18335( 2) - 04-Aug-2026 17:29:40 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18335 Version: 2 Check-in Time: 04-Aug-2026 17:29:40 STScI Edit Number: 0 Title The size, density, shape, and interior of mid-sized TNO Uni ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Benjamin Proudfoot University of Central Florida Board of Y Trustees CoI: Dra. Flavia Luane Rommel University of Central Florida Board of N Trustees CoI: Dr. Will M. Grundy Northern Arizona University N CoI: Prof. Darin Ragozzine Brigham Young University N CoI: Dr. Jose Luis Ortiz Instituto de Astrofisica de Andalucia N (IAA) ------------------------------------------------------------------------------------ Abstract The densities of trans-Neptunian objects (TNOs) provide insight into the formation of planetary building blocks in the earliest epochs of the solar system. The largest TNOs have high densities, implying a large rock fraction and a history of melting/differentiation, while the smallest TNOs have low densities, implying porous interiors. Surprisingly, the transition between those extremes happens at diameters of 600-800 km, allowing for relatively large bodies to maintain high porosity. The mid-sized TNO Uni is in this group, with a diameter estimated from thermal measurements of ~650 km and a density of 0.82 g/cm^3, requiring Uni to maintain a ~50% porosity. Recent evidence, however, has shown that Uni's satellite, named Tinia, is smaller than expected, calling into question the thermally-derived diameter for Uni. Thankfully, two upcoming stellar occultations provide a rare opportunity to measure both Uni and Tinia's sizes and shapes. To support observations of these occultations, and to significantly enhance the scientific return from this rare opportunity, we propose a 5 orbit program to observe Uni and Tinia using WFC3/UVIS. In addition to improving planning of the occultation campaigns, our observations will measure the mass ratio of Uni and Tinia and detect non-Keplerian precession of Tinia's orbit. These investigations will provide deep insight into the shape, orientation, and interior of Uni, allowing us to better understand the important mid-sized TNO population. ------------------------------------------------------------------------------------ Observations Description ------------------------ Our observations call for 5 single-orbit visits to image the Uni-Tinia system using the WFC3/UVIS instrument. All imaging will use the F350LP filter to maximize throughput and astrometric precision. The 5 visits are divided into two groups. The first three visits will be carefully timed to provide updates for stellar occultations occuring in Dec 2026. The last two visits will be scheduled in Jul-Oct 2027 to provide a maximal baseline for understanding the precession of the satellite's orbit. Occultation prep. observations: 18335( 2) - 04-Aug-2026 17:29:40 - [ 2] These visits will consist of 5 dithered images using the full UVIS aperture. The large aperture is used to capture as many Gaia reference stars in the images as possible for astrometric calibration. Exposure times are 350 seconds to allow for memory buffer dumps to occur during subsequent exposures. Scheduling of these observations is important. The timing windows (encoded in betweens) are to take place an integer number of satellite orbits before the occultations on Dec 05 and 15. The windows for Visits 1 and 2 are not flexible (although the selection between the available windows is flexible), but Visit 3 can have additional flexibility (above that included in the betweens) if needed. Long baseline observations: These visits will consist of 12 dithered images, 10 with the UVIS2-C512C-SUB aperture and 2 with the UVIS aperture. Exposures times are ~100 seconds to achieve SNR>50 on the satellite. The 2 full UVIS images will be taken at the end of the orbit with the memory buffer dump pushed into occultation. To maximize the return from these visits, we will initially place them on hold pending analysis of visits 1-3 (and the archival dataset of astrometric data). The visits will be scheduled in Uni's next HST visibility window (Jul-Oct 2027). Observing windows will last ~1 day and repeat once per orbit period (8.3095 days). As such, there should be ~10 scheduling opportunities for both these visits. ------------------------------------------------------------------------------------ 18335( 2) - 04-Aug-2026 17:29:40 - [ 3] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 TARGET NAME: UNI ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: ASTEROID uni ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 TYPE=ASTEROID, A=42.92456947386417, E=0.1462441869199175, I=19.42327844654774, O=204.6157936874821, W=276.0993480674751, M=310.1542385365317, EQUINOX=J2000, EPOCH=09-JUN-2026:00:00:00, EpochTimeScale=TDB ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 20 Ephemeris Center: EARTH ------------------------------------------------------------------------------------------------------------------------------------ 18335( 2) - 04-Aug-2026 17:29:40 - [ 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 ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 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 ------------------------------------------------------------------------------------------------------------------------------------ 18335( 2) - 04-Aug-2026 17:29:40 - [ 5] Visit: 01 (Occ. prep. 1) Visit Requirements: BETWEEN 01-NOV-2026:20:00:00 AND 02-NOV-2026:14:00:00; BETWEEN 10-NOV-2026:03:30:00 AND 10-NOV-2026:21:30:00; BETWEEN 18-NOV-2026:11:00:00 AND 19-NOV-2026:05:00:00 On Hold Comments: Additional Comments: Scheduled to take place an integer number of orbit periods (8.3095 days) before the occultation that will take place on 2026-12-05 19:50. There are three scheduling opportunities before Uni leaves HST's FOR. 