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M. Kaasalainen, J. Torppa, K. Muinonen (Observatory, Univ. of Helsinki)
The rotation period, pole direction, and scattering parameters of an asteroid can be deduced from its lightcurves simultaneously with the shape. Our inversion procedure is quite simple and automatic since it is general rather than based on modifications of some model types. Even very complex lightcurves can be modelled easily and consistently using convex inversion. If the data set covers sufficiently many geometries, the resulting pole, period, and shape solutions are stable regardless of noise or the scattering law(s) used. The potential insufficiency of the data shows during the inversion procedure; even large data sets may not be as informative as smaller but more varied ones. Often the pole and the period can still be expected to be accurate even if no details can be obtained for the shape.
Nonconvex features and scattering properties of the surface have very little effect on the lightcurve morphology. Parameters of scattering models such as the Hapke law cannot be determined unambiguously. We recommend, at least for inversion purposes, a few-parameter empirical model that seems to be able to accommodate most photometric properties of asteroids.
We present shape, pole, and period solutions for asteroids some of which have also been observed by planetary probes or radar. We are now in the process of going systematically through the Uppsala APC data base, and it is our purpose to publish a collection of analyses of several asteroids in the near future. It is our intention to collaborate with other institutions to build a web site that would show the results so far, list the asteroids of which observations would be needed the most, and contain a standard report form to be used by observers.
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