18426( 2) - 18-Aug-2026 18:46:43 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18426 Version: 2 Check-in Time: 18-Aug-2026 18:46:43 STScI Edit Number: 0 Title Enabling New Horizons Observations of Distant Scattered Disk Objects ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Simon Bernard Porter Southwest Research Institute Y CoI: Dr. Wesley C Fraser Dominion Astrophysical Observatory N CoI: Dr. Anne J. Verbiscer The University of Virginia N CoI: Dr. Susan D. Benecchi Planetary Science Institute N CoI: Dr. J. J. Kavelaars National Research Council of Canada N CoI: Dr. Alan Stern Southwest Research Institute N ------------------------------------------------------------------------------------ Abstract This program would observe up to three Target of Opportunity Scattered Disk Objects (SDOs) discovered in 2025 observations by the Vera C. Rubin Observatory (Rubin) in order to enable them to be observed up close by the New Horizons spacecraft. The Scattered Disk occupies a vast area of solar system beyond 55 AU, and the New Horizons spacecraft is the only facility now or in the foreseeable future that observe SDOs at distances less than 1 AU and phase angles up to 120 degrees (SDOs can only be seen at <2 degrees phase from Earth). These high-phase observations of SDOs would enable studies of their shapes and surfaces, critical for understanding both their origin in the Giant Planet zone and how SDOs presently evolve to become comets. The 2025 Rubin observations are presently being analyzed for any objects that may be observable by New Horizons. The ground based astrometry from Rubin and follow up from Subaru is sufficient to tell if an SDO is possibly observable from New Horizons, but not sufficient to actually enable those observations. The proposed HST observations would provide sufficient quality astrometry to enable the New Horizons observations of these SDOs without further follow up. Adding to the urgency, New Horizons is moving rapidly out of the solar system, and every year these SDO observations become harder to perform. ------------------------------------------------------------------------------------ Observations Description ------------------------ The purpose of this proposal is to observe up to three Kuiper Belt Objects (KBOs) to obtain astrometry to improve their orbit enought that can be observed by New Horizons in 2027-2030. Those targets are not currently known, but will be deifned by the end of the calendar year 2026. Nominally, two orbits are allocated per target, for a max of three targets. Any targets would be roughly in the same area of the sky (within a degree or so) as the one already confirmed 2027 New Horizons target, 2020 KV11, and so we used the orbit of that KBO as nominal placeholder for planning purposes. We would update the Phase II with the correct orbits (by entering the orbital elements directly) before triggering the TOO. The nominal plan has the objects separated by 30 days, though there could be some flexibility in that timing (depending on the sky motion of the KBO) if required for schedulability. All the visits are 18426( 2) - 18-Aug-2026 18:46:43 - [ 2] the same, using a 4-point box dither with the full frame WFC3/UVIS and the F350LP broadband filter. We need a few Gaia DR3 stars in the field for absolute astrometry, but the expected star density is high enough that this is unlikely be a problem. ------------------------------------------------------------------------------------ 18426( 2) - 18-Aug-2026 18:46:43 - [ 3] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 TARGET NAME: NHKBO1 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: ASTEROID New Horizons KBO 1 ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 TYPE=ASTEROID, A=100.783874222854, E=0.099773153155, I=3.314572022986, O=166.914905426798, W=331.176766747966, M=141.121051314833, EQUINOX=J2000, EPOCH=29-MAY-2020:11:22:33, EpochTimeScale=UTC ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 26.0 Ephemeris Center: EARTH ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 2 TARGET NAME: NHKBO2 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: ASTEROID New Horizons KBO 2 ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 TYPE=ASTEROID, A=100.783874222854, E=0.099773153155, I=3.314572022986, O=166.914905426798, W=331.176766747966, M=141.121051314833, EQUINOX=J2000, EPOCH=29-MAY-2020:11:22:33, EpochTimeScale=UTC ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 26.0 Ephemeris Center: EARTH ------------------------------------------------------------------------------------------------------------------------------------ 18426( 2) - 18-Aug-2026 18:46:43 - [ 4] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 3 TARGET NAME: NHKBO3 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: ASTEROID New Horizons KBO 3 ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 TYPE=ASTEROID, A=100.783874222854, E=0.099773153155, I=3.314572022986, O=166.914905426798, W=331.176766747966, M=141.121051314833, EQUINOX=J2000, EPOCH=29-MAY-2020:11:22:33, EpochTimeScale=UTC ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 26.0 Ephemeris Center: EARTH ------------------------------------------------------------------------------------------------------------------------------------ 18426( 2) - 18-Aug-2026 18:46:43 - [ 5] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type BOX Pattern_Purpose OTHER 