Ringworld

Pierson's puppeteers, three-leg two-head aliens find immense structure in unexplored part of the universe. Frightened of meeting the builders, they send a team of two humans, a puppeteer and a kzin, eight-foot red-fur catlike alien. Ringworld is 180 million miles across, sun at center. But the expedition crashes, and crew face a disastrously long trek.