842 AU. From these data the variation with heliocentric distance of the CO(+) production rate is investigated along with the size of the contact surface (the diameter of the onset of the tail). A photoionization model is proposed for the production of CO(+) in Comet West in which CO photoionization is considered to be the principle source of CO(+); the model is consistent with the observational data and the MHD theories.

Combi, M. R.↗
NASA NTRS1980

Radio observation of comet Meier /1978f/ in 18-cm OH lines

Observations of 18-cm OH spectral lines in comet Meier (1978f) with the 1000-ft. Arecibo telescope show spatial resolution of the OH coma by the 2.9 arcmin beam (=3.7 x 10 to the 5th km). The data agree with predictions of the solar Fraunhofer spectrum-pumping theory of comet OH excitation. On the assumption that the OH parent molecule (e.g., H2O) has a Haser-model scale length of about 1.0 x 10 to the 5th km at heliocentric distance 1 AU, an OH scale length of less than 10 to the 6th km, and probably near 1.0 x 10 to the 5th km is derived. Assuming a recently calculated value of the OH lifetime and the solar radiative pumping model, the results indicate an OH production rate of 10 to the 29th Kayser at heliocentric distance 2 AU.

Giguere, P. T.↗
NASA NTRS1980

Ultraviolet spectroscopy of comets

The UV spectra of a comet, Comet Seargent 1978m, were obtained two weeks after the comet's discovery before any ground-based spectroscopic observations were made. The second comet observed with IUE was Comet Bradfield 1979e. It was first observed on January 10 and 11, 1980, and subsequently on several occasions until the beginning of March of 1980. The comets were observed at various heliocentric distances in the low and high dispersion mode. In the long wavelength region, emission bands of OH, CS, CO2(+), NH and CO(+) were identified, and the rotational structures of the (0, 0) and (1, 0) bands of OH were resolved. In the short wavelength region, emission lines of H(L alpha), O I, C I, C II, and C 1 and the CO fourth positive bands appeared. The results of these observations are the detection of S I; indications that CS exists as "Parent Molecules" in the cometary ice; the resolution of the (0, 0) and (1, 0) bands of OH; the determination of the spatial variations of various emissions; the determination of the variation of the production rates of various species, especially of H and OH, with heliocentric distance.

Rahe, J.↗
NASA NTRS1981

An outburst of comet Tempel-2 observed spectrophotometrically

Periodic comet Tempel-2, first discovered in 1873, is a possible target of a spacecraft flyby in the 1980s. Accurate physical measurements of this comet have only recently been attempted. Few spectrophotometric data exist for any faint comet, but such data are presented for Tempel-2 at a heliocentric distance of 2.97 AU. The data were obtained during an outburst in which the comet exhibited heightened CN and C3 emission over a quiescent phase. Comet flares of such magnitude are exceptional and are particularly surprising for a periodic comet at such a large heliocentric distance.

Johnson, P. E.↗
NASA NTRS1981

Effect of the size distribution on the thermal emission from cometary dust

The IR thermal emission from cometary dust is analyzed by two size distribution functions, Sekanina A and a modified Sekanina Miller distribution. The resulting thermal emission spectra are illustrated for heliocentric distances 0.15 to 1.6 AU. The Sekania A distribution does not predict the observed heliocentric variation in the flux ratio I (4.8 micron) I (3.5 micron) for bright comets. A size distribution intermediate between the two presented is a better representation of the dust in recently observed comets.

Hanner, M. S.↗
NASA NTRS1981

Comet Bradfield 1979 X - The gassiest comet

The Comet Bradfield 1979 X provided the most recent opportunity to obtain quantitative photometric data on a comet. Although the comet was not unusually luminous, its very close approach to earth made it possible to observe it from the time of discovery at a heliocentric distance of just over 0.5 AU until it was beyond 1.5 AU from the Sun. The observations conducted are discussed along with aspects of data reduction. It was found that the OH production of comets can be experimentally studied on the basis of a routine procedure. The Comet Bradfield 1979 X was one of the gassiest comets ever observed, comparable to Comet P/Encke. The production rates of all molecular species were found to decline sharply with heliocentric distance.

Ahearn, M. F.↗
NASA NTRS1981

On the detectability of icy grains in the comae of comets

While there is no direct evidence for the presence of icy grains in cometary comae, they are assumed in order to explain observed phenomena. The evaporation of icy grains over the distance scale of the visible cometary coma sets specific limits on their temperature. Unless the grains are pure water ice, maximum icy grain halo size will be limited to a few hundred km at heliocentric distances not greater than about 2.5 AU. While it is unlikely that the 1.5- or 2.0-micron ice band could be detected in the scattering by icy grains, detection of the 3.0-micron ice band may be possible in comets displaying a coma at large heliocentric distances.

Hanner, M. S.↗
NASA NTRS1981

IUE observations of faint comets

Ultraviolet spectra of seven comets taken with the same instrument are given. The comets P/Encke (1980), P/Tuttle (1980 h), P/Stephan-Oterma (1980 g), and Meier (1980 q) were observed in November and December 1980 with the IUE satellite, and comets P/Borrelly (1980 i) and Panther (1980 u) were observed with the IUE on March 6, 1981. The spectra of these comets are compared with one another, as well as with comet Bradfield (1978 X), which was extensively studied earlier in 1980 with the IUE. To simplify the interpretation of the data and to minimize the dependence upon a specific model, the spectra are compared at approximately the same value of heliocentric distance whenever possible. Effects arising from heliocentric velocity, geocentric distance, and optical depth are also discussed. All of the cometary spectra are found to be remarkably similar, suggesting that these comets may have a common composition and origin.

Weaver, H. A.↗