The rotation state of 67P/Churyumov-Gerasimenko from approach observations with the OSIRIS cameras on Rosetta^⋆

¹ Institute of Planetary Research, DLR, Rutherfordstrasse 2, 12489 Berlin, Germany
e-mail: stefano.mottola@dlr.de
² Centre for Astrophysics and Planetary Science, School of Physical Sciences, The University of Kent, Canterbury, CT2 7NH, UK
³ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg, 3 37077 Göttingen, Germany
⁴ Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astro-physique de Marseille) UMR 7326, 13388 Marseille, France
⁵ Konkoly Observatory, MTA CSFK CSI, 1525 Budapest, Hungary
⁶ Department for Astronomy, University of Maryland, College Park, MD 20742-2421, USA
⁷ University of Padova, Department of Physics and Astronomy, vicolo dell’Osservatorio 3, 35122, Padova, Italy
⁸ LESIA, Obs. de Paris, CNRS, Univ Paris 06, Univ. Paris-Diderot, 5 place J. Janssen, 92195 Meudon, France
⁹ LATMOS, CNRS/UVSQ/IPSL, 11 boulevard d’Alembert, 78280 Guyancourt, France
¹⁰ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹¹ CNR-IFN UOS Padova LUXOR, via Trasea 7, 35131 Padova, Italy
¹² Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden
¹³ UNITN, Università di Trento, via Mesiano, 77, 38100 Trento, Italy
¹⁴ Department of Mechanical Engineering – University of Padova, via Venezia 1, 35131 Padova, Italy
¹⁵ INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
¹⁶ Instituto de Astrofí-sica de Andalucí-a – CSIC, 18080 Granada, Spain
¹⁷ Institute for Space Science, National Central University, 32054 Chung-Li, Taiwan
¹⁸ Institute for Geophysics and Extraterrestrial Physics, TU Braunschweig, 38106 Braunschweig, Germany
¹⁹ Research and Scientific Support Department, European Space Agency, 2201 Noordwijk, The Netherlands
²⁰ ESA/ESAC, PO Box 78, 28691 Villanueva de la Cañada, Spain
²¹ Dept. Physics, University of Padova, 35131 Padova, Italy
²² Institut für Datentechnik und Kommunikationsnetze, 38106 Braunschweig, Germany
²³ Department of Information Engineering – University of Padova, via Gradenigo 6, 35131 Padova, Italy
²⁴ Centro de Astrobiologia (INTA-CSIC), 28850 Torrejan de Ardoz, Madrid, Spain
²⁵ International Space Science Institute, 3012 Bern, Switzerland
²⁶ Instituto Nacional de Técnica Aeroespacial, 28850 Torrejon de Ardoz, Spain
²⁷ Physikalisches Institut, Sidlerstrasse 5, University of Bern, 3012 Bern, Switzerland
²⁸ University of Padova, CISAS, via Venezia 15, 35100 Padova, Italy

Received: 11 July 2014
Accepted: 21 August 2014

Abstract

Aims. Approach observations with the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) experiment onboard Rosetta are used to determine the rotation period, the direction of the spin axis, and the state of rotation of comet 67P’s nucleus.

Methods. Photometric time series of 67P have been acquired by OSIRIS since the post wake-up commissioning of the payload in March 2014. Fourier analysis and convex shape inversion methods have been applied to the Rosetta data as well to the available ground-based observations.

Results. Evidence is found that the rotation rate of 67P has significantly changed near the time of its 2009 perihelion passage, probably due to sublimation-induced torque. We find that the sidereal rotation periods P₁ = 12.76129 ± 0.00005 h and P₂ = 12.4043 ± 0.0007 h for the apparitions before and after the 2009 perihelion, respectively, provide the best fit to the observations. No signs of multiple periodicity are found in the light curves down to the noise level, which implies that the comet is presently in a simple rotation state around its axis of largest moment of inertia. We derive a prograde rotation model with spin vector J2000 ecliptic coordinates λ = 65° ± 15°, β = + 59° ± 15°, corresponding to equatorial coordinates RA = 22°, Dec = + 76°. However, we find that the mirror solution, also prograde, at λ = 275° ± 15°, β = + 50° ± 15° (or RA = 274°, Dec = + 27°), is also possible at the same confidence level, due to the intrinsic ambiguity of the photometric problem for observations performed close to the ecliptic plane.