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Zellner, B.

Publications and source records attributed to Zellner, B..

35 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

UBV photometry of small and distant asteroids

Photoelectric magnitudes and colors on the ultraviolet (UBV) system are presented for 65 minor planets, including four Mars crossers, six Trojans, and main-belt objects down to 6 km in diameter. The Trojans all have very similar colors not characteristic of the main-belt population. A paucity of S-type asteroids at the smallest diameters, predicted from trends seen at larger sizes, is not observed. The newly available color data for small objects ranging from 1.0 to 5.2 astronomic units in heliocentric distance show the main belt to be a transition zone between predominantly silicate and carbonaceous compositions.

Zellner, B.↗
NASA NTRS1977

Asteroid families - Observational evidence for common origins

Colors of minor planets in the UBV system indicate compositions quite distinct from those of the field population in each of three Hirayama families. The Eos and Koronis families apparently originated from the collisional fragmentation of undifferentiated silicate bodies, and the Nysa group from a geochemically differentiated parent body

Gradie, J.↗
NASA NTRS1977

Asteroid compositional types and their distributions

A table is presented listing the compositional types chosen for 336 main-belt asteroids, twelve Mars-crossers, two Hildas, and nine Trojans. There are about 100 asteroids with polarimetric data, 126 with reflection spectra, 165 with radiometric diameters, and 300 with UBV colors. Absolute visual magnitudes V(1,0 were computed from the blue magnitudes listed by Gehrels and Gehrels (1977) and B-V colors either known or estimated according to the type. The sample of main-belt asteroids classified in the table is complete to mean opposition apparent magnitude B(a,0) = 12. The frequencies of the compositional types are discussed. Attention is also given to the distributions over diameter and the distributions over heliocentric distances.

Zellner, B.↗
NASA NTRS1977

The asteroid albedo scale. I - Laboratory polarimetry of meteorites

Laboratory reflection polarimetry is presented for crushed rocks, lunar soils, and 24 meteoritic samples. The polarization-phase curves found for the meteorites closely resemble telescopic results for minor planets, but only if the samples are crushed finely enough to coat the larger particles with dust. The results generally confirm the identifications previously made from reflection spectroscopy for the meteoritic analogues of various types of asteroids; the more metal-rich ordinary chondrites, for example, match telescopic observations of the S-type minor planets well enough, as do the basaltic achondrites for Vesta. No meteorite measured, however, has a geometric albedo less than 0.05 as required for many of the C asteroids. A calibration of the polarimetric slope-albedo law, now determined strictly for meteoritic samples, confirms the asteroid albedo and diameter scale derived by the independent method of thermal radiometry.

Zellner, B.↗
NASA NTRS1977

The asteroid albedo scale. II - Laboratory polarimetry of dark carbon-bearing silicates

Laboratory reflection polarimetry is presented for eight samples of artificial, poorly crystalline magnesian silicates with varying admixtures of carbon black. The polarimetric slope-albedo law saturates for geometric albedos lower than about 0.05, and good agreement with the telescopic polarization-phase curves of C-type asteroids is found for albedos as low as 0.02. Thus the conclusion from thermal radiometry is confirmed that the C objects are very dark, darker than any known carbonaceous chondrite.

Zellner, B.↗
NASA NTRS1977

Optical polarimetry of particulate surfaces

Reflection polarimetry is a remote-sensing tool sensitive to the refractive index and microtexture of solid surfaces. Mechanisms producing linear polarization within a rough, porous, or particulate surface are discussed, and laboratory techniques for precise measurements are described in some detail. Polarimetry of atmosphereless solar-system bodies provides valuable indications of albedo for objects too small and distant for direct diameter determinations, and reveals that even quite small asteroids and satellites have particulate surfaces

Zellner, B.↗
NASA NTRS1976

Polarization of the reflected light of asteroid 433 Eros

Linear polarizations measured for asteroid 433 Eros at various wavelengths and at solar phase angles ranging from 9 to 53 deg are presented. The polarization results are entirely typical of main-belt S asteroids, and indicate a dusty surface with geometric albedo 0.20. The derived effective diameter at photometric maximum is 21 km. Eros is quite uniform polarimetrically; no dependence on aspect is detected, and the polarization is shown to be constant during a single rotation with a precision of one part in forty.

Zellner, B.↗
NASA NTRS1976

Physical properties of asteroid 433 Eros

Newly available photometric, polarimetric, spectroscopic, thermal-radiometric, radar, and occultation results are synthesized in order to derive a coherent model for Eros. The geometric albedo is 0.19 plus or minus 0.01 at the visual wavelength, and the overall dimensions are approximately 13 by 15 by 36 km. The rotation is about the short axis, in the direct sense, with a sidereal period of 5 hr 16 min 13.4 sec. The pole of rotation lies within a few degrees of ecliptic coordinates 16 deg longitude and beta = +11 deg latitude. Eros is uniformly coated with a particulate surface layer several millimeters thick. It has an iron-bearing silicate composition, similar to that of a minority of main-belt asteroids, and probably identifiable with H-type ordinary chondrites.

