(S (S (BASENP (NNP NASA)) (VP (VBZ has) (VP (VBN held) (NP (BASENP (DT a) (JJ memorial) (NN ceremony)) (SBAR (S (VP (TO to) (VP (VB mark) (NP (BASENP (DT the) (JJ fifth) (NN anniversary)) (PP (IN of) (BASENP (DT the) (NNP Space) (NNP Shuttle) (NNP Columbia) (NN disaster)))))))))))) (PERIOD .))