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Gehrels, T.

Publications and source records attributed to Gehrels, T..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Wavelength dependence of polarization. XXXIV - Observations of Venus

Polarimetric observations of linear polarization on Venus are presented. The observations, which cover nine small disk areas as well as the whole disk, were made with seven filters between 0.3 and 1 micron and also with three filters in the range from 1.2 to 2.2 microns. A polarization wavelength dependence is determined which appears to confirm Landau's (1975) discovery of a steep rise in polarization near 3 microns. The dark UV markings on Venus are shown to be regions of more positive UV polarization and are interpreted as a general feature of a gaseous atmosphere in which sulfuric acid droplets are suspended.

Gehrels, T.↗
NASA NTRS1978

Conference on Protostars and Planets, University of Arizona, Tucson, Ariz., January 3-7, 1978, Proceedings

These papers deal with star formation, the origin of the solar system, and the possible formation of planets associated with other stars. Specific topics include chemical and isotopic anomalies in meteorites, laboratory analogs of interstellar dust, properties of dark globules, locations of newly formed stars in molecular clouds, O-star formation, observations of star-formation regions near supernova remnant W44, and equilibrium condensation of supernova ejecta. Other contributions discuss radio observations of molecular clouds associated with T Tauri stars, rotational velocities of pre-main-sequence stars, observational studies of star formation, characteristics of a possible preplanetary disk around the pre-main-sequence star MWC 349, calculations of the collapse of a rotating dusty protostellar cloud, a theory on the dynamics of dust particles in an incompressible turbulent fluid, and an analysis of the dynamical collapse of a model presolar nebula.

Gehrels, T.↗
NASA NTRS1977

Observations of Uranus occultation events

An analysis of photometric data obtained during the Uranus appulse to SAO 158687 shows three occultation events which appear to coincide with portions of the Uranian ring system. Two occultation events were associated with the alpha ring and one with the beta ring of the planet; the alpha ring event suggests a ring model with a random distribution of occulting bodies of about 1 km diameter, and an areal density of about 50% over a band 10 km wide. Calculations indicate that neither the inner nor outer rings of the planet are truly concentric circles (to within their radius). In addition, determination of the ring centers can be used to locate the center of mass of the planet, and an upper limit of 26,300 km can be assigned to its radius.

Hubbard, W. B.↗
NASA NTRS1977

Minor planets and related objects. XXII - Phase functions for /6/ Hebe

Twenty-two light curves of Hebe are analyzed along with results of UBV photometry and photometric astrometry. A mean synodic period of approximately 7 hr 16 min 28.7 sec is adopted, and it is found that the amplitude of Hebe appears to vary directly with phase. The asteroid's magnitudes and colors are plotted against phase, its pole orientation is estimated, and the sidereal period is determined to be about 7 hr 16 min 28.01 sec from photometric astrometry. A color curve is plotted which shows that Hebe has a reddened region on its surface near the secondary minimum and that the asteroid is nearly spherical. Opposition effects are compared and found to be closely similar for Hebe, Ceres, Lydia, Massalia, and Vesta.

Gehrels, T.↗
NASA NTRS1977

Picture of Ganymede

Spin-scan images of Ganymede obtained by the imaging photopolarimeter of Pioneer 10 in December 1973 are described and presented. Intensity measurements in the blue and red scans are provided. The width of the scan elements is 0.5 mrad which is the size of the aperture in the focal plane of the telescope; the height is also 0.5 mrad (1 millisec integration - or dwell-time at 5 rpm spin rate). The distance of Pioneer 10 from Ganymede was 7.8 x 10 to the fifth km, and the resolution is therefore 390 km. The spin-scan image is in general agreement with Lyot's visual observations (Dollfus, 1961). Preliminary results on polarimetry of Ganymede obtained during the flyby of Pioneer 10 are discussed with attention to the question of whether water frost, NaCl powdered crystals, or other substances is responsible for the observed pattern.

Gehrels, T.↗
NASA NTRS1976

Minor planets and related objects. XXI - Photometry of eight asteroids

Light curves, UBV colors, rotational periods, phase coefficients, and absolute magnitudes are presented. The asteroids studied are (1) Ceres, (4) Vesta, (16) Psyche, (78) Diana, (281) Lucretia, (451) Patientia, (1212) Francette, and (1362) Griqua. The rotation axis of Lucretia is nearly perpendicular to the plane of the ecliptic. Ceres appears to be nearly spherical with an absolute magnitude of B(1,0)=4.42, which is 0.3 mag fainter than previously reported. The determination of the absolute magnitude for an asteroid depends on its aspect, and for each opposition there is, therefore, a different absolute magnitude.

