13475( 8) - 11-Sep-2013 17:20:09 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 13475 Version: 8 Check-in Time: 11-Sep-2013 17:20:09 STScI Edit Number: 0 Title Hubble Imaging of a Newly Discovered Main Belt Comet ------------------------------------------------------------------------------------ Type Cycle GO 21 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. David Jewitt University of California - Los Angeles Y CoI: Dr. Harold A. Weaver The Johns Hopkins University Applied Y Physics Laboratory CoI: Dr. Jessica Agarwal Max Planck Institute for Solar System N Research CoI: Dr. Stephen M. Larson University of Arizona N CoI: Mr. Max Mutchler Space Telescope Science Institute Y ------------------------------------------------------------------------------------ Abstract Main-belt comets (MBCs, or ``active asteroids'') have the orbital characteristics of asteroids but also show transient, comet-like activity. Examples of mass-loss likely caused by ice sublimation and by impact have been established, while numerous additional processes are capable of launching material from asteroids. We propose two-orbits of non-disruptive, target-of-opportunity observations of the next MBC discovered in order to help determine the process driving mass-loss. ------------------------------------------------------------------------------------ Observations Description ------------------------ This proposal is a straightforward continuation of our (so-far un-triggered) ToO approved in Cycle 20. We request two single-orbit HST visits to image a newly-discovered MBC. The basic objective is to characterize the early-time morphology and establish the initial rates of change in the appearance, so that the need for additional HST observations and their optimum cadence can be realistically determined. In some cases (as with P/2010 A2, for instance), the initial observations alone will be enough to determine whether the morphology is more consistent with sublimation or another origin. In others, careful study of the photometry and evolution over long intervals will be needed for this purpose. Our observing strategy is to take both short and long exposures using WFC3 and a single filter (F606W). Asteroid Scheila, with its ultra-bright nucleus V~13.7, represents an end-member case. We used an exposure time of 0.5 sec (the shortest available) to obtain S/N~200 on the nucleus itself in search of structure at ~100 km scales. An intermediate exposure time of ~40 s brought out the near-nucleus environment without saturating too many pixels, while long exposures of ~400 s provided the deepest search for debris. We plan to take a sequence of at least 3 images (one for each exposure time) at one location on the CCD and then repeat that sequence of images at a dithered location (offset by 0.2'' in each dimension). Dithered, multiple images provide protection from bad pixels and cosmic ray strikes that otherwise might compromise the photometry. Previous HST observations show that the timescale for substantial change ranges from several weeks (Scheila) to several months (P/2010 A2). 13475( 8) - 11-Sep-2013 17:20:09 - [ 2] Therefore, while the first observation should be scheduled as soon as possible after discovery of activity, the science does not require that the first visit be secured within the first two weeks. So ours is a non-disruptive Target of Opportunity program, which will not require heroic scheduling. The first visit will set the scene, by establishing the high-resolution morphology of the object (at ~60 km/pixel at 2 AU geocentric distance). The second visit, ideally separated from the first by ~14 days, will reveal the changes. The two visits together will allow us to make a rational decision about the need for further observations with HST. Main-belt targets have apparent rates of motion typically ~50''/hr, which are easily within Hubble's 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 numbered asteroids is negligible (sub-arcsecond), compared to the WFC3 field-of-view of 162x162''. For newly discovered objects the uncertainty can be larger but, as we showed even for the low surface brightness and morphologically complex example set by P/2010 A2, attaining 1" to 2" accuracy is straightforward. Ephemeris issues are of no concern to this observation. We understand that we will 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. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- The trigger for these observations is the discovery of an object in the main-belt having a Tisserand parameter TJ >= 3.1 and showing a coma or tail. Comets have TJ < 3, asteroids have TJ > 3. The parameter is useful only in the context of the circular, restricted three-body approximation. As a result, objects with TJ very close to 3 can be either cometary or asteroidal in nature. In practice, we take TJ > 3.1 as the dividing line, since objects with larger TJ cannot be dynamically linked to the classical comets. The Tisserand constraint is quite stringent, and avoids any possibility of confusion with classical comets (the blue dots in Figure 1). With one exception, the known MBCs have been discovered serendipitously by sky surveys conducted for other purposes. We expect this mode of discovery to continue. Co-I Larson is PI of the Catalina Sky Survey (CSS) which discovered coma at (596) Scheila. The CSS will be used to obtain astrometric and photometric data on objects discovered elsewhere (or by CSS itself) in support of the HST observations in this proposal. Over the last 6 yrs MBCs have been discovered by on-going surveys at the average rate of about one per year. From this, and bearing in mind statistical fluctuations, we estimate that the probability that an MBC will be discovered in the next HST Cycle is 50% to 100%. 