We actually know a lot more about the asteroid belt today than when the Pioneers ventured forth in the early '70s. Although there are thousands of asteroids, they occupy a large space and are not close together--in fact, typical spacing between objects is about 5 million km. In 1991, David Morrison of NASA/Ames Research Center calculated that there is about 1 chance in 10 million billion that a randomly aimed spacecraft will collide with an asteroid that is 1 km or more in diameter. Planetary spacecraft are covered with thermal blanketing, which also helps dissipate the energy from any micrometeorites that might hit the spacecraft.