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## 3.4. Ephemerides 

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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 

