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 \sim 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 .