17830( 9) - 03-Nov-2025 17:00:14 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 17830 Version: 9 Check-in Time: 03-Nov-2025 17:00:14 STScI Edit Number: 1 Title The Next Interstellar Interloper ------------------------------------------------------------------------------------ Type Cycle GO 32 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. David Jewitt University of California - Los Angeles Y CoI: Dr. Jessica Agarwal Technische Universitat Braunschweig N CoI: Dr. Jing Li University of California - Los Angeles N CoI: Dr. Yoonyoung Kim University of California - Los Angeles N CoI: Max Mutchler Space Telescope Science Institute Y ------------------------------------------------------------------------------------ Abstract The recent detection of two interstellar interlopers (1I/Oumuamua and 2I/Borisov) in the solar system has generated enormous scientific and popular interest. A particular surprise is the extreme physical difference between the two objects, with 1I appearing asteroid-like and 2I being much more similar to a normal outgassing, solar system comet. These differences remain unexplained, although speculation abounds. In this proposal we request a 4 orbit disruptive ToO with WFC3 to carry out the earliest possible high resolution assessment of the next interloper object, "3I", so that more intensive follow-up observations can be rationally planned. Nucleus isolation, coma morphology and activity assessment, and initial search for nucleus rotation, are the objectives. We request a 0 month proprietary period so the observations can be of maximum benefit to the community. ------------------------------------------------------------------------------------ Observations Description ------------------------ We request four orbits with WFC3 to image the next ISO at the highest sensitivity using the broad F350LP filter. These four orbits will be dithered and combined to optimize the rejection of field object trails, especially from hard-to-remove low surface brightness galaxies. The proposed observations will be triggered upon the identification of the next ISO (e > 1.1) object by the Minor Planet Center and/or NASA's Horizons ephemeris service. We will secure six 270 s integrations in each orbit, with dithering to provide protection from chip defects. According to the WFC3 ETC, a single exposure of this length reaches V = 26.5 at SNR = 5. Combining 6 images from one orbit will take us 1 magnitude fainter and will also provide protection from cosmic ray strikes on individual images. Combining data from 4 orbits will take us another 0.75 magnitude deeper, to a point-source sensitivity V ~ 28.2. Ephemeris uncertainty is unlikely to present a problem given the 160 arcsec field of WFC3, and even if we opt to use a subarray. The evolution of the ephemeris uncertainty on the next ISO can be judged by reference to the examples set by 1I and 2I. For both 1I and 2I, several weeks of ground-based astrometry were needed for the e >> 1 diagnosis to become irrefutable. With such an arc, the orbits were sufficiently accurately known to predict the position to about 2 arcsecond for about a month. Moreover, high-interest targets such as ISOs attract a continuous stream of astrometric measurements, resulting in steady improvement of the ephemeris 17830( 9) - 03-Nov-2025 17:00:14 - [ 2] accuracy as time goes on. Ephemeris uncertainties will be of no concern for observations using WFC3. Sub-arcsecond astrometry from HST will enable much more accurate orbit fitting and the measurement of non-gravitational accelerations, should they exist. Image saturation is unlikely to be a problem. The wide-field surveys detect most new objects close to their limiting magnitudes, all fainter than V ~ 20 to 21 (Borisov was brighter at discovery, but only because of its extended coma; the central source brightness was faint and HST saturation was not a problem). We request 4 HST orbits triggered upon the discovery of the next interstellar object. This is a disruptive ToO request, with observations desired within a week of the trigger. The orbits should be as nearly consecutive as possible in order to assess possible rotational variation of the object. We understand, however, that the need for a gyro-reset limits the number of consecutive orbits allocated to a solar system target to be at most two. Therefore, our observations would be acquired as two pairs of orbits separated by an orbit gap, spanning ~8 hours in total. All observations are with WFC3 and the F350LP filter to provide the highest sensitivity. We request four orbits to be dithered and combined to optimize the rejection of field object trails. Two interlopers were discovered in the last 7 years suggesting that the likelihood of discovery of the next example in Cycle 32 is 30%. This is consistent with the published estimate by Do et al. (2018). We understand that we will have essentially no control over the spacecraft roll angle, which means that we will not be able to optimize the orientation of the dust coma on the CCD (e.g., to orient the tail along the longest dimension of the detector). However, the field-of-view of the camera is large enough and the target coma small enough that we should obtain excellent data, no matter what spacecraft roll angle is used. Finally, we note that our team can prepare and submit a Phase 2 proposal within a day of activation of this program. ------------------------------------------------------------------------------------ 17830( 9) - 03-Nov-2025 17:00:14 