SHAP8 version of the 67P/Churyumov-Gerasimenko nucleus shape model ================================================================== The SPC versions of the dataset represent the shape models of the nucleus of comet 67P/Churyumov-Gerasimenko as derived using photoclinometry techniques. Images obtained with the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) instrument (including both the Narrow Angle Camera and the Wide Angle Camera) were used in constructing these models The shape models presented here were developed by Robert Gaskell (PSI, Tucson, USA) with support from Laurent Jorda (LAM, Marseille, France) from the Stereophotoclinometry (SPC) technique. Details about the SPC technique can be found in Gaskell et al., 'Characterizing and navigating small bodies with imaging data', Meteoritics and Planet. Sci. 43, 1049-1061, 2008 [GASKELLETAL2008]. Details about the full reconstruction process of comet 67P/Churyumov-Gerasimenko from clinometry techniques can be found in Jorda et al., 'The Global Shape, Density and Rotation of Comet 67P/Churyumov-Gerasimenko from Pre--Perihelion Rosetta/OSIRIS Observations', Icarus 277, 257-278, 2016 [JORDAETAL2016]. Additional information about the nucleus of 67P can be found in Sierks et al., 'On the nucleus structure and activity of comet 67P/Churyumov-Gerasimenko', Science 347, aaa1044, 2014 [SIERKSETAL2014]. SHAP8 Version of the model --------------------------- The SHAP8 SPC model represents a much more advanced model compared to the SHAP2 and SHAP5 models for the following reasons. a) It is based on a large data set of 20679 OSIRIS/NAC images and 6072 OSIRIS/WAC images acquired between July 11, 2014 and September 30, 2016 (see below). b) The model covers the whole surface of the nucleus at high-resolution, including the south pole region which was not imaged at the time the SHAP2 was reconstructed. The post-perihelion mapping images were not included in the reconstruction of the SHAP5 model. c) The SPC shape reconstruction has been performed directly in the Cheops frame rather than in an intermediate SPC frame. The SHAP8 model is thus aligned in this frame with a higher accuracy compared to the SHAP5 model. The model is based on a tiling of the nucleus surface with 12734 "maplets" with a sampling of 1 m and 2161 "maplets" with a resolution of 2.5 m. In the SPC technique, a "maplet" is a 99x99 surface elements digital elevation model. The 2.5 m "maplets" covers the whole nucleus surface while the 1 m "maplets" covers 97 % of the nucleus surface. The images used for the reconstruction of the model were acquired during several OSIRIS observational phases including: the initial mapping phases (SHAP2, SHAP4, SHAP4S, SHAP5, SHAP6) in 2014, the post-perihelion mapping phases (SHAP7 and SHAP8) and most nucleus observation sequences until the end of the mission in September 2016. TABLE: Parameters of all images used to reconstruct the SHAP5 SPC model. UTC date of first NAC image: 2014 JUL 11 09:22:05.118 UTC date of last NAC image: 2016 SEP 30 10:22:45.343 UTC date of first WAC image: 2014 AUG 05 19:45:07.958 UTC date of last WAC image: 2016 SEP 30 10:39:10.010 Number of NAC images: 20679 Number of WAC images: 6072 Minimum number of images per maplet: 6 Maximum number of images per maplet: 2048 Average number of images per maplet: 251 Lowest image resolution: 5.6 m/pixel Highest image resolution: 0.016 m/pixel Average image resolution: 4.2 m/pixel Reference Frame and Coordinate System --------------------------------------- The spin axis orientation was derived from a stereophotogrammetric analysis of OSIRIS images acquired in between August and early September 2014. The orientation of the models in the J2000 Equatorial frame (EME2000) is described in Scholten, F. et al., 'Reference Frames and Mapping Schemes of Comet 67P/C-G' in the PDF