Using Modules
By going with a modular design, the trips to the asteroids could be made with several extra modules which then could be left in place for future and extended missions. The geology of a single asteroid could be complex enough to spend entire lifetimes studying! And there are thousands of astroids to study!
If we get sick of studying a given asteroid, then we could pick-up the modules when the time is right and move them somewhere else! It could be something similair to Rauol's Moon Bus concept, but in some ways more limited, and in others more unlimited.
Scott Evil