On the puppeteer home world, they are told that the expedition's goal is to investigate the Ringworld, a gigantic artificial ring, to see if it poses any threat. The Ringworld is about one million miles (1.6 million km) wide and approximately the diameter of Earth's orbit (which makes it about 584.3 million miles or 940.4 million km in circumference), encircling a sunlike star. It rotates to provide artificial gravity 99.2% as strong as Earth's from centrifugal force. The Ringworld has a habitable, flat inner surface (equivalent in area to approximately three million Earths), a breathable atmosphere and a temperature optimal for humans. Night is provided by an inner ring of shadow squares which are connected to each other by thin, ultra-strong wire. When the crew completes their mission, they will be given the starship in which they travelled to the puppeteer home world; it is orders of magnitude faster than any possessed by humans or Kzinti.