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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 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Chemical composition of cometary nuclei

Observational evidence pertaining to the origin and composition of cometary material is reviewed. Arguments favoring the undifferentiated character of the icy conglomerate are summarized. Theoretical descriptions of the sublimation of a cometary nucleus and the velocity field of the expanding gas are presented and compared with observations. The nature of cometary dust and the atomic abundances of H, C, N, O, and S in the volatile fraction are examined, and data on the dust and volatile fractions are combined to derive elemental abundances. It is shown that O, N, and S in cometary nuclei appear to have essentially cosmic abundances but that both H and C are drastically depleted with respect to the cosmic abundances. The apparent depletion of C by a factor of more than three is discussed. It is suggested that the missing carbon might be hidden in the dust fraction in the form of heavy organic molecules or might have remained in either the primeval solar nebula or interstellar space.

Delsemme, A. H.↗
NASA NTRS1983

To Uranus and beyond

Known characteristics of the planet Uranus are reviewed, together with a status report on the Voyager spacecraft. Uranus was discovered in 1781, and has been found to have a north pole that is pointed directly at the sun. Data is still needed on the Uranus atmosphere, its possible magnetic field, and its rotation frequency. Five moons have been sighted, accompanied by nine rings. The Voyager 2 spacecraft will encounter Uranus in December 1986 at a 14.7 km/sec velocity, imposing limitations on the observation sequence. The spacecraft carries 11 scientific packages, including a camera, and will pass within 50,000 km of the moon Miranda and 107,000 km of Uranus. Radio signals will be broadcast through the rings, which will be imaged by a UV spectrometer. Voyager 2 will then continue on to a Neptune encounter in 1989 before moving toward interstellar space.

Kerridge, S. J.↗
NASA NTRS1985

Composition and energy spectra of cosmic ray nuclei above 500 GeV/nucleon from the JACEE emulsion chambers

The composition and energy spectra of charge groups (C - 0), (Ne - S), and (Z approximately 17) above 500 GeV/nucleon from the experiments of JACEE series balloonborne emulsion chambers are reported. Studies of cosmic ray elemental composition at higher energies provide information on propagation through interstellar space, acceleration mechanisms, and their sources. One of the present interests is the elemental composition at energies above 100 GeV/nucleon. Statistically sufficient data in this energy region can be decisive in judgment of propagation models from the ratios of SECONDARY/PRIMARY and source spectra (acceleration mechanism), as well as speculative contributions of different sources from the ratios of PRIMARY/PRIMARY. At much higher energies, i.e., around 10 to the 15th power eV, data from direct observation will give hints on the knee problem, as to whether they favor an escape effect possibly governed by magnetic rigidity above 10 to the 16th power eV.

Burnett, T. H.↗
NASA NTRS1985

Propagation of injected cosmic rays under distributed reacceleration

The cosmic ray acceleration model in which the particles gain continuously energy from stochastic processes in the interstellar space by encountering shocks leads to a logarithmic increase in the secondary to primary ratios with energy, which is inconsistent with observations. The observed decrease of the secondary to primary ratios may therefore suggest that a considerable part of the acceleration has taken place rapidly at the sources. Astronomical observations on the other hand indicate that shocks from supernovae do exist in the galaxy and observations in the interplanetary space show clear evidence that shock acceleration works.

Simon, M.↗
NASA NTRS1985

Propagation of cosmic rays and new evidence for distributed acceleration

The origin and propagation of cosmic rays in terms of conventional and supplementary newer assumptions were explored. Cosmic rays are considered to be accelerated by supernoava shock waves and to traverse clouds in the source region. After rigidity-dependent escape from these clouds into interstellar space, cosmic rays are further accelerated by the weakened shocks of old supernova remnants and then pass through additional material. The distributed acceleration hypothesis is discussed with emphasis on recent data on the abundances of cosmic-ray isotopes of N above 1 GeV/u and of He near 6 GeV/u.

