16310( 10) - 15-Mar-2021 15:52:50 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 16310 Version: 10 Check-in Time: 15-Mar-2021 15:52:50 STScI Edit Number: 3 Title Interstellar Comet 2I/Borisov ------------------------------------------------------------------------------------ Type Cycle GO 28 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. David Jewitt University of California - Los Angeles Y CoI: Max Mutchler Space Telescope Science Institute Y CoI: Dr. Harold A. Weaver The Johns Hopkins University Applied Y Physics Laboratory CoI: Dr. Jing Li University of California - Los Angeles N CoI: Dr. Jessica Agarwal Technische Universitaet Braunschweig N CoI: Dr. Yoonyoung Kim Technische Universitaet Braunschweig N CoI: Dr. Man-To Hui Macau University of Science and Technology N ------------------------------------------------------------------------------------ Abstract C/2019 Q4 (Borisov) is the first interstellar comet and only the second interstellar object (after 'Oumuamua) ever identified within the solar system. We propose high resolution observations with WFC3 and HST in order to probe the development of this body in Cycle 28. During this time we will use HST and WFC3 to constrain the nucleus and dust properties. Observation as the Earth passes through the projected orbit plane early in the Cycle will provide a model-free estimate of the dust extent and velocity perpendicular to the plane. Additional, spaced observations provide perspectives as the object recedes from about 9 to about 13 AU. The high resolution of HST is needed to resolve faint near-nucleus structures (jets, fans) and fragments at the growing distance to the comet. The proposed observations extend the temporal coverage of the evolution of this unique object and take advantage of new perspectives. ------------------------------------------------------------------------------------ Observations Description ------------------------ We request 8 HST orbits spread over Cycle 28. The proposed observations span the outbound heliocentric distance range 9.0 AU to 13.7 AU. 2I emerges from the HST Sun-exclusion zone on 2021 January 28, at rH = 9.0, delta = 9.5 AU, and with phase angle 5 deg. This date also corresponds to orbit plane crossing, needed to detect ultra-low surface brightness trail material consisting of the largest, most slowly moving particles. In this geometry, the direction of trailing of field objects is parallel to the direction of the extended dust emission, placing especially stringent constraints on the need for accurate background removal. For this reason, we request four orbits with WFC3 and the F350LP filter to image 2I on the plane-crossing date at the highest sensitivity using the broad F350LP filter (effective center 6230A, FWHM 4760A). These four orbits will be dithered and combined to optimize the rejection of field object trails, especially from hard-to-remove low surface brightness galaxies. Subsequent visits will be used to measure the brightness and morphological evolution of 2I, as well as to obtain sub-arcsecond astrometry. These single-orbit visits will secure six 270s integrations each, with dithering to 16310( 10) - 15-Mar-2021 15:52:50 - [ 2] provide protection from chip defects. According to the WFC3 ETC, a single exposure of this length reaches V = 26.5 at SNR = 5. Combing 6 images from one orbit will take us 1 magnitude fainter and will also provide protection from cosmic ray strikes on individual images. Combining data from 4 orbits will give us another 0.6 magnitudes, taking us to a point-source sensitivity V = 28.1 for the plane-crossing V1. As noted above, the