accuracy of the model, Chesley and Yeomans (2005) already used the RJM to quite successfully estimate the spin pole of 67P/Churyumov-Gerasimenko: the difference between prediction and the actual spin pole orientation (Sierks et al., 2015) is only 13 $^{◦}$. The analysis of the images acquired during the close encounter will possibly provide an independent estimate of the spin pole to be compared to that obtained from the orbital dynamics. Future work will be related to further refinements of the nongravitational perturbation. For instance, the standard g ( r ) function used by the Marsden et al. (1973) model and the RJM is driven by the sublimation rate of water ice as a function of heliocentric distance. Directly estimating the H$_{2}$O and CO$_{2}$ production rates of C/2013 A1 and future astrometric observations could allow one to derive a more C/2013 A1 specific g ( r ). The authors are grateful D. Bodewits, L. Buzzi, P. Camilleri, D. Herald, K. R. Hills, E. I. Kardasis, J.-L. Li, A. Maury, M. J. Mattiazzo, J. Oey, J.-F. Soulier, D. Storey, H. B. Throop, and H. Williams for sharing their astrometric images. We thank M. S. P. Kelley and C. M. Lisse for supporting our request of astrometric images. Part of this research was conducted at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. Copyright 2015 California Institute of Technology. Acknowledgments 32