Travelling Between Extrasolar Planets

If in the future humans will reach other stars, or better extrasolar planets, they will face the problem of travelling between the new planets. Many planetary systems are much more complex than the solar system, particularly in case their star is double or multiple, and there is a variety of possible cases. In most cases the restricted two body assumption cannot be used and even in case of impulsive propulsion the trajectories cannot be obtained in closed form. The problem of finding the impulsive trajectory can be reduced to the solution of a boundary value problem, as is the case for continuous thrust solved through indirect methods. A few examples for an hypothetical extrasolar system are dealt with, with the only aim of discussing possible way to tackle the problem of traveling between exoplanets.