Despite his age, Louis turns out to be in perfect physical condition owning to a combination of advanced medical technology and boosterspice, a drug that extends human life. However, though healthy, rich and intelligent, it is becoming clear Louis is utterly bored. Having lived for two centuries, he has seen it all many times over and people in general are getting on his nerves. Between transfer booths he considers another sabbatical — a trip to and beyond the reaches of Known Space, all alone in a single ship for a year or more, until he begins to yearn for people's company again — when all of a sudden the transfer booth materializes him in a sunlit hotel room, rather than the nocturnal Seville he had set its control for. Facing him is an alien with three legs, no arms and two heads.
The alien introduces himself as Nessus and Louis recognizes him for a Pierson's Puppeteer, a species that had the most advanced technology in Known Space but vanished from the region before Louis was born. Being descended from herbivorous herd animals, their morality is essentially based on cowardice. Puppeteers that display any signs of bravery are considered insane by their peers, and in fact are insane since this bravery is accompanied by other symptoms of mental illness, such as manic-depressive cycles. With aliens being potentially dangerous, space ships exposed to vacuum and their being distrustful of faster-than-light space travel, only a "brave" (insane) Puppeteer would leave home and go to a planet like Earth. These ones are in turn are still mostly cowards by human standards, so Nessus has been ordered to hire three mercenaries to do the things he himself dare not. Louis is on top of his list of candidates.
With Nessus being secretive about the mission, Louis is reluctant to join, but when the Puppeteer eventually shows Louis a blurry picture of a distant star with a ring around it, the bored Louis immediately signs up: this ring turns out to be the Ringworld, an artificial circular strip of world with spin for surface gravity, orbiting the star. The Puppeteers, being on the run from the galaxy, have spotted this artifact in their path and being cowards, the sheer power of whatever has created such a structure frightens them profoundly. Hence, Nessus' mission is to assemble a team, visit the Ringworld and see whether it poses a threat to his species. Payment to the expedition's members will be the Long Shot, the extremely fast ship depicted in the story At the Core, that Beowulf Shaeffer rode to the galactic core and back, centuries earlier.
Eventually the team is assembled. The third member, Speaker-to-Animals (Speaker) is a Kzin, a ferocious felinoid predator species which has, in the recent past, fought a series of brutal wars with humanity, eventually losing every time because of a tendency to attack before being quite ready. The Kzin who is a translator, a low-ranking official at the Kzinti embassy to Earth, reckons obtaining the Long Shot for the Kzinti Empire is enough of an achievement to (literally) give him a name ("Speaker-to-Animals" being a description rather than a name), and therefore signs on too, as the expedition's security chief.
Finally, Teela Brown is a young human female whose role in the mission is not immediately clear. But Puppeteers do not do anything without a very good reason, and her significance is revealed as the plot unfolds. She is the result of a secret Puppeteer experiment in selective breeding for luck among humans, which generally helps her and her descendants. The Puppeteers reckon her luck will increase the probability of a successful mission, however it soon turns out that Teela's personal luck and the luck of the expedition seldom go hand in hand.
As they approach their target, The Ringworld turns out to be an awesome sight: a huge, circular strip of land, teeming with life and with entire oceans bigger than Earth. Between the Ringworld and its star, a series of squares (dubbed shadow squares by the expedition) are suspended in another ring, orbiting the sun faster than the Ringworld itself, thus providing the artificial world below with a day/night cycle. However, when their ship is hit by a powerful, automated meteor defense system and then strikes one of the near-invisible shadow-square wires, the expedition crash-land on the Ringworld with their vessel severly damaged. They now have to set out to find a way to get back into space, as well as fulfilling their original mission. They cross vast distances, witness strangely evolved ecosystems originating from many different planets, including Earth, and interact with some of the Ringworld's varied primitive civilizations. They attempt to discover what caused the Ringworld's inhabitants to lose their technology, and puzzle over who created the Ringworld and why.
|Radius||9.5×107 miles (~1.5×108 km) (~1 AU)|
|Circumference||6×108 miles (~9.7×108 km)|
|Width||997,000 miles (1,600,000 km)|
|Height of rim walls||1,000 miles (1,600 km)|
|Mass||2×1027 kg (1.8×1024 short tons) (1,250,000 kg/m², e.g. 250 m thick, 5,000 kg/m³)|
|Surface area||6×1014 sq mi (1.6×1015 km²); 3 million times the surface area of Earth.|
|Surface gravity||0.992 gee (~9.69 m/s²)|
|Spin velocity||770 miles/second (~1,200,000 m/s)|
|Sun's spectral class||G3 verging on G2; "barely smaller and cooler than Sol".|
|Day length||30 hours|
|Rotational time||7.5 Ringworld days (225 hours, 9.375 Earth days)|
|On Ringworld, time longer than a day is measured in falans, with 1 falan being 10 turns or 75 Ringworld days (93.75 Earth days), so 4 falans are slightly longer than 1 Earth year.|
The "Ringworld" is an artificial ring about one million miles wide and approximately the diameter of Earth's orbit (which makes it about 600 million miles in circumference), encircling a Sol-type star. It rotates, providing an artificial gravity that is 99.2% as strong as Earth's gravity through the action of centrifugal force. Ringworld has a habitable flat inner surface equivalent in area to approximately three million Earth-sized planets. The majority of the surface is land interspersed with shallow, freshwater seas. On opposite sides of the ring are two large deep saltwater oceans, placed in counterbalance to one another. One of the large oceans, known as the "Great Ocean", contains one-to-one maps of all of the inhabited worlds of known space. The "Other Ocean" has many maps of a single world: the Pak Homeworld. Walls 1000 miles tall along the edges retain the atmosphere. The Ringworld could be regarded as a thin, rotating slice of a Dyson sphere, with which it shares a number of characteristics. Niven himself thinks of the Ringworld as "an intermediate step between Dyson spheres and planets."
Scrith is a milky-gray translucent, nearly frictionless material. The fairly thin layer of scrith that forms the floor of the Ringworld blocks the passage of 40% of the neutrinos that encounter it, equivalent to almost a light year of lead. It also absorbs nearly 100% of all other radiation and subatomic particles and rapidly dissipates heat. The tensile strength of scrith is similar to the strong nuclear force, with the Ringworld foundation only about 30m (100 ft) deep. Also, it is transparent to large magnetic fields.
Due to its enormous strength, scrith is impervious to most weapons. A body (such as a comet or asteroid) striking with enough kinetic energy may be able to deform the Ringworld floor and punch a hole. The Ringworld engineers used a device, called the cziltang brone in their language, to pass from the vacuum of their spaceports right through the scrith to the habitable surface of the Ringworld.
The physical composition of scrith is unclear, but it appears to share some of the properties of a metal (albeit in a greatly exaggerated form): for instance, the high tensile strength, the ability to conduct heat and the ability to retain an induced magnetic field. Scrith is said to have been artificially produced through the transmutation of matter.
Thus, large thrusters must be incorporated into the design to keep it centered about its star. This point gave Niven some difficulty after he published his first Ringworld novel; he was deluged with letters pointing out that "the Ringworld isn't stable" and dedicated the first sequel to a resolution of this problem. He notes in the dedication of Ringworld Engineers that at the 1971 World Science Fiction Convention MIT students crowded the hotel hallways chanting "The Ringworld is Unstable!" In this first sequel, he also tackled how to prevent all the soil from ending up in the oceans. In the fourth book in the series, Ringworld's Children, he creates backplot explanations for several of the imperfections in his original design of the Ringworld — and wholly glosses over others, such as that Louis Wu is worried about his dietary intake of salt since only the Great Oceans are described as being saline.