0 Assigned ID: 3948 ----- Attempting Submission 1 (Wed May 17 21:57:28 GMT 2023) ----- HST Phase I Proposal 3948 successfully submitted. Receipt: # 3948-1 ----- Attempting Submission 2 (Tue May 23 23:48:13 GMT 2023) ----- ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN We resubmit with later windows for visit 1 due to an HST safing event. We can find even later windows if HST recovery requires it, but sooner is better for the orbit plane crossing of our target. An overrun of a few seconds occurs in the first orbit of each visit which, if real, could be addressed by trimming a few seconds off the exposure time. -- Max Mutchler 1367575681 ----- Attempting Submission 1 (Fri Aug 25 20:10:34 GMT 2023) ----- ----- Attempting Submission 2 (Mon Feb 12 19:38:56 GMT 2024) ----- ----- Attempting Submission 3 (Mon Apr 29 13:46:02 GMT 2024) ----- Small overrun in first orbit of each visit can be addressed by PC and schedulers if real, perhaps by trimming last exposure. Extraordinary asteroid 331P/Gibbs consists of a rapidly rotating primary body about 1.6 km in diameter, a debris trail, and an embedded chain of 19 fragments, evidently all produced by a breakup in 2012. It is the best and most accessible example of rotational instability in the asteroid belt. HST observations in the 2015 - 2018 period have established the basic properties, revealing that one fragment has a large amplitude lightcurve consistent with a contact binary, that the fragments combined carry about 1 percent of the mass of the primary, and have an incredibly small (10 cm/s) velocity dispersion. Here, we propose follow-up observations with HST to define the fragment dynamics by extending the observational arc from 3 to 8 years. The new data will allow accurate orbital elements to be computed for each resolved object and, from these, accurate determination of the time and speed of release from the primary. The degree to which the release times are staggered, the speeds of release, and speed-size correlations, will enable the first comparisons with numerical models of asteroid rotational breakup. The longer astrometric arc will yield sensitive limits to non-gravitational acceleration, as might be caused by the sudden exposure and sublimation of previously buried asteroid ice. If present, ice could spin-up the primary by sublimation torques, rather than radiation (YORP) torque as presently assumed; this new mechanism would have implications for rotational destruction timescales across the asteroid belt. These high resolution, high sensitivity observations lie far beyond the realm offered by even the largest ground-based telescopes, requiring the use of HST. We request 6 orbits divided into two separate visits in order to determine the positions and morphology of the fragment train in 331P. These 6 orbits should be divided into visits of 4 and 2 orbits, one coinciding with the Earth crossing the projected orbit plane of 331P and the other about two months later. The later visit benefits from smaller geocentric distance and brighter apparent magnitudes, hence requires only 2 orbits to reach comparable sensitivity. We will combine the images from within each visit to reveal the (very faint) 19 fragments. The need for 2 visits is to remove ambiguity in the fragment identifications resulting from ephemeris uncertainties caused by the extrapolation from the previous HST observations, as described below. We request observations at two epochs in Cycle 31. As discussed below, we have selected these epochs in order to maximize differences in the perspective as seen from HST, in order to set the strongest constraints on the orbits. One important perspective is obtained as the Earth passes through the projected orbit plane of 331P, when the out-of-plane distribution is uniquely revealed. Our basic observing strategy is to take multiple long exposures (280 s) using WFC3 and the wide bandpass filter F350LP. We will use the F350LP filter because it provides a 1.6x sensitivity advantage relative to other commonly used WFC3 broadband filters for a target with a solar-type spectrum. This matters because most of the identified fragments are exceedingly faint (V > 27) and data from 4 orbits must be combined to detect and measure a majority of them with confidence. We used the on-line exposure tool to calculate the signal-to-noise ratio (SNR) for various target fragments, using the absolute magnitudes from Table 2 of Jewitt et al. 2021) and assuming a G2V Kurucz solar spectrum model. We will reach the SNR indicated in Table 1 given 4 orbits at the April plane-crossing and 2 orbits with the more favorable geometry in July. We will dither the exposures to mitigate the effects from bad pixels, cosmic rays, and the inter-chip gap. The orbital visibility for a solar system target near the ecliptic plane is 54 min (Section 6.3 of the HST Primer document). In each orbit, we will take 8 exposures of 280 s using the C512C subarray of WFC3 (to save readout time) and perform a 2-point dither pattern to mitigate hot pixels and cosmic ray hits, obtaining 4 exposures at each dither point. This provides a total exposure time of 2240s = 37 min. We will average the exposures obtained at each dither point rejecting hot pixels and cosmic ray hits, and determine the component distance and brightness by least-squares-fitting of the two PSFs sub-sampled at an even smaller pixel scale to each of the two stacked images individually. This method can measure distances with uncertainties under 0.04 arcsec. The timing of the two visits is specified in order to maximize the observational leverage on the locations of the fragments in three dimensions. Visit 1 (4 orbits): The Earth passes through the projected orbit plane of 331P on UT 2024 April 27, offering a unique view of the out-of-plane distribution of fragments. Observations within a