( It ( ( means ( that ( ( ( the ( small sample ) ) ( of ( ( 40 asteroids ) ( used ( by Dahlgren ) ) ) ) ) ( ( was already ) ( ( covering ( ( a ( significant fraction ) ) ( of ( the volume ) ) ) ) ( in ( ( the ( phase space ) ) ( occupied ( by ( the ( entire ( Hilda population ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ means) (SBAR (IN that) (S (NP (NP (DT the) (JJ small) (NN sample)) (PP (IN of) (NP (NP (CD 40) (NNS asteroids)) (VP (VBN used) (PP (IN by) (NP (NNP Dahlgren))))))) (VP (VBD was) (ADVP (RB already)) (VP (VBG covering) (NP (NP (DT a) (JJ significant) (NN fraction)) (PP (IN of) (NP (DT the) (NN volume)))) (PP (IN in) (NP (NP (DT the) (NN phase) (NN space)) (ADJP (VBN occupied) (PP (IN by) (NP (DT the) (JJ entire) (NNP Hilda) (NN population))))))))))) (. .)))
It means that the small sample of 40 asteroids used by Dahlgren was already covering a significant fraction of the volume in the phase space occupied by the entire Hilda population.
(ROOT (S (NP (DT The) (NN term) (NN volume)) (VP (VBZ is) (VP (VBN used) (S (VP (TO to) (VP (VB describe) (S (NP (NP (DT the) (NN amount)) (PP (IN of) (NP (NN space)))) (ADJP (VBN occupied) (PP (IN by) (NP (NP (DT a) (NN sample)) (PP (IN of) (NP (NN matter)))))))))))) (. .)))