Figure 5: Out-of-plane nongravitational perturbation for three orbital solutions: 1) Mars- den et al. (1973) model, best fit to the full arc, 2) Marsden et al. (1973) model, best fit to the pre-encounter arc, and 3) RJM model with two jets and diurnal lag angle, best fit to the full arc. 3.4. Ephemerides As our best estimate of C/2013 A1’s trajectory we computed JPL solution 102 by using the two jets + θ model for nongravitational perturbations. Table 6 shows the corresponding orbital elements, RJM parameters, and close approach data along with their formal uncertainties. Solution 102 provides a good fit to the astrometric data, e.g., the square root of the reduced χ 2 is 0.29. In particular, Table 2 contains the HiRISE astrometry residuals with respect to solution 102, which are well within the uncertainties used to weight the data, 2 ′′ for last October 18 observation, 1 ′′ otherwise. The left column of Fig. 2 shows the RA and DEC residuals again solution 102 as a function of the observation epoch. Table 5 reports the b -plane coordinates ξ and ζ for solution 102, which are 27 2014 2014.5 2015 2015.5 −4 −3 −2 −1 0 1 2 3 x 10 −9 Time [yr] Out−of−plane nongravitational perturbation [au/d 2 ] A$_{1}$, A$_{2}$, A$_{3}$ full arc A$_{1}$, A$_{2}$, A$_{3}$ pre−enc 2 jets +  θ