About the image. Drawn after the description in Niven's novel: a ribbon around a Sun-like star, land and sea on its inner face, and an inner ring of shadow squares that makes night. The ribbon is drawn far wider than its real proportions; at true scale it would be a hair-thin line around the star.
In 1970 Larry Niven published a novel about an artificial world shaped like a ribbon. It circles a star like ours at roughly the distance of Earth’s orbit, and it is so large that its inner face has about three million times the surface area of Earth. The book won the Nebula Award in 1970 and both the Hugo and Locus Awards the following year, and the Ringworld became one of the best-known structures in science fiction.
The numbers
The ring is 299 million kilometres across and about 1.6 million kilometres wide. More than a hundred Earths could sit side by side across its width. It is not in orbit in the usual sense. It spins, and the spin presses everything on the inner surface outward against the floor, giving 99% of Earth’s gravity. Walls along both rims hold the air in.
The ring faces its star all the time, so on its own it would have no night. Niven solved this with an inner ring of shadow squares, huge panels joined by thin, ultra-strong wire, that circle closer to the star. As they pass between the star and the ground, they cast the shadows that make night.
“The Ringworld is unstable!”
Readers soon found a flaw. A rigid ring around a star is not truly in orbit, and the smallest nudge sideways makes it drift further off-centre until it hits the star. At the 1971 World Science Fiction Convention, students from MIT chanted “The Ringworld is unstable!” in the hallways.
Niven answered in his sequel, The Ringworld Engineers, first published in a limited edition in 1979. In it the builders had fitted attitude jets along the rim to keep the ring centred, but only a small fraction of them still work.
Its descendants
The Ringworld gave its name to a whole class of imagined structures, sometimes called Niven rings. The ring-shaped worlds of the Halo video games took inspiration from it, although they are far smaller and orbit a star or planet rather than encircling it. The Orbitals in Iain M. Banks’s Culture novels are often described as small Ringworlds.
Nobody has proposed a material strong enough to build one. The idea endures as one of the most vivid answers to a simple question: if you could build anything, how much living space could one star support?
Gallery
A vast curved band with land and cloud on its inner face, arcing around a bright star, an impression of the Ringworld. AI-generated painting.Ringworld, 3D artist's impression, rendered from the interactive model.Ringworld, 3D artist's impression, from another angle, rendered from the interactive model.Ringworld, 3D artist's impression, close-up, rendered from the interactive model.
Timeline
1970Larry Niven publishes Ringworld; it wins the Nebula Award.
1971Hugo and Locus Awards. MIT students chant "The Ringworld is unstable!" at the World Science Fiction Convention.
1979The Ringworld Engineers adds attitude jets to keep the ring centred (wide release 1980).
1996The Ringworld Throne.
2004Ringworld's Children.
Questions people ask
Is the Ringworld real?
No. It is a structure from Larry Niven's 1970 novel. No known material is strong enough to build one.
How big is the Ringworld?
299 million kilometres across and about 1.6 million kilometres wide, with an inner surface of about three million times Earth's area.
How fast does the Ringworld spin?
To give 99% of Earth's gravity at a radius of about 150 million kilometres, it must turn at roughly 1 200 kilometres per second.
Why is the Ringworld unstable?
A rigid ring around a star is not truly in orbit. Any small sideways push makes it drift further until it hits the star, which is why Niven gave it attitude jets in the sequel.
Is Halo based on the Ringworld?
Partly. Halo's rings were inspired by it, but they are far smaller and orbit a star or planet instead of encircling a star.