plus/minus3 day window about this date negate the effects of projection into the orbit plane and provide a model-free measure of the out-of-plane positions of fragments and the perpendicular profile of the debris trail (consisting of particles 10 cm and larger). Visit 2 (2 orbits): The most negative orbit plane angle is attained on UT 2023 July 23, about 80 days after the plane crossing, roughly coinciding with the minimum geocentric distance (when image scale 60 km pixel-1). Off-plane observations on this date therefore provide a complement to the in-plane data from April 27 and benefit from a more favorable geometry requiring only 2 orbits to reach comparable SNR. The apparent rates of motion in late April are under 30 arcsec hr-1, which are easily within Hubbles tracking capabilities. This rate of motion is also slow enough to keep a single pair of guide stars within the FGS pickles for an entire visibility window. The ephemeris uncertainty of primary body 331P is < 1 arcsec, which is negligible compared to the WFC3 field-of-view of 162 x 162 arcsec. The length of the fragment train is, from our analysis of archival data, still < 40 arcsec in Cycle 30, fitting comfortably within the field of view. Ephemeris uncertainties are of no concern to this observation. We understand that the need in Cycle 31 for a gyro-gap is uncertain. A gyro-gap, if such exists, will not undermine the goals of this proposal. We also understand that we may have essentially no control over the spacecraft roll angle, which means we will not be able to optimize the orientation of the dust tail on the CCD (i.e., to orient the tail along the longest dimension of the detector). However, the field-of-view of the camera is large enough that we should obtain excellent data on a portion of the tail, no matter what spacecraft roll angle is used. As described above, we request that the observations for Visit 1 be obtained as close as possible to UT 2024 April 27. Dates within 3 days of 2024 April 27 are sufficient to remove the effects of projection. The requested date of Visit 2, when the geocentric distance is minimized, is UT 2023 July 23 plus/minus 10 days. David Jewitt is a long-time user of HST and ground-based telescopes to explore the nature of the solar system's primitive body population. Yoonyoung Kim is experienced with Monte Carlo modeling of dust and the production of simulated comet images used to infer dust ejection properties. Max Mutchler is an expert in optimization of the use of HSt for solar system science. Jing Li is experienced with handling large datasets and coordinating measurements. Jessica Agarwal is an expert on the near-nucleus dynamics of cometary debris and on subsequent motions under the action of radiation pressure and solar gravity Solar System Astronomy 6 false false false false false false false false SUPPORTED SOLAR SYSTEM YES ASTEROID Disintegrating asteroid TYPE=ASTEROID,A=3.005169327070824,E=0.04162092936179756,I=9.738048614887637 ,O=216.8567409364987,W=177.8652913669414,M=234.3925641937084,EQUINOX=J2000,EPOCH=20-AUG-2013:00:00:00,EpochTimeScale=TDB EARTH WFC3/UVIS Imaging No No F350LP POS-TARG DITHER 0.2 LINE false false 4 45 LINE 1 false POS-TARG DITHER 0.4 LINE false false 2 45 LINE 2 false 1714397978470 Using ephemeris for target 17540-01-331P ephemeris data for target 17540-01-331P does not cover planning interval. Physical constraints are assumed to be unschedulable outside of the ephemeris data. Planning interval: 23.274 to 25.091. ephemeris interval: 24.091 to 26.091 The scheduling data was generated by CASM 32.0. CASM was run in Phase II mode. 1715808402000 252.70580162155392 (Tic-Visit :proposal-id 17540 :visit-id "01" :proposal-version -1 :trans-version "20230626" :date "29-Apr-2024 09:39:38.0" :trans-errors '(:WARNINGS 1 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 4 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 128144 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17540 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17540 :visit-id "01" :orbit-number 1 :pcs-time 443 :si-vis-utilization 2351 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2776 :pcs-gap 0 ) (Tic-Orbit :proposal-id 17540 :visit-id "01" :orbit-number 2 :pcs-time 392 :si-vis-utilization 2334 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2776 :pcs-gap 0 ) (Tic-Orbit :proposal-id 17540 :visit-id "01" :orbit-number 3 :pcs-time 392 :si-vis-utilization 2319 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2776 :pcs-gap 0 ) (Tic-Orbit :proposal-id 17540 :visit-id "01" :orbit-number 4 :pcs-time 392 :si-vis-utilization 2319 :al-overhead-at-end 0 :internal-dur-at-end 177 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2776 :pcs-gap 0 ) true 4 1 1719493676576 Using ephemeris for target 17540-02-331P ephemeris data for target 17540-02-331P does not cover planning interval. Physical constraints are assumed to be unschedulable outside of the ephemeris data. Planning interval: 23.274 to 25.091. ephemeris interval: 24.091 to 26.091 The scheduling data was generated by CASM 32.0. CASM was run in Phase II mode. /private/var/folders/q9/0jb3sqc10kx2dw3wnl21b_dw000702/T/OPVisit8620081905862710734.xml (Tic-Visit :proposal-id 17540 :visit-id "02" :proposal-version -1 :trans-version "20240604" :date "27-Jun-2024 09:07:56.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 64080 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17540 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17540 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2260 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2645 :pcs-gap 0 ) (Tic-Orbit :proposal-id 17540 :visit-id "02" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2262 :al-overhead-at-end 0 :internal-dur-at-end 107 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2645 :pcs-gap 0 ) true 2 0