( ( if ( ( an asteroid ) ( does ( collide ( with earth ) ) ) ) ) ( ( ( let it ) be ) . ) )
(ROOT (S (SBAR (IN if) (S (NP (DT an) (NN asteroid)) (VP (VBZ does) (ADJP (JJ collide) (PP (IN with) (NP (NN earth))))))) (VP (VB let) (NP (PRP it)) (VP (VB be))) (. .)))
if an asteroid does collide with earth let it be.
(ROOT (S (PP (IN In) (NP (DT the) (NN past))) (, ,) (NP (NP (NNS asteroids)) (VP (VBG colliding) (PP (IN with) (NP (NN planet) (NN earth))))) (VP (VBZ has) (VP (VBN caused) (NP (JJ many) (NNS organisms)) (S (VP (TO to) (VP (VB die) (PRT (RP off))))))) (. .)))