16904( 2) - 18-Jan-2022 20:30:53 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 16904 Version: 2 Check-in Time: 18-Jan-2022 20:30:53 STScI Edit Number: 0 Title Disintegrating Comet 108P/Ciffreo ------------------------------------------------------------------------------------ Type Cycle GO/DD 29 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. David Jewitt University of California - Los Angeles Y CoI: Max Mutchler Space Telescope Science Institute Y 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 ------------------------------------------------------------------------------------ Abstract As was first noticed on 2022 January 2, the short-period (Kuiper belt) comet 108P/Ciffreo is undergoing a fragmentation event. In low-resolution ground-based data, the ejected fragments appear only as a diffuse, low surface brightness blob separated from the main nucleus. Given such data, neither photometric nor dynamical studies are possible. We propose early-time measurements at the high angular resolution offered by HST to characterize these embedded fragments in terms of their brightness and velocity distributions, with the aim being to determine the process behind the fragmentation. Three orbits are requested to determine the sky-plane motion of the fragments so that their velocities and ejection dates can be determined. Cometary fragmentation events are both unpredictable and short-lived and, as a result, few have been studied in detail. DDT observations are needed to obtain data while the fragment swarm is fresh and before it dissipates and fades. ------------------------------------------------------------------------------------ Observations Description ------------------------ We request three orbits with WFC3 spread over about a week in order to achieve our two main science objectives: fragment size distribution and dynamics. This timing will permit the measurement of fragment velocities and their extrapolation to determine the release dates, assuming nominal ejection speeds of order 1 m/s. The ejected fragments may themselves be sublimating. Recoil forces from asymmetric sublimation drive a non-gravitational (rocket) acceleration of magnitude inversely proportional to fragment size. If the rocket force from sublimation in the subsolar region dominates, the fragments will remain in the orbital plane, because like radiation pressure this force is directed away from the Sun and acts only to reduce the effective solar gravity. If outgassing is negligible and instead the fragment motion is governed by the initial velocity, the fragments will generally occupy a more three- dimensional distribution, as in active asteroid P/2010 A2. Analysis of the dynamical evolution of the fragments will therefore provide a separate indication of whether sublimation played a major role in the break-up event, or if other (e.g., tidal or centrifugal) forces were prominently involved. Our basic observing strategy is to take multiple long exposures (348-400s) using WFC3 and the wide bandpass filter (F350LP) for maximum sensitivity. We use F350LP because it provides a count rate 1.6 higher for a target with a solar-type 16904( 2) - 18-Jan-2022 20:30:53 - [ 2] spectrum than other commonly used broadband filters (e.g., F606W). We will also dither the exposures to mitigate the effects from bad pixels, cosmic rays, and the inter-chip gap. The WFC3 Exposure Time Calculator indicates that, for a point source, solar-type spectrum, a signal-to-noise ratio of 6 is reached in 348 s at V = 26.5 in the F350LP filter (ETC request ID WFC3UVIS.im.765972; optimal extraction yields SNR=13). We gain an additional 0.9 magnitudes by combining five images from each orbit, giving V = 27.4. For example, on UT 2021 March 1, when heliocentric distance rH = 2.31 AU, geocentric distance 1.68 AU and phase angle 22, magnitude V = 27.4 corresponds to an object radius 65 m (geometric albedo 0.04 and C-type asteroid phase function 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. Gas emission from 108P/Ciffreo is likely to be negligible, as it is in all low activity comets at 2 AU. Even if gas drives the expulsion of matter from the nucleus, its rapid expansion velocity and intrinsically small resonance fluorescence scattering cross- section (compared to continuum scattering by dust) renders gas optically unimportant in observations of comets much beyond Earths orbit. The apparent rates of motion relative to sidereal are easily within Hubbles tracking capabilities. This rate of motion is also slow enough to keep a single pair of guide stars within the FGS pickles for an entire visibility window. The ephemeris uncertainties (from JPL Horizons) on 108P/Ciffreo are sub-arcsecond and therefore of no consequence for these observations. We understand that we will have essentially no control over the spacecraft roll angle. Observations are requested as early as possible in order to capture the early-time development of the ejected material, before it has had a change to fade and disperse. However, we do not request disruptive scheduling of these observations. In the February - March period, 108P/Ciffreo will be about 2.2 AU from the Sun 1.5 AU from the Earth and at a phase angle near 20 degrees. Observations from about -8 below the plane of the orbit of 108P will provide a suitable perspective view of the distribution of bodies and dust in the orbit plane, which we will interpret using an existing