In the spirit of challenging the reader, here's an easy one: If Petey displaces as much water as a heavier-than-water sphere of 1000 meters diameter, what's the LEAST he can weigh? (And don't give me any excuses along the lines of "things in space don't weigh anything." If he's displacing water, we can safely assume he's not in space. Think Earth-normal gravity, you bozos.)
If that problem's not difficult enough for you, here's a hard one: Extrapolate Petey's blueprints and superstructure from the drawings in the comic, estimate the mass of the hypertensile materials with which he's constructed, average out the mass of his fittings and crew, and tell us all how much of the rest of his mass is neutronium.
Show your work.