  Comet C/2013 A1 (siding Spring) will experience a high velocity encounter
with Mars on October 19, 2014 at a distance of 135,000 km +- 5000 km from the
planet center. We present a comprehensive analysis of the trajectory of both
the comet nucleus and the dust tail. The nucleus of C/2013 A1 cannot impact on
Mars even in the case of unexpectedly large nongravitational perturbations.
Furthermore, we compute the required ejection velocities for the dust grains of
the tail to reach Mars as a function of particle radius and density and
heliocentric distance of the ejection. A comparison between our results and the
most current modeling of the ejection velocities suggests that impacts are
possible only for millimeter to centimeter size particles released more than 13
au from the Sun. However, this level of cometary activity that far from the Sun
is considered extremely unlikely. The arrival time of these particles spans a
20-minute time interval centered at October 19, 2014 at 20:09 TDB, i.e., around
the time that Mars crosses the orbital plane of C/2013 A1. Ejection velocities
larger than currently estimated by a factor >2 would allow impacts for smaller
particles ejected as close as 3 au from the Sun. These particles would reach
Mars from 43 to 130 min after the nominal close approach epoch of the purely
gravitational trajectory of the nucleus.