13475( 8) - 11-Sep-2013 17:20:09 - [ 3] ------------------------------------------------------------------------------------ 13475( 8) - 11-Sep-2013 17:20:09 - [ 4] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 TARGET NAME: MBC-2013P5 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: ASTEROID Main Belt Comet ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 TYPE=COMET, Q=1.93384934892737, E=0.1173619073956487, I=4.981097557209049, O=279.3844915611511, W=144.9317439481429, T=17-APR-2014:21:25:27, TTimeScale=TDT, EQUINOX=J2000, EPOCH=22-AUG-2013:00:00:00, EpochTimeScale=TDT ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 20 +/- 5 Ephemeris Center: EARTH ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 2 TARGET NAME: MBC-2013P5-V2 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: COMET Main Belt Comet ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 TARGET POSITION LEVEL 2 TYPE=COMET, Q=1.936403971304516, E=0.1152027633258743, I=4.967974595638397, O=279.2852123089732, W=144.2069632687044, T=15-APR-2014:17:39:45, TTimeScale=TDT, EQUINOX=J2000, EPOCH=28-AUG-2013, EpochTimeScale=TDT ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 WINDOWS ------------------------------------------------------------------------------------------------------------------------------------ DATA COMMENTS V = 20 +/- 1 Updated to JPL #2 orbital elements Ephemeris Center: EARTH ------------------------------------------------------------------------------------------------------------------------------------ 13475( 8) - 11-Sep-2013 17:20:09 - [ 5] Visit: 01 Visit Requirements: VISIBILITY INTERVAL 52.5 M 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 MBC-2013P5 WFC3/UVI ACCUM UVIS1 F350LP CR-SPLIT=NO 1 348 S (348 S S) ------------------------------------------------------------------------------------------------------------------------------------ 2 MBC-2013P5 WFC3/UVI ACCUM UVIS1 F350LP CR-SPLIT=NO 1 348 S (348 S S) ------------------------------------------------------------------------------------------------------------------------------------ 3 MBC-2013P5 WFC3/UVI ACCUM UVIS1 F350LP CR-SPLIT=NO 1 348 S (348 S S) ------------------------------------------------------------------------------------------------------------------------------------ 4 MBC-2013P5 WFC3/UVI ACCUM UVIS1 F350LP CR-SPLIT=NO 1 348 S (348 POS TARG 0.2, 2.41 S S) ------------------------------------------------------------------------------------------------------------------------------------ 5 MBC-2013P5 WFC3/UVI ACCUM UVIS1 F350LP CR-SPLIT=NO 1 348 S (348 POS TARG 0.2, 2.41 S S) ------------------------------------------------------------------------------------------------------------------------------------ 6 MBC-2013P5 WFC3/UVI ACCUM UVIS1 F350LP CR-SPLIT=NO 1 233 S (233 POS TARG 0.2, 2.41 S 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 | ------------------------------------------------------------------------------------------------------------------------------------ MBC-2013P5 1 WFC3/UVI ACCUM UVIS1 F350LP none none none none 1 N/A S MBC-2013P5 2 WFC3/UVI ACCUM UVIS1 F350LP none none none none 1 N/A S MBC-2013P5 3 WFC3/UVI ACCUM UVIS1 F350LP none none none none 1 N/A S MBC-2013P5 4 WFC3/UVI ACCUM UVIS1 F350LP none none none none 1 N/A S MBC-2013P5 5 WFC3/UVI ACCUM UVIS1 F350LP none none none none 1 N/A S MBC-2013P5 6 WFC3/UVI ACCUM UVIS1 F350LP none none none none 1 N/A S 13475( 8) - 11-Sep-2013 17:20:09 - [ 6] Visit: 02 Visit Requirements: BETWEEN 12-SEP-2013 AND 13-OCT-2013; VISIBILITY INTERVAL 52.5 M 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 MBC-2013P5- WFC3/UVI ACCUM UVIS1 F350LP CR-SPLIT=NO 1 348 S (348 V2 S S) ------------------------------------------------------------------------------------------------------------------------------------ 2 MBC-2013P5- WFC3/UVI ACCUM UVIS1 F350LP CR-SPLIT=NO 1 348 S (348 V2 S S) ------------------------------------------------------------------------------------------------------------------------------------ 3 MBC-2013P5- WFC3/UVI ACCUM UVIS1 F350LP CR-SPLIT=NO 1 348 S (348 V2 S S) ------------------------------------------------------------------------------------------------------------------------------------ 4 MBC-2013P5- WFC3/UVI ACCUM UVIS1 F350LP CR-SPLIT=NO 1 348 S (348 POS TARG 0.2, 2.41 V2 S S) ------------------------------------------------------------------------------------------------------------------------------------ 5 MBC-2013P5- WFC3/UVI ACCUM UVIS1 F350LP CR-SPLIT=NO 1 348 S (348 POS TARG 0.2, 2.41 V2 S S) ------------------------------------------------------------------------------------------------------------------------------------ 6 MBC-2013P5- WFC3/UVI ACCUM UVIS1 F350LP CR-SPLIT=NO 1 233 S (233 POS TARG 0.2, 2.41 V2 S 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 | ------------------------------------------------------------------------------------------------------------------------------------ MBC-2013P5-V2 1 WFC3/UVI ACCUM UVIS1 F350LP none none none none 1 N/A S MBC-2013P5-V2 2 WFC3/UVI ACCUM UVIS1 F350LP none none none none 1 N/A S MBC-2013P5-V2 3 WFC3/UVI ACCUM UVIS1 F350LP none none none none 1 N/A S MBC-2013P5-V2 4 WFC3/UVI ACCUM UVIS1 F350LP none none none none 1 N/A S MBC-2013P5-V2 5 WFC3/UVI ACCUM UVIS1 F350LP none none none none 1 N/A S MBC-2013P5-V2 6 WFC3/UVI ACCUM UVIS1 F350LP none none none none 1 N/A S 13475( 8) - 11-Sep-2013 17:20:09 - [ 7] Summary Form for Proposal 13475 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BETWEEN 12-SEP-2013 AND 13-OCT-2013, POS TARG 0.2, 2.41, VISIBILITY INTERVAL 52.5 M ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F350LP ------------------------------------------------------------------------------------------------------------------------------------ Target Names MBC-2013P5, MBC-2013P5-V2 ------------------------------------------------------------------------------------------------------------------------------------