Possible interpretation of the precession of comet 67P/Churyumov-Gerasimenko

¹ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n., 18080 Granada, Spain
² Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
³ Planetary Science Institute, Tucson, Arizona 85719, USA
⁴ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
⁵ German Aerospace Center (DLR), Institut für Planetenforschung, Rutherfordstraße 2, 12489 Berlin, Germany
⁶ Institut für Geophysik und extraterrestrische Physik (IGEP), Technische Universität Braunschweig, Mendelssohnstraße 3, 38106 Braunschweig, Germany
⁷ IMCCE/Observatoire de Paris, 61 avenue de l’Observatoire, 75014 Paris, France
⁸ Centro de Astrobiología (INTA-CSIC), European Space Agency (ESA), European Space Astronomy Centre (ESAC), PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁹ International Space Science Institute, Hallerstraße 6, 3012 Bern, Switzerland
¹⁰ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
¹¹ University of Padova, Department of Physics and Astronomy, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹² Scientific Support Office, European Space Research and Technology Centre/ESA, Keplerlaan 1, Postbus 299, 2201 AZ Noordwijk ZH, The Netherlands
¹³ PAS Space Research Center, Bartycka 18A, 00716 Warszawa, Poland
¹⁴ University of Maryland, Department of Astronomy, College Park, MD 20742-2421, USA
¹⁵ LESIA-Observatoire de Paris, CNRS, Université Pierre et Marie Curie, Université Paris Diderot, 5 place J. Janssen, 92195 Meudon Principal Cedex France
¹⁶ LATMOS, CNRS/UVSQ/IPSL, 11 boulevard d’Alembert, 78280 Guyancourt, France
¹⁷ Centro di Ateneo di Studi ed Attivitá Spaziali “Giuseppe Colombo” (CISAS), University of Padova, via Venezia 15, 35131 Padova, Italy
¹⁸ INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁹ CNR-IFN UOS Padova LUXOR, Via Trasea, 7, 35131 Padova, Italy
²⁰ Department of Information Engineering, University of Padova, via Gradenigo, 6/B, 35131 Padova, Italy
²¹ University of Trento, via Sommarive, 9, 38123 Trento, Italy
²² Physikalisches Institut der Universität Bern, Sidlerstr. 5, 3012 Bern, Switzerland
²³ INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34014 Trieste, Italy
²⁴ Graduate Institute of Astronomy, National Central University, 300 Chung-Da Rd, 32054 Chung-Li, Taiwan
²⁵ Operations Department, European Space Astronomy Centre/ESA, PO Box 78, 28691 Villanueva de la Cañada (Madrid), Spain
²⁶ Solar System Exploration Research Virtual Institute, Southwest Research Institute, 1050 Walnut St., Suite 300, Boulder, CO 80302, USA
²⁷ Instituto Nacional de Tecnica Aeroespacial, Carretera de Ajalvir, p.k. 4, 28850 Torrejon de Ardoz, Madrid, Spain
²⁸ Center for Space and Habitability, University of Bern, 3012 Bern, Switzerland
²⁹ MTA CSFK Konkoly Observatory, 1121 Budapest, Hungary

Received: 22 December 2015
Accepted: 15 March 2016

Abstract

Context. Data derived from the reconstruction of the nucleus shape of comet 67P/Churyumov-Gerasimenko (67P) from images of the OSIRIS camera onboard ROSETTA show evidence that the nucleus rotates in complex mode. First, the orientation of the spin axis is not fixed in an inertial reference frame, which suggests a precessing motion around the angular momentum vector with a periodicity of approximately 257 h ± 12 h.Second, periodograms of the right ascension and declination (RA/Dec) coordinates of the body-frame Z axis show a very significant (higher than 99.99%) periodicity at 276 h ± 12 h, different from the rotational period of 12.40 h as previously determined from light-curve analysis.

Aims. The main goal is to interpret the data and associated periodicities of the spin axis orientation in space.

Methods. We analyzed the spin axis orientation in space and associated periodicities and compared them with solutions of Euler equations under the assumption that the body rotates in torque-free conditions. Statistical tests comparing the observationally derived spin axis orientation with the outcome from simulations were applied to determine the most likely inertia moments, excitation level, and periods.

Results. Under the assumption that the body is solid-rigid and rotates in torque-free conditions, the most likely interpretation is that 67P is spinning around the principal axis with the highest inertia moment with a period of about 13 h. At the same time, the comet precesses around the angular momentum vector with a period of about 6.35 h. While the rotating period of such a body would be about 12.4 h, RA/Dec coordinates of the spin axis would have a periodicity of about 270 h as a result of the combination of the two aforementioned motions.

Conclusions. The most direct and simple interpretation of the complex rotation of 67P requires a ratio of inertia moments significantly higher than that of a homogeneous body.