18133( 10) - 15-Dec-2025 17:23:29 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18133 Version: 10 Check-in Time: 15-Dec-2025 17:23:29 STScI Edit Number: 4 Title Probing the TNO Density Transition with Mid-Size 2013 FY27 ------------------------------------------------------------------------------------ Type Cycle GO 33 ------------------------------------------------------------------------------------ Investigators Contact? PI: Prof. Darin Ragozzine Brigham Young University Y CoI: Dr. Benjamin Proudfoot University of Central Florida Board of Y Trustees CoI: Dr. Will M. Grundy Lowell Observatory N CoI: Dr. Scott Sander Sheppard Carnegie Institution of Washington N ------------------------------------------------------------------------------------ Abstract The density distribution of Trans-Neptunian Objects (TNOs) provides key insights into the composition and interiors of outer solar system bodies, with broad implications for planet formation and evolution. Only ~20 TNOs have measured densities which seem to fall into three categories: small TNOs have similar low densities and large TNOs have similar high densities. The "transition region" (diameters of 400-900 km) only has 5 known densities which show a surprisingly rapid transition between these two regimes. While the density-size relationship was thought to be linear, a recent measurement breaks this trend, suggesting the need to explore other interpretations. This proposal targets 2013 FY27, a rare TNO in the transition region with a known satellite and a diameter of ~760 km. Previous HST observations were insufficient to determine its orbit due to a small near edge-on orbit. We propose a focused 5-orbit program to obtain precise relative astrometry of FY27 and its satellite to measure the density of FY27 and to determine the possibility of information-rich mutual events. Previous observations are used to design an optimal observing program, including iterative updates to the observing schedule using state-of-the-art orbit fitting and prediction tools. Success in these goals is only plausible with HST. This program will add a valuable new data point to the poorly-understood density distribution of TNOs in the density transition, helping to resolve whether composition or porosity drives the transition. The resulting insights on composition and interiors have broad importance in astrometry through improved constraints on planet formation and evolution. ------------------------------------------------------------------------------------ Observations Description ------------------------ Our observations aim to recover and determine the orbit of 2013 FY27's satellite. Observations focus on high SNR and astrometric precision. Each single-orbit visit will consist of 5 F350LP exposures, each of ~420 seconds. The first 4 exposures will use the UVIS2-C512C-SUB aperture, which will limit data volume and reduce overheads. The last exposure will use the full UVIS2 18133( 10) - 15-Dec-2025 17:23:29 - [ 2] aperture, pushing the final data buffer dump into Earth occultation. Observations will be dithered to optimally sample the PSF and provide resilience to per-pixel effects. Our program relies on precise observation timing to fulfill our science goals efficiently. Since the orbit phase of the satellite is ~unknown, our first three observations will be dedicated to blindly recovering the satellite. These observations should take place in a sequence with 2.5 +/- 0.5 days between each visit. The first visit should be carried out soon after 2013 FY27 becomes visible, which allows us the maximum time to carry out our carefully planned sequence of observations. Visits 4 and 5 will remain on hold until preliminary analysis is able to predict optimal times to observe the satellite. If we accomplish our science goals before all visits are carried out, we will release allocated orbits back to STScI. ------------------------------------------------------------------------------------ 18133( 10) - 15-Dec-2025 17:23:29 - [ 3] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 TARGET NAME: 2013FY27 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: DWARF-PLANET FY27 and satellite ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 