document CHEOPS_REF_FRAME_V1.PDF in the DOCUMENTS directory. The SPC technique led to a spin axis such that the North (positive) pole points to a right ascension of 69.4+/-0.2 deg and a declination of +64.1+/-0.2 deg (J2000), compatible with the more accurate definition given in the above document. The body's coordinate system was defined with the +Z axis in the direction of the spin axis and the prime meridian (+X axis) is defined such that the center of the large boulder named Cheops is at a longitude +142.35 degrees, following the IAU definition presented in the document CHEOPS_REF_FRAME_V1.PDF. The +Y axis completes the right-hand coordinate system. The body center is not exactly coincident with its center of gravity, but the offset is within the uncertainties derived for the surface positions. Data Formats -------------- The models are presented in the standard PDSSBN vertex/triangular plate format (see PDSSBN_PLATE_SHAPE_DEF.ASC in the documents directory) with dimensions of km in cartesian coordinates. The files are presented with VRML wrappers that allow the model to be displayed with existing VRML viewers that are freely available (e.g., INSTANT PLAYER, OCTAGA, CORTONA, etc.). In addition to the PDS formatted files, the models have also been converted to DSK kernels that can be used with the SPICE utilities. For the complex shape of 67P, spherical coordinates (lat/long/radius) result in multiple values in some regions of the nucleus, so the model is not presented in this form. Model Resolutions ------------------- Two different SPC-derived models are included, known as SPC and MSPCD. Each of these models is presented in its highest resolution form, with additional versions degraded in resolution (in steps of ~2). The different resolutions are included to allow calculations to be optimized for a given problem, when the highest resolution is not needed. Additional information on how the high-resolution shape model was degraded into lower resolution versions can be found in H. Hoppe, 'Progressive meshes', Conference SIGGRAPH 96; 23rd International Conference on Computer Graphics and Interactive Techniques. New Orleans, LA, USA — August 04 - 09, 1996, pp. 99-108. DOI: 10.1145/237170.237216 Information contained in the filenames: CG_SPC_SHAP8_006K_CART.WRL ^ ^ ^ ^ ^ ^---- File format (VRML, SPICE DSK) | | | | |-------- Cartesian coordinates | | | |-------------- Resolution (# triangular plates) | | |------------------- Shape model generation (SHAP2, SHAP5 or SHAP8) | |------------------------ Production Technique (SPC, SPCQMAPS or MSPCD) |---------------------------- Comet C-G Details about the SPC SHAP8 files CG_SPC_SHAP8_003K_CART.WRL - 1467 vertices forming 2926 triang. plates CG_SPC_SHAP8_006K_CART.WRL - 2994 vertices forming 5978 triang. plates CG_SPC_SHAP8_012K_CART.WRL - 5805 vertices forming 11596 triang. plates CG_SPC_SHAP8_024K_CART.WRL - 11953 vertices forming 23902 triang. plates CG_SPC_SHAP8_049K_CART.WRL - 24557 vertices forming 49107 triang. plates CG_SPC_SHAP8_096K_CART.WRL - 47803 vertices forming 95602 triang. plates CG_SPC_SHAP8_400K_CART.WRL - 200035 vertices forming 400022 triang. plates CG_SPC_SHAP8_786K_CART.WRL - 393048 vertices forming 785824 triang. plates CG_SPC_SHAP8_002M_CART.WRL - 779133 vertices forming 1557796 triang. plates CG_SPC_SHAP8_003M_CART.WRL - 1571389 vertices forming 3142629 triang. plates TABLE: Shape Model Characteristics (for the SPC SHAP8 model) Surface Area: 49.5 km^2 Volume: 18.7 +/- 0.3 km^3 Mean diameter: 3.48 +/- 0.01 km (diameter of sphere of equivalent vol.) Dimensions along the principal axes of inertia: A: (4.35 +/- 0.02) km B: (2.60 +/- 0.02) km C: (2.09 +/- 0.06) km Axis orientation: RA: 69.54 deg (Cheops frame definition) Dec: +64.11 deg (Cheops frame definition) Rotation Period: 12.4041 +/- 0.0001 hr (pre-perihelion)