expected apparent magnitude of a 0.5 km radius nucleus (albedo 0.04) is V = 28 AU at V1, so that detection of the nucleus will be challenging. We do expect to find the largest, slowest particles aligned in a trail behind the nucleus; the 0.04 arcsec PSF of HST projects to 300 km at the comet. Sub-pixel astrometry will assist with orbit fitting and the determination of non-gravitational acceleration. The currently predicted 1 arcsec ephemeris uncertainty (from JPL Horizons) is 2 arcsec on UT 2021 January 28. This is already small enough compared to the WFC3 field of view that ephemeris uncertainty poses no threat to the proposed observations. Moreover, as with all high profile targets, continuous additions to the astrometric database by astronomers around the world will shrink this uncertainty month-by-month up until the comet reaches elongations < 50 degree (in 2020 October). Astrometry from this work will also be used to update the orbit. The ephemeris accuracy for HST observations is expected to be of no concern to these observations. All observations are with WFC3 and the F350LP (effective center 6230A, FWHM 4760A) filter to provide the highest sensitivity. The division of the requested orbits is as follows. For visits V1 and V2, 2I emerges from the 55 deg HST exclusion zone on UT 2021 January 28 at rH = 9.0, delta = 9.5 AU and with phase angle 5 deg. Fortuitously, the Earth will pass through the orbit plane on this date providing us with the maximum sensitivity to large particles confined to the orbit plane of the body. Thus, early observation of 2I in Cycle 28 provides our best chance to detect a large particle trail. In this geometry, the direction of trailing of field objects is parallel to the direction of the extended dust emission, placing especially stringent constraints on the need for accurate background removal. For this reason we request three orbits to be dithered and combined to optimize the rejection of field object trails. The four orbits in V1 and V2 should be as close together as possible, although they do not need to be consecutive. (We realize that no more than two consecutive orbits are possible on a moving target because of the gyro problem). In fact, spaced orbits would be an advantage by giving enough time for background objects to completely leave the field of view. Image alignment between two consecutive HST orbits is excellent but alignment after a dither or other alignment change is less secure, particularly on a faint target that might not be readily identified in a single, heavily cosmic-ray impacted orbit. From experience, we feel confident in achieving good alignment in four plane-crossing orbits separated by a gyro gap. The in-plane angle changes by about 0.08 deg/day near plane crossing. Therefore, we need to be within 2 to 3 days of January 28 in order to attain the in-plane geometry and the closer to this time the better. Visit V3 is a single orbit visit near UT 2021 April 1 (rH = 10.1 AU), requested to sample the changing morphology and photometry of the comet. Visit V4 is a single orbit visit near UT 2021 June 1 (rH = 11.4 AU), requested to sample the changing morphology and photometry of the comet. Visit V5 is a single orbit visit near UT 2021 August 1 (rH = 12.6 AU), requested to sample the changing morphology and photometry of the comet. The Earth again passes through the orbital plane of 2I at this time but the increased distance will substantially reduce our sensitivity to a large particle trail. Therefore, we have focused 16310( 10) - 15-Mar-2021 15:52:50 - [ 3] our in-plane attention primarily on V1 and V2. Visit V6 is a single