The story starts with a comet that touches the Earth in its flight and collects a few small chunks of it. Some forty people of various nations and ages are condemned to a two-year-long journey on the comet. They form a mini-society and cope with the hostile environment of the comet (mostly the cold). The size of the 'comet' is about 2300 kilometers in diameter - far larger than any comet or asteroid that actually exists.
