( ( At ( half gravity ) ) ( , ( ( ( a ( hovering light ) ) ( with ( an ( energy pack ) ) ) ) ( ( can ( ( stay aloft ) indefinitely ) ) . ) ) ) )
(ROOT (S (PP (IN At) (NP (DT half) (NN gravity))) (, ,) (NP (NP (DT a) (JJ hovering) (NN light)) (PP (IN with) (NP (DT an) (NN energy) (NN pack)))) (VP (MD can) (VP (VB stay) (ADVP (RB aloft)) (ADVP (RB indefinitely)))) (. .)))
At half gravity, a hovering light with an energy pack can stay aloft indefinitely.
(ROOT (S (NP (NNP Birds)) (VP (VBP need) (NP (DT a) (JJ light-weight) (NN body)) (PP (IN for)) (S (VP (TO to) (VP (VB stay) (ADVP (RB aloft)))))) (. .)))