Spacers were the fictional first humans to emigrate to space in Isaac Asimov's Foundation and related Robot and Empire series. In these stories, about a millennium thereafter, they severed political ties with Earth, and embraced low population-growth and extreme longevity (with lifespans reaching 400 years) as a means for a high standard of living, in combination with using large numbers of robots as servants. At the same time, they also became militarily dominant over Earth.
Asimov's novels chronicle the gradual deterioration of the Spacer worlds, and the disappearance of robots from human society. The exact details vary from book to book, and in at least one case — the radioactive contamination of Earth — later scientific discoveries forced Asimov to retcon his own future history. The general pattern, however, is as follows:
In the vague period between Asimov's near-future Robot short stories (of the type collected in I, Robot) and his novels, immigrants from Earth establish colonies on fifty worlds, the first being Aurora, the last Solaria, and the Hall of the Worlds located on Melpomenia, the nineteenth. Sociological forces possibly related to their sparse populations and dependence on robot labor lead to the collapse of most of these worlds; their dominance is replaced by new, upstart colonies known as "Settler" worlds. Unlike their Spacer predecessors, the Settlers detested robots, and so by the time of the Empire novels, robotics is almost an unknown science.
Known Spacer worlds 
Although Asimov never issued a full list of the fifty Spacer worlds, some of them can be inferred from the author's novels and short stories. Some of them are:
- Acrisia (from Mark W. Tiedemann's Mirage)
- Capella (from Mark W. Tiedemann's Chimera)
- Inferno (from Roger MacBride Allen's Caliban trilogy)
- Keresia (from Mark W. Tiedemann's Chimera)
- Osiris (from Mark W. Tiedemann's Chimera)
- Pallena (from Mark W. Tiedemann's Mirage)
- Saon (from Mark W. Tiedemann's Mirage)
- Theia (from Mark W. Tiedemann's Chimera)
Wider connections 
In Asimov's short-story collection I, Robot, it is mentioned that the first extra-solar Earth colonies had recently been settled, in the wake of Gregory Powell and Mike Donovan's initial historic hyperspatial jump. In the novel The Robots of Dawn, it is established that the planet Aurora was originally known as "New Earth," during the first few centuries following its colonization by Earthmen (in the Tau Ceti star system), and was the first planet settled by Man outside of Earth's solar system. Therefore, it can be inferred that Aurora/New Earth is that same very first extrasolar colony settled during the timeframe of the stories featured within I, Robot, during the mid-21st Century.
Asimov's novel Nemesis hints that the Spacers may have been descendants of human beings selected by a non-human intelligence for their mental characteristics. However, except for a brief mention in Forward the Foundation, the Nemesis plotline is entirely unlinked with the rest of Asimov's science-fiction canon. (The internal logic of the Robot-Empire-Foundation saga demands that robots be present on Earth prior to the Spacer worlds' colonization, yet Nemesis contains no robots, making the continuity difficult to accept.)
Further, another story within the story arc establishes the Spacers' mastery of myco-food (food derived from fungi), which they then retain all through history up to their inclusion in the Imperium on Trantor in the sector of Mycogen. The Spacers' control of myco-food makes the farming operations of Solaria seem more puzzling, until we remember that Solaria was aberrant even by Spacer standards and remained so in the later book Foundation and Earth as a real example of menace to the Second Foundation itself.
In a somewhat similar vein, Mark W. Tiedemann's "Robot Mystery" trilogy also portrays the Spacers as a group genetically distinct from Earthpeople and their Settler descendants. Tiedemann's trilogy, set between The Robots of Dawn and its sequel Robots and Empire, attempts to update Asimov's work to reflect more recent scientific and science-fictional speculation, for example explaining the lack of nanotechnology in Asimov's robot-ridden society. According to Tiedemann's Aurora (2002), the cumulative effects of genetic alterations (due partly to nanotech devices since abandoned) separated Spacers from the rest of humanity, to such an extent that the word "human" in the Three Laws of Robotics may no longer apply to them.