Target C2019Y4: There is no need or capability to test the scheduling of observations of moving targets in Phase I. This warning will go away the next time the proposal is loaded. 0 Assigned ID: 1101 ----- Attempting Submission 1 (Mon Apr 06 18:32:20 GMT 2020) ----- Software bug: Warning (Visit Planner): Target C2019Y4: There is no need or capability to test the scheduling of observations of moving targets in Phase I. This warning will go away the next time the proposal is loaded. In this resubmission, I made changes now knowing that we will get scheduled April 20-23 with a nominal roll of 243-250 degrees. Although the Visit Planner still would not work, I could use Aladin to create BETWEEN windows that avoid spoiler stars for both visits. I also adjusted the small POS TARG offsets to optimally place the comet within the 2k subarray at this orient. -- Max Mutchler 1657631093 ----- Attempting Submission 1 (Wed Apr 08 23:33:56 GMT 2020) ----- ----- Attempting Submission 2 (Thu Apr 09 13:41:39 GMT 2020) ----- Visit Planner not working. Breakup of Long-Period Comet C/2019 Y4 (ATLAS) We seek HST time with the WFC3 camera in order to study the on-going break-up of long-period comet C/2019 Y4 (ATLAS). Break-up of comets may be the dominant destruction process of these bodies, yet is poorly studied because breakups occur unpredictably and evolve rapidly, require timely observations which are difficult to secure. We propose a 4 orbit allocation to measure the temporal evolution of the fragments, in order to measure the brightness and velocity distributions and to measure fragment nn-gravitational acceleration caused by outgassing forces (the latter are a measure of fragment size). We will also seek evidence for rotational and other modulation of the primary nucleus and fragments, indicative of patchy distribution of surface volatiles. A Monte-Carlo dust model will be applied to imaged of the diffuse envelope. C/2019 Y4 has the potential to be one of only a handful of disrupting comets ever studied at HST resolution, and to set exciting constraints on models of the breakup mechanism. We request observations of comet ATLAS with HST and WFC3, using the F350LP broadband filter. We request 2 to 3 day spacing between visits in order to connect fragments and their motion between orbits, thereby to measure the velocities and accelerations. In each orbit, we will obtain six 260s integrations on the comet, dithered to provide protection from chip defects. The six images, when combined, will also provide adequate protection from cosmic rays. The WFC3 Exposure Time Calculator indicates that, for a point source, solar-type spectrum, a signal-to-noise ratio of 6 is reached in 230 s at V = 26 in the F350LP filter (optimal extraction yields SNR=13). For example, at opposition and heliocentric distance R = 1 AU, magnitude V = 26 corresponds to an object radius 25 m (geometric albedo 0.04 assumed). These are upper limits to the point source thresholds because at least some of the fragments are embedded in their own dust comae. However, it is clear that HST offers unprecedented sensitivity to faint fragments. We use F350LP because the F350LP count rate is 1.6 higher for a target with a solar-type spectrum than other commonly used broadband filters (e.g., F606W), providing a better count rate and signal-to-noise ratio. We also plan to dither the exposures to mitigate the effects from bad pixels, cosmic rays, and the inter-chip gap. Angular non-sidereal rates peak at about 10000 hour1 and are well within the range of HST capabilities. The ephemeris uncertainty is 3 sigma < 1 arcsec, of no concern given the field of view of WFC3. We request Disruptive Scheduling for these observations. This is necessary because of the expected rapid photometric and morphological development of the fragments as seen in other disintegrating comets. Comet ATLAS will be in the HSTs 55 Sun exclusion zone from UT 2020 April 30 to UT 2020 September 7. Therefore, we must observe with HST before 2020 April 30, but earlier dates are preferred. The apparent rates of motion relative to sidereal are < 1 arcsec/hour, which are easily within Hubble's tracking capabilities. These rates of motion are also slow enough to keep a single pair of guide stars within the FGS pickles for an entire visibility window. The ephemeris uncertainties are typically at the 1 arcsec level. We understand that we will have essentially no control over the spacecraft roll angle. Jewitt is an experienced planetary astronomer and long-time user of HST, with many resulting publications on a range of small body targets. Max Mutchler is expert in the use of HST for studying solar system targets by designing optimal observing strategies. Jessica Agarwal and Yoonyoung Kim bring expertise in the dynamics of dust under the action of solar gravity and radiation pressure forces. Harold Weaver is an expert planetary astronomer with a record of achievment in both ground based and space based observations Solar System Astronomy 0 false SUPPORTED SOLAR SYSTEM YES COMET disintegrating comet TYPE=COMET,Q=0.2528326026905927,E=0.9992444279234513,I=45.38507225915464 ,O=120.5732078366555,W=177.4075353245124,T=31-MAY-2020:00:22:46,TTimeScale=TDB,EQUINOX=J2000,EPOCH=06-MAR-2020:00:00:00,EpochTimeScale=TDB,R0=2.808 ,DT=0. ,A1=4.282845687866E-7,A2=0.,A3=0. ,ALN=0.1112620426,NM=2.15,NN=5.093,NK=4.6142,AMRAT=0. 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