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This is the term, in both sf and popular culture, used to indicate sentient extraterrestrial beings – creatures from other worlds endowed with reason, consciousness, thought, Intelligence (the terms for and conceptions of this vital but slippery quality vary). Aliens may have minds somewhat less capable than ours, of comparable capacity, of greater (even vastly greater) power, or minds so different that comparison becomes impossible. They may appear as invaders (see Invasion) or teachers, as allies or enemies, as victims of human exploitation or judges of human civilization, as Secret Masters guiding human history (see Uplift), or as utterly indifferent forces paying no attention to humanity at all. Aliens may look like us, resemble (more or less) any number of Earthly species, or take on shapes we have never seen or imagined, forms so strange we sometimes fail to recognize them (and they us) as fellow beings at all.

While the overwhelming majority of aliens hail from outer space, aliens may occasionally emerge from little-explored zones on our own planet, from Under the Sea or the rarefied layers of the upper atmosphere. They may be found Underground or in the hollow interior of our planet (see Hollow Earth). Some aliens come from other Dimensions, while others have their origins in Parallel Worlds or Alternate Histories. In most (though not all) cases, non-extraterrestrial aliens function in sf texts in much the same way that extraterrestrials do, and their nearer origins make little difference.

Approaches to the subject in sf fall into two broad (though often overlapping) categories. On one hand, writers have long exploited the rich literary opportunities offered by the figure of the alien, whether or not the aliens they imagine differ radically from human beings or other Earthly models. Occasionally they have striven to convey the sense of an alien mind which thinks in ways unlike those of human beings. (This has frequently been acknowledged as one of the most difficult challenges an sf writer can face.) On the other hand,writers have variously attempted to depict truly alien beings, the products of environments and histories unlike Earth's, sometimes even based on physical substrates other than familiar matter (e.g., radio waves or stellar plasma). The very possibility of an alien intelligence is an existential provocation, the stuff of identity crisis, and writers of fiction have been exploring the implications of alien beings since their existence began to be seriously considered in the Proto SF of the early seventeenth century. Over that time – and particularly since the turn of the twentieth century – writers have expanded and complicated our conception of alien biologies and societies, deployed aliens in critiques of the chauvinistic and parochial assumptions embedded in human culture, and carried on a complex intertextual meditation on the likely effects of an encounter with alien beings on human consciousness and civilization.

The idea of Life on Other Worlds dates back to antiquity. Epicurus (341-270 BCE) taught that there were an infinity of other life-supporting worlds; the poet Lucretius (circa 95-circa 50 BCE) reiterated the notion in his De rerum natura; and the Pythagoreans (c. fifth century BCE) proclaimed the existence of life (including intelligent life) on the Moon. Plutarch (circa 45-120 CE) also speculated about the inhabitants of the moon, and Lucian imagined a journey there in his True History (second century CE), a Parody of the Fantastic Voyage; but Plato (429-347 BCE) and Aristotle (384-322 BCE) both opposed the notion that other worlds as such existed at all, and their influence predominated throughout the medieval period. The heliocentric cosmological model proposed by Copernicus (1473-1543) and the 1610 telescopic observations of the Moon by Galileo (1564-1642) brought the question back into Western thought, and a lively debate grew up almost immediately. The philosophers and theologians of the seventeenth and eighteenth centuries quickly recognized the challenge aliens posed to a range of precious received notions. Early thinkers such as Tommaso Campanella, John Wilkins, and Athanasius Kircher frequently focused on matters which rarely crop up in sf – such as the status of aliens vis-à-vis original sin and their connection to the lines of descent presented in Genesis – but they fully appreciated the fundamental threat that the idea of aliens posed to humanity's self-image as the crown of Creation with a unique place in the cosmos. Responses, then and since, varied dramatically. Some (such as Cyrano de Bergerac, Robert Wittie [1613?-1684], and Voltaire) fairly revelled in the possibility of humanity's dethronement, while others (such as Kircher, Charles Sorel [c. 1602-1674], Emanuel Swedenborg, and Bernardin de Saint-Pierre [1737-1814]) denied the existence of other worlds altogether, or strove to retain a measure of cosmic prestige for the Earth and its inhabitants in a universe filled with innumerable counterparts. (Swedenborg, for instance, dedicated a chapter of his De Telluribus [1758] to explaining that Christ appeared on Earth alone because it was only here, out of all the countless worlds populated with human beings, that humanity developed the art of writing, and so was able to preserve the story of the Incarnation for future generations.) Efforts at denial did little to stem the tide of public and scientific opinion, and the likelihood of extraterrestrial counterparts to humanity who might even be superior to us in some or many ways became undeniable to most thinkers of the eighteenth and early nineteenth centuries.

