content_image/322625.jpg
the encounter (see Table 3). 

<img src='content_image/322625.jpg'>

To quantify the discrepancy between the pre-encounter solution and the post-encounter data, we measured how much extending the data arc after the encounter changes the orbital solution. As shown by the fourth row in Table 5, there is a statistically unacceptable > 10 σ correction from the pre-encounter best-fit solution to the full arc best-fit solution. 

## 3.3. Dynamical models 

The observed discrepancy points to the need of refining the nongravita- tional perturbation model. The Marsden et al. (1973) model does a good job at fitting the pre-encounter observations of C/2013 A1, actually none of the other models discussed below improves the fit to the pre-encounter arc. How- ever, this model proves inadequate as the trajectory is further constrained by post-encounter data. 

