Pretty fun, I like those physics. But the game does feel a bit silent. (Reads about the optional challenge.) Oh, I guess that's on purpose.
The art looks great! (sees holes in shadows.) Well, I imagine it would be hard to get the swinging crane and rope to cast shadows in a realistic way. As for the buildings, the animals and the top of the spaceship, I guess you didn't have time to make better shadows for those.
And I think this is one of the most fun ways to "collect" stuff I've come across so far in the submissions. That and the silence (you can't hear anything in space anyway) makes me think you really nailed the theme.