We propose to conduct the first HST imaging of Asteroid 2 Pallas with WFPC2-PC over 4 HST orbits. We will image the asteroid in five filters: F336w, F439w, F555w, F675w and F814w. We will utilize these observations to drastically improve the knowledge of Pallas' shape, spin pole position and surface properties, including roughness and albedo, parameters that are poorly determined by previous study. These observations will result in high signal-to-noise, high resolution surface maps from the visible to the UV. A satellite search will also be conducted for objects within the stability field of up to 21st magnitude, or about 900m in diameter. It is demonstrated in our proposal that significant scientific opportunity exists in Cycle 16 because Pallas is at both a low-phase, 3.9 degree opposition and near its closest approach to Earth, conditions that do not occur simultaneously in the next twenty years. This window represents the best chance to answer long standing, fundamental questions about Pallas, the main asteroid belt, and the formation of the solar system.

We will use the WFPC2 Planetary Camera (PC1) to observe 2 Pallas in five broad band filters--F439W (B), F555W (V), F675W (R), F814W (I), and F336W (UV)--through four orbits, over most of a single rotation period of 7.1832 hrs (JPL Horizons). PC1's pixel scale gives 6-7 resolution elements across Pallas' diameter, allowing for spatially-resolved images. We will acquire approximately global longitudinal coverage for surface mapping. Shape models will be produced using representative images from each perspective. Pole determination will follow from shape models and tracking the motion of surface features through Pallas' rotation. Filters were selected for their diagnostic capabilities with respect to mineralogy and hydration, primarily to sample the full range of Pallas' spectrum available to us with WFPC2. The B, V, R and I filters cover the visible portion of Pallas' spectrum. The UV filter is in Pallas' UV spectral drop off , and so provides us with an opportunity to constrain Pallas' spectral behavior in this previously unexplored region. Pallas' surface will be mapped in each filter to determine the nature and the spatial behavior of the asteroid's mineralogy and albedo. Longer exposures in the V band and coadded frames will be used to search for satellites up to 21st magnitude (R~900m). We seek to observe Pallas during opposition on September 4. At this time, Pallas reaches a minimum phase angle of 3.98 degrees. Pallas is also at an angular diameter 0.326 arcseconds, the maximum for this opposition. From September 1-9, the asteroid is visible at phase angles below 4.5 degrees. Before August 9 and after September 30, the phase increases to 10 degrees, resulting in the loss of half a resolution element across the asteroid's surface. This comprimises both the spatial resolution of our images and the accuracy of the shape models. Ideally, we will be scheduled on or just after September 4. SOLAR SYSTEM 12 false 0.0 false false false false false false SUPPORTED SOLAR SYSTEM ASTEROID 2 Pallas, B-type asteroid TYPE=ASTEROID,A=2.77166846462974,E=0.23075882817256,I=34.841828581513,O=173.191804366333,W=310.3448192659582,M=199.72616,EQUINOX=J2000,EPOCH=10-APR-2007:00:00:00 SOLAR SYSTEM ASTEROID 2 Pallas, B-type asteroid TYPE=ASTEROID,A=2.77166846462974,E=0.23075882817256,I=34.841828581513,O=173.1358033133696,W=310.3448192659582,M=199.72616,EQUINOX=J2000,EPOCH=10-APR-2007:00:00:00 WFPC2 Imaging No F336W WFPC2 Imaging No F439W WFPC2 Imaging No F555W WFPC2 Imaging No F675W WFPC2 Imaging No F814W It is important that this visit be scheduled as close to the opposition of Pallas as possible in order to illuminate the greatest fraction of the asteroid as well as provide the best spatial resolution. Pallas is at minimum phase 3.98 on 2007-Sep-04 00:00 UT. Pallas is below 4.5 degrees from Aug 29-Sept 9, however, its largest angular size of 0.326 arcseconds occurs from Sept 4-9. Ideally, we will be scheduled on or just after Sept 4. (Tic-Visit :proposal-id 11115 :visit-id "01" :proposal-version -1 :trans-version "20070326" :date "21-Jun-2007 16:26:42.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 4 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 3047488 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11115 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11115 :visit-id "01" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2675 :al-overhead-at-end 94 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11115 :visit-id "01" :orbit-number 2 :pcs-time 355 :si-vis-utilization 2675 :al-overhead-at-end 94 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11115 :visit-id "01" :orbit-number 3 :pcs-time 355 :si-vis-utilization 2675 :al-overhead-at-end 94 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11115 :visit-id "01" :orbit-number 4 :pcs-time 355 :si-vis-utilization 2649 :al-overhead-at-end 120 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) false 4 0 It is important that this visit be scheduled as close to the opposition of Pallas as possible in order to illuminate the greatest fraction of the asteroid as well as provide the best spatial resolution. Pallas is at minimum phase 3.98 on 2007-Sep-04 00:00 UT. Pallas is below 4.5 degrees from Aug 29-Sept 9, however, its largest angular size of 0.326 arcseconds occurs from Sept 4-9. Ideally, we will be scheduled on or just after Sept 4. (Tic-Visit :proposal-id 11115 :visit-id "01" :proposal-version -1 :trans-version "20070326" :date "21-Jun-2007 16:26:42.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 4 :saa-model "26" :si-bright-earth-limb-angle 20.0 :data-volume 3047488 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11115 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11115 :visit-id "01" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2675 :al-overhead-at-end 94 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11115 :visit-id "01" :orbit-number 2 :pcs-time 355 :si-vis-utilization 2675 :al-overhead-at-end 94 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11115 :visit-id "01" :orbit-number 3 :pcs-time 355 :si-vis-utilization 2675 :al-overhead-at-end 94 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) (Tic-Orbit :proposal-id 11115 :visit-id "01" :orbit-number 4 :pcs-time 355 :si-vis-utilization 2649 :al-overhead-at-end 120 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3132 :pcs-gap 0 ) false 4 0