0 Assigned ID: 1390 ----- Attempting Submission 1 (Wed Feb 10 20:20:20 GMT 2021) ----- updated orbital elements from email from PI 258534638 ----- Attempting Submission 1 (Tue Feb 16 18:10:37 GMT 2021) ----- ----- Attempting Submission 2 (Wed Feb 17 13:11:22 GMT 2021) ----- 323P/SOHO is a near-Sun comet with perihelion activity driven by mass-loss mechanism most likely completely different from that of typical comets, and thus is possibly unique in the comet population of the solar system. Postperihelion observations from CFHT in early February 2021 showed that the comet has morphology consistent with ongoing disintegration in rapid evolution, in stark contrast to the preperihelion morphology, which was simply asteroidal in Subaru data from late December 2020. Thus, 323P becomes the first short period near-Sun object that has disintegrated. The ongoing event is offering us an unprecedented opportunity to study a near-Sun object in great detail, and to constrain properties of the interior as well as the disintegration process. We hereby request four DD orbits, taking advantage of HST/WFC3's ultra-high resolution, sensitivity, and photometric stability to monitor and characterize the disintegration event at 323P. We request four orbits to observe disintegrating near-Sun comet 323P/SOHO using HST's WFC3 through the F350LP filter. The four orbits are split as two pairs, with each pair of the visits separated by a day or two in order to cross-link debris and measure their motions between visits, and the two pairs should be a week or so apart, in order to detect any nongravitational effects that the debris are subject to, and to monitor changes in the morphology of the comet from different observing geometries. In each orbit, we request that consecutive exposures of 405 s on the comet should be taken, with image dithering performed in the middle to mitigate CCD defects including bad pixels and the gap between the two WFC3 gaps after stacking the images. During each entire visibility window, five such consecutive exposures can be obtained. According to the WFC3 Exposure Time Calculator, a point source with a solar-type color at mV = 27.4 will reach signal-to-noise ratio (SNR) ~3 in a single F350LP image. Combining five such images will help removal of cosmic ray hits effectively, and the SNR can be improved by a factor of ~2.2. Using the viewing geometry of 323P on 2021 March 15 (heliocentric distance 1.48 au, cometocentric distance 1.18 au, phase angle 42 deg) as an example, the HST observations will be able to reveal debris from the disintegration event down to ~30 m in radius (comet-like geometric albedo 0.04 assumed) in a combined image from a single visit. This clearly manifests the supreme sensitivity of HST that is unparalleled by any other facilities. The nonsidereal apparent motion of 323P will be <4 arcmin/hr, well within the tracking capacity of HST. The current 3-sigma ephemeris uncertainty during the observable window is at the subarcsec level, much smaller than the field of view (FOV) of WFC3, and so will not be a concern to telescope pointing. In order to follow up the disintegration process in rapid evolution seen in our CFHT data from early February 2021, we request disruptive scheduling for the first two visits of observations. The other pair of orbits should be executed roughly a week after the first pair of visits. The comet is currently swiftly receding from both the Sun and Earth. Observations from much later epochs will have sensitivity way lower than what should have been achieved in earlier epochs, and the disintegration may have faded to a degree where the morphology may have been altered considerably as the rapid evolution of the disintegration continues, making investigation of the initial disintegration mechanism difficult, and co-moving remnants may have dispersed beyond WFC3's FOV. In addition, the comet will enter HST's Sun exclusion zone (solar elongation <50 deg) starting from mid May 2021. Therefore, we must observe 323P and complete the program in March 2021, the earlier the better. We understand that the HST roll angle is not within our control, although ideally the extension of the debris cloud should be placed along the diagonal of WFC3's FOV. Man-To Hui is experienced in observational and morphological studies of comets, and have participated in multiple HST programs on comets and asteroids. Rainer Kracht is skilled in orbit determination. Max Mutchler