Looks like the pass of the comet Siding Spring by Mars on October 19th will not in fact bring along a dust cloud of fast moving particles thick enough to damage the spacecraft orbiting the planet:

By the middle of this summer, published estimates (based on new images and additional modelling) were indicating a flux of around 10-6 particles per m2, which, for Mars Express, very roughly equates to a 1-in-300,000 chance of being hit. It’s starting to look like our comet C/2013 A1/Siding Spring will manifest itself as a more friendly passer-by than initially thought and that it won’t be hurling clouds of large particles at unthinkable speeds towards Mars and its man-made satellites.

More at

An image of the comet Siding Spring

Comet C/2013 A1 Siding Spring seen on 6 September 2014 from Argentina. Image credit: César Nicolás Fornari https://www.facebook.com/cesar.fornari

Comet C/2013 A1 Siding Spring seen on 6 September 2014 from Argentina.
Image credit: César Nicolás Fornari https://www.facebook.com/cesar.fornari

A diagram illustrating the relative closeness of the pass of comet Siding Spring to Mars vs going by the Earth:

Closest approach: If Mars were Earth. Credit: NASA/JPL

Closest approach: If Mars were Earth. Credit: NASA/JPL