Brightness variation distributions among main belt asteroids from sparse   light curve sampling with Pan-STARRS 1
  The rotational state of asteroids is controlled by various physical mechanisms including collisions, internal damping and the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect. We have analysed the changes in magnitude between consecutive detections of approximately 60,000 asteroids measured by the PanSTARRS 1 survey during its first 18 months of operations. We have attempted to explain the derived brightness changes physically and through the application of a simple model. We have found a tendency toward smaller magnitude variations with decreasing diameter for objects of 1 < D < 8 km. Assuming the shape distribution of objects in this size range to be independent of size and composition our model suggests a population with average axial ratios 1 : 0.85 \pm 0.13 : 0.71 \pm 0.13, with larger objects more likely to have spin axes perpendicular to the orbital plane. 
A. McNeill, A. Fitzsimmons, R. Jedicke, R. Wainscoat, L. Denneau, P.   Veres, E. Magnier, K.C. Chambers, N. Kaiser, C. Waters
astro-ph.EP