0 Assigned ID: 6905 ----- Attempting Submission 1 (Sun Jul 13 09:56:05 GMT 2025) ----- FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser updated target ephem 1836338811 ----- Attempting Submission 1 (Thu Jul 24 16:32:10 GMT 2025) ----- ----- Attempting Submission 2 (Fri Sep 12 19:25:34 GMT 2025) ----- ----- Attempting Submission 3 (Thu Oct 16 20:11:34 GMT 2025) ----- ----- Attempting Submission 4 (Tue Nov 25 18:19:53 GMT 2025) ----- ----- Attempting Submission 5 (Tue Dec 02 14:48:50 GMT 2025) ----- HST Phase II Proposal 18152 successfully submitted. Receipt: # 18152-5 ----- Attempting Submission 6 (Tue Dec 02 15:41:27 GMT 2025) ----- ----- Attempting Submission 7 (Mon Jan 26 15:49:23 GMT 2026) ----- The Visit Planner now says all remaining visits have scheduling problems (even the unchanged ones) which is probably because of the new Visit 22 (or a server is down). We expect to iterate with our Program Coordinator to schedule and optimize all 10 epochs. FLASH may be too low for Visits 7-10 but time is very limited there. Monitoring Interstellar Object 3I/ATLAS Departing the Solar System While numerical models predict a large population of interstellar objects (ISOs) in the interstellar neighborhood of the solar system, few have been found. Discovered in early July 2025, 3I/ATLAS is only the third ISO identified in our solar system, following 1I/`Oumuamua and 2I/Borisov. These ISOs exhibited distinct properties that remain poorly understood, possibly stemming from diverse origins in their parent circumplanetary systems or varying evolutionary stages. To better understand the ISO population, we propose ten HST DD orbits using the WFC3 camera with the F350LP filter to study 3I on its outbound heliocentric trajectory. Our primary scientific objectives are: (1) to obtain high-precision astrometry to evaluate nongravitational effects, (2) to monitor and resolve near-nucleus morphology, (3) to extract the nucleus signal to constrain its size, and (4) to search for potential fragments. No ground-based facility can achieve our goals because of HST's superior angular resolution, sensitivity, and imaging stability. We request ten DD orbits using HST/WFC3 with the F350LP filter to image 3I postperihelion. The broad F350LP filter provides maximum sensitivity to faint coma, enabling detailed study of 3I's near-nucleus dust morphology and better constraints on its nucleus. Individual exposures from the same orbit will be dithered and combined to facilitate the removal of background sources. 3I is projected close to the midplane of the Milky Way and the field star density is correspondingly high. Accurate photometry and astrometry will be obtained only by using multiple images to eliminate contamination by field objects. Similar techniques have been successfully applied to HST observations of 1I and 2I. We propose scheduling the ten orbits starting from late November 2025, when 3I emerges from the solar exclusion zone, with each visits spaced at heliocentric distance intervals of 0.5 au. The final orbit should be completed before the target reenters the solar exclusion zone in early May 2026. The time coverage and high-precision astrometry are both crucial for determining the trajectory of 3I and, in particular in determining the nongravitational acceleration and its heliocentric distance dependency. According to the WFC3 ETC, a single 260 s exposure achieves a signal-to-noise ratio (SNR) of 5 at V-band magnitude 26.5. In each orbit, five such exposures can be obtained. Combining these exposures will further increase the SNR by a factor of 2.2, corresponding to a gain of nearly 1 magnitude. Using the viewing geometry on 2025 December 1 as an example, a 1 km diameter bare nucleus with geometric albedo 0.04 is expected to have apparent magnitude V = 23, well within the capacity of HST. The combined HST image from the same orbit can be used to search for fragments down to 700 m in radius. JPL Horizons predicts the 1-sigma ephemeris uncertainty to be 10 at the time of the HST observations, small enough compared to the WFC3's field of view. As the target remains observable preperihelion, the ephemeris uncertainty will further decrease. Astrometry from the proposed observations will also be included to update the orbit. The accuracy of the HST astrometric measurements is expected to be at the 0.1" level. Therefore, there is no concern to the HST pointing for the proposed program. We understand that we cannot control the spacecraft roll angle, meaning that we will not be able to optimize the orientation such that the tail is directed along the diagonal of the detector. However, WFC3's