5843( 12) - 01/02/96 11:19 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 5843 Version: 12 Check-in Date: 02-Jan-1996 11:08:34 1.Proposal Title: First Ultraviolet Spectroscopic Survey of 2060 Chiron ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle Solar System GO 5 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Alan Stern Southwest Research Institute CoI: Michel Festou Southwest Research Institute N CoI: Ted Bowell Lowell Observatory N CoI: Bobby Bus MIT N CoI: Michael A'Hearn University of Maryland Y CoI: David Weintraub Vanderbilt University N ------------------------------------------------------------------------------------ 6. Abstract Chiron's coma has been imaged by HST, but the object has never been spectroscopically surveyed in the ultraviolet. UV spectroscopy is a powerful tool for studying the surface and atmospheric properties of comets, planetary satellites, and planets. We request time to obtain the first-ever UV survey spectrum of the highly interesting outer solar system object. These spectra will bear on critical questions concerning the nature of Chiron's surface, its coma, and perhaps the source of its activity. Specifically, we will use the FOS/BL with G270H to: (i) establish the UV albedo & color slope, (ii) quantify evidence for non-icy materials on Chiron's surface, (iii) gain a better handle on the scattering properties of the particulate coma, and (iv) search for emissions from gaseous coma species. We will obtain groundbased CCD images in conjunction with each FOS spectrum to establish the coma morphology and level of coma activity. We will combine all of these observations to provide a comprehensive picture or set of constraints on Chiron's nuclear properties, source of activity, and coma composition, which we want to compare to conventional comets. This is a critical time to conduct these observations, since Chiron will pass through its perihelion during Cycle 5. Chiron is much too faint for IUE to adequately study; the only practical approach to obtaining UV spectroscopy is HST. ------------------------------------------------------------------------------------ 5843( 12) - 01/02/96 11:19 - [ 2] Observations Description ------------------------ With the FOS, we propose to obtain the first UV spectroscopy of Chiron. On the same days, we will also obtain groundbased CCD images to gauge Chiron's level of activity. Because the FOS spectra will be time-resolved, we will be able to bin them in wavelength to also establish both an integrated-UV lightcurve and, after temporal binning, information on the variegation of Chiron's nuclear reflectance spectrum with rotational phase. Two kinds of observations are planned: 4 orbits (6.4 hrs) using the FOS with a 0.5" aperture to study Chiron's nucleus as it rotates (P~5.9 hrs), and 4 orbits using the FOS with the 4.3" acquisition aperture to study gases and particulates in the inner coma. The first coma observation will be made in conjunction with the nucleus observations. The later 3 coma observations will be made on 3 other dates during Cycle 5 in order to allow us to study variations in Chiron's coma with varying levels of activity. Our total request is for 8 orbits. Although this corresponds to a significant amount of HST time, we believe this is appropriate given the fact that Chiron's perihelion passage is centered on Cycle 5, and will not reoccur until 2047. Still, we stress to the TAC that total allocations as small as 2 orbits would be preferable to missing the perihelion year altogether. The first orbit of nucleus observations will acquire 26 min of integrations; the following 3 orbits will obtain 41 minutes each because the reacquisition time is smaller. The 4 orbits of coma observations using the large, 3.7x3.7" (designated the 4.3") FOS ACQ aperture will obtain 26-35 minutes of integration time each, depending on whether a blind acquisition can be used with this large aperture. All data will be obtained using the FOS/BL with the G270H grating; we will use the FOS in ACCUM mode, which provides 4 minute time sampling. We chose the BL Digicon detector to minimize the effects of instrument-scattered light at short wavelengths. At a typical geocentric distance of 9 AU during Cycle 5, the 3.7" dimension of the ACQ aperture will correspond to a linear dimension of ~24,000 km in Chiron's coma. Since the coma is typically optically thin (Buratti & Dunbar 