The persistence of theological and philosophical assumptions, and the lack of a robust notion of adaptive development, meant that visitors to other worlds in the literature of ancient and early modern times met no genuine alien beings; instead they found men and animals, sometimes wearing strange forms but always filling readily recognizable roles. In his Somnium (1634), Johannes Kepler imagines his moon-dwellers traveling by boat, planning towns and fortifications, and devising various means of coping with the less congenial lunar climate. Gonsales, the voyager in Francis Godwin's The Man in the Moone (1638), finds that lunar lifeforms are gigantic versions of those on Earth (just as the Pythagoreans had believed); likewise for Cyrano de Bergerac's stand-in hero Dyrcona in Les estats et empires de le lune (1657) [for Cyrano's complex bibliography, see his entry]; his moon folk pointedly reprise many Earthly faults (the Lunar priests even put Dyrcona on trial for claiming that Earth is a world like their own). The inhabitants of the moon and the other solar planets differ from Earthlings culturally but not physically in the Traüme (1754) of Johan Gottlob Krüger (1715-1759), and all the countless worlds of the universe have human populations in the epic poem Der Messias (1748; variously expanded until 1773 2vols) by Friedrich Gottlieb Klopstock (1724-1803). The pattern of life on Earth was reproduced with minor amendments: Utopian improvement or satirical (see Satire) exaggeration. Godwin's Lunarians enjoy extended longevity and are paragons of virtue. A rare few writers of the period ventured beyond the strictly anthropomorphic in imagining Extraterrestrials: the freethinking German scholar Christlob Mylius (1722-1754) speculated that the inhabitants of other planets might fly through the air, swim through the waters, or burrow in the ground like worms; in Cyrano de Bergerac's Les estats et empires du soleil (1662), Dyrcona finds in the Sun a world of intelligent birds (who again put him on trial); and the titular hero of The History of Israel Jobson, the Wandering Jew (1757), by Miles Wilson writing as M W, meets bellicose Moon dwellers made of metal (their surgeons, says Jobson's angel guide, are plumbers), mysterious scarlet-coloured Martians who stand still as trees, and Saturnians with one eye in front and one in the back of their heads. But the concept of a differently determined pattern of life, and thus of a lifeform quite alien to Earthly biology and habits of thought, did not fully emerge until the late nineteenth century, as a natural consequence of the concepts of Evolution and of the process of adaptation to available environments promulgated by Jean-Baptiste Lamarck and later by Charles Darwin.

The idea of genuinely alien lifeforms – beings shaped by adaptation to extraterrestrial environments – was first popularized by Camille Flammarion in his nonfictional Les mondes imaginaires et les mondes reels (1864; trans 1865 as Real and Imaginary Worlds) and in his short philosophical novel Lumen (1887; trans anon 1892) [for further publication details see Flammarion]. His account of Life on Other Worlds describes a world of sentient, gentle-mannered trees, another of tentacled seal-like creatures who must like sharks push themselves constantly through their world's atmosphere in order to breathe, and another where the flora and fauna are composed of silicon and magnesium. Flammarion's imagination ran in more mystical channels as well: The idea that divinely created souls could experience serial Reincarnation in an infinite variety of extraterrestrial physical forms – a notion that attained surprising popularity in the later eighteenth and nineteenth centuries – is featured in his Uranie (1889; trans 1890 as Urania); among the alien beings encountered here are the dragonfly-like inhabitants of a planet orbiting the multiple star system of Gamma Andromedae and the winged, six-limbed folk of Mars, and yet a great many are plainly humanoid, with minor variant features such as ears with a closable covering like eyelids or the bioluminescent display of emotions. This was, nevertheless, far more authentic alienness than most nineteenth-century tales of other worlds had to offer. The inhabitants of Mars, for example, are literally human in Mr. Stranger's Sealed Packet (1889) by Hugh MacColl, and in Robert Cromie's A Plunge into Space (1890) the Martians differ physically from Earthlings only in their oversized heads (perhaps the earliest occurrence of what would become a standard alien feature, particularly among those arriving in UFOs). In Six Thousand Years Hence (1891), Milton Worth Ramsey gives us Venusians distinguished from Earthlings chiefly by the peculiar fashion of their clothes. Among such company, Flammarion's aliens were a revelation.

