One of those "doom and gloom" shows on the History Channel last weekend covered the topic of near earth objects. One of the "scientists" stated that on any given day we have a 1 in 20,000 chance of being hit by an object large enough to cause catastrophe on Earth. This one seems minuscule by comparison....
That number is what the error in their calculations would have to be for it to hit the earth. Or in other words, their calculations would have to be more accurate than that in order to know it wouldn't hit the earth. That's a strikingly small error to have to calculate things to. So small that it leaves a lot of doubt that it is calculated that accurately.