The Smoke Ring is a 1987 science fiction novel by Larry Niven. Like much of Niven's work, the story is heavily influenced by the setting: a gas torus, a ring of air around a neutron star. It is a sequel to The Integral Trees. Setting The story is set at the fictional neutron star Levoy's Star (abbreviated "Voy"). The gas giant Goldblatt's World (abbreviated "Gold") orbits the star just outside its Roche limit. While Gold's gravity is enough to keep it from being pulled apart by Voy's tidal forces, it is insufficient to hold its atmosphere, which has been pulled loose into an independent orbit around Voy. This orbiting air forms a ring known as a gas torus. The gas torus is huge—one million kilometers thick—but most of it is too thin to be habitable. The central part of the Gas Torus, where the air is thicker, is known as the Smoke Ring. The Smoke Ring supports a wide variety of life. Robert L. Forward helped Niven calculate the parameters of the ring. There is no "ground" in the Smoke Ring; it is a world consisting entirely of sky. Thus, most animals can fly, even the fish. Furthermore, since the Smoke Ring is in orbit, it is in free fall. There is no "up" or "down", only "in" or "out" from Voy. Humans moving in the Smoke Ring use a poetic adage to aid their understanding of orbital mechanics - "East takes you Out, Out takes you West, West takes you In, In takes you East. Port and Starboard bring you back". (In the novel, the characters also say: "North and South bring you back"). Most animals have trilateral symmetry, allowing them to see in all directions. Most plants in the Smoke Ring are quite fragile, as they don't have to support their own weight. A notable exception to this rule are the Integral Trees. These are trees that are up to 100 kilometers long. Tidal locking causes them to be oriented radially, with one end pointing in toward Voy and one end pointing out. The ends of the tree experience a tidal force of up to 1/5 g. Each end consists of a leafy "tuft", which is where photosynthesis occurs. Each tuft of a tree is 50 kilometers from the tree's center of mass. Thus, a tuft is either orbiting too slowly (the in tuft) or too quickly (the