FOV is far larger than the apparent size of the target that we should obtain exquisite data regardless of the HST roll angle. Man-To Hui is experienced in observational and morphological studies of comets and has led and participated in multiple HST programs on small bodies. Jessica Agarwal specializes in dust dynamics for comets. David Jewitt has abundant experience in performing data analyses with HST data on various small bodies. Yoonyoung Kim is skilled at modelling cometary dust morphology through Monte Carlo processes. Max Mutchler is an expert in HST cameras and observing planning, particularly for high-quality imaging of moving targets. We will collaborate on the proposed HST program and aim to promptly publish our findings upon obtaining results from the HST observations. Solar System Astronomy 0 false false SUPPORTED SOLAR SYSTEM YES COMET interstellar object TYPE=COMET,Q=1.356065570996796,E=6.137350157289094,I=175.1128577937168 ,O=322.1522849649193,W=128.0116082517727,T=29-OCT-2025:11:36:17,TTimeScale=TDB,EQUINOX=J2000,EPOCH=28-JUL-2025:00:00:00,EpochTimeScale=TDB EARTH SOLAR SYSTEM YES COMET interstellar object TYPE=COMET,Q=1.35640569052028,E=6.139362564105249,I=175.1130622033269 ,O=322.1561849782609,W=128.0095067807252,T=29-OCT-2025:11:34:25,TTimeScale=TDB,EQUINOX=J2000,EPOCH=30-JUL-2025:00:00:00,EpochTimeScale=TDB,R0=2.808 ,DT=0. ,A1=4.264217376709E-7,A2=2.000436019897E-7,A3=0. ,ALN=0.1112620426,NM=2.15,NN=5.093,NK=4.6142,AMRAT=0. EARTH SOLAR SYSTEM YES COMET interstellar object TYPE=COMET,Q=1.356404858629546,E=6.139346138450772,I=175.113090485184 ,O=322.1566074986231,W=128.0099236586739,T=29-OCT-2025:11:34:27,TTimeScale=TDB,EQUINOX=J2000,EPOCH=04-AUG-2025:00:00:00,EpochTimeScale=TDB,R0=1. ,DT=0. ,A1=7.199572086334E-8,A2=4.569792747498E-8,A3=-3.925215303898E-9 ,ALN=1.,NM=2.,NN=0.,NK=0.,AMRAT=0. EARTH SOLAR SYSTEM YES COMET interstellar object TYPE=COMET,Q=1.356418761995381,E=6.139422831829797,I=175.1130917268881 ,O=322.1566239181344,W=128.0096924001076,T=29-OCT-2025:11:34:23,TTimeScale=TDB,EQUINOX=J2000,EPOCH=07-AUG-2025:00:00:00,EpochTimeScale=TDB,R0=1. ,DT=0. ,A1=4.572104930878E-8,A2=1.862444758415E-8,A3=-5.019464492798E-9 ,ALN=1.,NM=2.,NN=0.,NK=0.,AMRAT=0. EARTH SOLAR SYSTEM YES COMET interstellar object TYPE=COMET,Q=1.356421182919772,E=6.139423907689657,I=175.1130967647645 ,O=322.1567016652489,W=128.0097235988275,T=29-OCT-2025:11:34:27,TTimeScale=TDB,EQUINOX=J2000,EPOCH=11-AUG-2025:00:00:00,EpochTimeScale=TDB,R0=1. ,DT=0. ,A1=4.474215507507E-8,A2=1.700624108315E-8,A3=-5.324076414108E-9 ,ALN=1.,NM=2.,NN=0.,NK=0.,AMRAT=0. EARTH SOLAR SYSTEM YES COMET interstellar object TYPE=COMET,Q=1.356424890557981,E=6.139409161771214,I=175.1131128947683 ,O=322.1569360020026,W=128.0098740554459,T=29-OCT-2025:11:34:38,TTimeScale=TDB,EQUINOX=J2000,EPOCH=20-AUG-2025:00:00:00,EpochTimeScale=TDB,R0=1. ,DT=0. ,A1=4.533728599548E-8,A2=1.745203018188E-8,A3=-5.953273177147E-9 ,ALN=1.,NM=2.,NN=0.,NK=0.,AMRAT=0. EARTH SOLAR SYSTEM YES COMET interstellar object TYPE=COMET,Q=1.356481057231181,E=6.141351449317625,I=175.1164570850441 ,O=322.1696089290778,W=128.0228697185194,T=29-OCT-2025:11:53:10,TTimeScale=TDB,EQUINOX=J2000,EPOCH=19-FEB-2026:00:00:00,EpochTimeScale=TDB,R0=1. ,DT=9.478815 ,A1=5.320206165314E-8,A2=1.148166060448E-8,A3=-6.854491829872E-9 ,ALN=1.,NM=2.,NN=0.,NK=0.,AMRAT=0. EARTH WFC3/UVIS Imaging No No F350LP Using ephemeris for target 18152-01-3I-ATLAS-OCT10 ephemeris data for target 18152-01-3I-ATLAS-OCT10 does not cover planning interval. Physical constraints are assumed to be unschedulable outside of the ephemeris data. Planning interval: 25.303 to 26.305. ephemeris interval: 25.334 to 27.335 The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. 1764534449000 295.15272331242727 (Tic-Visit :proposal-id 18152 :visit-id "01" :proposal-version -1 :trans-version "20250929" :date "02-Dec-2025 09:31:46.0" :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 422320 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18152 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18152 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2698 :al-overhead-at-end 0 :internal-dur-at-end 498 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3078 :pcs-gap 0 ) false 1 0 Using ephemeris for target 18152-02-3I-ATLAS-NOV21 ephemeris data for target 18152-02-3I-ATLAS-NOV21 does not cover planning interval. Physical constraints are assumed to be unschedulable outside of the ephemeris data. Planning interval: 25.303 to 26.305. ephemeris interval: 25.334 to 27.335 The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. 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