18495( 4) - 04-Nov-2026 15:49:25 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18495 Version: 4 Check-in Time: 04-Nov-2026 15:49:25 STScI Edit Number: 0 Title The debris field of recently activated asteroid 6993 ------------------------------------------------------------------------------------ Type Cycle GO/DD 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Jessica Agarwal Technische Universitat Braunschweig Y CoI: Jens Richter Technische Universitat Braunschweig N CoI: Oriel A Humes Technische Universitat Braunschweig N CoI: Max Mutchler Space Telescope Science Institute N CoI: Dr. David Jewitt University of California - Los Angeles N CoI: Dr. Yoonyoung Kim University of California - Los Angeles N ------------------------------------------------------------------------------------ Abstract We request time-critical observations of asteroid 6993, which on 2026 October 26 was reported by Clark et al. (2026) to have activated during 2026 summer. We seek to explore the morphology of the debris field at high resolution to search for traces of an impact ejecta cone and/or for signs of sublimation-driven (re)activation as the target approaches perihelion. If impact traces such as arc-shaped features, bright filaments or individual fragments are found, we will reconstruct the magnitude and geometry of the impact through computer similations of the debris motion. This will yield a constraint on the depth of a potential crater and by inference on the depth of ice below the pre-impact surface if sublimation has been triggered, or a lower limit to it if not. If sublimating, 6993 would have the largest diameter (~12km) and perihelion distance (2.85au) of the known main-belt comets, significantly expanding the diversity of the known population. Activated asteroids represent unique opportunities to look below the surfaces of these objects and to constrain the depth and abundance of ice in the main-belt population, which ultimately informs on their formation conditions inside or outside the snowline and on their subsequent evolution. The target is visible to HST until 2026 November 15 according to APT, wherefore we request disruptive scheduling. The next HST visibility window opens in 2027 October, when the current debris will have been dispersed by solar radiation pressure. ------------------------------------------------------------------------------------ Observations Description ------------------------ We request WFC3/UVIS imaging with F350LP, a 2-point-dither pattern, repeated as often as can be fit into one orbit. If schedulability is given only for fewer exposures, that will still allow us to address our science goal, at lower sensitivity. We request a minimum of 2 exposures to mitigate cosmic ray hits and bad pixels. The target is an asteroid (expected point source brightness g=16.3, which drives the short exposure time). The asteroid is expected to be embedded in a debris field and/or dust cloud of much fainter surface brightness. Avoiding point source saturation and minimizing diffraction spikes is crucial to obtain 18495( 4) - 04-Nov-2026 15:49:25 - [ 2] a clear view of the debris field, which is the actual science target. The brightness of the point source target may help to mitigate the low background. For an optimum trade-off between saturation risk (due to the intrinsic uncertainty of the target brightness), CTE loss mitigation and sensitivity to faint dust far from the bright point source, we request the following set of exposures per each of the two dither stations: 10x 15s without FLASH 5x 10s with FLASH=20 2x 5s with FLASH=20 We request maximum scheduling flexibility (100), as the target ceases to be observable on 2026-Nov-15. We use the C512C-subarray to mitigate CTE losses and optimize readout time. ------------------------------------------------------------------------------------ 18495( 4) - 04-Nov-2026 15:49:25 - [ 3] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 TARGET NAME: 6993 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: ASTEROID Active Asteroid ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 TYPE=ASTEROID, A=3.108106318045111, E=0.0821350829480771, I=2.400639198261841, O=128.0752077754092, W=293.8456083453971, M=322.0076192507526, EQUINOX=J2000, EPOCH=09-JUN-2026:00:00:00, EpochTimeScale=TDB ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 16.3 +/- 0.3 Ephemeris Center: EARTH Extended: YES ------------------------------------------------------------------------------------------------------------------------------------ 18495( 4) - 04-Nov-2026 15:49:25 - [ 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 ------------------------------------------------------------------------------------------------------------------------------------ 18495( 4) - 04-Nov-2026 15:49:25 - [ 5] Visit: 01 (6993_imaging) Visit Requirements: BETWEEN 09-NOV-2026:00:00:00 AND 12-NOV-2026:17:55:00; BETWEEN 12-NOV-2026:19:45:00 AND 16-NOV-2026:00:00:00 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 6993 WFC3/UVI ACCUM UVIS2-C51 F350LP FLASH=14 10 15 S (300 PATTERN 1 1-3 S 2C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 2 6993 WFC3/UVI ACCUM UVIS2-C51 F350LP FLASH=21 5 10 S (100 S 2C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 3 6993 WFC3/UVI ACCUM UVIS2-C51 F350LP FLASH=21 3 5 S (30 S) S 2C-SUB ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ 6993 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none 1 none 1 N/A S 12C-SUB 6993 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none 2 none 1 N/A S 12C-SUB 6993 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none 3 none 1 N/A S 12C-SUB 6993 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none 4 none 1 N/A S 12C-SUB 6993 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none 5 none 1 N/A S 12C-SUB 6993 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none 6 none 1 N/A S 12C-SUB 6993 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none 7 none 1 N/A S 12C-SUB 6993 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none 8 none 1 N/A S 12C-SUB 6993 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none 9 none 1 N/A S 12C-SUB 6993 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none 10 none 1 N/A S 12C-SUB 6993 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none 1 none 1 N/A S 12C-SUB 6993 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none 2 none 1 N/A S 12C-SUB 6993 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none 3 none 1 N/A S 12C-SUB 18495( 4) - 04-Nov-2026 15:49:25 - [ 6] 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 | ------------------------------------------------------------------------------------------------------------------------------------ 6993 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none 4 none 1 N/A S 12C-SUB 6993 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none 5 none 1 N/A S 12C-SUB 6993 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none 6 none 1 N/A S 12C-SUB 6993 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none 7 none 1 N/A S 12C-SUB 6993 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none 8 none 1 N/A S 12C-SUB 6993 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none 9 none 1 N/A S 12C-SUB 6993 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none 10 none 1 N/A S 12C-SUB 6993 2 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none 1 none 1 N/A S 12C-SUB 6993 2 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none 2 none 1 N/A S 12C-SUB 6993 2 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none 3 none 1 N/A S 12C-SUB 6993 2 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none 4 none 1 N/A S 12C-SUB 6993 2 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none 5 none 1 N/A S 12C-SUB 6993 2 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none 1 none 1 N/A S 12C-SUB 6993 2 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none 2 none 1 N/A S 12C-SUB 6993 2 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none 3 none 1 N/A S 12C-SUB 6993 2 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none 4 none 1 N/A S 12C-SUB 6993 2 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none 5 none 1 N/A S 12C-SUB 6993 3 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none 1 none 1 N/A S 12C-SUB 6993 3 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none 2 none 1 N/A S 12C-SUB 6993 3 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none 3 none 1 N/A S 12C-SUB 6993 3 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none 1 none 1 N/A S 12C-SUB 6993 3 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none 2 none 1 N/A S 12C-SUB 6993 3 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none 3 none 1 N/A S 12C-SUB 18495( 4) - 04-Nov-2026 15:49:25 - [ 7] Summary Form for Proposal 18495 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2-C512C-SUB ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=14, FLASH=21 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BETWEEN 09-NOV-2026:00:00:00 AND 12-NOV-2026:17:55:00, BETWEEN 12-NOV-2026:19:45:00 AND 16-NOV-2026:00:00:00, PATTERN 1 1-3 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F350LP ------------------------------------------------------------------------------------------------------------------------------------ Target Names 6993 ------------------------------------------------------------------------------------------------------------------------------------