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 UNI WFC3/UVI ACCUM UVIS F350LP 1 350 S (350 S S) ------------------------------------------------------------------------------------------------------------------------------------ 2 UNI WFC3/UVI ACCUM UVIS F350LP 1 350 S PATTERN 1 2 S (1400 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 | ------------------------------------------------------------------------------------------------------------------------------------ UNI 1 WFC3/UVI ACCUM UVIS F350LP none none none none 1 N/A S UNI 2 WFC3/UVI ACCUM UVIS F350LP 1 none none none 1 N/A S UNI 2 WFC3/UVI ACCUM UVIS F350LP 2 none none none 1 N/A S UNI 2 WFC3/UVI ACCUM UVIS F350LP 3 none none none 1 N/A S UNI 2 WFC3/UVI ACCUM UVIS F350LP 4 none none none 1 N/A S 18335( 2) - 04-Aug-2026 17:29:40 - [ 6] Visit: 02 (Occ. prep. 2) Visit Requirements: BETWEEN 03-NOV-2026:20:30:00 AND 04-NOV-2026:20:30:00; BETWEEN 12-NOV-2026:04:00:00 AND 13-NOV-2026:04:00:00; BETWEEN 20-NOV-2026:11:00:00 AND 21-NOV-2026:11:00:00 On Hold Comments: Additional Comments: Scheduled to take place an integer number of orbit periods (8.3095 days) before the occultation that will take place on 2026-12-15 21:28. There are three scheduling opportunities before Uni leaves HST's FOR. 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 UNI WFC3/UVI ACCUM UVIS F350LP 1 350 S (350 S S) ------------------------------------------------------------------------------------------------------------------------------------ 2 UNI WFC3/UVI ACCUM UVIS F350LP 1 350 S PATTERN 1 2 S (1400 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 | ------------------------------------------------------------------------------------------------------------------------------------ UNI 1 WFC3/UVI ACCUM UVIS F350LP none none none none 1 N/A S UNI 2 WFC3/UVI ACCUM UVIS F350LP 1 none none none 1 N/A S UNI 2 WFC3/UVI ACCUM UVIS F350LP 2 none none none 1 N/A S UNI 2 WFC3/UVI ACCUM UVIS F350LP 3 none none none 1 N/A S UNI 2 WFC3/UVI ACCUM UVIS F350LP 4 none none none 1 N/A S 18335( 2) - 04-Aug-2026 17:29:40 - [ 7] Visit: 03 (Occ. prep. 3) Visit Requirements: BETWEEN 01-NOV-2026:00:00:00 AND 01-NOV-2026:10:00:00; BETWEEN 08-NOV-2026:17:30:00 AND 09-NOV-2026:17:30:00; BETWEEN 17-NOV-2026:01:00:00 AND 18-NOV-2026:01:00:00 On Hold Comments: Additional Comments: Scheduled to take place at an orbit phase maximally separated from visits 1 and 2. As with those visits, there are three scheduling opportunities, each an orbit period (8.3095 days) apart. If needed, a slightly different orbit phase can be targeted to provide more scheduling flexibility with only a minimal loss in scientific return. 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 UNI WFC3/UVI ACCUM UVIS F350LP 1 350 S (350 S S) ------------------------------------------------------------------------------------------------------------------------------------ 2 UNI WFC3/UVI ACCUM UVIS F350LP 1 350 S PATTERN 1 2 S (1400 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 | ------------------------------------------------------------------------------------------------------------------------------------ UNI 1 WFC3/UVI ACCUM UVIS F350LP none none none none 1 N/A S UNI 2 WFC3/UVI ACCUM UVIS F350LP 1 none none none 1 N/A S UNI 2 WFC3/UVI ACCUM UVIS F350LP 2 none none none 1 N/A S UNI 2 WFC3/UVI ACCUM UVIS F350LP 3 none none none 1 N/A S UNI 2 WFC3/UVI ACCUM UVIS F350LP 4 none none none 1 N/A S 18335( 2) - 04-Aug-2026 17:29:40 - [ 8] Visit: 04 (Precession baseline 1) Visit Requirements: ON HOLD On Hold Comments: On hold for Visits 1-3 to be analyzed. Will be scheduled when the target is visible with HST in July-Oct 2027. Scheduling windows should be similar to visits 1-3, although there will be more opportunities. Additional Comments: Initially on hold pending analysis of visits 1-3. Scheduling windows are expected to last ~1 day and recur every orbit period (8.3095 days). 