Fig. 3

a For large spherical planetary bodies (radius R \({}_{\rm T}\) \(>\,\sim\)1000 km), observation can be conducted at a close stable orbit (R \({}_{\rm O}\)) (i.e. R \({}_{\rm T}\) \(\sim\) R \({}_{\rm O}\)), where a simple collimator can be sufficient for identifying large surface variation. b, c For small targets (R \({}_{\rm T}\) \(<\,\sim\)100 km), low gravity or irregular shape prevents observation at a close steady orbit (i.e. R \({}_{\rm T}\) \(<\,<\) R \({}_{\rm O}\)), and the size of the imaging footprint on the surface can change dramatically over time or even in a single orbit (Hong and Romaine 2014)