Siding Spring and Mars

[james_davis_nicoll]

Our study of the comet and models of the dust environment led to some very interesting conclusions:

Even with this close approach, very little, if any, dust from the comet is likely to hit Mars.
Any dust that does hit Mars will be in the form of relatively large particles, a few millimeters in size.
The highest risk of impact is not at the time the comet is closest to Mars, but instead occurs around 100 minutes after close approach.
Any dust grains that are going to hit Mars would have been ejected from the comet over a year ago.

As I noted, these results are fascinating and counterintuitive. How can the comet get so close to Mars, with so few impacts? Why is it that only grains of a particular size can reach Mars, and any impacts will happen, not around close approach, but over an hour later when the comet is more than twice as far away? And how is it that any impacting dust had to have left the nucleus so long ago, while the comet's activity throughout 2014 is irrelevant to the encounter?