More Phase Curves from the Kuiper Belt: Photometric Properties of Subaru-Discovered KBOs From New Horizons LORRI and HST Observations

Since 2020, NASA’s New Horizons team has been conducting a ground-based search using Subaru Telescope’s Hyper Suprime-Cam dedicated to finding additional targets for the spacecraft’s LOng Range Reconnaissance Imager (LORRI) to observe, either via distant or close flybys (Fraser et al. 2024; Yoshida et al. 2024). To date, the search (near RA/Dec 288,-21) has found and reported to the Minor Planet Center over 240 new Kuiper Belt Objects (KBOs). Among these new KBOs, 7 were located close enough to the spacecraft’s trajectory that they could be observed by New Horizons’ LORRI (V<21); however, LORRI’s narrow (0.29° x 0.29°) field of view required astrometric refinement provided by Hubble Space Telescope (HST) observations (Programs 16183 and 16924, S. B. Porter, PI) to ensure accurate spacecraft pointing. In addition to their astrometric refinements, these HST images also provide photometric observations at small solar phase angles. Because accurate photometric modeling requires observations that span the widest range possible in solar phase angle, we combine these ‘low-phase’ HST WFC3 observations with those at ‘high-phase’ from New Horizons’ LORRI to construct disk-integrated solar phase curves that extend their phase curves to phase angles as high as 92 degrees. The Subaru-discovered KBOs already observed by New Horizons sample all but the dynamically coldest populations, and we compare their photometric properties with those of other KBOs already observed by New Horizons to reveal any correlations between surface textural and scattering properties among the dynamical classes. Preliminary results indicate that phase curves of Subaru-discovered KBOs have shallower slopes and therefore different shapes from those of cold classical KBOs. From a heliocentric distance of 60 au in October 2024, New Horizons can no longer observe Classical KBOs, but future LORRI observations can continue to sample Resonant KBOs as well as those from the Scattered Disk. Furthermore, these high-phase LORRI observations can sample objects whose apoapses lie beyond the termination shock and the heliopause, enabling studies of space weathering processes via photometric modeling of surfaces in the outermost regions of the Solar System.