Just because the universe is going to be distroyed doesent mean that other races can't be saved and that the rapture can't happen symaltaneously on other plannets, and the rapture wont happen untill every nation has had a chance to hear the gospel, a nation is a group of people, if aliens exiist they wopuld be a nation or a collection of nations, prehaps it is our duty to reach out to them? (Prehaps missionary journeys into outer space sounds farfetched, but its not impossible and the bible doesent say anything against it does it?)