2.2. Dynamical models To model nongravitational perturbations we first considered the classical formulation by Marsden et al. (1973): where r is the heliocentric distance  m = 2 . 15  n = 5 . 093  k = 4 . 6142  r$_{0}$ = 2 . 808 au  and α is such that g (1 au) = 1. A$_{1}$   A$_{2}$   and A$_{3}$ are free parameters that give the nongravitational acceleration at 1 au in the radial-transverse- normal reference frame defined by ˆ r = r /r   ˆ t = ˆ n × ˆ r   ˆ n = r × v / | r × v |   where r and v are the heliocentric position and velocity of the comet. It is common practice to ignore the out-of-plane component  i.e.  A$_{3}$ = 0. As of 2014 October 3  it was clear that nongravitational perturbations were needed to fit the observed data. We computed two different orbital solutions: Table 1 shows the values of the nongravitational parameters for solutions 95 and 97  and Fig. 1 shows the corresponding b -plane predictions. Solution 95 corresponds to the more conservative approach where no assumptions are 11 • Solution 95  where A$_{1}$   A$_{2}$   and A$_{3}$ are all determined as part of the least squares orbital fit; • Solution 97  where A$_{1}$ and A$_{2}$ are determined from the orbital fit and A$_{3}$ is set to zero. a$_{NG}$ = g ( r )( A$_{1}$ ˆ r + A$_{2}$ ˆ t + A$_{3}$ ˆ n )   g ( r ) = α$^{(}$ r r$_{0}$$^{)}$$^{−}$$^{m}$$^{[}$ 1 +$^{(}$ r r$_{0}$$^{)}$$^{n}$$^{]}$$^{−}$$^{k}$ (1)