So, throwing a spacecraft at the Asteroid Belt, with the asteroids on average being a million kilometres apart, your chances of hitting one are not exactly high, which is why the spacecraft we have sent out that way got through unscathed. However, what is an asteroid? From what I can gather they are the objects we can see through our telescopes and have counted. Beyond that the number is estimated and estimations vary widely. I also have to wonder what we can see. I’m guessing that objects the size of a football or a pea aren’t picked up. The chances of something the size of the Pioneer or Voyager craft hitting one of these was probably negligible, but they’d certainly have to be taken into account if you’re presenting a profile 5 kilometres across and travelling at 20,000 kilometres per hour.