Larry Niven

The artefact is a circular ribbon of matter six hundred million miles long and ninety million miles in radius. Pierson's puppeteers, the aliens who discovered it, are understandably wary of encountering the builders of such an immense structure and have assembled a team of two humans, a mad puppeteer and a kzin, a huge cat-like alien, to explore it. But a crash landing on the vast edifice forces the crew on a desperate and dangerous trek across the Ringworld.

"This rousing sequel to the classic Ringworld continues the adventures of Louis Wu and Speaker-to-Animals on that fantastic planet."--School Library Journal

They first appear as a series of dots on astronomical plates, heading from Saturn directly toward Earth. Since the ringed planet carries no life, scientists deduce the mysterious ship to be a visitor from another star.The world's frantic efforts to signal the aliens go unanswered. The first contact is hostile: the invaders blast a Soviet space station, seize the survivors, and then destroy every dam and installation on Earth with a hail of asteriods. Now the conquerors are descending on the American heartland, demanding servile surrender--or death for all humans.

THE LUCKY ONES WENT FIRST… The gigantic comet has slammed into Earth, forging earthquakes a thousand times too powerful to measure on the Richter scale, tidal waves thousands of feet high. Cities were turned into oceans; oceans turned into steam. It was the beginning of a new Ice Age and the end of civilization But for the terrified men and women chance had saved, it was also the dawn of a new struggle for survival—a struggle more dangerous and challenging than any they had ever known….

In 3016, the 2nd Empire of Man spans hundreds of star systems, thanks to faster-than-light Alderson Drive. Intelligent beings are finally found from the Mote, an isolated star in a thick dust cloud. The bottled-up ancient civilization, at least one million years old, are welcoming, kind, yet evasive, with a dark problem they have not solved in over a million years.