Another major late nineteenth and early twentieth-century French-language writer, J H Rosny aîné, presented more startlingly strange aliens embedded in a more explicitly evolutionary framework than even Flammarion (from whom he drew inspiration). Prehistoric humans (see Prehistoric SF) battle inscrutable, geometrically-shaped mineral lifeforms in Les Xipéhuz (in L'Immolation ["The Sacrifice"] coll 1887; 1888; trans as "The Shapes" in One Hundred Years of Science Fiction, anth 1968, ed Damon Knight), while a race of vampiric "ferromagnetic" beings supplant a dwindling humanity on a depleted future Earth in La Mort de la Terre (1910; trans 1928 as "The Death of the World", new trans Danièle Chatelain and George Slusser 2012). Rosny's Un autre monde (1895 Revue Parisienne #5; exp as coll 1898; trans as "Another World" in A Century of Science Fiction, anth 1962, ed Damon Knight) reveals two alien species who live alongside modern humans in an unseen parallel dimension. Though most of Rosny's aliens had terrestrial origins (the case of the Xipéhuz is ambiguous), his Les navigateurs de l'infini ["Navigators of Infinity"] (December 1925 Les Œuvres Libres; 1927) features a love affair between a human and a six-eyed tripedal Martian. In the tradition of the French evolutionary philosophers Lamarck and Henri Bergson, Flammarion and Rosny fitted both humans and aliens into a great evolutionary scheme, and Rosny displayed a remarkable sympathy for alien beings, even when they were engaged in a fatal struggle with humanity. The human hero who studies the Xipéhuz in order to defeat them observes all manner of recognizable and even admirable behavior among them, and bitterly laments the fact that humanity's survival requires the extinction of this other intelligent species. Likewise, the ferromagnetics are not presented as simple Monsters but as creatures better adapted to the conditions of the future Earth, and their replacement of humanity evokes neither the glee of the satirist nor the horror of the anthropocentrist, only a note of fatalistic sorrow.

In the UK, evolutionary philosophy was dominated by the Darwinian idea of the survival of the fittest, so it is perhaps not surprising that the most notable of the early depictions of aliens in the British form of the Scientific Romance – H G Wells's The War of the Worlds (1898) – casts the alien as a Darwinian competitor, a genocidal invader aiming to conquer and colonize the Earth (see Invasion). Wells's Martians are unmistakably nonhuman – big bulbous leathery-skinned creatures dragging themselves about on "Gorgon groups of tentacles" – and have just as clearly evolved in an alien environment: They struggle in Earth's higher gravity and thicker atmosphere, and in the end, famously, they succumb in exemplary Darwinian fashion to the infectious bacteria of Earth, to which humans have long ago developed immunity. Wells thus deploys the cold logic of Darwinism for and against humanity. A superior alien competitor may well threaten the survival of the species (and in particular its place at the top of the planetary heap), but the ruthless process which has culled the weak over many generations has given humans a powerful advantage over any such challenger.

The role of aliens as rapacious invaders rapidly became a Cliché, and Wells's novel also set the pattern by which alien beings are frequently imagined as loathsome Monsters, though in his work the instinctive disgust felt by human observers is complicated by repeated musings on their vast intellectual superiority and the deeper implications of the Martians in the text. He employs his aliens as agents of social criticism, explicitly inverting the colonialism upon which British society rested and challenging the comforting illusions of imperial destiny cherished by his countrymen (see Imperialism). In the tradition of Cyrano de Bergerac and Voltaire, Wells focuses sharply on the existential threat posed by his alien beings, the radical demotion of humanity in the scheme of creation, as Earthlings are forced to assume the role of hunted animals. The War of the Worlds is the first fictional exploration of the terror inspired by mankind's "dethronement", and sf has both exhibited and countered this fearful response to the alien down to the present day. Wells took a less xenophobic tack in his elaborate description of an alien society in The First Men in the Moon (1901). He based it on the model of the ant-nest, with its array of diverse physical types adapted for specific functions, and thus inaugurated another frequently revisited motif, though his Selenites have individual names and personalities and a complex technological civilization (their vast physiognomic and functional diversity is more a product of conscious engineering than genetic determination). The civilization of the Selenites was the most fully developed alien society of its time, though the satiric impulse still guided the conception of many elements. In this it had more in common with the imaginary countries of Jonathan Swift's Gulliver's Travels than with the intricately conceived alien cultures found in later sf.

