The rotational state of asteroids is controlled by various physical mechanisms including collisions, internal damping and the Yarkovsky-OKeefe-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.
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