Gold in the Sky — Alan E. Nourse

When Greg and Tom Hunter learn of their father's accidental death during his exploration of the asteroid belt, they are certain that it was no accident. They know that Jupiter Equilateral, a mining corporation, was behind it, but they can't prove it. They bring their suspicions to the authorities who tell them that without evidence they can't even investigate. The brothers vow to get that evidence, and set out on an adventure more grand than any they could have imagined.