Other
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........16r&link_type=abstract
Thesis (PH.D.)--RENSSELAER POLYTECHNIC INSTITUTE, 1995.Source: Dissertation Abstracts International, Volume: 56-11, Section: B,
Other
2
Lightcurves, Planets
Scientific paper
Asteroids have never been imaged adequately by Earth-based telescopes due to their small angular size. Lightcurves, measured flux varying over an asteroid's rotation, indicate changes in projected area towards the observer (and thus shape), changes in brightness (albedo) over the surface, or a combination of the two. Lightcurve data relating asteroid shape with possible surface albedo variations have always been ambiguous. One method to alleviate this ambiguity is through combining thermal and visible flux measurements. A major source of systematic error in absolute determinations of asteroid diameter and albedo has been the modeling of the thermal output of an asteroid without knowing in detail its thermophysical properties. The measurement of thermal and reflected flux are inherently more precise than the use of the thermal models used to calculate total thermal output. Factoring out the effects of the model to find relative changes in projected area and albedo will minimize the effect of the large systematic errors. Concurrent measurements of visible and thermal flux are used to calculate relative changes in projected area and albedo for asteroids 45 Eugenia, 532 Herculina, 15 Eunomia, and 4 Vesta. Data from Lebofsky and others were used for 45 Eugenia and 532 Herculina. Similar data were obtained for 15 Eunomia and 4 Vesta. The results of the analyses for 45 Eugenia and 532 Herculina are in agreement with previous qualitative findings. Their respective lightcurves are mostly shape -driven with very little, if any, albedo variation across their surfaces. There is an indication that there is a possible albedo spot on 45 Eugenia, though the data are too incomplete to be certain. 532 Herculina is a slightly oblong asteroid whose lightcurve variations are mainly due to shape changes. 15 Eunomia is an ovoid asteroid whose surface composition changes from metalliferous with major olivine and lesser pyroxene in subequal metal/silicate ratios for the more "pointed" end, to a higher concentration of high-Fe pyroxene with a higher general concentration of silicate for the "blunt" end. 15 Eunomia is seen as a being a stratigraphic section through its parent body, showing evidence for planetary-scale differentiation in that parent body. The surface of the asteroid 4 Vesta shows evidence of a complex history. One hemisphere exhibits evidence for large impact features that may be correlated with topography. There is evidence for an impact basin structure surrounded by a basin ring mountain range. The eastern side of this impact basin may be partially filled with a low-albedo material, possibly basalt. To the west of the impact basin is an increase in albedo, possibly due to the Southern Diogenite Feature as seen by Gaffey (1995), which may be a shallow crater, uncovering fresh, unweathered material. The remainder of the surface of 4 Vesta appears to be a low albedo, possibly space-weathered material.
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