(S (S (BASENP (DT The) (NN propulsion) (NN technology)) (VP (VBZ is) (VP (ADVP (RB currently)) (VP (VBN believed) (S (VP (TO to) (VP (VB be) (NP (BASENP (DT the) (ADJP (RBS most) (JJ efficient))) (ADJP (JJ available) (PP (IN for) (BASENP (JJ interstellar) (NN space) (NN travel)))))))))))) (PERIOD .))