Question

How do the spacecraft we send past mars avoid the asteroid belt? Are they so spread out that spacecraft is unlikely to be hit or do they have to guide it manually away from asteroids that are close?

The real asteroid belt is nothing like the depictions you see in movies or on TV. Although there are millions of asteroids, they are spread out over a vast distance. You could make many trips across the asteroid belt without even seeing one, let alone having a close encounter. In fact, it has taken considerable effort when sending a spacecraft to the outer planets to find any asteroids within reach; usually we can manage only one asteroid flyby. Before we had sent any spacecraft we were worried about the effects of dust that might be present in the asteroid belt, but Pioneer 10 and 11 showed that dust is not a problem either. One reason these two spacecraft were named Pioneer is that they pioneered the route across the asteroid belt to the outer planets.

David Morrison
NAI Senior Scientist
October 26, 2009