0 Assigned ID: 329 ----- Attempting Submission 1 (Thu Mar 05 22:39:10 GMT 2020) ----- HST Phase I Proposal 329 successfully submitted. Receipt: # 329-1 ----- Attempting Submission 2 (Fri Mar 06 23:01:39 GMT 2020) ----- HST Phase I Proposal 329 successfully submitted. Receipt: # 329-2 ----- Attempting Submission 3 (Fri Mar 06 23:29:10 GMT 2020) ----- 1. Initial submission 2. Resubmitted with APT 2020.2.1 3. Shortened orbit 2 to resolve orbit visibility overrun 480131194 ----- Attempting Submission 1 (Wed Jun 03 06:45:47 GMT 2020) ----- HST Phase II Proposal 16214 successfully submitted. Receipt: # 16214-1 ----- Attempting Submission 2 (Wed Jun 03 22:49:03 GMT 2020) ----- HST Phase II Proposal 16214 successfully submitted. Receipt: # 16214-2 ----- Attempting Submission 3 (Thu Jun 04 08:37:06 GMT 2020) ----- C/2017 K2 is intrinsically the brightest Oort cloud comet found in recent history, and presents a unique opportunity to efficiently characterize the constituent properties of a still primitive comet on its way in toward the inner solar system for the very first time. Its exceptional brightness in the outer solar system suggests it to be much larger than typical Oort cloud comets---normally too faint to observe at these distances---and makes it a particularly compelling target for early polarimetric characterization, which requires a high signal-to-noise ratio to usefully constrain its dust properties. Imaging polarimetry of nearby comets has previously revealed varied polarized structures, such as halos and jets, that reflect spatial variation in grain size and structure. Jets are of particular interest as they entrain and expose fresh grains from beneath the surface, previously shielded from the space environment since the formation of the solar system, and are too small/narrow to be resolved in the outer solar system by ground-based imaging polarimetry. We propose to use high resolution HST/ACS polarimetry and color imaging of C/2017 K2 at a pre-perihelion epoch 6.5--7 au from the Sun, well before the onset of water ice sublimation, to constrain the properties of refractory grains in the jets and surrounding coma, and their physical evolution as they move outward from the nucleus. These observations, in concert with analogous results from more evolved comets in the inner solar system, will probe the formation environments and evolutionary histories of these assorted comets, and their implications on the formation and evolution of the solar system as a whole. We request one epoch of two orbits for polarimetry and color imaging of C/2017 K2, ideally scheduled on or within a few days of 2021 March 21. Each orbit will contain three polarized F775W exposures, one through each of the POL0V, POL60V, and POL120V polarizers, as well as one F775W and one F475W non-polarized exposure. Exposures will be distributed to approximately equalize the signal-to-noise of all individual exposures, and their order will be reversed between the orbits to mitigate the impact of any brightness variations linear on ~2 h timescales. Repeating each exposure across the two orbits provides redundancy against field stars, cosmic rays, and detector defects in the final polarization and color maps. Additionally, scheduling windows where the comet nucleus passes near bright stars or any extended background objects (e.g., galaxies, nebulae) should be avoided, if possible, and will be identified in early planning, and subsequently validated and possibly refined 1--2 months in advance of the target epoch when more reliable comet and HST ephemerides become available. On 2021 March 21, the comet trails the Sun by ~80 deg in both R.A. and ecliptic longitude while at a solar elongation of ~85 deg. This position is broadly consistent with inclusion in the Reduced Gyro Mode (RGM) field of regard, suggesting that the observation likely remain schedulable if RGM is place. Furthermore, while the epoch is optimally scheduled on or near 2021 March 21, scheduling it elsewhere in 2021 March and April is still acceptable for achieving the science objectives, if necessary by a less flexible schedule. Finally, the comet will only be moving at ~13 arcsec/h, which is well within the RGM tracking limits. We therefore anticipate limited impacts to this program from entering RGM, beyond scheduling flexibility and acquisition overheads. Q. Zhang is experienced in observing and characterizing small bodies in the solar system, and led a recent HST/ACS polarimetry program targeting interstellar comet 2I/Borisov (HST GO 16044). He was involved with the observational and theoretical characterization of interstellar object 1I/`Oumuamua (e.g., Zhang 2018, ApJL, 852, L13), and is familiar with the behavior of small bodies under solar irradiation, including from his work with the Sungrazer project characterizing the orbits and morphology of near-Sun objects with space-based telescopes. Dr. Kolokolova is an expert in photopolarimetric studies of cosmic dust, specifically cometary dust and asteroid regolith. Her main interest is interpretation of the photometric and polarimetric data using computer modeling of light scattering by dust particles, thus revealing their size, shape, structure, and composition. As the archive manager of the PDS Small Bodies Node, she is well familiar with ground-based and space data resulting from observations of comets and asteroids. Dr. Ye has led and/or participated a number of ground-based or HST-involved projects on small solar system bodies, such as (3200) Phaethon (HST GO 15357, PI Q. Ye; Ye et al. 2018, ApJL, 864, 9), (6478) Gault (Ye et al. 2019, ApJL, 874, 16), and 1I/`Oumuamua (Ye et al. 2017, ApJL, 851, 5). His area of expertise is in the analysis of visible and infrared imaging of comets and asteroids, as well as in planetary dynamics. S. Vissapragada is an expert near-infrared observer, and is experienced with the WIRC+Pol spectropolarimeter (installed on the Palomar 200-inch Hale Telescope), for which he helped develop data reduction techniques. Dr. Millar-Blanchaer is an expert in near-infrared polarimetry, which he typically uses to characterize extrasolar planets and associated debris. He is experienced with ground-based polarimeters, including with their operation, calibration, and data reduction, and recently led an upgrade of the WIRC+Pol spectropolarimeter that improved its precision by over an order of magnitude. Prof. Hallinan is an expert in time domain astronomy, and has spearheaded and supervised numerous instrumentation and observing programs targeting time varying sources at frequencies across the electromagnetic spectrum both from the ground and from space. Solar System Astronomy 6 false false false false false false false SUPPORTED SOLAR SYSTEM YES COMET C/2017 K2 (PANSTARRS) TYPE=COMET,Q=1.80529237551517,E=1.000365195836125,I=87.5435551941697 ,O=88.25971276371043,W=236.0824969444859,T=20-DEC-2022:14:54:52,TTimeScale=TDB,EQUINOX=J2000,EPOCH=20-DEC-2018:00:00:00,EpochTimeScale=TDB EARTH SOLAR SYSTEM YES COMET C/2017 K2 (PANSTARRS) - Offset (2nd Dither Position) TYPE=COMET,Q=1.80529237551517,E=1.000365195836125,I=87.5435551941697 ,O=88.25971276371043,W=236.0824969444859,T=20-DEC-2022:14:54:52,TTimeScale=TDB,EQUINOX=J2000,EPOCH=20-DEC-2018:00:00:00,EpochTimeScale=TDB TYPE=POS_ANGLE,RAD=0.5,ANG=180,REF=SUN EARTH ACS/WFC Imaging No Yes F775W ACS/WFC Imaging No No F775W ACS/WFC Imaging No No F475W 1591259704338 /tmp/OPVisit16527285293536838033.xml The scheduling data was generated by CASM 28.2. CASM was run in Phase II mode. 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