Complex sentence: The comet will pass extremely close to Mars on 19 October 2014 with a small possibility of an impact event on the planet The observation arc of 171 days, the nominal pass is from the center-point of Mars.
Split into simpler sentences: The comet will pass extremely close to Mars on 19 October 2014; there is even a small possibility that it could impact the planet. With an observation arc of 171 days, the nominal pass is from the center-point of Mars.