ISON or whatever is left of it IS NEVER EVER COMING BACK. IT DOESN’T HAVE AN ORBIT. It is HEADING OUT INTO INTERSTELLAR SPACE. Its trajectory cannot have changed, it is moving too fast, it sling-shot around the sun. ISON is/was moving at 151,088 mph. The only thing that could change its trajectory is a collision with another object going as fast. The trajectory is well established, it’s never coming back. ISON orbits nothing, unlike Halley’s comet which orbits our sun. There is nothing to worry about.
42 miles per second, that’s bookin’ along! ISON came in on a hyperbolic trajectory, which used to be considered diagnostic of having an origin outside the Solar System.