Taylor, R. C.↗
NASA NTRS1976

Jupiter: Studies of the interior, atmosphere, magnetosphere and satellites; Proceedings of the Colloquium, Tucson, Ariz., May 19-21, 1975

This volume is a comprehensive technical text covering all scientific aspects of Jupiter's interior, atmosphere, magnetosphere, and satellites. The chapters deal with such general subjects as the origin and interior structure of Jupiter, the planet's atmosphere and ionosphere, as well as its magnetosphere, radiation belts, and satellites. Specific topics include a review of theories on the origin and structure of Jupiter and its satellites, interior models of Jupiter, the planet's gravity field, its thermal and atmospheric structure, model ionospheres, chemistry and spectroscopy of the atmosphere, the IR spectrum of the planet, and the meteorology of the atmosphere. Other chapters discuss radio observations of Jupiter, the dynamics of the Jovian magnetosphere, structural and thermal models of the icy Galilean satellites and Io's atmosphere and optical emissions. Results are evaluated for the IR radiometer experiment on Pioneers 10 and 11, radio occultation measurements from the Pioneer probes, Pioneer 10 UV photometric observations of the planet and its satellites, Pioneer 10 and 11 imaging polarimetry, the plasma analyzer experiment on the two probes, and observations of energetic Jovian electrons and protons. Individual items are announced in this issue.

Gehrels, T.↗
NASA NTRS1974

Minor planets and related objects. XVI - Polarimetric diameters

Polarimetric observations of 43 asteroids are presented. All objects show a well-developed negative polarization branch as an indicator of unconsolidated surface regoliths. The empirical slope-albedo law for diffusely reflecting solid surfaces is reexamined and used to compute polarimetric albedos and diameters for 30 asteroids. In many cases the results are in good agreement with infrared-radiometric diameters; the older visual diameter measurements were systematically too small. Radiometric albedos below 5%, however, are not confirmed by the polarimetry. Bimodal frequency distributions are noted for asteroid color, albedo, and the depth of the negative polarization branch. Correlations between B - V color and polarimetric parameters suggest that most of the asteroid population can be divided into silicaceous and carbonaceous opacity classes.

Zellner, B.↗
NASA NTRS1974

Jupiter's clouds - Equatorial plumes and other cloud forms in the Pioneer 10 images

The imaging photopolarimeter experiment aboard the Pioneer 10 spacecraft produced two-dimensional maps of intensity and polarization in red and blue light at high resolution during flyby of Jupiter in December 1973. The present article describes cloud forms seen in the equatorial zone and compares them with rotational periods as a function of latitude derived from earth-based observations of features on Jupiter. A striking new feature consists of a bright, well-defined nucleus in the equatorial zone, with a plume apparently drawn out from the core of the nucleus.

Fountain, J. W.↗
NASA NTRS1973

Minor planets and related objects. IX - Photometry and polarimetry of /1685/ Toro.

UBV photometry, lightcurves, a polarization-phase relation, and rotational polarization curves of Toro were obtained, June-August 1972. The average colors are B-V = 0.87 and U-B = 0.47 mag, showing some reddening with phase. The geometric albedo of Toro in the B filter is 0.14 which corresponds to an effective mean radius of 2.2 km plus or minus 0.4 (p.e.). The rotational polarization curves indicate that the lightcurves are mostly due to shape, but some albedo variation over the surface is detected as well. The shape is elongated, and probably irregular, with a cross-sectional area ratio of about 3.2, the maximum effective radius is 2.8 km and the minimum, 1.6 km.

Dunlap, J. L.↗
NASA NTRS1972

Shape and Orientation of Nucleus

The shape and rate of rotation, as determined from brightness-time variations of reflected sunlight, for various asteroid nuclei are presented. The possible application of such a technique to the nucleus of comets is discussed.

Gehrels, T.↗
NASA NTRS1971

Asteriods and comets

Asteroids and comets bibliography and review, considering celestial mechanics and astrometry, photometry, spectra, polarimetry and radar measurements

Gehrels, T.↗
NASA NTRS1971

Physical studies of minor planets

Astronomical observations of the asteroids are discussed. The origin of asteroids and their interrelationships with comets, meteorites, and meteors are considered. Possible space missions and further topics of research are also discussed.

Gehrels, T.↗