! Proposal 5842, submission 1 ! PI: Alan Stern ! Received Tue Mar 7 17:36:04 EST 1995 ! From: pi22@stsci.edu ! Hubble Space Telescope Cycle 5 (1995) Phase II Proposal Template ! $Id: 5842,v 11.1 1995/11/27 18:39:47 pepsa Exp $ ! ! Refer to the HST Phase II Proposal Instructions to fill this out ! ! Anything after a "!" is ignored, and may be deleted ! ! All keywords with multiple entries are comma delimited except the ! Visit_Requirements and Special_Requirements keywords which can be ! delimited with carriage returns or semi-colons, but not commas ! ! For help call your Program Coordinator: Andy Lubenow ! Phone: 410 338-4928 , E-mail: lubenow@stsci.edu ! ! This partially completed template was generated from a Phase I proposal. ! Name of Phase I Proposal: stern-330.prop ! Date generated: Fri Dec 16 15:15:29 EST 1994 ! Proposal_Information ! Section 4 Title: A Search for Ice at the Poles of Ceres Proposal_Category: GO Scientific_Category: Solar System Cycle: 5 Investigators PI_name: Alan Stern PI_Institution: Southwest Research Institute CoI_Name: Rick Binzel CoI_Institution: MIT Contact: ! Y or N (designate at most one contact) CoI_Name: Michel Festou CoI_Institution: Southwest Research Institute Contact: ! Y or N (designate at most one contact) CoI_Name: Brian Flynn CoI_Institution: Southwest Research Institute Contact: ! Y or N (designate at most one contact) CoI_Name: Larry Lebofsky CoI_Institution: University of Arizona Contact: ! Y or N (designate at most one contact) Abstract: ! Free format text (please update) One of the most intriguing suggestions about the largest asteroid, 1 Ceres, is that it may display water-ice on its surface. IR evidence for this hypothesis was presented over 12 years ago in the form of H_2O-ice absorption on Ceres' surface. Recently, however, King et al., have suggested the IR absorption attributed to water-ice may instead be due to NH_4- bearing minerals. IUE searches for OH emission resulting from the dissociation of water vapor above Ceres have yielded contradictory results. We propose to address this long-standing controversy using a new approach. We will obtain maps of Ceres in NUV, MUV, and FUV filters that together will discriminate for the presence or absence of significant quantities of surface ice. As a byproduct, these images will also provide the first-ever global maps of the albedo distribution on Ceres. Questions ! Free format text (please update) Observing_Description: \medskip We are proposing to map Ceres using the FOC/f96 chain in the near-UV (F342W), mid-UV (F275W+F278M), and far- UV (F152M+F175W). The selection of two in-series filters in the mid- and far-UV was motivated to effectively eliminate red leaks. We have tentatively selected the FOC 512x512 format, with a FOV of 7.2x7.2", 25x25Mum 0.028x0.028") square pixels, a 16 bit word length, and N_ max=0.12 Hz pixel^-1. We plan to use a standard, FOC Mode III (Blind Pointing) Acquisition, which is clearly capable of putting Ceres in this aperture. \medskip For the June 1995 visit, Ceres spans a diameter near 0.55", or many FOC/96 pixels. Although the FOC's imaging resolution is best in the UV, the OTA+COSTAR+FOC will obtain an effective resolution that is somewhat coarser than a pixel, or about 0.04" (FOC V5.0 Handbook, 1994). This corresponds to a linear resolution of ~70 km. It also means the images will be oversampled, which is advantageous for building S/N. This effective resolution will put ~12 resolution elements across Ceres' diameter, and ~240 resolution elements on the disk. \medskip Our nominal observation program involves obtaining images of two opposite hemispheres of Ceres at all three program wavelengths. S/N calculations reveal that 90 minutes of exposure time are required to obtain a S/N of 12 for an FOC resolution element with an albedo of 0.20 in F152M+F175W images (this is albedo one might see in an icy patch). By comparison, Ceres' mean visible albedo is 0.10; the lunar albedo at 1400 Angstrom\ is 0.05. Our S/N estimate assumes a standard UV solar spectrum (Mount & Rottman 1983), the instrument and filter parameters presented in the V5.0 FOC Handbook (1994), and a background count (i.e., noise) rate of 0.002 Hz pixel^-1. \medskip The longer-wavelength F275W+F278M and F342W images will be count rate (i.e., N max) limited, and therefore will require the use of neutral density filters. At N_ max=0.12 Hz, attaining S/N=25 per resolution element will require about 1000 seconds (17 minutes) each. We can obtain such well-exposed images of Ceres in both the F342W and F275W+F278M bands in a single orbit (since ~45 minutes of integration time are available on these orbits). However, three F152M+F175W images taken over 2 consecutive orbits will be required to obtain the desired far -UV S/N. Since 3 HST orbits take 4.8 hours and Ceres rotates in 9.1 hours, in some circumstances it may be necessary to hemispherically-average images of Ceres. However, small far-UV features with albedo~0.20 (twice the Ceres mean) can be detected in a single orbit at S/N=7 with F152M+F175W. The total, 4-orbit FOC program we have initially designed to cover a single hemisphere on Ceres is summarized as follows: orbits 1 & 3: 1 image each in F275W+F278M and F342W+F346M; orbits 2 & 4 2 images each orbit in F152M+F175W. 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. There are no supporting observations required for this program, however, we wish to note that Michel Festou will be leaving Observatoire Midi-Pyrenees for a year to be a visiting scientist at Southwest Research Institute in 1995- 1996. As such, he will be as full a participant as the other CoIs in the analysis of these HST data. THE MAGNTIUDE OF CERES VARIES AS IT MOVES AROUND THE SUN, PRIMARILY DUE TO HELIOCENTRIC AND GEOCENTRIC DISTANCE EFFECTS. THE SUB-EARTH LATITUDE IS ~30 DEG NORTH DURING VISIT 1, SO THE SOUTH POLE WILL NOT BE VISIBLE. A SECOND VISIT IN 96 IS REQUIRED FOR THAT OBJECTIVE, AND DD TIME HAS BEEN REQUESTED. THE ONLY TIME IN CY 5 WHEN BOTH POLES CAN BE SEEN WITH AN EQUATORIAL SUB-EARTH LAT IS DURING SOLAR CONJUNCTION. THE WHOLE VISIT 1 IMAGING SEQUENCE MUST GROUP WITHIN 6 HOURS. Calibration_Justification: ! Move appropriate text from Real_Time_Justification Additional_Comments: Solar_System_Targets ! Section 5.2 Target_Number: 1 Target_Name: 1-CERES Description: ASTEROID CERES, ASTEROID 1 Level_1: STD=1 (CERES) ! Satellite of Sun Level_2: ! Satellite of Level_1 Level_3: ! Satellite of Level_2 Window: Flux: V=8.7, B-V=+0.72 ! Include at least V and B-V Comments: ! This is a template for a single visit containing a single exposure ! Repeat exposure and visit blocks as needed Visits ! Section 6 Visit_Number: 1 Visit_Requirements: ! Section 7.1 ! Uncomment or copy visit level special requirements needed ! Most of these requirements (including ORIENT) will limit scheduling PCS MODE Fine ! GUIDing TOLerance ! ORIENTation TO ! ORIENTation TO FROM ! ORIENTation TO FROM NOMINAL ! SAME ORIENTation AS ! CVZ ! PARallel ! AFTER [BY [TO ]] ! AFTER ! BEFORE BETWEEN 08-JUN-95 AND 03-JUL-95 GROUP 1 WITHIN 6H ! PERIOD