with time annotations, Time[1] and Time[2], represent the proposed migration time for the boulder in El-Maarry et al. (2017) and Hasselmann et al. (2019), respectively. The perihelion passage and excursion period are present in vertical line.]

[Figure: Figure 3. Comparison of NAC surface resolution with Rosetta at different distances to the nucleus. NAC captured two images in different time. The image in rich detail was taken after perihelion while another showing the spacecraft was traveling away from 67P. The axes are set in logarithm scale.]

3. MORPHOLOGICAL ANALYSIS To compare observations acquired under different imaging conditions, and to perform quantitative topographic and photometric analysis, we utilised the stereo-photogrammetric (SPG) SHAP7 shape model of 67P (Preusker et al. 2017). Specifically, we use the shape model version comprising 1 million vertices and 2 million triangular facets, corresponding to a horizontal resolution of ∼4 meter. The SHAP7 shape model was derived from images acquired between August 2014 and February 2016, covering almost the entire mission time of Rosetta at comet 67P. As a result, it presents dual existence of the migrating boulder at both its ‘before’ and ‘after’ locations (Figure 1a). This allows us to identify the boulder’s position by matching synthetic images with actual images, compensating for the low resolution of images taken during the excursion. Synthetic images were produced by simulating the view of the camera according to the observational geometry at the time when actual images were captured. Basically, two types of synthetic images were produced: the first with artificial lighting, where all surface features are visible, even those actually in shadow; the second with realistic illumination condition by applying ray-tracing technique. The second type was directly compared with actual images to identify any difference concerning the boulder, and the first type was used for locating the boulder, or other features of interest when they are in the shadow. We first visually inspected candidate images with pre- and post-migration views of the area to identify possible movement of the boulder relative to common surface features. We then performed geometric calculation to infer the boulder’s coordinates on 3D shape model using the position and pointing parameters of the camera provided by the SPICE library (Acton et al. 2016). In the same way, we also identified other surface changes as well as local dust activities.
3.1. Time of the migration We discovered that the migration occurred on 2015 October 3, roughly fifty days after 67P’s perihelion point. Since the Rosetta spacecraft was at a distance over one thousand kilometers from the nucleus, images taken during that time have typical resolutions of ∼20 m px−1 (Table 1), in which the boulder extends over only two pixels. However, the presence of boulder shadows on the surface helped us identifying the boulder (Oklay et al. 2016). The two NAC images taken at UTC 09:26 and 22:49 (Figure 4 a0 & c0) presents the comparative view closest in time before and after the boulder’s migration. By blinking between real and synthetic images, we identify the boulder was at its original location at 09:26 with its lengthened shadow (Figure 4 a0 & a1), while at 22:49, the boulder had disappeared from its initial location, but leaving faint shadow at it new position (Figure 4 c0 & c1). This constrains the migration time to within 14 hours, slightly longer than one rotation, indicating that the migration event was not due to long-term evolution of the boulder, but rather a sudden and rapid displacement likely related to local activity. Only one other OSIRIS image covering this area was acquired in between, at 19:30 on October 3. Unfortunately, the region was at night at that time and surface features are poorly illuminated (Figure 4b). The observational conditions in original image cannot directly provide detect any clues for the migration event. Though this image actually recorded information of local dust activities (see subview in Figure 4b, and Sec. 3.3). Another image acquired later at 21:18 on October 4 shows the area under favourable illumination (Figure 4d), and the boulder could be confirmed to be unmistakably at its new location.
3.2. Boulder and its surroundings 9 We compare pairs of images with similar observation geometry acquired before and after the migration event to investigate changes in the morphology and positions of the boulder and its surroundings (Figure 5).