Monte-Carlo dust dynamics model. ------------------------------------------------------------------------------------ 16904( 2) - 18-Jan-2022 20:30:53 - [ 3] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 2 TARGET NAME: COMET-108P-CIFFREO ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: COMET Disintegrating comet ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 TYPE=COMET, Q=1.676339083838281, E=0.549930518846976, I=13.31246416501656 , O=52.92407308081733, W=358.246135975604, T=21-OCT-2014:22:14:10, TTimeScale=TDB, EQUINOX=J2000, EPOCH=29-APR-2017:00:00:00, EpochTimeScale=TDB ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 20 +/- 5 Ephemeris Center: EARTH Extended: YES ------------------------------------------------------------------------------------------------------------------------------------ 16904( 2) - 18-Jan-2022 20:30:53 - [ 4] Visit: 01 Visit Requirements: SCHED 100%; BETWEEN 09-FEB-2022:15:00:00 AND 10-FEB-2022:20:00:00; BETWEEN 31-JAN-2022:00:00:00 AND 05-FEB-2022:18:00:00; BETWEEN 13-FEB-2022:09:00:00 AND 15-FEB-2022:15:00:00; BETWEEN 16-FEB-2022:02:00:00 AND 16-FEB-2022:20:00:00; VISIBILITY INTERVAL NO GYRO BIAS UPDATE ON MOVING TARGET 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 COMET-108P- WFC3/UVI ACCUM UVIS2-FIX F350LP 3 360 S SEQ 1-2 NON-INT CIFFREO S (1080 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 COMET-108P- WFC3/UVI ACCUM UVIS2-FIX F350LP 2 360 S (720 POS TARG 0.4, 2.5 CIFFREO 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 | ------------------------------------------------------------------------------------------------------------------------------------ COMET-108P-CIFFREO 1 WFC3/UVI ACCUM UVIS2-FI F350LP none none 1 none 1 N/A S X COMET-108P-CIFFREO 1 WFC3/UVI ACCUM UVIS2-FI F350LP none none 2 none 1 N/A S X COMET-108P-CIFFREO 1 WFC3/UVI ACCUM UVIS2-FI F350LP none none 3 none 1 N/A S X COMET-108P-CIFFREO 2 WFC3/UVI ACCUM UVIS2-FI F350LP none none 1 none 1 N/A S X COMET-108P-CIFFREO 2 WFC3/UVI ACCUM UVIS2-FI F350LP none none 2 none 1 N/A S X 16904( 2) - 18-Jan-2022 20:30:53 - [ 5] Visit: 02 Visit Requirements: SCHED 100%; AFTER 01 BY 2 D TO 5 D; BETWEEN 09-FEB-2022:15:00:00 AND 10-FEB-2022:20:00:00; BETWEEN 31-JAN-2022:00:00:00 AND 05-FEB-2022:18:00:00; BETWEEN 13-FEB-2022:09:00:00 AND 15-FEB-2022:15:00:00; BETWEEN 16-FEB-2022:02:00:00 AND 16-FEB-2022:20:00:00; VISIBILITY INTERVAL NO GYRO BIAS UPDATE ON MOVING TARGET 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 COMET-108P- WFC3/UVI ACCUM UVIS2-FIX F350LP 3 360 S SEQ 1-2 NON-INT CIFFREO S (1080 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 COMET-108P- WFC3/UVI ACCUM UVIS2-FIX F350LP 2 360 S (720 POS TARG 0.4, 2.5 CIFFREO 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 | ------------------------------------------------------------------------------------------------------------------------------------ COMET-108P-CIFFREO 1 WFC3/UVI ACCUM UVIS2-FI F350LP none none 1 none 1 N/A S X COMET-108P-CIFFREO 1 WFC3/UVI ACCUM UVIS2-FI F350LP none none 2 none 1 N/A S X COMET-108P-CIFFREO 1 WFC3/UVI ACCUM UVIS2-FI F350LP none none 3 none 1 N/A S X COMET-108P-CIFFREO 2 WFC3/UVI ACCUM UVIS2-FI F350LP none none 1 none 1 N/A S X COMET-108P-CIFFREO 2 WFC3/UVI ACCUM UVIS2-FI F350LP none none 2 none 1 N/A S X 16904( 2) - 18-Jan-2022 20:30:53 - [ 6] Visit: 03 Visit Requirements: SCHED 100%; AFTER 01 BY 7 D TO 10 D; BETWEEN 09-FEB-2022:15:00:00 AND 10-FEB-2022:20:00:00; BETWEEN 31-JAN-2022:00:00:00 AND 05-FEB-2022:18:00:00; BETWEEN 13-FEB-2022:09:00:00 AND 15-FEB-2022:15:00:00; BETWEEN 16-FEB-2022:02:00:00 AND 16-FEB-2022:20:00:00; VISIBILITY INTERVAL NO GYRO BIAS UPDATE ON MOVING TARGET 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 COMET-108P- WFC3/UVI ACCUM UVIS2-FIX F350LP 3 360 S SEQ 1-2 NON-INT CIFFREO S (1080 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 COMET-108P- WFC3/UVI ACCUM UVIS2-FIX F350LP 2 360 S (720 POS TARG 0.4, 2.5 CIFFREO 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 | ------------------------------------------------------------------------------------------------------------------------------------ COMET-108P-CIFFREO 1 WFC3/UVI ACCUM UVIS2-FI F350LP none none 1 none 1 N/A S X COMET-108P-CIFFREO 1 WFC3/UVI ACCUM UVIS2-FI F350LP none none 2 none 1 N/A S X COMET-108P-CIFFREO 1 WFC3/UVI ACCUM UVIS2-FI F350LP none none 3 none 1 N/A S X COMET-108P-CIFFREO 2 WFC3/UVI ACCUM UVIS2-FI F350LP none none 1 none 1 N/A S X COMET-108P-CIFFREO 2 WFC3/UVI ACCUM UVIS2-FI F350LP none none 2 none 1 N/A S X 16904( 2) - 18-Jan-2022 20:30:53 - [ 7] Summary Form for Proposal 16904 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2-FIX ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements AFTER 01 BY 2 D TO 5 D, AFTER 01 BY 7 D TO 10 D, BETWEEN 09-FEB-2022:15:00:00 AND 10-FEB-2022:20:00:00, BETWEEN 13-FEB-2022:09:00:00 AND 15-FEB-2022:15:00:00, BETWEEN 16-FEB-2022:02:00:00 AND 16-FEB-2022:20:00:00, BETWEEN 31-JAN-2022:00:00:00 AND 05-FEB-2022:18:00:00, POS TARG 0.4, 2.5, SCHED 100%, SEQ 1-2 NON-INT, VISIBILITY INTERVAL NO GYRO BIAS UPDATE ON MOVING TARGET ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F350LP ------------------------------------------------------------------------------------------------------------------------------------ Target Names COMET-108P-CIFFREO ------------------------------------------------------------------------------------------------------------------------------------