Zellner, B.↗
NASA NTRS1976

Minor planets and related objects. XX - Polarimetric evidence for the albedos and compositions of 94 asteroids

Polarimetric observations are presented and analyzed for a total of 94 asteroids. The data include numerical parameters describing the polarization-plane curve, the maximum depth of the negative branch, geometric albedo, and diameter. With few exceptions, the polarizations are found to be repeatable from hour to hour, night to night, and apparition to apparition at the same solar phase angle within an observational accuracy of plus or minus 0.05%. It is shown that over a range of more than a factor of 50 in diameter or a factor of 100,000 in mass, all the asteroids seem to have microscopically very rough or particulate surface textures. In terms of their polarimetric properties, the asteroids are classified as follows: 48 in the broad S class, with compositions corresponding to stony-iron meteorites or ordinary chondrites; 34 in the C class, probably corresponding to carbonaceous chondrites; three to five M asteroids with surfaces rich in free metal; two with low-opacity surfaces (class E) attributable to pure enstatite; and five of other types.

Zellner, B.↗
NASA NTRS1975

Minor planets and related objects. XVIII - UBV photometry and surface composition

New photometric observations on the UBV system are presented for 45 asteroids, bringing the total to 91 for which complete UBV colors are available. The reduced magnitudes B (1, 0) scatter about the adopted catalog values by + or - 0m, 25, with negligible systematic difference. The UBV colors fall into three distinct groups which correspond almost perfectly to the S (silicaceous or stony), C (carbonaceous), and U (unclassifiable) asteroid types recently identified from polarimetry, infrared radiometry, and narrow-band spectrophotometry. It becomes possible to classify asteroids from UBV observations alone. The S asteroids have colors very similar to those of L and LL ordinary chondrites, and some of the U objects are consistent in color with nickel-iron or enstatite-rich meteorites, but no meteorites appear to have the UBV colors of the C asteroids. The asteroid C and S compositional types have no exact counterparts among the surfaces of natural satellites.

Zellner, B.↗
NASA NTRS1975

44 Nysa - An iron-depleted asteroid

The minor planet 44 Nysa has a unique combination of photopolarimetric parameters, with the most nearly neutral UBV colors, the shallowest negative polarization branch, and by far the highest polarimetric albedo yet obtained for any asteroid. Its surface apparently consists of a low-opacity, iron-free silicate strongly suggestive of enstatite achondrites.

Zellner, B.↗
NASA NTRS1975

Surface properties of asteroids - A synthesis of polarimetry, radiometry, and spectrophotometry

Surface compositions of 110 asteroids are analyzed using data obtained by polarimetry and visible and near-IR spectrophotometry. A comparison of albedos and diameters determined by polarimetry and radiometry indicates that a slight upward revision of the radiometric albedo scale is necessary. Reliable adopted albedos and diameters are presented for 56 objects, and rough diameters are obtained for 52 additional objects. More than 90% of the asteroids are shown to fall into two broad groups (C and S) which are compositionally similar to carbonaceous and stony meteorites, respectively. C-type asteroids are found to predominate in the asteroid belt, and the largest of these (Ceres, Pallas, Hygiea, Davida) may be metamorphosed. It is shown that Vesta may be the only differentiated asteroid remaining intact in the belt. The results support the conclusions that asteroids and meteorites have similar classes of mineral assemblages, but the distribution of asteroid types contrasts with the distribution of major meteorite types found on earth.

Chapman, C. R.↗
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

Polarizations of asteroids and satellites

Polarization data for Mercury, the moon, asteroids, and satellites of Mars, Jupiter, and Saturn are compiled along with the relevant laboratory results. It is shown that the slope of the ascending branch of the polarization-phase curve is a discriminant of the bulk albedo only, while the maximum polarization and the depth of the negative branch are sensitive also to the microscale roughness and opacity. Recently published data are used for new calibrations of the laws relating these polarimetric parameters to geometric albedo. It is demonstrated that the albedos of asteroids and small satellites derived from their polarization properties are in good agreement with infrared diameters, and that a microscopically intricate surface is characteristic of atmosphereless solar-system bodies.

Bowell, E.↗
NASA NTRS1972

On the nature of Iapetus.

For solar system bodies with little or no atmosphere, a linear relation exists between the depth of the negative polarization branch and the reciprocal of the geometric albedo. Considered in this fashion, the polarization data for Iapetus imply an albedo difference by about a factor of 6 between its leading and trailing hemispheres, and are consistent with the amplitude of the light curve for Iapetus. The brightness of the satellite at any particular maximum or minimum is unpredictable by a certain factor because of inadequately understood variations in solar phase angle and rotation. Although other measurements of the Iapetus albedos give results different from those in the presented paper, it is still clear that the two hemispheres have very different albedos.

Zellner, B.↗
NASA NTRS1972

Minor planets and related objects. VIII - Deimos.

Evaluation of 12 polarization measurements of Deimos in blue light were obtained at phase angles between 5 and 31 deg. A well-developed negative branch, with minimum polarization approximately -1.5% near phase angle 11 deg, indicates that Deimos is not bare rock but has a particulate surface layer of dark dust or powder. The phase angle of inversion is 19.5 deg, and the slope of the positive branch is 0.26 plus or minus 0.01% per degree. On the basis of the slope-albedo law for the moon and the major asteroids, the geometrical albedo of Deimos in blue light should be 0.07 plus or minus 0.01, and the radius 5.4 plus or minus 0.8 km. These values are in good agreement with preliminary Mariner 9 data.

Zellner, B.↗