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 UNI WFC3/UVI ACCUM UVIS2-C51 F350LP FLASH=10 1 100 S PATTERN 1 1 S 2C-SUB see below (428 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 UNI WFC3/UVI ACCUM UVIS2-C51 F350LP FLASH=10 1 100 S PATTERN 1 2 S 2C-SUB see below (428 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 UNI WFC3/UVI ACCUM UVIS2-C51 F350LP FLASH=10 1 100 S PATTERN 2 3 S 2C-SUB see below (214 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 UNI WFC3/UVI ACCUM UVIS F350LP FLASH=10 1 100 S PATTERN 2 4 S see below (214 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 | ------------------------------------------------------------------------------------------------------------------------------------ UNI 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none none none 1 107.0 S S 12C-SUB UNI 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none none none 1 107.0 S S 12C-SUB UNI 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 3 none none none 1 107.0 S S 12C-SUB UNI 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 4 none none none 1 107.0 S S 12C-SUB UNI 2 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none none none 1 107.0 S S 12C-SUB UNI 2 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none none none 1 107.0 S S 12C-SUB UNI 2 WFC3/UVI ACCUM UVIS2-C5 F350LP 3 none none none 1 107.0 S S 12C-SUB UNI 2 WFC3/UVI ACCUM UVIS2-C5 F350LP 4 none none none 1 107.0 S S 12C-SUB 18335( 2) - 04-Aug-2026 17:29:40 - [ 9] 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 | ------------------------------------------------------------------------------------------------------------------------------------ UNI 3 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none none none 1 107.0 S S 12C-SUB UNI 3 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none none none 1 107.0 S S 12C-SUB UNI 4 WFC3/UVI ACCUM UVIS F350LP 1 none none none 1 107.0 S S UNI 4 WFC3/UVI ACCUM UVIS F350LP 2 none none none 1 107.0 S S 18335( 2) - 04-Aug-2026 17:29:40 - [ 10] Visit: 05 (Precession baseline 2) Visit Requirements: ON HOLD On Hold Comments: On hold for Visits 1-3 to be analyzed. Will be scheduled when the target is visible with HST in July-Oct 2027. Scheduling windows should be similar to visits 1-3, although there will be more opportunities. Additional Comments: Initially on hold pending analysis of visits 1-3. Scheduling windows are expected to last ~1 day and recur every orbit period (8.3095 days). 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 UNI WFC3/UVI ACCUM UVIS2-C51 F350LP FLASH=10 1 100 S PATTERN 1 1 S 2C-SUB see below (428 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 UNI WFC3/UVI ACCUM UVIS2-C51 F350LP FLASH=10 1 100 S PATTERN 1 2 S 2C-SUB see below (428 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 UNI WFC3/UVI ACCUM UVIS2-C51 F350LP FLASH=10 1 100 S PATTERN 2 3 S 2C-SUB see below (214 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 UNI WFC3/UVI ACCUM UVIS F350LP FLASH=10 1 100 S PATTERN 2 4 S see below (214 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 | ------------------------------------------------------------------------------------------------------------------------------------ UNI 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none none none 1 107.0 S S 12C-SUB UNI 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none none none 1 107.0 S S 12C-SUB UNI 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 3 none none none 1 107.0 S S 12C-SUB UNI 1 WFC3/UVI ACCUM UVIS2-C5 F350LP 4 none none none 1 107.0 S S 12C-SUB UNI 2 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none none none 1 107.0 S S 12C-SUB UNI 2 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none none none 1 107.0 S S 12C-SUB UNI 2 WFC3/UVI ACCUM UVIS2-C5 F350LP 3 none none none 1 107.0 S S 12C-SUB UNI 2 WFC3/UVI ACCUM UVIS2-C5 F350LP 4 none none none 1 107.0 S S 12C-SUB 18335( 2) - 04-Aug-2026 17:29:40 - [ 11] 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 | ------------------------------------------------------------------------------------------------------------------------------------ UNI 3 WFC3/UVI ACCUM UVIS2-C5 F350LP 1 none none none 1 107.0 S S 12C-SUB UNI 3 WFC3/UVI ACCUM UVIS2-C5 F350LP 2 none none none 1 107.0 S S 12C-SUB UNI 4 WFC3/UVI ACCUM UVIS F350LP 1 none none none 1 107.0 S S UNI 4 WFC3/UVI ACCUM UVIS F350LP 2 none none none 1 107.0 S S 18335( 2) - 04-Aug-2026 17:29:40 - [ 12] Summary Form for Proposal 18335 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS, UVIS2-C512C-SUB ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=10 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BETWEEN 01-NOV-2026:00:00:00 AND 01-NOV-2026:10:00:00, BETWEEN 01-NOV-2026:20:00:00 AND 02-NOV-2026:14:00:00, BETWEEN 03-NOV-2026:20:30:00 AND 04-NOV-2026:20:30:00, BETWEEN 08-NOV-2026:17:30:00 AND 09-NOV-2026:17:30:00, BETWEEN 10-NOV-2026:03:30:00 AND 10-NOV-2026:21:30:00, BETWEEN 12-NOV-2026:04:00:00 AND 13-NOV-2026:04:00:00, BETWEEN 17-NOV-2026:01:00:00 AND 18-NOV-2026:01:00:00, BETWEEN 18-NOV-2026:11:00:00 AND 19-NOV-2026:05:00:00, BETWEEN 20-NOV-2026:11:00:00 AND 21-NOV-2026:11:00:00, ON HOLD , PATTERN 1 1, PATTERN 1 2, PATTERN 2 3, PATTERN 2 4 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F350LP ------------------------------------------------------------------------------------------------------------------------------------ Target Names UNI ------------------------------------------------------------------------------------------------------------------------------------