TYPE=ASTEROID, A=58.99363145348065, E=0.3916647205101159, I=32.99057781326125 , O=187.1703133045093, W=138.6060391018931, M=212.7151024578246, EQUINOX=J2000, EPOCH=21-APR-2017:00:00:00, EpochTimeScale=TDB ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 22.5 Ephemeris Center: EARTH Extended: NO ------------------------------------------------------------------------------------------------------------------------------------ 18133( 10) - 15-Dec-2025 17:23:29 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.173 Line_Spacing 0.112 Coordinate_Frame POS-TARG Pattern_Orient 23.884 Angle_Between_Sides 81.785 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 18133( 10) - 15-Dec-2025 17:23:29 - [ 5] Visit: 01 (Phase recovery 1) Visit Requirements: ORIENT 285D TO 296.1 D; BEFORE 10-DEC-2025:00:00:00 On Hold Comments: Additional Comments: Should take place soon after target becomes visible. 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 2013FY27 WFC3/UVI ACCUM UVIS2-C51 F350LP 1 420 S SEQ 1-2 NON-INT; PATTERN S 2C-SUB see below 1 1 (1672 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 2013FY27 WFC3/UVI ACCUM UVIS2 F350LP 1 420 S S see below (418 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 | ------------------------------------------------------------------------------------------------------------------------------------ 2013FY27 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none none none 1 418.0 S S 12C-SUB 2013FY27 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none none none 1 418.0 S S 12C-SUB 2013FY27 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 3 none none none 1 418.0 S S 12C-SUB 2013FY27 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 4 none none none 1 418.0 S S 12C-SUB 2013FY27 2 WFC3/UVI ACCUM UVIS2 F350LP none none none none 1 418.0 S S 18133( 10) - 15-Dec-2025 17:23:29 - [ 6] Visit: 02 (Phase recovery 2) Visit Requirements: ORIENT 285D TO 296.1 D; AFTER 01 BY 2 D TO 3 D On Hold Comments: Additional Comments: Should take place 2-3 days after Visit 1. The timing of this sequence is critically important for our science goals. 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 2013FY27 WFC3/UVI ACCUM UVIS2-C51 F350LP 1 420 S SEQ 1-2 NON-INT; PATTERN S 2C-SUB see below 1 1 (1672 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 2013FY27 WFC3/UVI ACCUM UVIS2 F350LP 1 420 S S see below (418 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 | ------------------------------------------------------------------------------------------------------------------------------------ 2013FY27 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none none none 1 418.0 S S 12C-SUB 2013FY27 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none none none 1 418.0 S S 12C-SUB 2013FY27 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 3 none none none 1 418.0 S S 12C-SUB 2013FY27 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 4 none none none 1 418.0 S S 12C-SUB 2013FY27 2 WFC3/UVI ACCUM UVIS2 F350LP none none none none 1 418.0 S S 18133( 10) - 15-Dec-2025 17:23:29 - [ 7] Visit: 03 (Phase recovery 3) Visit Requirements: ORIENT 285D TO 296.1 D; AFTER 02 BY 2 D TO 3 D On Hold Comments: Additional Comments: Should take place 2-3 days after Visit 2. The timing of this sequence is critically important for our science goals. 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 2013FY27 WFC3/UVI ACCUM UVIS2-C51 F350LP 1 420 S SEQ 1-2 NON-INT; PATTERN S 2C-SUB see below 1 1 (1672 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 2013FY27 WFC3/UVI ACCUM UVIS2 F350LP 1 420 S S see below (418 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 | ------------------------------------------------------------------------------------------------------------------------------------ 2013FY27 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none none none 1 418.0 S S 12C-SUB 2013FY27 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none none none 1 418.0 S S 12C-SUB 2013FY27 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 3 none none none 1 418.0 S S 12C-SUB 2013FY27 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 4 none none none 1 418.0 S S 12C-SUB 2013FY27 2 WFC3/UVI ACCUM UVIS2 F350LP none none none none 1 418.0 S S 18133( 10) - 15-Dec-2025 17:23:29 - [ 8] Visit: 04 (Orbit finalize 1) Visit Requirements: BETWEEN 24-DEC-2025:06:00:00 AND 25-DEC-2025:06:00:00; BETWEEN 28-DEC-2025:00:00:00 AND 29-DEC-2025:12:00:00; BETWEEN 03-JAN-2026:00:00:00 AND 04-JAN-2026:00:00:00; BETWEEN 06-JAN-2026:18:00:00 AND 08-JAN-2026:06:00:00; BETWEEN 12-JAN-2026:18:00:00 AND 13-JAN-2026:18:00:00; BETWEEN 16-JAN-2026:12:00:00 AND 18-JAN-2026:00:00:00; BETWEEN 22-JAN-2026:12:00:00 AND 23-JAN-2026:12:00:00; BETWEEN 28-JAN-2026:06:00:00 AND 29-JAN-2026:18:00:00 On Hold Comments: Additional Comments: Orbit to disambiguate between the final two remaining orbit solutions. 