1991), the 0.5" aperture planned for nucleus studies will reduce the coma contamination by about an order of magnitude compared to the 4.3" acquisition aperture, without significantly vignetting the signal. We hope to use the new Moving Target Binary option developed during the preparations for Shoemaker-Levy 9 to make the 0.5" aperture acquisitions. We note that Chiron is far from the Galactic plane, so the chance of stellar contamination in either aperture is very small. The FOS/BLUE G270H yields a dispersion of 2.09 Angstroms/diode for a point source like Chiron itself; for coma emissions which fill the ACQ 4.3" aperture, the FOS Handbook shows that the effective resolution will be ~12 Angstrom, which is more than sufficient to study the coma. As noted above, we will use the ACCUM readout mode, with a 4 minute readout time. This will allow us to correct for small FOS detector magnetic drifts (as suggested in the Instrument Handbook) and will also give us good time resolution for the wavelength-integrated UV lightcurve data we will produce. To study the average nuclear spectrum we will (i) produce wavelength-integrated lightcurves, (ii) make orbit-averaged spectra, and (iii) make a grand-sum average nuclear spectrum. The exposure time required to measure Chiron's UV reflectance with G270H was computed assuming a 180 km diameter and a 10\\ Sykes 1991; Campins et al. 1994). This implies an expected 2900 Angstrom flux of 3*10^-16 ergs cm^-2 sec^-1 at Earth, which checks nicely with a simple scaling calculation based on our past FOS G270H spectra of other objects. Using the 1994 FOS Handbook (V5.0) we thus derive an expected 2900 Angstrom\\ count rate of 0.1 counts/sec/diode. Assuming an FOS noise rate of 0.003 counts/sec/diode, we find that after binning the surface reflectance spectra to 20 Angstrom, a typical 30 minute exposure should achieve S/N~22 at ~2900 Angstroms. This resolution and S/N are sufficient for our goals of establishing the albedo slope and searching for an albedo drop around 3000 Angstrom\\ due to the absorption edge that refractory surface materials produce, but ices do not. Wider wavelength bins will be used to obtain finer time resolution for UV lightcurves. In addition, we will also coadd low-coma activity spectra to provide better data on the average nuclear spectrum. Concerning coma gas, one important feature we will search for is the 3085 Angstrom\\ OH band system. During periods of activity, Chiron's coma will easily overfill the 4.3" aperture. To estimate exposure times, we assume a point source with escape velocities in the 200-300 m/s range, and an escaping OH outflow velocity of 1 km sec^-1 and resonance fluorescence efficiency factor g_OH= 3* 10^-4 at 1 AU. We convolved the OH band's natural width of 20-25 Angstrom\\ with a rectangular FOS slit function. Using the V5.0 FOS handbook we estimte the FOS/BL will yield a count rate of approx1.5*10^-3 counts/sec/Rayleigh/diode. Therefore, by summing over the OH band, the FOS can detect OH emissions as low as ~5 R in 30 minutes at S/N=6. An OH signal of 5 R in the 4.3 arcsec aperture corresponds to an OH production rate of sim10^28 sec^-1, which would be very low considering Chiron's nucleus has a surface area 10^2-10^3 times larger than a typical comet. The planned FOS observations can therefore, for the first time, set an important limit on the OH (and hence the H_2O) production rate at Chiron. \\medskip We will combine all of the results from these HST observations to provide a comprehensive picture or set of constraints on Chiron's nuclear properties, source of activity, and coma composition. One critical question we will address is whether Chiron is simply a large comet (e.g., an H_2O and CO rich body) or something else, perhaps a more Triton-like (i.e., N_2 and CH_4 dominated) object, as Pluto and many larger Kuiper Disk objects are suspected to be (Lunine 1993). Additionally, we will use these HST data to address which parts of Chiron's surface are more ice covered, and to better constrain the source regions and mechanisms that generate Chiron's activity. Real Time Justification ----------------------- \\medskip There are no special dark time, time-critical, continuous-viewing zone, or TOO observations required for this program. No special calibrations are needed. