SHAP4S version of the 67P/Churyumov-Gerasimenko nucleus shape model The SPG versions of the dataset represent the shape models of the nucleus of comet 67P/Churyumov-Gerasimenko as derived using stereo-photogrammetric methods. Images obtained with the Narrow Angle Camera of the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) instrument were used in constructing these models. The shape models presented here were developed at DLR (Berlin, Germany) by Frank Preusker and Frank Scholten using the stereo-photogrammetric (SPG) technique. Details about the SPG technique as well as details about the full reconstruction process of comet 67P/Churyumov-Gerasimenko from SPG techniques can be found in Preusker et al., 'Shape model, reference system definition, and cartographic mapping standards for comet 67P/Churyumov-Gerasimenko - Stereo-photogrammetric analysis of Rosetta/OSIRIS image data' A&A 583, A33, 2015 [PREUSKERETAL2015]. 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]. SHAP4S Version of the model --------------------------- The SHAP4S model represents the current version of the SPG shape models. It is developed from 218 OSIRIS/NAC images acquired between Aug 5 and Sep 3, 2014 (see below). During this time frame, the south pole region remained unilluminated, and so there are regions in the shape model that are unconstrained by observations. There are also a few small regions on the northern hemisphere that are not well constrained (e.g. some overhangs). These areas appear smooth in the model. The SHAP4S model was used to define the Cheops reference frame and the Local reference frames for the two lobes and the neck. It also revealed the 0.14 deg precession of the spin axis. [PREUSKERETAL2015] Model CG_DLR_SPG_SHAP4S_4M.WRL contains 4M facets, and has an average sampling distance of ~4 m and a vertical accuracy of <1 m. Different resolution versions have different sampling distances, scaling inversely with the number of facets. More detail regarding model resolution is provided in Preusker et al., 'Shape model, reference system definition, and cartographic mapping standards for comet 67P/Churyumov-Gerasimenko - Stereo-photogrammetric analysis of Rosetta/OSIRIS image data' A&A 583, A33, 2015 [PREUSKERETAL2015] TABLE: Parameters of images used to reconstruct the SPG SHAP4S model. UTC date of first NAC image: 2014 AUG 05 23:19:14.571 UTC date of last NAC image: 2014 SEP 03 06:44:22.578 Number of NAC images: 218 Lowest image resolution: 2.4 m/pixel Highest image resolution: 0.9 m/pixel Typical image resolution: ~1 m/pixel Reference Frame and Coordinate System --------------------------------------- The Cheops reference frame was defined using the SHAP4S shape model and has been adopted for the standard for all other formats of the 67P shape model. 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. Note that as the nucleus approached the Sun, cometary activity introduced changes in the nucleus' rotation, and a more complete description of the rotation state can be found in the Rosetta SPICE dataset (RO/RL-E/M/A/C-SPICE-6-V1.0). 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 ------------------- The SPG model is provided in multiple resolutions, allowing calculations to be optimized for a given problem when the highest resolution is not needed. The highest resolution model has 4M facets, with additional versions degraded to resolutions with 1M, 200k, 100k and 50k facets. The reduction is resolution was achieved using a quadratic edge-collapse decimation technique. Information contained in the filenames: CG_DLR_SPG_SHAP4S_200K.WRL ^ ^ ^ ^ ^ ^---- File format (VRML, SPICE DSK) | | | | |-------- Resolution (# triangular plates) | | | |-------------- Shape model generation SHAPXXX | | |------------------- Production Technique (SPG) | |------------------------ Site of model development |---------------------------- Comet C-G Details about the SPG SHAP4S files CG_DLR_SPG_SHAP4S_4M.WRL - 1999974 vertices forming 3999958 triang. plates CG_DLR_SPG_SHAP4S_1M.WRL - 499990 vertices forming 999992 triang. plates CG_DLR_SPG_SHAP4S_200K.WRL - 99996 vertices forming 199998 triang. plates CG_DLR_SPG_SHAP4S_100K.WRL - 49993 vertices forming 99990 triang. plates CG_DLR_SPG_SHAP4S_050K.WRL - 24997 vertices forming 49994 triang. plates TABLE: Shape Model Characteristics (for the SPG SHAP4S model) Surface Area: 48.1 km^2 Volume: 18.7 +/- 1.2 km^3 Mean diameter: 3.3 km (diameter of sphere of equivalent vol.) Axis orientation: RA: 69.54 +/- 0.35 deg Dec: +64.11 +/- 0.12 deg Precession cone angle: 0.14 deg Rotation Period: 12.4041 +/- 0.0001 hr (pre-perihelion) Precession period: 10.7 d