Time evolution of the boulder height differences ΔH^* (as defined in text). Each time step contains information related to 22 boulders and the values correspond to the weighted average. They-axis reports the average height measurements referring to the subsets described in Sect. 4. Subset 1 refers to 21–22 August 2014. According to the illumination and visibility conditions, the time interval between Subset 1 and the perihelion is about 4 months. The time interval between the perihelion and Subset 6 is about one year. Erosion during the inbound orbit until December, 2014, nearly balances the fallout from the southern hemisphere during perihelion cometary activity. The dotted red line indicates the perihelion.