Observations of the 2025 Geminid Lunar Impact Flashes with TILT

During the 2025 Geminids, which peaked between 2025-12-13 and 2025-12-14, the Moon was between 30-40% illuminated, with the radiant of the Geminid meteoroid stream on the unilluminated hemisphere of the Moon. This orbital geometry, coupled with the favourable observation conditions, prompted a global campaign to observe Lunar Impact Flashes (LIFs). As part of the commissioning phase of the TILT instrument (a dual 40 cm Newtonian telescope system based in Calern, France, built for coordinated LIF observations alongside lunar-based seismometers, see abstract EGU26-7444 for more detail), we took part in this observation campaign.

TILT operated for the totality of the observable period over these two nights, obtaining a total of 8.5 hours of LIF observations. Five hours of observation were performed on 2025-12-13, using two visible cameras (one ASI183MM, and one ASI174MM), and a further three and a half hours were performed on 2025-12-14, using one visible camera (ASI183MM) and one short-wave infrared camera (Ninox 640SU). From this data, we detected 56 events which could not be immediately rejected as false positives and were so far able to confirm nine of these events as true LIFs, through the LIF lasting more than one frame (4 events), and by observing the flashes in multiple simultaneous observations (5 events). While we are unable to confirm with certainty that these events were belonging to the Geminids (due to the constant presence of the sporadic background population), all the confirmed LIFs exhibited impact geometry compatible with the Geminid meteoroid stream. After performing photometric calibration of these events using stars observed at a similar airmass throughout the observations, we found that the confirmed events have magnitudes ranging between +7.5 and +10.4. These impacts are estimated to have formed craters ranging between 0.7 m and 1.7 m rim-to-rim diameter.

Preliminary results suggest a rate of impacts of 1.1 hr^-1 for confirmed events, and 6.6 hr^-1 events for all events. For the purposes of multi-messenger observations with lunar seismometers, confirmation of the events can be performed using the seismic signal of the impact, and therefore confirming the impacts occurrence based solely on LIF observations is not required. Hence, this observation campaign has demonstrated the importance of observing during high impact-rate streams, such as the Geminids, for the future operations of TILT.