''Cluster'' is a series of five novels by Creator/PiersAnthony that take place in a far future civilized galaxy. Each species has its own "sphere" of influence wherein their laws hold and their species is free to colonize planets without interference. Sphere Sol begins the series as a 100-light year diameter sphere in our galaxy. 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.

Much of the plot of the series concerns attempts by the Andromeda galaxy, using agents placed through Kirlian transfer, to siphon off the energy of the Milky Way galaxy, which will result in the eventual disintegration of the entire galaxy.

[[AC:Novels in the Cluster series:]]
* ''Cluster'' (known as ''Vicinity Cluster'' in the UK) - 1977
* ''Chaining the Lady'' - 1978
* ''Kirlian Quest'' - 1978
* ''Thousandstar'' - 1980
* ''Viscous Circle'' - 1982

Anthony's novel ''Tarot'' also takes place in the same universe but is otherwise unrelated to the others.
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!!Tropes in this series:
* AbsurdlyDedicatedWorker: During ''Chaining the Lady'' two-thirds of the Etamin fleet are taken over by Andromedans possessing their key officers, who then start a fight with the rest of the fleet. None of the crewmen on the hostage ships seem to question their orders to suddenly start attacking other ships of their fleet without explanation, and a massive space battle results.
* 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. Which sex becomes a mother depends entirely on the order in which they meet.
** The Mintakans change gender with each mating, going from female to male and back again, losing a foot each time.
* DeusSexMachina: In ''Chaining the Lady'' and ''Kirlian Quest'', it turns out that the only way to open an Ancient site is to have two high-Kirlian entities mate near it.
* DudeShesLikeInAComa: In ''Chaining the Lady'' Melody's empty original body is effectively raped by another Mintakan, while she watches from a £ host.
* EasilyForgiven: High Kirlian auras seem to be strongly attracted to each other, and often the protagonist is willing to forgive a great deal of dodgy behavior.
** In ''Cluster'', the female slash agent has tried to kill Flint multiple times, and murdered many other sentients in the process. [[spoiler:Flint marries her and they raise a family.]]
** In ''Chaining the Lady'', Melody fully forgives [[spoiler:Llume]] for her part in the plot to conquer and then disintegrate the Milky Way galaxy, during which hundreds of military men are killed in a fleet battle.
* FantasticMeasurementSystem: Kirlian auras can be precisely measured with an aura reader. A typical person has a Kirlian rating of 1, while the protagonists are over 200.
* GreenSkinnedSpaceBabe: The humans of Outworld are all green skinned, so these crop up a few times:
** Flint's girlfriend Honeybloom from ''Cluster'' is the first in the series.
** Yael of Dragon, Melody's host in ''Chaining the Lady'' is green-skinned, and certainly attractive.
** Psyche in ''Kirlian Quest'' is a blue-skinned space babe.
* GreyAndGrayMorality: In ''Cluster'' Flint eventually decides that neither galaxy should have the Ancient technology he discovered, because both are willing to act immorally if it ensures their survival.
* 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.
* IntimateHealing: In ''Kirlian Quest'', in order to get Herald's injured alien host body healthy enough to get moving again, Sixteen suggests mating, as it's the only way to tap an internal energy reserve in the jet species. It works.
* 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.
* MentalSpaceTravel: Kirlian transfer.
* PaintingTheFourthWall: At one point in ''Kirlian Quest'', Herald realizes that he's [[TranslationPunctuation speaking in quotation marks instead of slashes]].
* {{Precursors}}: The Ancients came long before any of the species in the series, and had far greater technology.
* ProtagonistCenteredMorality: On Spica Flint accidentally rapes two Spicans because he didn't understand that whenever the three sexes meet they ''must'' mate. Then he purposefully rapes two other Spicans (of the same gender) to prevent his capture for the first rape. ''Then'' he rapes two more Spicans to nullify an Andromedan agent who has transferred to one of them. When the local authorities finally catch up to him he tells them that their laws don't apply to him because he is a transfer from another sphere. ''They agree.''
* SpotTheImposter: With BodySurfing being a primary plot device, this turns up often:
** In ''Cluster'' Flint and representatives of other spheres determine one of them must be an Andromedan agent and try various schemes to spot which of them it is.
** In ''Chaining the Lady'' Melody uses tarot cards essentially like an InkblotTest to spot Andromedan agents.
* 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=].
* TarotMotifs: All over the place throughout the series:
** In ''Cluster'' Flint discovers that there is a "church of tarot" that believes all faiths are true and provides readings to any who wish.
** In ''Chaining the Lady'' the space ships of the Etamin segment fleet are all named after tarot cards and all even ''shaped'' like a tarot suit. The Solarian flagship ''Ace of Swords'' for example looks like a giant sword.
** The tarot decks used in the series are a futuristic set with 100 cards and the energy/aura suit alongside the traditional swords, cups, wands/rods and disks/coins.
* 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.
* VerbalWeakness: Because of his discovery, through most of ''Kirlian Quest'', the astronomer Hweeh cannot hear the word "amoeba" without going into shock.
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