8231( 3) - 04/21/00 07:39 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8231 Version: 3 Check-in Date: 15-Feb-1999 14:23:36 1.Proposal Title: Determination of the radius of comet 19P/Borrelly in support of the NMP DS1 Flyby ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 8 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Michel Festou Southwest Research Institute CoI: Dr. Frances Bagenal University of Colorado N CoI: Dr. Daniel C. Boice Southwest Research Institute N CoI: Dr. Daniel T. Britt University of AZ N CoI: Dr. Bonnie J. Buratti Jet Propulsion Laboratory N CoI: Dr. Wing-Huen Ip MPAe N CoI: Dr. Robert H. Brown University of AZ N CoI: Dr. Robert M. Nelson Jet Propulsion Laboratory N CoI: Dr. Juergen Oberst DLR N CoI: Dr. Joel Wm. Parker Southwest Research Institute N CoI: Dr. Tobias C. Owen University of Hawaii N CoI: Dr. Bill R. Sandel University of AZ N CoI: Dr. Larry Soderblom USGS N CoI: Dr. S. Alan Stern Southwest Research Institute N CoI: Dr. Nicholas Thomas Max Planck Institute fuer Aeronomie N CoI: Dr. Roger V. Yelle Boston University N ------------------------------------------------------------------------------------ 5. Abstract Comet 19P/Borrelly is the ultimate target of the New Mellinium Program (NMP) Deep Space 1 asteroid-comet flyby mission. The size of this comet's nucleus is a critical parameter needed for flyby planning activities. However, as we describe below, P/Borrelly's radius is not well established. We, the NMP DS1 Science Team, request 1 orbit of HST/STIS time in Cycle 8 to refine the nuclear size estimate. This program cannot wait until Cycle 9 because of mission planning constraints and the fact that the comet is likely to be producing a weak but nonetheless enhanced coma by the time of Cycle 9. We therefore propose to observe comet P/Borrelly in Cycle 8 when the activity level of its nucleus is near its minimum, and quite possibly negligible. From a clear separation of the contributions of the coma and the nucleus in the inner part of STIS images, we will extract the cross section of Borrelly's nucleus. In addition to its immediate value for NMP DS1 mission planning, when c oupled to existing data sets, the STIS data will then enable us to model the coma morphology to better infer the effect of the nucleus outgassing on the comet's motion. ------------------------------------------------------------------------------------ 8231( 3) - 04/21/00 07:39 - [ 2] Observations Description ------------------------ We will make a single visit to P/Borrelly when its heliocentric distance will be near 4 AU, and its water outgassing will therefore be extremely weak. The dust coma will be consequently also weak. The solar phase angle will be $\sim 10$ degrees. We will use STIS in the IMAGE mode. The comparison of the relative performances of STIS and WFPC2 favors STIS when better sensitivity through wide band filters is important. The small differences in the pixel sizes and field of views (PC camera vs. STIS) are not important for this study: both instruments undersampled the PSF and the smaller FOV of STIS is much larger than the predicted coma size which, does not need to be imaged beyond about 10 arc seconds. We plan to obtain images of the comet in 2 wide bandpass filters. From the 1994 HST observations of Lamy etal. (1998) when the comet was 1.4 AU from the Sun and 0.62 AU from Earth, with a phase angle of 38 degrees, it revealed itself to have a mean V of 17.3 (the exact valu e depends on the rotational phase). When we plan to observe in late Cycle 8, the comet will characteristically be 4.1 AU from the Sun and 3.4 AU from Earth, with a phase angle of $\sim 10$ degrees, which implies a mean V magnitude near 22.2 for the Cycle 8 (nearer to aphelion) observations. We adopt these reference parameters in what follows. We further assume that nucleus color is slightly redder (G 5V) than the Sun, as suggested by ground based investigations of the properties of comet nuclei (Jewitt 1991). For P/Borrelly one estimates 53 minutes of available observing time per orbit. Of this, 12 minutes are taken up by the Guide Star acquisition. Some 12*** additional minutes are consumed by STIS image overheads for*** 2 images (these values will be optimized in Phase II if possible). Thus, 29*** minutes are available for the two images. These will be obtained with STIS 50CCD CLEAR (lambda_c=5850 Angstrom), and F28X50LP (lambda_c=7300 Angstrom). We will split th e observing time in three parts of equal length, 14.5*** minutes each. Exposure time calculations using the STIS ETC reveal that the CCD/CLEAR image will obtain S/N = 60*** on the brightest pixel in 14.5*** min of exposure time, and an average S/N of 28*** in the 25 central pixels. Similarly, the STIS ETC estimates that the F28X50LP image will yield S/N=42*** on the brightest pixel in 14.5*** min of exposure time, and an average S/N of 21.5*** in the 25 central pixels. For both the image ETC calculations we adopted E(B-V)=0 and average zodiacal light and earthshine parameters. Observing at a less favourable rotational phase angle than that chosen here will only slightly degrade the performances announced above. These observations will not exceed any detector count rate limits. Real Time Justification ----------------------- None. The DS1 Team plans an active groundbased campaign of measurements of P/Borrelly in 1999--2001. Included in these will be lightcurve measurements around the time of the HST Cycle 8 observations, so as to pin down precisely where in the lightcurve the HST orbit of data was taken. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8231( 3) - 04/21/00 07:39 - [ 3] Data Distribution Paper Products: NO Media: ELECTRONIC Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 8231( 3) - 04/21/00 07:39 - [ 4] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 | TARGET NAME: COMET-19PBORRELLY ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: COMET 19P/Borrelly, nucleus ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | TYPE=COMET,Q=1.358975480,E=0.623738593,I | =30.3235473,O=75.4223109,W=353.3441485,T | = 14-Sep- | 2001:17:35:13.6,EQUINOX=J2000,EPOCH=01- | Jun-2000,A1=0.204E-8,A2=-0.0389E-8 | | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | V = 22+/-0.5 | The orbital elements have to be B-V = 0 | revised and given for an epoch close | to the observing date (up to 2 | months difference). This will be | provided by PI. | Ephemeris Uncertainty: 3 S | Acquisition Uncertainty: 3 " | ------------------------------------------------------------------------------------------------------------------------------------ 8231( 3) - 04/21/00 07:39 - [ 5] Visit: 01 Visit Priority: Visit Requirements: BETWEEN 05-Jun-2000 AND 01-Aug-2000 On Hold Comments: Additional Comments: The comet must be observed at a small phase angle to limit the extrapolation made towards zero phase angle to which all fluxes will be reduced before radius determination, and the limit is here set a 10 degrees. More important, the coma, if present, must be as weak as possible and we consequently set the heliocentric distance to be smaller than 3.8 AU. The optimal observing date is when the phase angle is close to its minimum value, i.e. between the end of June 2000 and the end of July 2000. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 COMET-19PBO STIS/C ACCUM 50CCD MIRROR CR-SPLIT=3 1 1251 S MAX DUR RRELLY CD ------------------------------------------------------------------------------------------------------------------------------------ 20 COMET-19PBO STIS/C ACCUM F28X50L MIRROR CR-SPLIT=3 1 1251 S MAX DUR RRELLY CD P ------------------------------------------------------------------------------------------------------------------------------------ 8231( 3) - 04/21/00 07:39 - [ 6] Summary Form for Proposal 8231 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 50CCD F28X50LP ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=3 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BETWEEN 05-Jun-2000 AND 01-Aug-2000 MAX DUR ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names COMET-19PBORRELLY ------------------------------------------------------------------------------------------------------------------------------------