Asteroid orbits can change quite a bit over time, because they are so close to the planet Jupiter that Jupiter perturbs their orbits very frequently. Also, as the changing tugs and pulls from the other planets do their relentless work, asteroid orbits are constantly being resculpted over the decades. So far, about 4000+ asteroids and other 'minor bodies' have been identified and their orbits determined. These can be put into a computer and, using nothing else than Newton's Law of Gravity, astronomers can make predictions for where asteroids will be up to 100 years in the future. Even some simple home digital planetaria have asteroid orbit simulators for hundreds of the largest ones known. The accuracy of these simple programs is quite inferior to professional programs, but they give you a good idea of how asteroids move 'as a cloud'.