Gold in the Sky by Alan Edward Nourse

Tom knew now that it was the right thing to do. There was no question, after the Major's story, of what Dad had been doing out in the Belt at the time he had been killed. He had been doing a job that was more important to him than asteroid mining. but he had found something more important than his own life, and had no chance to send word of what he had found back to Major Briarton on Mars. That had been the unforeseeable part of the trap.

But now, of course, the Major had to know.