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

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 P 1 = 12.76129 ± 0.00005 h and P 2 = 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• , corresponding to equatorial coordinates RA = 22• , Dec = +76• . However, we find that the mirror solution, also prograde, at λ = 275• ), 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.

The global shape, density and rotation of Comet 67P/Churyumov-Gerasimenko from preperihelion Rosetta/OSIRIS observations

“…It did produce a 3D model in short-axis mode of rotation fitting the whole set of LCs whose direction of the spin axis was compatible with other evidences. A similar solution was obtained by Mottola et al (2014) and three views are displayed in their Fig. 4.…”

“…These measurements were quite important as comet nuclei in general, and comet 67P/C-G in particular , are known to experience detectable changes of their rotational parameters, including the apparition of non-principal axis rotation. The new light curves analyzed by Mottola et al (2014) imply a reduction of the spin period from the 2009 perihelion passage from 12.76 h to 12.4043 ± 0.0007 h.…”

“…Observations of dust jets have been used by Vincent et al (2013) to retrieve a direction of the spin axis in close agreement with the prograde solution of Lamy et al (2007). Mottola et al (2014) used a set of light curves obtained by the OSIRIS instrument (see below) during the far approach phase with a very high signal-to-noise ratio to re-determine the rotational parameters of the comet. These measurements were quite important as comet nuclei in general, and comet 67P/C-G in particular , are known to experience detectable changes of their rotational parameters, including the apparition of non-principal axis rotation.…”

Time variability and heterogeneity in the coma of 67P/Churyumov-Gerasimenko

“…ROSINA/DFMS is a mass spectrometer that measures the in situ neutral and plasma coma composition at the position of the spacecraft [see Supplementary material]. During Rosetta's approach to 67P, ROSINA/DFMS measured the neutral coma composition with a time resolution (>10 measurements per rotation) much finer than the rotation period of the comet of ~12.4 hours (13). In August, the spacecraft scanned the comet at northern summer hemisphere (positive latitudes) from about 10º up to almost 90º (Coordinates: Cheops System (14)).…”

Rosetta: Making History!

“…The first touchdown was inside the predicted landing ellipse, confirmed using the lander's downwards-looking ROLIS descent camera in combination with the orbiter's OSIRIS images to match features [7,8].…”