The Rolling Stones — Robert A. Heinlein

When the Stone twins made up their minds to leave Lunar City in a secondhand spaceship, they hadn't planned on having their whole family accompany them. But the Stones were not your ordinary Lunar family -- no way! -- and their voyage through the solar system sure proved it.

What began as a simple business expedition to Mars soon mushroomed into a dangerous situation when Grandma Stone was lost in space. Then, just when everything seemed to be getting better, a Martian flatcat came aboard and fouled up the works.

But the real trouble didn't get underway until the Stones headed for the asteroid belt to take up a mining proposition they, somehow, couldn't refuse . . .