0 Assigned ID: 7413 ----- Attempting Submission 1 (Thu Apr 16 15:00:25 GMT 2026) ----- Initial submission of 18335 Phase II. Willing to change anything as needed. -1302223788 ----- Attempting Submission 1 (Tue Aug 04 14:43:43 GMT 2026) ----- The size, density, shape, and interior of mid-sized TNO Uni 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. 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: 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. Dr. Benjamin Proudfoot is a posdoctoral scholar at the University of Central Florida. His research centers on understanding all aspects of TNO binaries. He has published several papers and PI'd several HST programs concerning TNO binaries and moons. Dr. Darin Ragozzine is an expert in TNO binaries, especially in understanding the formation, evolution, and orbits. He has been PI of multiple HST programs focusing on understanding TNOs. Dr. Will Grundy is an researcher at Northern Arizona Univeristy. 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. Flavia L. Rommel is a postdoctoral scholar at Florida Space Institute (University of Central Florida) with expertise in stellar occultations by small bodies. She led the research on the discovery and characterization of a huge topographic feature on the surface of 2002 MS4 and played a pivotal role in the discoveries of rings around Haumea and Quaoar. Dr. Rommel is also a key contributor to the development of the Stellar Occultation Reduction and Analysis (SORA) Python library, which streamlines the analysis of stellar occultation data. Dr. Jose Luis Ortiz is an expert in understanding TNOs. He is one of the leaders of the Lucky Star collaboration, which has radically reshaped our understanding of TNOs using stellar occultations. Dr. Ortiz will be involved in planning the occultation campaigns to understand the size and shape of Uni and Tinia. Solar System Astronomy 6 false false false false false false false SUPPORTED SOLAR SYSTEM ASTEROID uni 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 EARTH WFC3/UVIS Imaging No No F350LP WFC3/UVIS Imaging No No F350LP WFC3/UVIS Imaging No No F350LP WFC3/UVIS Imaging No No F350LP WFC3/UVIS Imaging No No F350LP POS-TARG DITHER 0.173 LINE 0.112 false false 81.785 4 23.884 WFC3-UVIS-DITHER-BOX 1 false POS-TARG DITHER 0.145 LINE false false 2 46.84 WFC3-UVIS-DITHER-LINE 2 false /Users/benjaminproudfoot/Documents/research/proposals/hst34/18335/OP-cache/phaseii-v01.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. 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. (Tic-Visit :proposal-id 18335 :visit-id "01" :proposal-version -1 :trans-version "20260618" :date "04-Aug-2026 09:17:16.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1443744 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18335 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18335 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2551 :al-overhead-at-end 0 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 1 0 /Users/benjaminproudfoot/Documents/research/proposals/hst34/18335/OP-cache/phaseii-v02.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. 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. (Tic-Visit :proposal-id 18335 :visit-id "02" :proposal-version -1 :trans-version "20260618" :date "04-Aug-2026 09:41:22.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1443744 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18335 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18335 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2536 :al-overhead-at-end 0 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 1 0 /Users/benjaminproudfoot/Documents/research/proposals/hst34/18335/OP-cache/phaseii-v03.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. 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. (Tic-Visit :proposal-id 18335 :visit-id "03" :proposal-version -1 :trans-version "20260618" :date "04-Aug-2026 09:41:22.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1443744 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18335 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18335 :visit-id "03" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2536 :al-overhead-at-end 0 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 1 0 /Users/benjaminproudfoot/Documents/research/proposals/hst34/18335/OP-cache/phaseii-v04.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. 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). (Tic-Visit :proposal-id 18335 :visit-id "04" :proposal-version -1 :trans-version "20260618" :date "04-Aug-2026 09:49:59.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 623264 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18335 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18335 :visit-id "04" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2598 :al-overhead-at-end 0 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 1 0 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. /Users/benjaminproudfoot/Documents/research/proposals/hst34/18335/OP-cache/phaseii-v05.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. 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). (Tic-Visit :proposal-id 18335 :visit-id "05" :proposal-version -1 :trans-version "20260618" :date "04-Aug-2026 09:53:05.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 623264 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18335 :visit-id "05" :obset-id "05" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18335 :visit-id "05" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2598 :al-overhead-at-end 0 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 1 0 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.