of it. Topopolis (also known as Cosmic Spaghetti) is a large tube that rotates to provide artificial gravity. A Ringworld (or Niven Ring) is an artificial ring encircling a star, rotating faster than orbital velocity to create artificial gravity on its inner surface. A non-rotating variant is a transparent ring of breathable gas, creating a continuous microgravity environment around the star, as in the eponymous Smoke Ring. Related structures which might not be classified as individual stellar megastructures, but occur on a similar scale: A Dyson swarm is a Dyson sphere made up of separately orbiting elements (including large habitats) rather than a single continuous shell. A Dyson bubble is a Dyson sphere in which the individual elements are statites, non-orbital objects held aloft by the pressure of sunlight. Planetary scale A Bishop Ring, Halo or Orbital is a space habitat similar to but much smaller than a Niven Ring. Instead of being centered on a star, it is in orbit around the star and its diameter is typically on the order of magnitude of a planet. By tilting the ring relative to its orbit, the inner surface would experience a nearly conventional day and night cycle. Due to its enormous scale, the habitat would not need to be fully enclosed like the Stanford torus, instead its atmosphere would be retained solely by centripetal gravity and side walls, allowing an open sky. Globus Cassus is a hypothetical proposed project for the transformation of Planet Earth into a much bigger, hollow, artificial world with the ecosphere on its inner surface. This model serves as a tool to understand the World's real functioning processes. Shellworlds or paraterraforming are inflated shells holding high pressure air around an otherwise airless world to create a breathable atmosphere. The pressure of the contained air supports the weight of the shell. Completely hollow shell worlds can also be created on a planetary or larger scale by contained gas alone, also called gravitational balloons, as long as the outward pressure from the contained gas balances the gravitational contraction of the entire structure, resulting in no net force on the shell. The scale is limited only by the mass of gas enclosed, the shell can be made of any mundane material. The shell can have an additional atmosphere on the outside. It can also refer to terraformed or artificial planets with multiple concentric layers. Orbital structures An orbital ring