orbit visit within 2 weeks of the end of Cycle 28 (UT 2021 September 30, rH = 13.7 AU), requested to complete the study. The end of Cycle 28 is followed by entry of 2I into the Sun exclusion zone from 2021 October 26 - 2022 February 7. V6 marks the likely end of any possible observations of 2I/Borisov. We understand that we will have essentially no control over the spacecraft roll angle, which means that we will not be able to optimize the orientation of the dust coma on the CCD (e.g., to orient the tail along the longest dimension of the detector). However, the field-of-view of the camera is large enough and the target coma small enough that we should obtain excellent data, no matter what spacecraft roll angle is used. ------------------------------------------------------------------------------------ 16310( 10) - 15-Mar-2021 15:52:50 - [ 4] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 TARGET NAME: 2I-BORISOV ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: COMET interstellar comet ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 TYPE=COMET, Q=2.006581893840375, E=3.356215101434632, I=44.05257068647377 , O=308.1487262895379, W=209.12367864468, T=08-DEC-2019:13:04:54, TTimeScale=TDB, EQUINOX=J2000, EPOCH=01-AUG-2020:00:00:00, EpochTimeScale=TDB, R0=2.808 , DT=87.2916 , A1=7.093444347382E-8, A2=-1.443811535835E-8, A3=6.534734368324E-10 , ALN=0.1112620426, NM=2.15, NN=5.093, NK=4.6142, AMRAT=0. ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 22 +/- 4 Ephemeris Center: EARTH Extended: NO ------------------------------------------------------------------------------------------------------------------------------------ 16310( 10) - 15-Mar-2021 15:52:50 - [ 5] Visit: 01 Visit Requirements: BETWEEN 31-JAN-2021:00:00:00 AND 31-JAN-2021:21:00:00; BETWEEN 01-FEB-2021:05:00:00 AND 02-FEB-2021:06:00:00; BETWEEN 02-FEB-2021:14:00:00 AND 03-FEB-2021:21:00:00; GROUP 01, 02 WITHIN 18H 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 2I-BORISOV WFC3/UVI ACCUM UVIS2-2K2 F350LP 4 500 S SEQ 1 NON-INT S C-SUB (2000 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 2I-BORISOV WFC3/UVI ACCUM UVIS2-2K2 F350LP 4 500 S POS TARG 0.4, 0.4; SEQ 2 S C-SUB (2000 S) NON-INT ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ 2I-BORISOV 1 WFC3/UVI ACCUM UVIS2-2K F350LP none none 1 none 1 N/A S 2C-SUB 2I-BORISOV 1 WFC3/UVI ACCUM UVIS2-2K F350LP none none 2 none 1 N/A S 2C-SUB 2I-BORISOV 1 WFC3/UVI ACCUM UVIS2-2K F350LP none none 3 none 1 N/A S 2C-SUB 2I-BORISOV 1 WFC3/UVI ACCUM UVIS2-2K F350LP none none 4 none 1 N/A S 2C-SUB 2I-BORISOV 2 WFC3/UVI ACCUM UVIS2-2K F350LP none none 1 none 2 N/A S 2C-SUB 2I-BORISOV 2 WFC3/UVI ACCUM UVIS2-2K F350LP none none 2 none 2 N/A S 2C-SUB 2I-BORISOV 2 WFC3/UVI ACCUM UVIS2-2K F350LP none none 3 none 2 N/A S 2C-SUB 2I-BORISOV 2 WFC3/UVI ACCUM UVIS2-2K F350LP none none 4 none 2 N/A S 2C-SUB 16310( 10) - 15-Mar-2021 15:52:50 - [ 6] Visit: 02 Visit Requirements: BETWEEN 31-JAN-2021:00:00:00 AND 31-JAN-2021:21:00:00; BETWEEN 01-FEB-2021:05:00:00 AND 02-FEB-2021:06:00:00; BETWEEN 02-FEB-2021:14:00:00 AND 03-FEB-2021:21:00:00 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 2I-BORISOV WFC3/UVI ACCUM UVIS2-2K2 F350LP 4 500 S POS TARG 0.8, 0.8; SEQ 1 S C-SUB (2000 S) NON-INT ------------------------------------------------------------------------------------------------------------------------------------ 2 2I-BORISOV WFC3/UVI ACCUM UVIS2-2K2 F350LP 4 500 S POS TARG 1.2, 1.2; SEQ 2 S C-SUB (2000 S) NON-INT ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ 2I-BORISOV 1 WFC3/UVI ACCUM UVIS2-2K F350LP none none 1 none 1 N/A S 2C-SUB 2I-BORISOV 