# # Program information file # PROGRAM_ID 2024B994 PROGRAM_TITLE Scattering Phase Function of Comet C/2023 A3 (Tsuchinshan-ATLAS) PROGRAM_INV1 Quanzhi Ye PROGRAM_INV2 PROGRAM_INV3 PROGRAM_INV4 PROGRAM_INV5 PROGRAM_SCICAT comets PROGRAM_ABSTRACT_BEG The approaching Oort cloud comet C/2023 A3 (Tsuchinshan-ATLAS) is following a trajectory that will permit observations from Earth at phase angles from 3 degrees to 173 degrees in 2024, providing a rare opportunity to constrain the scattering phase function of dust from a comet over such an exceptionally wide range of phase angles. The scattering phase function is dependent on the composition, structure and size distribution of dust grains, and is frequently used to compare photometry of cometary dust taken at different phase angles. Simultaneous optical and thermal/mid-infrared photometry, as enabled by MIRSI/MOC, provides the most reliable means of disentangling the phase dependence of dust brightness of interest from changes in the total dust cross section. The comet was observed at low phase angles in semester 2024A. In semester 2024B, the comet will be observable at high phase angles, attaining a maximum phase angle of 173 degrees at inferior conjunction on October 9. IRTF thermal observations surrounding conjunction will constrain the physical dust cross section which can be interpolated to derive scattering efficiency up to the maximum phase angle using optical photometry from planned observations with other telescopes, including the SOHO coronagraphs. Extrapolating from its current dust production activity, we estimate the comet will possess a total coma dust cross section on the order of 1e5 km2, corresponding to a total, integrated N band flux of 3 kJy spread over a coma ~3 arcmin wide with a central peak comparable in intensity to a point source of 20 Jy. We estimate 2 hours of clock time will be sufficient to gather S/N>10 photometry in the wide and narrow N-band filters. We request a total of five observation blocks of 2+ hr each on separate days, with two blocks before conjunction and three after conjunction to provide a robust baseline for sampling the comet's dust activity. The comet will be at <21 deg elongation from Oct 1-12, and we request the two observation blocks to be scheduled within a few days before and after this solar exclusion period. We also request one additional observing block approximately one week later to improve constraints on the activity trend. PROGRAM_ABSTRACT_END