Benjamin Zellner

Taxonomy of asteroids

Abstract A taxonomic system was introduced by C. R. Chapman, D. Morrison, and B. Zellner [Icarus 25, 104–130 (1975)], in which minor planets are classified according to a few readily observable optical properties, independent of specific mineralogical interpretations. That taxonomy is here augmented to five classes, now precisely defined in terms of seven parameters obtained from polarimetry, spectrophotometry, radiometry, and UBV photometry of 523 objects. We classify 190 asteroids as type C, 141 as type S, 13 as type M, 3 as type E, and 3 as type R; 55 objects are shown to fall outside these five classes and are designated U (unclassifiable). For the remaining 118, the data exclude two or more types but are insufficient for unambiguous classification. Reliable diameters, from radiometry or polarimetry or else from albedos adopted as typical of the types, are listed for 396 objects. We also compare our taxonomy with other ones and discuss how classification efforts are related to the interpretation of asteroid mineralogies.

The E asteroids and the origin of the enstatite achondrites

Abstract Color, polarization and albedo data are summarized for the three known minor planets of optical type E— 44 Nysa, 64 Angelina and 434 Hungaria. The inventory of E asteroids with dimensions > 50 km is shown to be essentially complete. The surfaces of the E objects evidently consist of colorless, translucent, iron-free silicates such as plagioclase, forstefite, or enstatite. Their possible identification as the source of enstatite achondrites is consistent with new laboratory polarimetry of the Norton County aubrite. Both Nysa and Hungaria seem to be rather favorably situated for the production of meteorites. Nysa has a highly non-spherical shape, and is dynamically associated with the metal-rich asteroid 135 Hertha and several small objects also apparently of the metal-depleted E type. The configuration is suggestive of the fragments of a differentiated parent body, in which Hertha originated as the iron core and Nysa as the largest surviving mantle fragment. The relative volumes, however, are not consistent with simple igneous differentiation from a chondritic composition.

Asteroid compositional types and their distributions

A sample of 359 minor planets with available colorimetric, spectropnotometric, thermal-radiometric, and/or polarimetrie data are classified into broadly defined compositional types C (carbonaceous), S (silicaceous), M (metal-rich), E (metal-free; enstatite?), O (ordinary-chondritic?), T (Trojan; unidentified composition), and U (unclassifiable; none of the above). The small asteroids in Mars- or earth-crossing orbits are almost invariably of type S or O . For the main belt between 2.2 and 3.5 AU., distributions of the various types over diameter and orbital parameters are derived with corrections for observational selection bias. For 560 main-belt asteroids with diameters >50 km, 76% are of type C , 16% of type S , 5% M , and 3% of other types. The S objects become progressively less common with distance. The shapes of the diammeter-frequency relations for primitive (C) and evolved ( S + M ) types are statistically indistinguishable, both showing a change of slope at 160-km diameter. While large asteroids avoid the Kirkwood gaps more strongly than small ones, we find no significant gap-related anomalies in the relative frequencies of compositional types. Thus most of the observational basis for models in which primitive and evolved asteroids respond differently to collisional evolution has been removed.

The Tucson revised index of asteroid data

Abstract Attention is called to the availability of the TRIAD computer file, a compilation of all reliable physical parameters for minor planets.

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.

Astrometry of Faint Planetary Satellites with WFPC2 of Hubble Space Telescope

Only the Hubble Space Telescope (HST) can detect about 20 of the faint satellites discovered with the two Voyager spacecraft. We describe here the techniques used in obtaining astrometric positions of the inner satellites of Uranus with the Wide Field Planetary Camera 2 (WFPC2) of HST, and those planned for our scheduled observations of the inner Neptunian satellites.