Results: No need to duck, asteroid won't hit Earth

NASA says an 150-foot wide asteroid will come 17,100 miles away from Earth when it zips by next Friday. They state no need to worry because it won't hit Earth. It will be the nearest known flyby for an object of this size. Did you hear about this?

Do you ever wonder how they can say with certainty it won't hit?

Do you ever wonder if they'll miss something or that they cannot know every circumstance regarding if two objects in space somehow hit each other causing a chain reaction they can't foresee that could eventually hit an asteroid they predict won't hit Earth, thereby putting the asteroid on a path that will hit Earth?