Target 2013FY27: There is no need or capability to test the scheduling of observations of moving targets in Phase I. This warning will go away the next time the proposal is loaded. 0 Assigned ID: 6377 ----- Attempting Submission 1 (Thu Apr 10 06:44:33 GMT 2025) ----- HST Phase I Proposal 6377 successfully submitted. Receipt: # 6377-1 ----- Attempting Submission 2 (Thu Apr 10 21:30:39 GMT 2025) ----- Moving target. First version of Phase II for 18133. Edit to Phase II for 18133. Changes made to scheduling windows for visits 1-3 to enable better scheduling. 12/14/2025: Edit to Phase II for 18133. Visits 4 and 5 taken off hold and given scheduling windows. 12/15/2025: Edit to Phase II for 18133. Visits 4 and 5 given more scheduling windows. 12/15/2025: Edit to Phase II for 18133. Visit 5 shortened to enable better scheduling. 689251183 ----- Attempting Submission 1 (Wed Aug 13 13:56:59 GMT 2025) ----- HST Phase II Proposal 18133 successfully submitted. Receipt: # 18133-1 ----- Attempting Submission 2 (Wed Aug 13 17:02:34 GMT 2025) ----- ----- Attempting Submission 3 (Sun Dec 14 14:36:04 GMT 2025) ----- ----- Attempting Submission 4 (Mon Dec 15 15:31:19 GMT 2025) ----- HST Phase II Proposal 18133 successfully submitted. Receipt: # 18133-4 ----- Attempting Submission 5 (Mon Dec 15 17:10:02 GMT 2025) ----- Probing the TNO Density Transition with Mid-Size 2013 FY27 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. 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 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. Dr. Darin Ragozzine is an expert in TNO satellite orbit fitting, with extensive experience in with dwarf planets moons. He has been PI of multiple HST programs focusing on understanding the formation, evolution, and physical characteristics of TNOs. He was the PI/Co-I of 5+ programs to study the orbits of other dwarf planet moons. He was the PI of the Program 15460 that observed FY27. His group has spent extensive time attempting to measure the density from which the current proposal for necessary new data is derived. Dr. Benjamin Proudfoot is a postdoctoral scholar at the University of Central Florida. He is an expert in orbit fitting of TNO moons and the lead developer of the MultiMoon code, which will be used to do all orbit fitting in this investigation. He has led several past studies focusing on understanding the orbits of TNO moons. Dr. Will Grundy is a tenured astronomer at Lowell Observatory. He has HST experience dating back to his time as a Hubble fellow in the late 1990s. He has observational experience as PI or Co-I in numerous HST programs using NICMOS, WFPC2, ACS, and WFC3 to study icy outer solar system targets, leading to publication of approximately three dozen papers in the peer-reviewed literature over the past 2 decades. He has also served on many Hubble review panels and on the Space Telescope User's Committee. Dr. Scott Sheppard is an expert in 2013 FY27, which he helped discover. He led the original discovery of its moon and supported the measurement of its diameter. He was Co-I on Program 15460. The proposal team are experts in the Kuiper belt in general and able to place the results of this proposal (FY27's density) in context of existing results. 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