Silberberg, R.↗
NASA NTRS1985

The detection of acetaldehyde in cold dust clouds

Observations of the 1(01)-0(00) rotational transitions of A and E state acetaldehyde are reported. The transitions were detected, for the first time in interstellar space, in the cold dust clouds TMC-1 and L134N, and in Sgr B2. This is also the first time acetaldehyde has been found in a dust cloud and is the most complex oxygen-bearing molecule yet known in this environment. A column density of 6 x 10 to the 12th/sq cm in TMC-1, comparable to many other species detected there, and an approximately equal column density in L134N are formed. In the direction of Sgr B2, the CH3CHO profile appears to consist of broad emission features from the hot molecular cloud core, together with absorption features resulting from intervening colder material. The possible detection of HC9N toward IRC + 10 deg 216 through its J = 33-32 transition is also reported. Implications for cold dust cloud chemistry and excitation are discussed.

Matthews, H. E.↗
NASA NTRS1985

X-ray secondary heating and ionization in quasar emission-line clouds

Accurate Monte Carlo computations of the X-ray secondary electron heating, ionization, and excitation of H and He gas in interstellar space and in quasar emission-line clouds, are presented. The fraction of energy deposited in each form is sensitive to the background ionization fraction, x = n(H+)/n(Htot), and can affect the temperature, ionization state, and line emissivities at large depths in X-ray photoionized clouds. Analytic fits are provided for these energy fractions over the range 0.0001-1 for primary electron energies up to many keV. In both broad-line and narrow-line clouds, emission lines sensitive to the energy budget and electron density may be strongly affected.

Shull, J. M.↗
NASA NTRS1985

Infrared spectral identification of complex organic molecules in interstellar grains

The chemical evolution of interstellar grains leads ultimately to a comparison consisting largely of complex organic molecules. Comparison of infrared absorption spectra of laboratory produced analogue materials with astronomical observations confirm the the presence of similar molecules in interstellar space. The abundance of this complex organic matter derived from the strength of the absorption bands is of the order of ten million solar masses and is almost certainly as large or larger than all conceivable planets.

Greenberg, J. M.↗
NASA NTRS1987

Interstellar polycyclic aromatic hydrocarbons and carbon in interplanetary dust particles and meteorites

Raman spectra of interplanetary dust particles (IDPs) and meteorites containing material similar to polycyclic aromatic hydrocarbons (PAHs) show features that are similar in position and relative strength to interstellar IR emission features attributable to vibrational transitions in free molecular-sized PAHs. In addition, these spectra sometimes show red photoluminescence that has elsewhere been attributed to PAHs, and a part of the carbonaceous phase in IDPs and meteorites contain a degree of deuterium enrichment anticipated in small, free PAHs that are exposed to ISM UV radiation. These observations suggest that some of the IDPs' carbonaceous material may have been produced in circumstellar dust shells, and only slightly modified in interstellar space.

Allamandola, L. J.↗
NASA NTRS1987

An initio calculations on C2, Si2, and SiC

Full-configuration-interaction (FCI) calculations have been performed for the Z 1Sigma(g)(+), a 3Pi(u), and b 3Sigma(g)(-) states of C2 and the X 3Sigma(g)(-) and A 3Pi(u) states of Si2 in valence double-zeta plus polarization one-particle basis sets. By carrying out the CASSCF multireference singles plus doubles CI (MRCI) procedure in a very large Gaussian basis set and correcting for errors with the FCI, the triplet-triplet separation in C2 is reproduced to within 47/cm. Analogous calculations on Si2 definitively determine the ground state to be 3Sigma(g)(-); the 3Pi(u) state is only 440 + or - 100/cm higher in energy. For the SiC molecule, which has never been observed spectroscopically but is believed to be an important component in carbon stars and to be present in interstellar space, the A 3Sigma(-) state is nearly 4000/cm above the X 3Pi ground state.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1988

