16111( 6) - 10-Apr-2020 18:51:29 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 16111 Version: 6 Check-in Time: 10-Apr-2020 18:51:29 STScI Edit Number: 3 Title Breakup of Long-Period Comet C/2019 Y4 (ATLAS) ------------------------------------------------------------------------------------ Type Cycle GO/DD 27 ------------------------------------------------------------------------------------ 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. Jessica Agarwal Max Planck Institute for Solar System N Research CoI: Dr. Yoonyoung Kim Max Planck Institute for Solar System N Research CoI: Dr. Jing Li University of California - Los Angeles N ------------------------------------------------------------------------------------ Abstract We seek HST time with the WFC3 camera in order to study the on-going break-up of long-period comet C/2019 Y4 (ATLAS). Break-up of comets may be the dominant destruction process of these bodies, yet is poorly studied because breakups occur unpredictably and evolve rapidly, require timely observations which are difficult to secure. We propose a 4 orbit allocation to measure the temporal evolution of the fragments, in order to measure the brightness and velocity distributions and to measure fragment nn-gravitational acceleration caused by outgassing forces (the latter are a measure of fragment size). We will also seek evidence for rotational and other modulation of the primary nucleus and fragments, indicative of patchy distribution of surface volatiles. A Monte-Carlo dust model will be applied to imaged of the diffuse envelope. C/2019 Y4 has the potential to be one of only a handful of disrupting comets ever studied at HST resolution, and to set exciting constraints on models of the breakup mechanism. ------------------------------------------------------------------------------------ Observations Description ------------------------ We request observations of comet ATLAS with HST and WFC3, using the F350LP broadband filter. We request 2 to 3 day spacing between visits in order to connect fragments and their motion between orbits, thereby to measure the velocities and accelerations. In each orbit, we will obtain six 260s integrations on the comet, dithered to provide protection from chip defects. The six images, when combined, will also provide adequate protection from cosmic rays. The WFC3 Exposure Time Calculator indicates that, for a point source, solar-type spectrum, a signal-to-noise ratio of 6 is reached in 230 s at V = 26 in the F350LP filter (optimal extraction yields SNR=13). For example, at opposition and heliocentric distance R = 1 AU, magnitude V = 26 corresponds to an object radius 25 m (geometric albedo 0.04 assumed). These are upper limits to the point source thresholds because at least some of the fragments are embedded in their own dust comae. However, it is clear that HST offers unprecedented sensitivity to faint fragments. We use F350LP because the F350LP 16111( 6) - 10-Apr-2020 18:51:29 - [ 2] count rate is 1.6 higher for a target with a solar-type spectrum than other commonly used broadband filters (e.g., F606W), providing a better count rate and signal-to-noise ratio. We also plan to dither the exposures to mitigate the effects from bad pixels, cosmic rays, and the inter-chip gap. Angular non-sidereal rates peak at about 10000 hour1 and are well within the range of HST capabilities. The ephemeris uncertainty is 3 sigma < 1 arcsec, of no concern given the field of view of WFC3. We request Disruptive Scheduling for these observations. This is necessary because of the expected rapid photometric and morphological development of the fragments as seen in other disintegrating comets. Comet ATLAS will be in the HSTs 55 Sun exclusion zone from UT 2020 April 30 to UT 2020 September 7. Therefore, we must observe with HST before 2020 April 30, but earlier dates are preferred. The apparent rates of motion relative to sidereal are < 1 arcsec/hour, which are easily within Hubble's tracking capabilities. These rates of motion are also slow enough to keep a single pair of guide stars within the FGS pickles for an entire visibility window. The ephemeris uncertainties are typically at the 1 arcsec level. We understand that we will have essentially no control over the spacecraft roll angle. ------------------------------------------------------------------------------------ 16111( 6) - 10-Apr-2020 18:51:29 - [ 3] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 TARGET NAME: C2019-Y4 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: COMET disintegrating comet ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 TYPE=COMET, Q=0.2528326026905927, E=0.9992444279234513, I=45.38507225915464 , O=120.5732078366555, W=177.4075353245124, T=31-MAY-2020:00:22:46, TTimeScale=TDB, EQUINOX=J2000, EPOCH=06-MAR-2020:00:00:00, EpochTimeScale=TDB, R0=2.808 , DT=0. , A1=4.282845687866E-7, A2=0., A3=0. , ALN=0.1112620426, NM=2.15, NN=5.093, NK=4.6142, AMRAT=0. ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 14 +/- 3 Core only (integrated over 1 degree the mag is 8) Ephemeris Center: EARTH Extended: YES ------------------------------------------------------------------------------------------------------------------------------------ 16111( 6) - 10-Apr-2020 18:51:29 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-LINE Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 0.145 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 46.84 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 16111( 6) - 10-Apr-2020 18:51:29 - [ 5] Visit: 01 Visit Requirements: BETWEEN 20-APR-2020:12:00:00 AND 21-APR-2020:21:00:00 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 C2019-Y4 WFC3/UVI ACCUM UVIS2-2K2 F350LP CR-SPLIT=NO 3 260 S POS TARG 10, -5; SEQ 1 S C-SUB (1560 S) NON-INT; PATTERN 1 1 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ C2019-Y4 1 WFC3/UVI ACCUM UVIS2-2K F350LP 1 none 1 none 1 N/A S 2C-SUB C2019-Y4 1 WFC3/UVI ACCUM UVIS2-2K F350LP 1 none 2 none 1 N/A S 2C-SUB C2019-Y4 1 WFC3/UVI ACCUM UVIS2-2K F350LP 1 none 3 none 1 N/A S 2C-SUB C2019-Y4 1 WFC3/UVI ACCUM UVIS2-2K F350LP 2 none 1 none 1 N/A S 2C-SUB C2019-Y4 1 WFC3/UVI ACCUM UVIS2-2K F350LP 2 none 2 none 1 N/A S 2C-SUB C2019-Y4 1 WFC3/UVI ACCUM UVIS2-2K F350LP 2 none 3 none 1 N/A S 2C-SUB 16111( 6) - 10-Apr-2020 18:51:29 - [ 6] Visit: 02 Visit Requirements: BETWEEN 23-APR-2020:06:00:00 AND 23-APR-2020:23:00:00 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 C2019-Y4 WFC3/UVI ACCUM UVIS2-2K2 F350LP CR-SPLIT=NO 3 260 S POS TARG 10, -5; SEQ 1 S C-SUB (1560 S) NON-INT; PATTERN 1 1 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ C2019-Y4 1 WFC3/UVI ACCUM UVIS2-2K F350LP 1 none 1 none 1 N/A S 2C-SUB C2019-Y4 1 WFC3/UVI ACCUM UVIS2-2K F350LP 1 none 2 none 1 N/A S 2C-SUB C2019-Y4 1 WFC3/UVI ACCUM UVIS2-2K F350LP 1 none 3 none 1 N/A S 2C-SUB C2019-Y4 1 WFC3/UVI ACCUM UVIS2-2K F350LP 2 none 1 none 1 N/A S 2C-SUB C2019-Y4 1 WFC3/UVI ACCUM UVIS2-2K F350LP 2 none 2 none 1 N/A S 2C-SUB C2019-Y4 1 WFC3/UVI ACCUM UVIS2-2K F350LP 2 none 3 none 1 N/A S 2C-SUB 16111( 6) - 10-Apr-2020 18:51:29 - [ 7] Visit: 11 Visit Requirements: BETWEEN 20-APR-2020:12:00:00 AND 21-APR-2020:21:00:00 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 C2019-Y4 WFC3/UVI ACCUM UVIS2-FIX F350LP CR-SPLIT=NO 3 396 S SEQ 1-2 NON-INT S (1188 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 C2019-Y4 WFC3/UVI ACCUM UVIS2-FIX F350LP CR-SPLIT=NO 2 397 S (794 POS TARG 0.5, -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 | ------------------------------------------------------------------------------------------------------------------------------------ C2019-Y4 1 WFC3/UVI ACCUM UVIS2-FI F350LP none none 1 none 1 N/A S X C2019-Y4 1 WFC3/UVI ACCUM UVIS2-FI F350LP none none 2 none 1 N/A S X C2019-Y4 1 WFC3/UVI ACCUM UVIS2-FI F350LP none none 3 none 1 N/A S X C2019-Y4 2 WFC3/UVI ACCUM UVIS2-FI F350LP none none 1 none 1 N/A S X C2019-Y4 2 WFC3/UVI ACCUM UVIS2-FI F350LP none none 2 none 1 N/A S X 16111( 6) - 10-Apr-2020 18:51:29 - [ 8] Visit: 12 Visit Requirements: BETWEEN 23-APR-2020:06:00:00 AND 23-APR-2020:23:00:00 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 C2019-Y4 WFC3/UVI ACCUM UVIS2-FIX F350LP CR-SPLIT=NO 3 396 S SEQ 1-2 NON-INT S (1188 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 C2019-Y4 WFC3/UVI ACCUM UVIS2-FIX F350LP CR-SPLIT=NO 2 397 S (794 POS TARG 0.5, -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 | ------------------------------------------------------------------------------------------------------------------------------------ C2019-Y4 1 WFC3/UVI ACCUM UVIS2-FI F350LP none none 1 none 1 N/A S X C2019-Y4 1 WFC3/UVI ACCUM UVIS2-FI F350LP none none 2 none 1 N/A S X C2019-Y4 1 WFC3/UVI ACCUM UVIS2-FI F350LP none none 3 none 1 N/A S X C2019-Y4 2 WFC3/UVI ACCUM UVIS2-FI F350LP none none 1 none 1 N/A S X C2019-Y4 2 WFC3/UVI ACCUM UVIS2-FI F350LP none none 2 none 1 N/A S X 16111( 6) - 10-Apr-2020 18:51:29 - [ 9] Summary Form for Proposal 16111 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2-2K2C-SUB, UVIS2-FIX ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BETWEEN 20-APR-2020:12:00:00 AND 21-APR-2020:21:00:00, BETWEEN 23-APR-2020:06:00:00 AND 23-APR-2020:23:00:00, PATTERN 1 1, POS TARG 0.5, -5, POS TARG 10, -5, SEQ 1 NON-INT, SEQ 1-2 NON-INT ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F350LP ------------------------------------------------------------------------------------------------------------------------------------ Target Names C2019-Y4 ------------------------------------------------------------------------------------------------------------------------------------