I think the reason you are leaving is crucial here. If you are leaving for a "good" reason that is beyond your control, I doubt a reasonable person would hold it against you. And since you say you've already thought of everything and it is absolutely not possible for you to stay through your contract period, I am assuming its an emergency that is out of your control.
Good luck - if you've done all you can and just can't manage to stay for the full contract, I think you need to be in respectful and responsible communication with your supervisors, so that they can plan accordingly. If you do that, there shouldn't be any reason to hold anything against you next year.
^Everything. You know the situation there best, so use your judgement as to when to tell them. Realistically you should probably give them the minimum 2 weeks notice, but if I were you and the reason for leaving was reasonable I would tell them ASAP, and really emhasize that you hate to leave and can't wait to return, if possible.