In an intriguing paper in the 11 June Nature, Jacques Laskar and Mickael Gastineau of the Paris Observatory report on their large-scale computer simulation, which explored orbital interactions among the nine planets in our solar system. Over timescales of billions of years, the accumulation of slight variations in the orbits very occasionally led to planetary disaster. Of the 2501 scenarios they ran, for instance, Earth collided with Mars 29 times.
So, OK, that would be bad. But look at it this way: it wouldn't happen for at least 3 billion years, and it sure would shorten the travel time to Mars.
You'll need a subscription to view the full paper, but Sid Perkins has a good summary at Science News.