Aromatic components in cometary materials

The Raman spectra of interplanetary dust particles (IDPs) collected in the stratosphere show that two bands at about 1350 and 1600 delta/cm and a broader feature between 2200 and 3300 delta/cm that are characteristic of aromatic molecular units with ordered domains smaller than 25 A in diameter. This suggests that the carbonaceous material in IDPs may be similar to the polymeric component seen in meteorites, where this material is thought to consist of aromatic molecular units that are randomly interlinked by short aliphatic bridges. The features in the Raman spectra of IDPs are similar in position, and relative strength to interstellar infrared emission features that have been attributed to vibrational transitions in free molecular polycyclic aromatic hydrocarbons. Taken together, these observations suggest that some fraction of the carbonaceous materials in IDPs may have been produced in circumstellar dust shells and only slightly modified in interstellar space.

Allamandola, Louis J.↗
NASA NTRS1989

Shocked carbon monoxide in G333.6-0.2

Rotationally excited carbon monoxide has been detected in the Galactic H II region/molecular cloud complex G333.6-0.2 in the 163-micron J = 16 to 15 and 186-micron J = 14 to 13 transitions. These detections, together with an upper limit to the J = 21 to 20 transition at 124 microns indicate that the excited CO emission comes from gas of kinetic temperature 200 to 800 K and pressure of about 5 x 10 to the 7th/cu cm. A high-resolution spectrum of the J = 14 to 13 transition shows CO emission over more than 80 km/s, possibly with a double peaked profile centered near the systemic velocity of G333.6-0.2. The far-infrared CO emission probably comes from shocked gas in the mass outflow from newly formed, massive stars. The detection of the J = 14 to 13 transition is the first to be made of this line in any source, and is the longest wavelength line yet detected in interstellar space by nonheterodyne techniques.

Storey, J. W. V.↗
NASA NTRS1989

Properties, detectability and origin of interstellar diamonds in meteorites

Laboratory studies suggest that microdiamonds will be quite elusive in interstellar space, being detectable only if more than 10 per cent of the carbon is present as diamond. The diamonds apparently formed by condensation in cool stellar atmospheres, and were then impregnated with isotopically anomalous noble gases, such as Xe, from a supernova.

Lewis, Roy S.↗
NASA NTRS1989

Direct determination of the morphology, structure and composition of cometary and interstellar ice analogs in the laboratory

Ices of various kinds (H2O, CO, CO2, CH4, NH3, CH3OH, etc.) comprise a volumetrically significant proportion of objects in the solar system (comets, outer planets, planetary rings, satellites) as well as in interstellar space (astrophysical ices). The refinement of analytical electron microscopy (AEM) procedures for storing, preparing and analyzing ices and other materials at cryogenic temperatures is discussed. These procedures will be essential to the successful analysis of returned comet nucleus samples.

Blake, D. F.↗
NASA NTRS1989

Effect of re-acceleration on cosmic ray components

Reacceleration of cosmic rays in interstellar space has been studied in detail in order to examine the behavior of the ratios of secondary to primary nuclei in cosmic radiation. It is found that modest acceleration in a confinement region, where particles escape more freely at high energies, provides a better fit to the observed data. The effect of reacceleration on the spectral shape of proton and helium components of cosmic rays has been studied. The examination of two different models has shown that reacceleration provides a poor fit to the observed proton data.

Stephens, S. A.↗
NASA NTRS1989

Deuterium and He-3 in cosmic rays

Observation of a large flux of antiprotons in cosmic rays prompted many to postulate new ideas relating to the origin and propagation of cosmic rays in the Galaxy, within the framework of the secondary hypothesis. Under this hypothesis, cosmic rays traverse a large amount of matter either in the source region or in the interstellar space. As a result, large amounts of deuterium and He-3 are also produced as a consequence of spallation of helium and heavier nuclei. In this paper, the spectra of these isotopes are derived, using various models for the propagation of cosmic rays and compare with the existing observations.

Stephens, S. A.↗