All of those authors usually have to resort to the idea of "shipping lanes" so if the heroes are stranded between two planets eventually someone else will pass close to them on the way from one of those planets to the other one. This is wrong in a number of ways (first of all, they keep going anyway) but without that and without magically powerful fuels plots would be "they launched from Mars to Neptune, forget about them for the next three seasons, they'll be there at the beginning of the fourth one".