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.Comment: 10 pages, 10 figures, accepted by MNRA
