SHAP5 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].
SHAP5 Version of the model
---------------------------
The SHAP5 SPC model represents a much more advanced model compared to the
SHAP2 models for the following reasons.
a) It is based on a large data set of 16182 OSIRIS/NAC images and 5138
OSIRIS/WAC images acquired between July 11, 2014 and February 16, 2016 (see
below).
b) The model covers the whole surface of the nucleus, including the south pole
region which was not imaged at the time the SHAP2 models were reconstructed.
c) The model has a global average sampling of about 6 m, with a typical
vertical resolution of 2-3 m.
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, from the initial mapping phases (SHAP2,
SHAP4, SHAP4S and SHAP5) in 2014 up to those obtained in Feb 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 FEB 13 19:09:27.158
UTC date of first WAC image: 2014 AUG 05 00:20:43.281
UTC date of last WAC image: 2016 FEB 16 22:35:12.636
Number of NAC images: 16182
Number of WAC images: 5138
Minimum number of images per maplet: 7
Maximum number of images per maplet: 567
Average number of images per maplet: 161
Distance to surface: 5.8-287 km(*)
Lowest image resolution: 5.4 m/pixel(*)
Highest image resolution: 0.11 m/pixel(*)
Average image resolution: 1.6 m/pixel(*)
(*)Data at the center of the 1 m sampling "maplets".
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.
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
-------------------
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_SHAP5_006K_CART.WRL
^ ^ ^ ^ ^ ^---- File format (VRML, SPICE DSK)
| | | | |-------- Cartesian coordinates
| | | |-------------- Resolution (# triangular plates)
| | |------------------- Shape model generation (SHAP2 or SHAP5)
| |------------------------ Production Technique (SPC or MPSCD)
|---------------------------- Comet C-G
Details about the SPC SHAP5 files
CG_SPC_SHAP5_006K_CART.WRL - 2842 vertices forming 5678 triang. plates
CG_SPC_SHAP5_012K_CART.WRL - 5861 vertices forming 11716 triang. plates
CG_SPC_SHAP5_024K_CART.WRL - 12070 vertices forming 24134 triang. plates
CG_SPC_SHAP5_050K_CART.WRL - 24877 vertices forming 49748 triang. plates
CG_SPC_SHAP5_097K_CART.WRL - 48420 vertices forming 96834 triang. plates
CG_SPC_SHAP5_199K_CART.WRL - 99729 vertices forming 199452 triang. plates
CG_SPC_SHAP5_384K_CART.WRL - 191997 vertices forming 383988 triang. plates
CG_SPC_SHAP5_788K_CART.WRL - 393963 vertices forming 787920 triang. plates
CG_SPC_SHAP5_001M_CART.WRL - 597251 vertices forming 1194496 triang. plates
CG_SPC_SHAP5_003M_CART.WRL - 1572866 vertices forming 3145728 triang. plates
TABLE: Shape Model Characteristics (for the SPC SHAP5 model)
Surface Area: 48.4 km^2
Volume: 18.8 +/- 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: (4.34 +/- 0.02) km
B: (2.60 +/- 0.02) km
C: (2.12 +/- 0.06) km
Axis orientation: RA: 69.57 +/- 0.35 deg
Dec: +64.01 +/- 0.12 deg
Rotation Period: 12.4041 +/- 0.0001 hr (pre-perihelion)