1 WFC3/UVI ACCUM UVIS2-2K F350LP none none 2 none 1 N/A S 2C-SUB 2I-BORISOV 1 WFC3/UVI ACCUM UVIS2-2K F350LP none none 3 none 1 N/A S 2C-SUB 2I-BORISOV 1 WFC3/UVI ACCUM UVIS2-2K F350LP none none 4 none 1 N/A S 2C-SUB 2I-BORISOV 2 WFC3/UVI ACCUM UVIS2-2K F350LP none none 1 none 2 N/A S 2C-SUB 2I-BORISOV 2 WFC3/UVI ACCUM UVIS2-2K F350LP none none 2 none 2 N/A S 2C-SUB 2I-BORISOV 2 WFC3/UVI ACCUM UVIS2-2K F350LP none none 3 none 2 N/A S 2C-SUB 2I-BORISOV 2 WFC3/UVI ACCUM UVIS2-2K F350LP none none 4 none 2 N/A S 2C-SUB 16310( 10) - 15-Mar-2021 15:52:50 - [ 7] Visit: 03 Visit Requirements: BEFORE 24-MAR-2021:00:00:00; GROUP 03, 04, 05, 06 WITHIN 18H 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 2I-BORISOV WFC3/UVI ACCUM UVIS2-2K2 F350LP 3 700 S SEQ 1 NON-INT S C-SUB (2100 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 | ------------------------------------------------------------------------------------------------------------------------------------ 2I-BORISOV 1 WFC3/UVI ACCUM UVIS2-2K F350LP none none 1 none 1 N/A S 2C-SUB 2I-BORISOV 1 WFC3/UVI ACCUM UVIS2-2K F350LP none none 2 none 1 N/A S 2C-SUB 2I-BORISOV 1 WFC3/UVI ACCUM UVIS2-2K F350LP none none 3 none 1 N/A S 2C-SUB 16310( 10) - 15-Mar-2021 15:52:50 - [ 8] Visit: 04 Visit Requirements: BEFORE 24-MAR-2021:00:00:00; GROUP 04, 05 WITHIN 18H 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 2I-BORISOV WFC3/UVI ACCUM UVIS2-2K2 F350LP 3 700 S POS TARG 0.4, 0.4; SEQ 1 S C-SUB (2100 S) NON-INT ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ 2I-BORISOV 1 WFC3/UVI ACCUM UVIS2-2K F350LP none none 1 none 1 N/A S 2C-SUB 2I-BORISOV 1 WFC3/UVI ACCUM UVIS2-2K F350LP none none 2 none 1 N/A S 2C-SUB 2I-BORISOV 1 WFC3/UVI ACCUM UVIS2-2K F350LP none none 3 none 1 N/A S 2C-SUB 16310( 10) - 15-Mar-2021 15:52:50 - [ 9] Visit: 05 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 2I-BORISOV WFC3/UVI ACCUM UVIS2-2K2 F350LP 3 700 S POS TARG 0.8, 0.8; SEQ 1 S C-SUB (2100 S) NON-INT ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ 2I-BORISOV 1 WFC3/UVI ACCUM UVIS2-2K F350LP none none 1 none 1 N/A S 2C-SUB 2I-BORISOV 1 WFC3/UVI ACCUM UVIS2-2K F350LP none none 2 none 1 N/A S 2C-SUB 2I-BORISOV 1 WFC3/UVI ACCUM UVIS2-2K F350LP none none 3 none 1 N/A S 2C-SUB 16310( 10) - 15-Mar-2021 15:52:50 - [ 10] Visit: 06 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 2I-BORISOV WFC3/UVI ACCUM UVIS2-2K2 F350LP 3 700 S POS TARG 1.2, 1.2; SEQ 1 S C-SUB (2100 S) NON-INT ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ 2I-BORISOV 1 WFC3/UVI ACCUM UVIS2-2K F350LP none none 1 none 1 N/A S 2C-SUB 2I-BORISOV 1 WFC3/UVI ACCUM UVIS2-2K F350LP none none 2 none 1 N/A S 2C-SUB 2I-BORISOV 1 WFC3/UVI ACCUM UVIS2-2K F350LP none none 3 none 1 N/A S 2C-SUB 16310( 10) - 15-Mar-2021 15:52:50 - [ 11] Summary Form for Proposal 16310 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2-2K2C-SUB ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BEFORE 24-MAR-2021:00:00:00, BETWEEN 01-FEB-2021:05:00:00 AND 02-FEB-2021:06:00:00, BETWEEN 02-FEB-2021:14:00:00 AND 03-FEB-2021:21:00:00, BETWEEN 31-JAN-2021:00:00:00 AND 31-JAN-2021:21:00:00, GROUP 01, 02 WITHIN 18H, GROUP 03, 04, 05, 06 WITHIN 18H, GROUP 04, 05 WITHIN 18H, POS TARG 0.4, 0.4, POS TARG 0.8, 0.8, POS TARG 1.2, 1.2, SEQ 1 NON-INT, SEQ 2 NON-INT ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F350LP ------------------------------------------------------------------------------------------------------------------------------------ Target Names 2I-BORISOV ------------------------------------------------------------------------------------------------------------------------------------