DPS 34th Meeting, October 2002
Session 2. Asteroids I
Oral, Chair(s): R.P. Binzel and H. Scholl, Monday, October 7, 2002, 9:30-11:00am, Ballroom

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[2.06] Obliquity Alignment and Spin Angular Momentum Clustering in the Koronis Family

S. M. Slivan, R. P. Binzel (MIT), M. Kaasalainen (U. Helsinki)

Rotation lightcurves of Koronis family asteroids obtained through 2001 November have been analyzed to deduce senses of rotation and spin-vector orientations for nine of the largest members of the family. Reductions using amplitude-magnitude-epoch methods based on those of Drummond {\em et al.} (1988) and reductions using the convex polyhedron inversion method of Kaasalainen and Torppa (2001) and Kaasalainen {\em at al.} (2001) were carried out independently; the quite good agreement between the two reductions increases our confidence in the results. Spin-vector determinations are now available for a total of ten members of the Koronis family, including (243) Ida, whose ground-based pole solution was confirmed by observations from the {\em Galileo} spacecraft.

The distribution of the north pole obliquities of family members is markedly bimodal, clustered near about 160\circ and 50\circ, and the anisotropy is statistically significant at greater than the 99% confidence level. Correlation between the sample objects' spin rates and obliquities forms a cluster of low-obliquity retrograde rotators with periods near 14 hours, and a second cluster of intermediate-obliquity prograde rotators with periods near 8.5 hours. Reconciling models of family formation and evolution with the unexpected alignments of spin obliquities and correlations with spin rates presents a new challenge in understanding asteroid collisional processes.

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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.