The Rolling Stones

by Robert A. Heinlein

The Stones, a family of "Loonies" (residents of the Moon, known as "Luna" in the book, from the Roman goddess), purchase and rebuild a used spaceship and go sightseeing around the Solar System.

The twin teenage boys, named Castor and Pollux after the half-brothers of classical legend, buy used bicycles on Luna to sell on Mars, their first stop, where they run afoul of local regulations but are freed by their grandmother Hazel Stone. While on Mars, the twins buy their brother Buster a native Martian creature called a flat cat, which produces a soothing vibration, as a pet.

In preparation for the asteroid belt, where the equivalent of a gold rush is in progress prospecting for "core material" and radioactive ores, the twins obtain supplies and luxury goods on Mars to sell at their destination, on the principle that it is shopkeepers, not miners, who get rich during gold rushes. En route, the flat cat and its offspring overpopulate the ship so the family places them in hibernation and later sells them to the miners.

The novel ends with the family setting out to see the rings of Saturn.