[M-ZZ45] Planetary defense - what should we do?

[MZZ45-08] A Megaton-class impact revealed by PONCOTS Jovian flash observations

*Ko Arimatsu¹, Kohji Tsumura², Fumihiko Usui³, Jun-ichi Watanabe⁴ (1.Kyoto University, 2.Tokyo City University, 3.Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), 4.National Astronomical Observatory of Japan)

Keywords:Jupiter, impact flash, planetary defense, solar system astronomy, trans-Neptunian object

We report the detection and observation of an optical flash event on Jupiter on October 15, 2021 (see figure) by an observation system, the Planetary ObservatioN Camera for Optical Transient Surveys (PONCOTS). Jovian optical flashes caused by impacts of decameter-sized outer Solar System objects have been observed by amateur astronomers. However, their accidental detections have hindered our understanding of the detailed characteristics of the flashes and the impacting objects. The low-cost PONCOTS 0.279 m aperture observing system provides multi-wavelength (three bands), high cadence (10 or 40 fps), and well-calibrated photometric data sets of the Jovian flash for the first time. The apparent peak brightness (approximately 4.7 mag in V-band) and the extremely long duration (approximately 5.5 seconds) of the observed flash indicate that a significant amount of optical energy was released during the impact. The total kinetic energy of the impact object was estimated to be approximately two megatons of TNT, an order of magnitude larger than previously detected flashes on Jupiter and comparable to the 1908 Tunguska impact on Earth. Our observation is the first recorded optical detection of a Tunguska-class impact moment. The observed flash exhibited a single-temperature blackbody spectrum with an effective temperature of approximately 8300 K, with no clear temporal variation. These radiative characteristics may represent common features of terrestrial Chelyabinsk- to Tunguska-class superbolides and would be useful for estimating the damage caused by their radiative consequences. The single detection of the megaton-class flash in Jupiter monitoring observations worldwide suggests that Tunguska-like impact events on Jupiter occur approximately once a year, two to three orders of magnitude more frequently than terrestrial impacts. We also discuss the impact frequency of decameter-sized outer Solar System objects on Earth, which has not been studied in detail.

Reference: Ko Arimatsu, et al., 2022, ApJL, 933, L5

Figure: Color composite image of the flash on Jupiter on 2021 October 15 obtained with the PONCOTS three-band observations. Blue, green, and red channels of the image are the PONCOTS V (505−650 nm), Gh (680−840 nm), and CH4 (880−900 nm) band images, respectively.