Shape model, reference system definition, and cartographic mapping standards for comet 67P/Churyumov-Gerasimenko - Stereo-photogrammetric analysis of Rosetta/OSIRIS image data

Abstract

We analyzed more than 200 OSIRIS NAC images with a pixel scale of 0.9 - 2.4 m/pixel of comet 67P/Churyumov-Gerasimenko (67P/C-G) that have been acquired from onboard the Rosetta spacecraft in August/September 2014 using stereo-photogrammetric methods (SPG). We derived improved spacecraft position and pointing data for the OSIRIS images and a high-resolution shape model that consists of about 16 million facets (2 m horizontal sampling) and a typical vertical accuracy at the decimeter scale. From this model, we derive a volume for the northern hemisphere of 9.35 km^3 ± 0.1 km^3. With the assumption of a homogeneous density distribution and taking into account the current uncertainty of the position of the comet’s center-of-mass, we extrapolated this value to an overall volume of 18.7 km^3 ± 1.2 km^3, and, with a current best estimate of 1.0 × 10^13 kg for the mass, we derive a bulk density of 535 kg/m^3 ± 35 kg/m^3. Furthermore, we used SPG methods to analyze 67P/C-G’s rotational elements. The rotational period for August/September 2014 was determined to be 12.4041 ± 0.0004 h. For the orientation of the rotational axis (z-axis of the body-fixed reference frame) we derived a precession model with a half-cone angle of 0.14°, a cone center position at 69.54° /64.11° (RA/DEC J2000 equatorial coordinates), and a precession period of 10.7 days. For the definition of zero longitude (x-axis orientation), we finally selected the boulder-like Cheops feature on 67P/C-G’s big lobe and fixed its spherical coordinates to 142.35° right-hand-rule eastern longitude and -0.28° latitude. This completes the definition of the new “Cheops reference frame” for 67P/C-G. Finally, we defined cartographic mapping standards for common use and combined analysis of scientific results that have been obtained not only within the osiris team, but also within other groups of the rosetta mission.