Example of the cardinality reduction step for asteroid (87) Sylvia against Herschel observation id 1342266670, based on the comparison among the HEALPix cells representing the maximum position uncertainty of the asteroid (87) Sylvia over the ten-day period (blue dotted tile), and the HEALpix cells representing the observation footprint in red (orange tiles). The latter are computed as the HEALpix cells overlapping with the smallest circle containing the observation footprint (dotted red circle). The position of Sylvia at the t_start of the observation is displayed in blue.