I am not saying that Dr. Astro is not correct but we must be cautious of anyone or any organization saying that the trajectory of this Comet is known with certainty.

Comet C/2013 A1 is a non-periodic Comet. Here is what Wikipedia say about non-periodic comets:

"Non-periodic comets (or long-period comets) are comets that do not have confirmed observations at more than one perihelion passage,[1] and thus generally have orbital periods of 200 years or more. They include single-apparition comets that pass through the Inner Solar System only once. They are on unstable near-parabolic orbits that will not return to the vicinity of the Sun for hundreds, thousands, or even millions of years – if ever."

So, if NASA knew that a comet was on a collision course with Earth in 2014, would they say anything? No, instead they would say that the Comet would hit Mars in 2014.