Tom Corbett Space Cadet #8: The Robot Rocket:

Captain Strong and the three cadets arrived on the control deck simultaneously. Strong wasted no time with preliminaries. He faced them and pointed to the asteroid belt clearly seen through the viewport. "There's a maverick planetoid acting up in this sector. Something has happened to its atomic structure and the planetoid has changed its nature from a positive or attracting magnetism-to a negative or repelling magnetism. It's been drifting in an irregular orbit for about two weeks now, bum...