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 5843( 12) - 01/02/96 11:19 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 5843( 12) - 01/02/96 11:19 - [ 4] TARGET LIST b)Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 | TARGET NAME: 2060-CHIRON ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: ASTEROID CHIRON, ASTEROID 2060 ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD=2060(CHIRON) | | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | V = 15.5+/-1 | Chiron is variable, the brightnesses B-V = +0.65 | and colors given are for the | nucleus. | ------------------------------------------------------------------------------------------------------------------------------------ 5843( 12) - 01/02/96 11:19 - [ 5] Visit: 01 Visit Requirements: BETWEEN 01-MAY-95 AND 31-DEC-95 On Hold Comments: Additional Comments: This visit is to (i) obtain the rotationally-resolved nuclear spectrum, & (ii) to obtain the first coma spectrum Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 2060-CHIRON FOS/BL ACCUM 4.3 G270H 1 20M ------------------------------------------------------------------------------------------------------------------------------------ 2 2060-CHIRON FOS/BL ACQ 4.3 MIRROR 1 360S END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 3 2060-CHIRON FOS/BL ACCUM 4.3 G270H 1 15M ------------------------------------------------------------------------------------------------------------------------------------ 4 2060-CHIRON FOS/BL ACQ/PEA 4.3 MIRROR SEARCH-SIZE- 1 8S ONBOARD ACQuisition FOR 5 K X=1,SEARCH-SIZE -Y=3,SCAN-STEP- Y=1.204 ------------------------------------------------------------------------------------------------------------------------------------ 5 2060-CHIRON FOS/BL ACQ/PEA 1.0 MIRROR SEARCH-SIZE- 1 8S ONBOARD ACQuisition FOR 6 END ORBIT K X=6,SEARCH-SIZE -Y=2,SCAN-STEP- X=0.602,SCAN- STEP-Y=0.602 ------------------------------------------------------------------------------------------------------------------------------------ 6 2060-CHIRON FOS/BL ACQ/PEA 0.3 MIRROR SEARCH-SIZE- 1 17S ONBOARD ACQuisition FOR 7-11 K X=5,SEARCH-SIZE -Y=5,SCAN-STEP- X=0.172,SCAN- STEP-Y=0.172 ------------------------------------------------------------------------------------------------------------------------------------ 7 2060-CHIRON FOS/BL ACCUM 0.5 G270H 1 12M END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 8 2060-CHIRON FOS/BL ACCUM 0.5 G270H 1 15M ------------------------------------------------------------------------------------------------------------------------------------ 9 2060-CHIRON FOS/BL ACCUM 0.5 G270H 1 20M END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 10 2060-CHIRON FOS/BL ACCUM 0.5 G270H 1 15M ------------------------------------------------------------------------------------------------------------------------------------ 11 2060-CHIRON FOS/BL ACCUM 0.5 G270H 1 20M END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 5843( 12) - 01/02/96 11:19 - [ 6] Visit: 02 Visit Requirements: BETWEEN 01-FEB-96 AND 29-FEB-96 On Hold Comments: Additional Comments: This visit is to obtain coma spectra for variability studies near perihelion Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 20 2060-CHIRON FOS/BL ACCUM 4.3 G270H 1 20M EXPAND MAXimum DURation 125% SEQuence 20-21 NON-INTerruptible ------------------------------------------------------------------------------------------------------------------------------------ 21 2060-CHIRON FOS/BL ACQ 4.3 MIRROR 1 360S MINimum DURation 80% SEQuence 20-21 NON- INTerruptible ------------------------------------------------------------------------------------------------------------------------------------ 5843( 12) - 01/02/96 11:19 - [ 7] Visit: 03 Visit Requirements: BETWEEN 10-MAR-96 AND 31-MAR-96 On Hold Comments: Additional Comments: This visit is to obtain coma spectra for variability studies near perihelion. Schedule on the same day as Meech's 5835 Cycle 5 program of HST Chiron imaging if at all possible. Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 30 2060-CHIRON FOS/BL ACCUM 4.3 G270H 1 20M EXPAND MAXimum DURation 125% SEQuence 30-31 NON-INTerruptible ------------------------------------------------------------------------------------------------------------------------------------ 31 2060-CHIRON FOS/BL ACQ 4.3 MIRROR 1 360S SEQuence 30-31 NON-INTerruptible ------------------------------------------------------------------------------------------------------------------------------------ 5843( 12) - 01/02/96 11:19 - [ 8] Visit: 04 Visit Requirements: BETWEEN 10-APR-96 AND 10-MAY-96 On Hold Comments: Additional Comments: This visit is to obtain coma spectra for variability studies near perihelion. Schedule on the same day as Meech's 5835 Cycle 5 program of HST Chiron imaging if at all possible. Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 40 2060-CHIRON FOS/BL ACCUM 4.3 G270H 1 20M EXPAND MAXimum DURation 125% SEQuence 40-41 NON-INTerruptible ------------------------------------------------------------------------------------------------------------------------------------ 41 2060-CHIRON FOS/BL ACQ 4.3 MIRROR 1 360S MINimum DURation 80% SEQuence 40-41 NON- INTerruptible ------------------------------------------------------------------------------------------------------------------------------------ 5843( 12) - 01/02/96 11:19 - [ 9] Visit: 05 Visit Requirements: BEFORE 01-FEB-96 On Hold Comments: Additional Comments: This visit is to (i) obtain the rotationally-resolved nuclear spectrum, & (ii) to obtain the first coma spectrum Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 2060-CHIRON FOS/BL ACCUM 4.3 G270H 1 20M ------------------------------------------------------------------------------------------------------------------------------------ 2 2060-CHIRON FOS/BL ACQ 4.3 MIRROR 1 360S END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 3 2060-CHIRON FOS/BL ACCUM 4.3 G270H 1 15M ------------------------------------------------------------------------------------------------------------------------------------ 4 2060-CHIRON FOS/BL ACQ/PEA 4.3 MIRROR SEARCH-SIZE- 1 8S ONBOARD ACQuisition FOR 5 K X=1,SEARCH-SIZE -Y=3,SCAN-STEP- Y=1.204 ------------------------------------------------------------------------------------------------------------------------------------ 5 2060-CHIRON FOS/BL ACQ/PEA 1.0 MIRROR SEARCH-SIZE- 1 8S ONBOARD ACQuisition FOR 6 END ORBIT K X=6,SEARCH-SIZE -Y=2,SCAN-STEP- X=0.602,SCAN- STEP-Y=0.602 ------------------------------------------------------------------------------------------------------------------------------------ 6 2060-CHIRON FOS/BL ACQ/PEA 0.3 MIRROR SEARCH-SIZE- 1 17S ONBOARD ACQuisition FOR 7-11 K X=5,SEARCH-SIZE -Y=5,SCAN-STEP- X=0.172,SCAN- STEP-Y=0.172 ------------------------------------------------------------------------------------------------------------------------------------ 7 2060-CHIRON FOS/BL ACCUM 0.5 G270H 1 12M END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 8 2060-CHIRON FOS/BL ACCUM 0.5 G270H 1 15M ------------------------------------------------------------------------------------------------------------------------------------ 9 2060-CHIRON FOS/BL ACCUM 0.5 G270H 1 20M END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 10 2060-CHIRON FOS/BL ACCUM 0.5 G270H 1 15M ------------------------------------------------------------------------------------------------------------------------------------ 11 2060-CHIRON FOS/BL ACCUM 0.5 G270H 1 20M END ORBIT ------------------------------------------------------------------------------------------------------------------------------------ 5843( 12) - 01/02/96 11:19 - [ 10] Summary Form for Proposal 5843 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations FOS/BL ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ACQ ACQ/PEAK ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters SEARCH-SIZE-X=1 SEARCH-SIZE-Y=3 SCAN-STEP-Y=1.204 SCAN-STEP-Y=0.602 SCAN-STEP-Y=0.172 SCAN-STEP-X=0.602 SCAN-STEP-X=0.172 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category Solar System ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BETWEEN 01-MAY-95 AND 31-DEC-95 END ORBIT ONBOARD ACQuisition FOR 5 ONBOARD ACQuisition FOR 6 END ORBIT ONBOARD ACQuisition FOR 7-11 BETWEEN 01-FEB-96 AND 29-FEB-96 EXPAND MAXimum DURation 125% SEQuence 20-21 NON-INTerruptible MINimum DURation 80% SEQuence 20-21 NON-INTerruptible BETWEEN 10-MAR-96 AND 31-MAR-96 EXPAND MAXimum DURation 125% SEQuence 30-31 NON-INTerruptible BETWEEN 10-APR-96 AND 10-MAY-96 EXPAND MAXimum DURation 125% SEQuence 40-41 NON-INTerruptible MINimum DURation 80% SEQuence 40-41 NON-INTerruptible BEFORE 01-FEB-96 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G270H MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names 2060-CHIRON ------------------------------------------------------------------------------------------------------------------------------------