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 2013FY27 WFC3/UVI ACCUM UVIS2-C51 F350LP 1 420 S SEQ 1-2 NON-INT; PATTERN S 2C-SUB see below 1 1 (1672 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 2013FY27 WFC3/UVI ACCUM UVIS2 F350LP 1 420 S S see below (418 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 | ------------------------------------------------------------------------------------------------------------------------------------ 2013FY27 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none none none 1 418.0 S S 12C-SUB 2013FY27 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none none none 1 418.0 S S 12C-SUB 2013FY27 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 3 none none none 1 418.0 S S 12C-SUB 2013FY27 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 4 none none none 1 418.0 S S 12C-SUB 2013FY27 2 WFC3/UVI ACCUM UVIS2 F350LP none none none none 1 418.0 S S 18133( 10) - 15-Dec-2025 17:23:29 - [ 9] Visit: 05 (Orbit finalize 2) Visit Requirements: AFTER 04 BY 2 D TO 365 D; BETWEEN 24-DEC-2025:06:00:00 AND 25-DEC-2025:06:00:00; BETWEEN 28-DEC-2025:00:00:00 AND 29-DEC-2025:12:00:00; BETWEEN 03-JAN-2026:00:00:00 AND 04-JAN-2026:00:00:00; BETWEEN 06-JAN-2026:18:00:00 AND 08-JAN-2026:06:00:00; BETWEEN 12-JAN-2026:18:00:00 AND 13-JAN-2026:18:00:00; BETWEEN 16-JAN-2026:12:00:00 AND 18-JAN-2026:00:00:00; BETWEEN 22-JAN-2026:12:00:00 AND 23-JAN-2026:12:00:00; BETWEEN 26-JAN-2026:06:00:00 AND 27-JAN-2026:18:00:00 On Hold Comments: Additional Comments: Orbit to disambiguate between the final two remaining orbit solutions. 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 2013FY27 WFC3/UVI ACCUM UVIS2-C51 F350LP 1 350 S SEQ 1-2 NON-INT; PATTERN S 2C-SUB (1400 S) 1 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 2013FY27 WFC3/UVI ACCUM UVIS2 F350LP 1 300 S (300 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 | ------------------------------------------------------------------------------------------------------------------------------------ 2013FY27 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none none none 1 N/A S 12C-SUB 2013FY27 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none none none 1 N/A S 12C-SUB 2013FY27 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 3 none none none 1 N/A S 12C-SUB 2013FY27 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 4 none none none 1 N/A S 12C-SUB 2013FY27 2 WFC3/UVI ACCUM UVIS2 F350LP none none none none 1 N/A S 18133( 10) - 15-Dec-2025 17:23:29 - [ 10] Summary Form for Proposal 18133 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2, UVIS2-C512C-SUB ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements AFTER 01 BY 2 D TO 3 D, AFTER 02 BY 2 D TO 3 D, AFTER 04 BY 2 D TO 365 D, BEFORE 10-DEC-2025:00:00:00, BETWEEN 03-JAN-2026:00:00:00 AND 04-JAN-2026:00:00:00, BETWEEN 06-JAN-2026:18:00:00 AND 08-JAN-2026:06:00:00, BETWEEN 12-JAN-2026:18:00:00 AND 13-JAN-2026:18:00:00, BETWEEN 16-JAN-2026:12:00:00 AND 18-JAN-2026:00:00:00, BETWEEN 22-JAN-2026:12:00:00 AND 23-JAN-2026:12:00:00, BETWEEN 24-DEC-2025:06:00:00 AND 25-DEC-2025:06:00:00, BETWEEN 26-JAN-2026:06:00:00 AND 27-JAN-2026:18:00:00, BETWEEN 28-DEC-2025:00:00:00 AND 29-DEC-2025:12:00:00, BETWEEN 28-JAN-2026:06:00:00 AND 29-JAN-2026:18:00:00, ORIENT 285D TO 296.1 D, PATTERN 1 1, SEQ 1-2 NON-INT ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F350LP ------------------------------------------------------------------------------------------------------------------------------------ Target Names 2013FY27 ------------------------------------------------------------------------------------------------------------------------------------