PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "T. Farnham; 29 Sept 2004 T. Farnham; 15 Nov 2005 14 Apr 2006 A.C.Raugh Changed DATA_SET_NAME " INSTRUMENT_HOST_NAME = "STARDUST" OBJECT = DATA_SET DATA_SET_ID = "SDU-C-NAVCAM-5-WILD2-SHAPE-MODEL-V2.0" OBJECT = DATA_SET_INFORMATION DATA_SET_NAME = "COLLECTED STARDUST/NAVCAM SHAPE MODELS OF 81P/WILD 2, V2.0" DATA_SET_COLLECTION_MEMBER_FLG = "N" DATA_SET_TERSE_DESC = "Basic tri-axial ellipsoid shape model and detailed plate shape model of comet 81P/Wild 2" START_TIME = "N/A" STOP_TIME = "N/A" DATA_SET_RELEASE_DATE = 2005-12-12 PRODUCER_FULL_NAME = {"Tony Farnham", "Tom Duxbury", "Jianyang Li"} DETAILED_CATALOG_FLAG = "N" DATA_OBJECT_TYPE = "TABLE" ABSTRACT_DESC = "Shape models of comet 81P/Wild 2, as derived from the Stardust Navcam images obtained around the time of closest approach to the comet. Includes both the basic tri-axial ellipsoid shape and the detailed plate model shape." ARCHIVE_STATUS = "LOCALLY ARCHIVED" CITATION_DESC = "Farnham, T.L., T. Duxbury and J.-Y. Li, SHAPE MODELS OF COMET WILD 2, SDU-C-NAVCAM-5-WILD2-SHAPE-MODEL-V2.0, NASA Planetary Data System, 2005." DATA_SET_DESC = " Data Set Overview ================= This data set presents the different shape models of comet 81P/Wild 2, as derived from the Stardust navigation camera images that were obtained around the time of closest approach to the comet. The first model is a basic tri-axial ellipsoid shape describing the axial ratios and their orientation. The second is a detailed plate shape model. Additional details and information about the ellipsoidal model can be found in Duxbury et al. J. Geophys. Res., Vol. 109, No. E12, E12S02 10.1029/2004JE002316 (2004) [DUXBURYETAL2004]. Additional information about the plate model can be found in Kirk et al., 36th Annual Lunar and Planetary Science Conference, March 14-18, 2005, #2244 (2005) [KIRKETAL2005]. On 2 January 2004, the STARDUST spacecraft flew past comet 81P/Wild 2 and took 72 images. In these images, the phase angle started at 70 deg during the approach, dropped to 3 deg near closest approach, and then increased to 110 deg during the departure. Most images were obtained within a range of 2,000 km, with the closest at 237 km. The camera took the images using a broadband filter and a CCD that had an angular resolution of 57 microrad/pixel. These images are archived in the PDS Stardust NAVCAM Data for comet Wild 2 (Data_Set_Name: STARDUST NAVCAM IMAGES OF WILD 2, Data_Set_ID: SDU-C-NAVCAM-2-EDR-WILD2-V1.0) TRI-AXIAL ELLIPSOID MODEL A tri-axial ellipsoid model was fit to the limb and terminator in all the Wild 2 nucleus images. The model was constrained by the fact that the viewing geometry changed by 180 degrees during the encounter and essentially the entire illuminated hemisphere of Wild 2 was viewed in these images (or ~50% of the surface, since there was no noticeable rotation during the few minutes that the images were taken). The axial dimensions determined from the visual fit of the model to the limb and terminator were 1.65 x 2.00 x 2.75 km +/-0.05 km (1 sigma). The illuminated limb is reasonably smooth, with the exception of a few large depressions. It is understood that half of the surface was not illuminated and that these unseen surfaces could deviate from the model derived from the illuminated regions. However the model fit to the terminator, even though quite rough due to topographic variations, does give some evidence that the model probably extrapolates to the unseen part of the nucleus to at least a km or better. A body-fixed coordinate system was established with the shortest axis assumed to be the rotation pole and the longest axis used to define the prime meridian. The posigrade axis is defined to match the corresponding poles determined by Sekanina (2003) [SEKANINA2003] and Farnham and Schleicher (2005) [FARNHAM&SCHLEICH2005] and is directed toward the position (J2000) RA=110 deg, dec=-13 deg (+/-3 deg). PLATE MODEL A detailed plate model was derived from the same set of images. Pairs of these images were used to generate stereo views if the comet and to determine the cartesian XYZ coordinates of points on the surface. Excel was then used to perform translations to convert the model to coordinates of latitude, longitude and elevation. Results are given in planetocentric coordinates. The model itself consists of 6432 points forming 12514 plates, covering the portions of the nucleus that were both illuminated and visible during the encounter (about 50% of the total surface). A best fitting triaxial ellipsoid was fit to the plate model using a least squares fit. This ellipsoid had the dimensions of 1.350 x 2.002 x 2.607 km (this is slightly different from the ellipsoidal model described above because the earlier model merely reproduced an envelope that enclosed the visible surface). Elevations relative to this best-fitting ellipsoid range from -600 m to +250 m, with an RMS range of heights of 93 m. The orientation of the minor axis (positive pole) of this ellipsoid is at a right ascension of 112 degrees and a declination of -17 degrees (J2000.0). Four versions of the plate model are included here. The primary version (WILD2_CART_VIS.TAB) is given in Cartesian coordinates based on a body-centered system. A second version (WILD2_PLAN_VIS.TAB) is given in planetocentric coordinates, with distances given from the center of the ellipsoid. In both of these versions, only the visible and illuminated portions of the nucleus are included. Two additional versions of the model (WILD2_CART_FULL.TAB and (WILD2_PLAN_FULL.TAB, respectively) again provide the shape models in Cartesian and planetocentric coordinates, but include full coverage of the surface of the nucleus. In these versions, the non-visible portions of the surface are represented by the best-fitting ellipsoid. For detailed analysis of the shape, routines such as the SURFACE and POLYSHADE routines in IDL can be used. To simply view the shape model, go to the browse directory, where the shape model has been converted to VRML format. There are two versions, one with the illumination that mimics that during the encounter, and one that uses a headlight to reveal the structure in a different manner. These models can be viewed using any standard VRML viewer. Author ------ The descriptions in this file were written by Tony Farnham. " CONFIDENCE_LEVEL_NOTE = " Confidence Level Overview ========================= A visual fit was used to match the ellipsoid model to the observations. Uncertainties were estimated by changing the parameters until an overlay of the model clearly deviated from the corresponding images. Estimated errors are 0.05 km in the axial dimensions, and 3 degrees in the angular measurements. The plate model has a horizontal resolution of approximately 50 meters and a vertical precision of about 6 meters. Review ====== The ellipsoid model was peer reviewed on Oct 15, 2004 and accepted pending a few minor corrections and additions, which have been applied. The plate model is sceduled for review on Dec 12, 2005. " END_OBJECT = DATA_SET_INFORMATION OBJECT = DATA_SET_MISSION MISSION_NAME = "STARDUST" END_OBJECT = DATA_SET_MISSION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "FARNHAM&SCHLEICH2005" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "DUXBURYETAL2004" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "SEKANINA2003" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "KIRKETAL2005" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_TARGET TARGET_NAME = "81P/WILD 2 (1978 A2)" END_OBJECT = DATA_SET_TARGET OBJECT = DATA_SET_HOST INSTRUMENT_HOST_ID = "SDU" INSTRUMENT_ID = "NAVCAM" END_OBJECT = DATA_SET_HOST END_OBJECT = DATA_SET END