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 11 ISO-AMBUSH-J SOLAR SYSTEM, OTHER RA=17H14M30.8538S, J2000 V = 12 +/- 4 ULY23 DEC=-18D02'13.54" Reference Frame: ICRS Extended: NO ------------------------------------------------------------------------------------------------------------------------------------ 17830( 9) - 03-Nov-2025 17:00:14 - [ 4] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 3 TARGET NAME: 3I-ATLAS ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: COMET interstellar object ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 TYPE=COMET, Q=1.355854073726217, E=6.132907187787578, I=175.1123912604994 , O=322.1448470069185, W=128.0103753852145, T=29-OCT-2025:12:15:37, TTimeScale=TDB, EQUINOX=J2000, EPOCH=03-JUL-2025:00:00:00, EpochTimeScale=TDB ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 12 +/- 4 Ephemeris Center: EARTH Extended: NO ------------------------------------------------------------------------------------------------------------------------------------ 17830( 9) - 03-Nov-2025 17:00:14 - [ 5] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 4 TARGET NAME: 3I-ATLAS-OCT29 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: COMET interstellar object ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 TYPE=COMET, Q=1.356052192196407, E=6.13730558284557, I=175.1128819539053 , O=322.1526311088164, W=128.0121873542861, T=29-OCT-2025:11:36:07, TTimeScale=TDB, EQUINOX=J2000, EPOCH=26-JUL-2025:00:00:00, EpochTimeScale=TDB, R0=2.808 , DT=0. , A1=1.662339477539E-6, A2=7.089414978027E-7, A3=0. , ALN=0.1112620426, NM=2.15, NN=5.093, NK=4.6142, AMRAT=0. ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 12 +/- 5 Ephemeris Center: EARTH Extended: YES ------------------------------------------------------------------------------------------------------------------------------------ 17830( 9) - 03-Nov-2025 17:00:14 - [ 6] Visit: 01 (Orbit 1) Visit Requirements: TOO RESPONSE TIME 10.0D On Hold Comments: Additional Comments: 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 3I-ATLAS WFC3/UVI ACCUM UVIS2-FIX F350LP 2 40 S (80 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 | ------------------------------------------------------------------------------------------------------------------------------------ 3I-ATLAS 1 WFC3/UVI ACCUM UVIS2-FI F350LP none none 1 none 1 N/A S X 3I-ATLAS 1 WFC3/UVI ACCUM UVIS2-FI F350LP none none 2 none 1 N/A S X 17830( 9) - 03-Nov-2025 17:00:14 - [ 7] Visit: 02 (Orbit 2) Visit Requirements: TOO RESPONSE TIME 10.0D On Hold Comments: Additional Comments: 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 3I-ATLAS WFC3/UVI ACCUM UVIS2-FIX F350LP 2 25 S (50 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 | ------------------------------------------------------------------------------------------------------------------------------------ 3I-ATLAS 1 WFC3/UVI ACCUM UVIS2-FI F350LP none none 1 none 1 N/A S X 3I-ATLAS 1 WFC3/UVI ACCUM UVIS2-FI F350LP none none 2 none 1 N/A S X 17830( 9) - 03-Nov-2025 17:00:14 - [ 8] Visit: 03 (Orbit 3) Visit Requirements: BEFORE 08-DEC-2025:00:00:00; TOO RESPONSE TIME 20.0D On Hold Comments: Additional Comments: 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 3I-ATLAS-OC WFC3/UVI ACCUM UVIS2-2K2 F350LP 2 270 S (540 T29 S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 2 3I-ATLAS-OC WFC3/UVI ACCUM UVIS2-2K2 F350LP 2 270 S (540 POS TARG 0.2, 0.2 T29 S C-SUB 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 | ------------------------------------------------------------------------------------------------------------------------------------ 3I-ATLAS-OCT29 1 WFC3/UVI ACCUM UVIS2-2K F350LP none none 1 none 1 N/A S 2C-SUB 3I-ATLAS-OCT29 1 WFC3/UVI ACCUM UVIS2-2K F350LP none none 2 none 1 N/A S 2C-SUB 3I-ATLAS-OCT29 2 WFC3/UVI ACCUM UVIS2-2K F350LP none none 1 none 1 N/A S 2C-SUB 3I-ATLAS-OCT29 2 WFC3/UVI ACCUM UVIS2-2K F350LP none none 2 none 1 N/A S 2C-SUB 17830( 9) - 03-Nov-2025 17:00:14 - [ 9] Visit: 04 (Orbit 4) Visit Requirements: AFTER 03 BY 2 D TO 8 D; TOO RESPONSE TIME 20.0D On Hold Comments: Additional Comments: 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 3I-ATLAS-OC WFC3/UVI ACCUM UVIS2-2K2 F350LP 2 270 S (540 T29 S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 2 3I-ATLAS-OC WFC3/UVI ACCUM UVIS2-2K2 F350LP 2 270 S (540 POS TARG 0.2, 0.2 T29 S C-SUB 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 | ------------------------------------------------------------------------------------------------------------------------------------ 3I-ATLAS-OCT29 1 WFC3/UVI ACCUM UVIS2-2K F350LP none none 1 none 1 N/A S 2C-SUB 3I-ATLAS-OCT29 1 WFC3/UVI ACCUM UVIS2-2K F350LP none none 2 none 1 N/A S 2C-SUB 3I-ATLAS-OCT29 2 WFC3/UVI ACCUM UVIS2-2K F350LP none none 1 none 1 N/A S 2C-SUB 3I-ATLAS-OCT29 2 WFC3/UVI ACCUM UVIS2-2K F350LP none none 2 none 1 N/A S 2C-SUB 17830( 9) - 03-Nov-2025 17:00:14 - [ 10] Summary Form for Proposal 17830 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2-2K2C-SUB, UVIS2-FIX ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements AFTER 03 BY 2 D TO 8 D, BEFORE 08-DEC-2025:00:00:00, POS TARG 0.2, 0.2, TOO RESPONSE TIME 10.0D, TOO RESPONSE TIME 20.0D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F350LP ------------------------------------------------------------------------------------------------------------------------------------ Target Names 3I-ATLAS, 3I-ATLAS-OCT29 ------------------------------------------------------------------------------------------------------------------------------------