initial submission -395392600 Assigned ID: 1224 ----- Attempting Submission 1 (Fri Jan 17 22:15:16 GMT 2020) ----- ----- Attempting Submission 2 (Sat Jan 18 04:41:31 GMT 2020) ----- update eph, change exposure structure -1743454192 ----- Attempting Submission 1 (Mon Mar 16 06:25:03 GMT 2020) ----- ----- Attempting Submission 2 (Tue Apr 07 10:49:22 GMT 2020) ----- The recently discovered long-period comet C/2019 Y4 (ATLAS) has an orbit resembling comet C/1844 Y1 (Great Comet), suggesting a common origin. This is only the third identified long-period comet group. Here we request 1 (one) HST orbit to constrain the nucleus size of C/2019 Y4 and to search for smaller, young fragments in its vicinity, with the goal being to understand the mechanism that drives the disintegration of its progenitor. We will spend one HST/WFC3 orbit to image C/2019 Y4. The observation will be conducted right before the comet enters the HST solar exclusion zone (UT 2020 Apr 30 04:00) at a heliocentric distance of 0.9 au. Since the situation is highly dynamic and the brightness of the comet around the observation date is uncertain, we will use two different exposure schemes: a large number of short, sub-array exposures aiming for near-nucleus features and fragments, and a small number of full-frame exposures aiming for fragments far from the nucleus. In case the comet disintegrates well before the time of observation, we will only conduct long, full-frame exposures to characterize the debris field, as the nucleus no longer exists and there is no concern of saturation. PI Quanzhi Ye has led and/or participated a number of HST programs on small solar system bodies, with expertise in observation planning and analysis, dynamical modellings of cometary dust, and comet morphology simulations. Co-I Man-To Hui is highly experienced with analysing both space and ground observations of comets, his expertise includes performing high-quality astrometry, interpreting comet morphologies, and extracting nucleus signal by removal of coma contamination. Solar System 3 false false false false false false false SUPPORTED SOLAR SYSTEM COMET Comet TYPE=COMET,Q=0.2528606942259004,E=0.9992079311528602,I=45.37858785518691 ,O=120.567065117029,W=177.4112843664408,T=31-MAY-2020:00:25:43,TTimeScale=TDB,EQUINOX=J2000,EPOCH=06-MAR-2020:00:00:00,EpochTimeScale=TDB TYPE=POS_ANGLE,RAD=30,ANG=90,REF=NORTH EARTH WFC3/UVIS Imaging No No F350LP 1586366582812 Using ephemeris for target 16089-01-C2019Y4 ephemeris data for target 16089-01-C2019Y4 does not cover planning interval. Physical constraints are assumed to be unschedulable outside of the ephemeris data. Planning interval: 19.213 to 21.091. ephemeris interval: 20.091 to 22.091 The scheduling data was generated by CASM 28.0. CASM was run in Phase II mode. /private/var/folders/q9/0jb3sqc10kx2dw3wnl21b_dw000702/T/OPVisit7523721014785309367.xml (Tic-Visit :proposal-id 16089 :visit-id "01" :proposal-version -1 :trans-version "20191203" :date "08-Apr-2020 13:23:02.0" :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 16089 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 16089 :visit-id "01" :orbit-number 1 :pcs-time 393 :si-vis-utilization 2749 :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 ) true 1 0