0 This pattern is not used in this proposal. resubmission of APT file from Alison. Please check that the dither pattern is appropiate for small dither to mitigate possible bad pixels. 1228473350 ----- Attempting Submission 1 (Tue Sep 09 12:19:03 GMT 2025) ----- 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. 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. 6 false false false false false false SUPPORTED SOLAR SYSTEM YES COMET Active Centaur 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 EARTH POS-TARG DITHER 0.173 LINE 0.112 false false 81.785 4 23.884 WFC3-UVIS-DITHER-BOX 1 false /System/Volumes/Data/solaris/user1/sherwin/18196/OP-cache/18196-v01.xml 1757426567960 Using ephemeris for target 18196-01-C2023RS61 ephemeris data for target 18196-01-C2023RS61 does not cover planning interval. Physical constraints are assumed to be unschedulable outside of the ephemeris data. Planning interval: 24.259 to 26.244. ephemeris interval: 25.243 to 27.244 The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. 1757867177000 253.0 (Tic-Visit :proposal-id 18196 :visit-id "01" :proposal-version -1 :trans-version "20250624" :date "09-Sep-2025 10:02:47.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 563088 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18196 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18196 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2238 :al-overhead-at-end 0 :internal-dur-at-end 652 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2619 :pcs-gap 0 ) true 1 0