Cluster is a series of science fiction novels by Piers Anthony. Anthony originally conceived of and wrote the series as a trilogy but later added two additional volumes. Setting As the series opens, humanity has colonized an area of the galaxy roughly 100 light years in diameter. This area is called Sphere Sol, as it is the sphere of influence of the race from the star Sol, that is, the Sun. Sphere Sol's "neighborhood" is also home to other spheres, each centered around a particular star: Spheres Polaris, Canopus, Spica, Nath, Mirzam, and Bellatrix as well as the huge, decadent Spheres Sador and Mintaka. In the Cluster setting, races colonize other planets in three ways. The first is through instantaneous teleportation, called matter transmission or mattermission. This, however, is prohibitively expensive. Second, colonists travel to new worlds in "freezer" ships, where they are cryonically stored for the decades it takes for the voyage. Half of those frozen are lost due to failures either of the containment units or the ships themselves. The third and most common method of colonist travel is via lifeships, slower but safer multigenerational vessels that may take centuries to reach their destination. The drawback of the lifeships is that as they plod through space, their occupants regress in technical sophistication. In part due to this, and perhaps because colony worlds with small populations cannot support sophisticated social constructs, all spheres in the Cluster series suffer spherical regression—i.e., the farther a colony is from its home star, the less technology the inhabitants use, while the social order begins to resemble periods of the home planet's past. For instance, the inhabitants of Outworld, Sphere Sol's farthest colony, are paleolithic tribes. They hunt with flint spears and make fire, but their society has little technology beyond that level. Spherical regression does not create true copies of Earth or any world's past, however. Colonists know they are part of an interstellar empire, and they are aware of the technological and social differences on other planets. Furthermore, the home world mattermits some government and security personnel to all colony worlds. Therefore, individual students of history can often use their knowledge of the home world's past to make their own planets more or less primitive than one would expect when simply calculating the distance from the home star. Kirlian transfer The central plot mechanism of the Cluster novels is that of transfer.