16496( 8) - 12-Mar-2021 19:12:23 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 16496 Version: 8 Check-in Time: 12-Mar-2021 19:12:23 STScI Edit Number: 5 Title 323P/SOHO: The First Short Period Near-Sun Object Observed to Disintegrate ------------------------------------------------------------------------------------ Type Cycle GO/DD 28 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Man-To Hui Macau University of Science and Technology Y CoI: Rainer Kracht N CoI: Max Mutchler Space Telescope Science Institute Y CoI: Dr. David Tholen University of Hawaii N CoI: Prof. Paul Wiegert The University of Western Ontario N CoI: Dr. Quanzhi Ye University of Maryland N ------------------------------------------------------------------------------------ Abstract 323P/SOHO is a near-Sun comet with perihelion activity driven by mass-loss mechanism most likely completely different from that of typical comets, and thus is possibly unique in the comet population of the solar system. Postperihelion observations from CFHT in early February 2021 showed that the comet has morphology consistent with ongoing disintegration in rapid evolution, in stark contrast to the preperihelion morphology, which was simply asteroidal in Subaru data from late December 2020. Thus, 323P becomes the first short period near-Sun object that has disintegrated. The ongoing event is offering us an unprecedented opportunity to study a near-Sun object in great detail, and to constrain properties of the interior as well as the disintegration process. We hereby request four DD orbits, taking advantage of HST/WFC3's ultra-high resolution, sensitivity, and photometric stability to monitor and characterize the disintegration event at 323P. ------------------------------------------------------------------------------------ Observations Description ------------------------ We request four orbits to observe disintegrating near-Sun comet 323P/SOHO using HST's WFC3 through the F350LP filter. The four orbits are split as two pairs, with each pair of the visits separated by a day or two in order to cross-link debris and measure their motions between visits, and the two pairs should be a week or so apart, in order to detect any nongravitational effects that the debris are subject to, and to monitor changes in the morphology of the comet from different observing geometries. In each orbit, we request that consecutive exposures of 405 s on the comet should be taken, with image dithering performed in the middle to mitigate CCD defects including bad pixels and the gap between the two WFC3 gaps after stacking the images. During each entire visibility window, five such consecutive exposures can be obtained. According to the WFC3 Exposure Time Calculator, a point source with a solar-type color at mV = 27.4 will reach signal-to-noise ratio (SNR) ~3 in a single F350LP image. Combining five such images will help removal of cosmic ray hits effectively, and the SNR can be improved by a factor of ~2.2. Using the viewing geometry of 323P on 2021 March 15 (heliocentric distance 1.48 au, cometocentric distance 1.18 au, phase angle 42 deg) as an example, the HST observations will be able to reveal debris from the disintegration event down to ~30 m in radius (comet-like geometric 16496( 8) - 12-Mar-2021 19:12:23 - [ 2] albedo 0.04 assumed) in a combined image from a single visit. This clearly manifests the supreme sensitivity of HST that is unparalleled by any other facilities. The nonsidereal apparent motion of 323P will be <4 arcmin/hr, well within the tracking capacity of HST. The current 3-sigma ephemeris uncertainty during the observable window is at the subarcsec level, much smaller than the field of view (FOV) of WFC3, and so will not be a concern to telescope pointing. In order to follow up the disintegration process in rapid evolution seen in our CFHT data from early February 2021, we request disruptive scheduling for the first two visits of observations. The other pair of orbits should be executed roughly a week after the first pair of visits. The comet is currently swiftly receding from both the Sun and Earth. Observations from much later epochs will have sensitivity way lower than what should have been achieved in earlier epochs, and the disintegration may have faded to a degree where the morphology may have been altered considerably as the rapid evolution of the disintegration continues, making investigation of the initial disintegration mechanism difficult, and co-moving remnants may have dispersed beyond WFC3's FOV. In addition, the comet will enter HST's Sun exclusion zone (solar elongation <50 deg) starting from mid May 2021. Therefore, we must observe 323P and complete the program in March 2021, the earlier the better. We understand that the HST roll angle is not within our control, although ideally the extension of the debris cloud should be placed along the diagonal of WFC3's FOV. ------------------------------------------------------------------------------------ 16496( 8) - 12-Mar-2021 19:12:23 - [ 3] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 TARGET NAME: 323P ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: COMET Disintegrating comet ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 TYPE=COMET, Q=0.0392071121041, E=0.9848146391461, I=5.3699316736616, O=324.2293902822781, W=353.1741243642062, T=17-JAN-2021:15:09:08, TTimeScale=TDT, EQUINOX=J2000, EPOCH=17-FEB-2021:00:00:00, EpochTimeScale=TDT ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 25 +/- 3 Ephemeris Center: EARTH Extended: YES ------------------------------------------------------------------------------------------------------------------------------------ 16496( 8) - 12-Mar-2021 19:12:23 - [ 4] Visit: 01 Visit Requirements: On Hold Comments: Additional Comments: Schedule this visit as soon as possible. 