Occultation and Lightcurve Analysis: the Figure of 2 Pallas

Techniques for analysis of occultation and lightcurve observations of asteroids have been developed and applied to the determination of the figure of 2 Pallas. The procedure consisted of first solving for the pole using modified implementations of photometric astrometry and amplitude -aspect. The derived pole and the results from two stellar occultations were then used to identify the triaxial ellipsoid best approximating the figure of Pallas. A computer model, developed for the simulation of the lightcurves of irregularly shaped bodies with complex surface scattering functions, was used to compare theoretical lightcurves for the derived ellipsoid with observations. The photometric astrometry analysis draws heavily on the implementation developed by Taylor (1979). The method of computing the subearth point was changed to use the phase angle bisector rather than the line of sight and a more direct method of calculating the change in the subpoint between epochs was incorporated to remove restrictions on pole latitudes. An amplitude-aspect analysis was performed to select between the two resulting solutions, again, modified to utilize the phase angle bisector. Application of these techniques has resulted in a new pole orientation, (227(DEGREES), +20(DEGREES)), and sidereal period, 0.324785 (+OR-) 0.000003 days, for 2 Pallas. When the pole of an asteroid is known, an observed elliptical occultation profile could be produced only by a restricted set of triaxial ellipsoids. If, as for 2 Pallas, multiple occultation profiles are available, the triaxial ellipsoid best approximating the figure of the asteroid can be identified. The ellipsoid obtained from the analysis of the two Pallas occultations has equatorial dimensions of 580 (+OR-) 8 and 522 (+OR-) 2 Km. and a polar dimension of 478 (+OR-) 18 Km.