''Cluster'' is a series of five novels by PiersAnthony that take place in Sphere Sol, a human-colonized region of the galaxy roughly 100 light years in diameter, centered around Sol, our sun. Other such spheres exist in this setting, each centered around its own star: Spheres Polaris, Canopus, Spica, Nath, Mirzam, and Bellatrix as well as the huge, decadent Spheres Sador and Mintaka.

Three methods of colonization are used in this setting:

# Teleportation, known as ''matter transmission'' or ''mattermission'', the most expensive method.
# Freezer ships, wherein colonists are cryogenically frozen for the duration of the voyage (but roughly half the colonists traveling this way are lost due to mechanical failures).
# Lifeships, the most common method, which are slower but safer multigenerational vessels which may take generations to reach their destination. Unfortunately, as lifeships plod through space, their inhabitants lose technical sophistication. Partly because of this, all spheres tend to suffer ''spherical regression'', that is, the further from the center a colony world is, the less technology its inhabitants use.

A central plot mechanism of the ''Cluster'' novels is Kirlian transfer, by which the mind and personality of individuals with a high Kirlian aura can be projected to inhabit a distant body. This is a refinement of mattermission technology, but because only the aura is sent it is far less expensive.

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[[folder: Novels in the Cluster series: ]]

* ''Cluster'' (known as ''Vicinity Cluster'' in the UK)
* ''Chaining the Lady''
* ''Kirlian Quest''
* ''Thousandstar''
* ''Viscous Circle''

His novel ''Tarot'' also takes place in the same universe but is otherwise unrelated to the others.
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!!Tropes in this series:

* BizarreAlienBiology: The series has numerous biologically bizarre aliens, including a water-squirting ball that lives off atmospheric gasses, magnetically-levitating disks of metallic particles that communicate by laser, a teardrop-shaped being with a single tentacle who rolls on a track-ball instead of legs (said ball also serving as the egg for females) and tastes the ground as it rolls, and sentient slime-fish with three sexes.
* BizarreAlienLocomotion: One of the alien races rolls around on a single large sphere embedded at the bottom of their tear-shaped bodies.
* BizarreAlienReproduction: Every species has a different, exotic way of breeding, and of course the hero, as he {{Body Surf}}s between the species, [[BoldlyComing experiences them all]]. Perhaps the best example not covered by another trope is the Spicans: they have three sexes, and whenever all three are present in the same area, mating ''will'' occur- not might, ''will''. There are three roles that can each be assumed by any of the three sexes, and the gender of the offspring is determined by which sex takes which role.
* BizarreAlienSexes: The Spicans have three sexes - impact, undulant, and sibilant - of which all three are required for reproduction.
* HumansThroughAlienEyes: ''Viscous Circle'' involves a grotesque and disturbing description of an alien that one of the flying magnetic disk aliens sees; it's very easy not to realize that this is a description of a human being. The rest of the Cluster series often deals with "outsider" views of humanity, sometimes ''literally'' through human eyes as body-sharing technology is a major plot device.
* LethalHarmlessPowers: In ''Viscous Circle'', the Bands stage a mock fight for training in which they inadvertently kill (or possibly drive to suicide) those on the other side of the mock fight by using the light that transmits their emotions between individuals to transmit powerful HATE messages to the Bands on the other side. Afterward, they go through a HeroicBSOD and suicide themselves. It's pretty sad, really.
* {{Precursors}}: The Ancients came long before any of the species in the series.
* TacticalRockPaperScissors: ''Thousandstar'' has three sapient species compete for control of precursor technology: the spherical [=HydrO=], whose needle-like water jets can penetrate the flesh of an Erb, but are vulnerable to the claws of a Squam; the snake-like Squam, whose claws can cut the flesh of a [=HydrO=], but are vulnerable to the drilling action of an Erb; and the plant-like Erb, whose drilling action can penetrate the carapace of a squam, but are vulnerable to the water jets of a [=HydrO=].
* TranslationPunctuation: Human speech is represented with ordinary quotation marks, and each type of alien speech uses a different punctuation symbol as a quotation mark. By the end of the series pretty much every symbol on the keyboard has been pressed into service.
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