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 ------------------------------------------------------------------------------------------------------------------------------------ 18426( 2) - 18-Aug-2026 18:46:43 - [ 6] Visit: 01 (NHKBO1 Visit 1) Visit Requirements: TOO MIN RESPONSE TIME 30.0D; TOO MAX RESPONSE TIME 180.0D 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 NHKBO1 WFC3/UVI ACCUM UVIS2 F350LP 1 400 S SEQ 1 NON-INT; PATTERN 1 S see below 1 (1940 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 | ------------------------------------------------------------------------------------------------------------------------------------ NHKBO1 1 WFC3/UVI ACCUM UVIS2 F350LP 1 none none none 1 485.0 S S NHKBO1 1 WFC3/UVI ACCUM UVIS2 F350LP 2 none none none 1 485.0 S S NHKBO1 1 WFC3/UVI ACCUM UVIS2 F350LP 3 none none none 1 485.0 S S NHKBO1 1 WFC3/UVI ACCUM UVIS2 F350LP 4 none none none 1 485.0 S S 18426( 2) - 18-Aug-2026 18:46:43 - [ 7] Visit: 02 (NHKBO1 Visit 2) Visit Requirements: AFTER 01 BY 30 D TO 1000 D; TOO MIN RESPONSE TIME 30.0D; TOO MAX RESPONSE TIME 180.0D 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 NHKBO1 WFC3/UVI ACCUM UVIS2 F350LP 1 400 S SEQ 1 NON-INT; PATTERN 1 S see below 1 (1940 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 | ------------------------------------------------------------------------------------------------------------------------------------ NHKBO1 1 WFC3/UVI ACCUM UVIS2 F350LP 1 none none none 1 485.0 S S NHKBO1 1 WFC3/UVI ACCUM UVIS2 F350LP 2 none none none 1 485.0 S S NHKBO1 1 WFC3/UVI ACCUM UVIS2 F350LP 3 none none none 1 485.0 S S NHKBO1 1 WFC3/UVI ACCUM UVIS2 F350LP 4 none none none 1 485.0 S S 18426( 2) - 18-Aug-2026 18:46:43 - [ 8] Visit: 03 (NHKBO2 Visit 1) Visit Requirements: TOO MIN RESPONSE TIME 30.0D; TOO MAX RESPONSE TIME 180.0D 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 NHKBO2 WFC3/UVI ACCUM UVIS2 F350LP 1 400 S SEQ 1 NON-INT; PATTERN 1 S see below 1 (1940 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 | ------------------------------------------------------------------------------------------------------------------------------------ NHKBO2 1 WFC3/UVI ACCUM UVIS2 F350LP 1 none none none 1 485.0 S S NHKBO2 1 WFC3/UVI ACCUM UVIS2 F350LP 2 none none none 1 485.0 S S NHKBO2 1 WFC3/UVI ACCUM UVIS2 F350LP 3 none none none 1 485.0 S S NHKBO2 1 WFC3/UVI ACCUM UVIS2 F350LP 4 none none none 1 485.0 S S 18426( 2) - 18-Aug-2026 18:46:43 - [ 9] Visit: 04 (NHKBO2 Visit 2) Visit Requirements: AFTER 03 BY 30 D TO 1000 D; TOO MIN RESPONSE TIME 30.0D; TOO MAX RESPONSE TIME 180.0D 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 NHKBO2 WFC3/UVI ACCUM UVIS2 F350LP 1 400 S SEQ 1 NON-INT; PATTERN 1 S see below 1 (1940 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 | ------------------------------------------------------------------------------------------------------------------------------------ NHKBO2 1 WFC3/UVI ACCUM UVIS2 F350LP 1 none none none 1 485.0 S S NHKBO2 1 WFC3/UVI ACCUM UVIS2 F350LP 2 none none none 1 485.0 S S NHKBO2 1 WFC3/UVI ACCUM UVIS2 F350LP 3 none none none 1 485.0 S S NHKBO2 1 WFC3/UVI ACCUM UVIS2 F350LP 4 none none none 1 485.0 S S 18426( 2) - 18-Aug-2026 18:46:43 - [ 10] Visit: 05 (NHKBO3 Visit 1) Visit Requirements: TOO MIN RESPONSE TIME 30.0D; TOO MAX RESPONSE TIME 180.0D 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 NHKBO3 WFC3/UVI ACCUM UVIS2 F350LP 1 400 S SEQ 1 NON-INT; PATTERN 1 S see below 1 (1940 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 | ------------------------------------------------------------------------------------------------------------------------------------ NHKBO3 1 WFC3/UVI ACCUM UVIS2 F350LP 1 none none none 1 485.0 S S NHKBO3 1 WFC3/UVI ACCUM UVIS2 F350LP 2 none none none 1 485.0 S S NHKBO3 1 WFC3/UVI ACCUM UVIS2 F350LP 3 none none none 1 485.0 S S NHKBO3 1 WFC3/UVI ACCUM UVIS2 F350LP 4 none none none 1 485.0 S S 18426( 2) - 18-Aug-2026 18:46:43 - [ 11] Visit: 06 (NHKBO3 Visit 2) Visit Requirements: AFTER 05 BY 30 D TO 1000 D; TOO MIN RESPONSE TIME 30.0D; TOO MAX RESPONSE TIME 180.0D 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 NHKBO3 WFC3/UVI ACCUM UVIS2 F350LP 1 400 S SEQ 1 NON-INT; PATTERN 1 S see below 1 (1940 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 | ------------------------------------------------------------------------------------------------------------------------------------ NHKBO3 1 WFC3/UVI ACCUM UVIS2 F350LP 1 none none none 1 485.0 S S NHKBO3 1 WFC3/UVI ACCUM UVIS2 F350LP 2 none none none 1 485.0 S S NHKBO3 1 WFC3/UVI ACCUM UVIS2 F350LP 3 none none none 1 485.0 S S NHKBO3 1 WFC3/UVI ACCUM UVIS2 F350LP 4 none none none 1 485.0 S S 18426( 2) - 18-Aug-2026 18:46:43 - [ 12] Summary Form for Proposal 18426 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2 ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements AFTER 01 BY 30 D TO 1000 D, AFTER 03 BY 30 D TO 1000 D, AFTER 05 BY 30 D TO 1000 D, PATTERN 1 1, SEQ 1 NON-INT, TOO MAX RESPONSE TIME 180.0D, TOO MIN RESPONSE TIME 30.0D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F350LP ------------------------------------------------------------------------------------------------------------------------------------ Target Names NHKBO1, NHKBO2, NHKBO3 ------------------------------------------------------------------------------------------------------------------------------------