18196( 4) - 11-Sep-2025 15:04:04 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18196 Version: 4 Check-in Time: 11-Sep-2025 15:04:04 STScI Edit Number: 2 Title Characterizing an onging outburst of a rare centaur C/2023 RS61 with joint JWST and HST observations ------------------------------------------------------------------------------------ Type Cycle GO 32 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Eva Lilly Planetary Science Institute Y CoI: Dr. Charles Schambeau University of Central Florida Board of N Trustees CoI: Dr. Michael S Kelley University of Maryland N CoI: Dr. Nalin H. Samarasinha Planetary Science Institute N CoI: Dr. Carey Michael Lisse The Johns Hopkins University Applied N Physics Laboratory CoI: Dr. Dominique Bockelee-Morvan Observatoire de Paris N CoI: Dr. Adam McKay Appalachian State University N CoI: Dr. Audrey Thirouin Lowell Observatory N CoI: Dr. Robert Weryk The University of Western Ontario N CoI: Ms. Helen Usher Open University N CoI: Dr. Yanga Fernandez University of Central Florida Board of N Trustees CoI: Dr. Maria Womack National Science Foundation (NSF) N ------------------------------------------------------------------------------------ Abstract Centaurs are small bodies scattered from the trans-Neptunian region, carrying invaluable information on the composition of trans-Neptunian objects. About 10% of centaurs show cometary activity, poorly understood and often unpredictable. In early 2025 centaur C/2023 RS61 developed a wide tail not aligned with the anti-solar direction. Follow-up images from July 2025 show a weak, anti-solar oriented tail, while on August 10 the brightness of C/2023 RS61 suddenly increased by >4 magnitudes, indicative of a powerful outburst. Archival images reveal at least two more brightening events in the past, suggesting this object is indeed a rare distant outbursting Centaur with rapidly changing coma morphology. Moreover, it is a dynamically new centaur, meaning the nucleus has not yet been significantly thermally processed, therefore offering a unique opportunity to analyze volatiles driving a distant activity on a nearly-pristine small body, with the potential to trace back its formation location in the protosolar disc. We propose 5.5 hours and 1 orbit respectively of joint JWST and HST observations to characterize the rare centaur C/2023 RS61 (PANSTARRS) that has undergone at least four major outbursts and displays an unusually red dust coma. The proposed program will allow to acquire high-resolution spectra of the gas species driving the current outburst complemented with the detailed inner dust coma morphology linking the gas species to their nucleiographic sources. Time-critical DD time is necessary, as C/2023 RS61 is undergoing a powerful outburst, meaning the coma brightness will likely decrease rapidly and disappear before the start of a new regular observing cycle. 18196( 4) - 11-Sep-2025 15:04:04 - [ 2] ------------------------------------------------------------------------------------ Observations Description ------------------------ The observations proposed are to investige the gas species (e.g., CO, CH4, or CO2) driving the activity in the recently discovered active centaur C/2023 RS61 (PANSTARRS) using the JWST NIRSpec IFU Prism mode during Cycle 4. The observations have dedicated background observations due to the extended nature of the target's gas and dust comae. The science and background observations are sequenced together and non-interruptible to ensure the most accurate measure of the background. Also, HST WFC3/UVIS imaging data are to be collected close in time (+/- ~ 3 days) for high-resolution inner coma morphology data for a deeper understanding of the outburst vs. persistent low-level activity. ------------------------------------------------------------------------------------ 18196( 4) - 11-Sep-2025 15:04:04 - [ 3] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 TARGET NAME: C2023RS61 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: COMET Active Centaur ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 TYPE=COMET, Q=8.000601912855959, E=0.3276831041371935, I=19.94378143878478 , O=347.0154081198834, W=119.3522397524865, T=30-NOV-2028:05:20:02, TTimeScale=TDB, EQUINOX=J2000, EPOCH=02-FEB-2025:00:00:00, EpochTimeScale=TDB ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 18 Ephemeris Center: EARTH Extended: YES ------------------------------------------------------------------------------------------------------------------------------------ 18196( 4) - 11-Sep-2025 15:04:04 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.173 Line_Spacing 0.112 Coordinate_Frame POS-TARG Pattern_Orient 23.884 Angle_Between_Sides 81.785 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 18196( 4) - 11-Sep-2025 15:04:04 - [ 5] Visit: 01 Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 C2023RS61 WFC3/UVI ACCUM UVIS2-2K2 F350LP FLASH=14 1 60 S (240 PATTERN 1 1 S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 2 C2023RS61 WFC3/UVI ACCUM UVIS2-2K2 F350LP FLASH=14 1 60 S (240 PATTERN 1 2 S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ C2023RS61 1 WFC3/UVI ACCUM UVIS2-2K F350LP 1 none none none 1 N/A S 2C-SUB C2023RS61 1 WFC3/UVI ACCUM UVIS2-2K F350LP 2 none none none 1 N/A S 2C-SUB C2023RS61 1 WFC3/UVI ACCUM UVIS2-2K F350LP 3 none none none 1 N/A S 2C-SUB C2023RS61 1 WFC3/UVI ACCUM UVIS2-2K F350LP 4 none none none 1 N/A S 2C-SUB C2023RS61 2 WFC3/UVI ACCUM UVIS2-2K F350LP 1 none none none 1 N/A S 2C-SUB C2023RS61 2 WFC3/UVI ACCUM UVIS2-2K F350LP 2 none none none 1 N/A S 2C-SUB C2023RS61 2 WFC3/UVI ACCUM UVIS2-2K F350LP 3 none none none 1 N/A S 2C-SUB C2023RS61 2 WFC3/UVI ACCUM UVIS2-2K F350LP 4 none none none 1 N/A S 2C-SUB 18196( 4) - 11-Sep-2025 15:04:04 - [ 6] Summary Form for Proposal 18196 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2-2K2C-SUB ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=14 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 1, PATTERN 1 2 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F350LP ------------------------------------------------------------------------------------------------------------------------------------ Target Names C2023RS61 ------------------------------------------------------------------------------------------------------------------------------------