Comet C/2013 A1 Siding Spring. How treatment of data and NG effects can   change our predictions about close encounters with Mars ?
  We show that the estimates of close encounter of this comet with Mars depend on data treatment. Using the data taken in the two-year period, we derived that the comet will miss Mars on 2014 October 19 at the distance of about $140\,150\pm 55$ km or $140\,300\pm 45$ km from its center, depending on the method of data processing in the purely gravitational model of motion (based on non-weighted data or weighted data, respectively). Unfortunately, the non-gravitational model of motion is still very uncertain, thus we can only speculate about estimates of expected distances for non-gravitational orbital solutions. However, we did not obtain a significant differences in close encounter prediction between the non-gravitational solutions and the gravitational ones. 
Pawe{\l} Wajer and Ma{\l}gorzata Kr\'olikowska
physics.space-ph astro-ph.EP