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 323P WFC3/UVI ACCUM UVIS1-FIX F350LP CR-SPLIT=NO 3 405 S POS TARG 40, 0; SEQ 1-2 S (1215 S) NON-INT ------------------------------------------------------------------------------------------------------------------------------------ 2 323P WFC3/UVI ACCUM UVIS1-FIX F350LP CR-SPLIT=NO 2 405 S (810 POS TARG 40.2, 2.5 S 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 | ------------------------------------------------------------------------------------------------------------------------------------ 323P 1 WFC3/UVI ACCUM UVIS1-FI F350LP none none 1 none 1 N/A S X 323P 1 WFC3/UVI ACCUM UVIS1-FI F350LP none none 2 none 1 N/A S X 323P 1 WFC3/UVI ACCUM UVIS1-FI F350LP none none 3 none 1 N/A S X 323P 2 WFC3/UVI ACCUM UVIS1-FI F350LP none none 1 none 1 N/A S X 323P 2 WFC3/UVI ACCUM UVIS1-FI F350LP none none 2 none 1 N/A S X 16496( 8) - 12-Mar-2021 19:12:23 - [ 5] Visit: 02 Visit Requirements: AFTER 01 BY 20.3 H TO 2 D On Hold Comments: Additional Comments: Schedule this visit as soon as possible. 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 323P WFC3/UVI ACCUM UVIS1-FIX F350LP CR-SPLIT=NO 3 405 S POS TARG 40, 0; SEQ 1-2 S (1215 S) NON-INT ------------------------------------------------------------------------------------------------------------------------------------ 2 323P WFC3/UVI ACCUM UVIS1-FIX F350LP CR-SPLIT=NO 2 405 S (810 POS TARG 40.2, 2.5 S 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 | ------------------------------------------------------------------------------------------------------------------------------------ 323P 1 WFC3/UVI ACCUM UVIS1-FI F350LP none none 1 none 1 N/A S X 323P 1 WFC3/UVI ACCUM UVIS1-FI F350LP none none 2 none 1 N/A S X 323P 1 WFC3/UVI ACCUM UVIS1-FI F350LP none none 3 none 1 N/A S X 323P 2 WFC3/UVI ACCUM UVIS1-FI F350LP none none 1 none 1 N/A S X 323P 2 WFC3/UVI ACCUM UVIS1-FI F350LP none none 2 none 1 N/A S X 16496( 8) - 12-Mar-2021 19:12:23 - [ 6] Visit: 03 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 323P WFC3/UVI ACCUM UVIS1-FIX F350LP CR-SPLIT=NO 3 405 S POS TARG 40, 0; SEQ 1-2 S (1215 S) NON-INT ------------------------------------------------------------------------------------------------------------------------------------ 2 323P WFC3/UVI ACCUM UVIS1-FIX F350LP CR-SPLIT=NO 2 405 S (810 POS TARG 40.2, 2.5 S 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 | ------------------------------------------------------------------------------------------------------------------------------------ 323P 1 WFC3/UVI ACCUM UVIS1-FI F350LP none none 1 none 1 N/A S X 323P 1 WFC3/UVI ACCUM UVIS1-FI F350LP none none 2 none 1 N/A S X 323P 1 WFC3/UVI ACCUM UVIS1-FI F350LP none none 3 none 1 N/A S X 323P 2 WFC3/UVI ACCUM UVIS1-FI F350LP none none 1 none 1 N/A S X 323P 2 WFC3/UVI ACCUM UVIS1-FI F350LP none none 2 none 1 N/A S X 16496( 8) - 12-Mar-2021 19:12:23 - [ 7] Visit: 04 Visit Requirements: AFTER 03 BY 3 D TO 7 D 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 323P WFC3/UVI ACCUM UVIS1-FIX F350LP CR-SPLIT=NO 3 405 S POS TARG 40, 0; SEQ 1-2 S (1215 S) NON-INT ------------------------------------------------------------------------------------------------------------------------------------ 2 323P WFC3/UVI ACCUM UVIS1-FIX F350LP CR-SPLIT=NO 2 405 S (810 POS TARG 40.2, 2.5 S 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 | ------------------------------------------------------------------------------------------------------------------------------------ 323P 1 WFC3/UVI ACCUM UVIS1-FI F350LP none none 1 none 1 N/A S X 323P 1 WFC3/UVI ACCUM UVIS1-FI F350LP none none 2 none 1 N/A S X 323P 1 WFC3/UVI ACCUM UVIS1-FI F350LP none none 3 none 1 N/A S X 323P 2 WFC3/UVI ACCUM UVIS1-FI F350LP none none 1 none 1 N/A S X 323P 2 WFC3/UVI ACCUM UVIS1-FI F350LP none none 2 none 1 N/A S X 16496( 8) - 12-Mar-2021 19:12:23 - [ 8] Summary Form for Proposal 16496 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS1-FIX ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements AFTER 01 BY 20.3 H TO 2 D, AFTER 03 BY 3 D TO 7 D, POS TARG 40, 0, POS TARG 40.2, 2.5, SEQ 1-2 NON-INT ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F350LP ------------------------------------------------------------------------------------------------------------------------------------ Target Names 323P ------------------------------------------------------------------------------------------------------------------------------------