Michael Dahl 
Voyage to the Metal Moon [PDF ebook] 

Zak Nine and Erro have come up with a brilliant plan to escape. They’ll simply hide on board a prison ship and ride it to freedom on another planet! Unfortunately, the boys don’t get far when the ship docks at one of Planet Alcatraz’s metallic moons. The hollow sphere is magnetic, and the only way to walk on it is by wearing special magnetic boots. The boys are soon discovered and are forced to flee. But how will they escape when they’re stuck floating inside a giant metal ball?