( ( ( A ( key idea ) ) ( in ( ( O'Neill 's ) thinking ) ) ) ( ( was ( that ( ( such ( large structures ) ) ( could ( be ( ( built out ) ( of ( material ( ( mined ( from ( ( ( the Moon ) or ) asteroids ) ) ) ( to ( avoid ( ( the ( high cost ) ) ( of ( ( launching out ) ( ( of ( ( Earth 's ) ( ( much larger ) gravity ) ) ) well ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT A) (JJ key) (NN idea)) (PP (IN in) (NP (NP (NNP O'Neill) (POS 's)) (NN thinking)))) (VP (VBD was) (SBAR (IN that) (S (NP (JJ such) (JJ large) (NNS structures)) (VP (MD could) (VP (VB be) (VP (VBN built) (PRT (RP out)) (PP (IN of) (NP (NP (NN material)) (VP (VBN mined) (PP (IN from) (NP (NP (DT the) (NN Moon)) (CC or) (NP (NNS asteroids)))) (S (VP (TO to) (VP (VB avoid) (NP (NP (DT the) (JJ high) (NN cost)) (PP (IN of) (S (VP (VBG launching) (PRT (RP out)) (PP (IN of) (NP (NP (NNP Earth) (POS 's)) (ADJP (RB much) (JJR larger)) (NN gravity)) (ADVP (RB well))))))))))))))))))) (. .)))
A key idea in O'Neill's thinking was that such large structures could be built out of material mined from the Moon or asteroids to avoid the high cost of launching out of Earth's much larger gravity well.
(ROOT (S (NP (NNP High) (NNP gravity)) (VP (VBZ explains) (NP (NP (DT the) (JJ spherical) (NN shape)) (PP (IN of) (NP (NNS ceres)))) (, ,) (SBAR (IN once) (S (VP (VBD thought) (S (VP (TO to) (VP (VB be) (NP (DT the) (JJS largest) (NN asteroid))))))))) (. .)))