is an expert in HST cameras and observing plannings, especially for acquisition of high-quality imaging of moving targets. David Tholen has awarded ground telescope time and is skillful in astrometric observations of small bodies. Paul Wiegert is an expert in orbital dynamics. Quanzhi Ye has awarded ground telescope time and has expertise in dynamical modelling of cometary dust and their interactions with Earth. We will all collaborate for the proposed HST program, and write the paper expeditiously once we have results from the HST observations. Solar System Astronomy 0 false SUPPORTED SOLAR SYSTEM YES COMET Disintegrating comet TYPE=COMET,Q=0.0392071121041,E=0.9848146391461,I=5.3699316736616,O=324.2293902822781,W=353.1741243642062,T=17-JAN-2021:15:09:08,TTimeScale=TDT,EQUINOX=J2000,EPOCH=17-FEB-2021:00:00:00,EpochTimeScale=TDT EARTH WFC3/UVIS Imaging No No F350LP 1613497079185 Using ephemeris for target 16496-01-323P ephemeris data for target 16496-01-323P does not cover planning interval. Physical constraints are assumed to be unschedulable outside of the ephemeris data. Planning interval: 20.214 to 22.089. ephemeris interval: 21.031 to 23.032 Timing link suitability includes constraints ([02 After 01 by 1.0 to 2.0 days]) The scheduling data was generated by CASM 29.0. CASM was run in Phase II mode. Schedule this visit as soon as possible. 1614682146000 77.67873246401382 (Tic-Visit :proposal-id 16496 :visit-id "01" :proposal-version -1 :trans-version "20200619" :date "17-Feb-2021 01:37:58.0" :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 16496 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 16496 :visit-id "01" :orbit-number 1 :pcs-time 393 :si-vis-utilization 2741 :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 3142 :pcs-gap 0 ) false 1 0 1613497084345 Using ephemeris for target 16496-02-323P ephemeris data for target 16496-02-323P does not cover planning interval. Physical constraints are assumed to be unschedulable outside of the ephemeris data. Planning interval: 20.214 to 22.089. ephemeris interval: 21.031 to 23.032 Timing link suitability includes constraints ([02 After 01 by 1.0 to 2.0 days] [03 After 02 by 6.0 to 10.0 days]) The scheduling data was generated by CASM 29.0. CASM was run in Phase II mode. Schedule this visit as soon as possible. 1614756530000 80.63044231165861 (Tic-Visit :proposal-id 16496 :visit-id "02" :proposal-version -1 :trans-version "20200619" :date "17-Feb-2021 01:38:04.0" :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 16496 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 16496 :visit-id "02" :orbit-number 1 :pcs-time 393 :si-vis-utilization 2741 :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 3142 :pcs-gap 0 ) false 1 0 1613497087716 Using ephemeris for target 16496-03-323P ephemeris data for target 16496-03-323P does not cover planning interval. Physical constraints are assumed to be unschedulable outside of the ephemeris data. Planning interval: 20.214 to 22.089. ephemeris interval: 21.031 to 23.032 Timing link suitability includes constraints ([03 After 02 by 6.0 to 10.0 days] [04 After 03 by 1.0 to 2.0 days]) The scheduling data was generated by CASM 29.0. CASM was run in Phase II mode. (Tic-Visit :proposal-id 16496 :visit-id "03" :proposal-version -1 :trans-version "20200619" :date "17-Feb-2021 01:38:07.0" :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 16496 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 16496 :visit-id "03" :orbit-number 1 :pcs-time 393 :si-vis-utilization 2741 :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 3142 :pcs-gap 0 ) false 1 0 1613497091101 Using ephemeris for target 16496-04-323P ephemeris data for target 16496-04-323P does not cover planning interval. Physical constraints are assumed to be unschedulable outside of the ephemeris data. Planning interval: 20.214 to 22.089. ephemeris interval: 21.031 to 23.032 Timing link suitability includes constraints ([04 After 03 by 1.0 to 2.0 days]) The scheduling data was generated by CASM 29.0. CASM was run in Phase II mode. (Tic-Visit :proposal-id 16496 :visit-id "04" :proposal-version -1 :trans-version "20200619" :date "17-Feb-2021 01:38:10.0" :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 16496 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 16496 :visit-id "04" :orbit-number 1 :pcs-time 393 :si-vis-utilization 2741 :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 3142 :pcs-gap 0 ) false 1 0