Early US Pulp-magazine sf in the vein of Edgar Rice Burroughs usually populated other worlds with quasihuman inhabitants – almost invariably including beautiful women for the heroes to fall in love with – and frequently, for melodramatic purposes, placed such races under threat from predatory monsters. Such is the pattern of Ralph Milne Farley's The Radio Man (28 June-19 July 1924 Argosy; 1948; vt An Earthman on Venus 1950), in which Venus harbours an assortment of giant insects, including an advanced civilization of giant ants, and a kingdom of extraordinarily handsome humanoids (including an eligible princess) restive under ant domination. The specialist sf magazines inherited this tradition in combination with the Wellsian exemplars, and made copious use of monstrous alien invaders; the climaxes of such stories were often unapologetically genocidal, as in "Invaders from the Infinite" by John W Campbell Jr (Spring/Summer 1932 Amazing Stories Quarterly), which details the utter destruction of the diminutive Thessians after a complex arms race and battles ranging across time and space. Edmond Hamilton was a prolific author of stories in this vein: "The Other Side of the Moon" (Fall 1929 Amazing Stories Quarterly) culminates in the shattering of the glass dome that protects an alien civilization of turtle-men, and "The Universe Wreckers" (May-July 1930 Amazing) similarly ends in the annihilation of the disk-shaped Neptunians. In the early Space Operas of E E "Doc" Smith and others, alien physical forms might break down along broad-stroke moral lines in an echo of the historical conflation of aliens with angelic or demonic forces: spider-like and octopoid aliens were reliably nasty, reptilian aliens usually but not invariably so, while meek and benevolent aliens often had humanoid, mammalian and avian characteristics. Thus the canine-humanoid Ortolians of Campbell's "Invaders from the Infinite" are peaceful victims of aggression, while the bellicose natives of the Sirius system in J Schlossel's "The Second Swarm" (Spring 1928 Amazing Stories Quarterly) are irredeemably hostile arachnid creatures, and in Sewell Peaslee Wright's "The Forgotten Planet" (July 1930 Astounding) the interstellar Alliance spends centuries trying to civilize the aggressive reptile-people of a newly contacted planet, only to have the aliens threaten all the inhabited worlds with a superweapon they have developed in secret. Hamilton's "Locked Worlds" (Spring 1929 Amazing Stories Quarterly) serves up one species of each type: the giant intelligent spiders planning to conquer Earth, and the bird-people who created the spiders, only to be overthrown and exiled to a few polar fortresses. In extreme cases, alien allies and enemies became straightforwardly symbolic of Good and Evil: The Arisians and Eddorians of Smith's Lensman series are secular equivalents of angels and demons; the universal benevolence and near-omniscience of the Arisians and the insatiable lust for conquest that drives the Eddorians exude a sense of the metaphysical, and the vast, almost godlike power of both races – who rarely appear on stage, instead acting through human and alien proxies – enhances their likeness to spiritual entities. (At least one writer has noted that the Arisian-Eddorian conflict bears more than a passing resemblance to the struggle between Ahura Mazda and Angra Mainyu in Zoroastrian Mythology.)

Early Pulp-sf writers occasionally inverted the Wellsian Invasion formula and cast humans in the role of alien conquistadors. In Arthur K Barnes's "The Mole-Men of Mercury" (December 1933 Wonder Stories), the Interplanetary Legion eagerly wipes out the tunneling natives to clear the way for human mining efforts, and in Campbell's "The Voice of the Void" (Summer 1930 Amazing Stories Quarterly) humanity exterminates the inhabitants of the Betelgeuse system in order to acquire a new home when the Sun threatens to go nova. A few seized the opportunity to decry the imperialist spirit of the age: Hamilton's "A Conquest of Two Worlds" (February 1932 Wonder Stories) takes a critical view of the ethos which informs much of his other work, as a young Earthling sides with the last remnants of resisting Martians and, after their defeat, leads an equally doomed rebellion of the Jovians; in P Schuyler Miller's "The Forgotten Man of Space" (April 1933 Wonder Stories) a terrestrial prospector sacrifices his life to protect kindly Martian creatures from the depredations of a new human mining team. Meanwhile, stories focusing on the exoticism of alien beings in place of Invasions tended to take their inspiration from the works of A Merritt, who had described in compulsive detail a fascinating life-system of metallic beings, somewhat reminiscent of Rosny's Xipéhuz, in The Metal Monster (7 August-25 September 1920 Argosy; 1946) and had transcended conventional biological chauvinism in his portrayal of "The Snake Mother" (25 October-6 December 1930 Argosy; incorporated in The Face in the Abyss 1931). Though Merritt's vividly strange creatures were, like most of Rosny's, wholly terrestrial in origin, they provided a model for writers interested in conveying a richer sense of alien otherness. Jack Williamson clearly showed Merritt's influence in "The Alien Intelligence" (July-August 1929 Wonder Stories) – a Lost Race tale depicting an ancient human civilization besieged by highly-evolved, superscientific insects – and "The Moon Era" (February 1932 Wonder Stories), which depicts a fecund Moon of the remote past and one of the earliest fully sympathetic alien beings, the insectoid-serpentine Mother, last survivor of a once-great lunar civilization.

Throughout these early years – indeed, well into the 1940s – aliens predominantly originated on the other planets of our own solar system, as well as under the surface (see Underground) or Under the Sea, and they were still quite often human or humanoid. The Pulps were filled with tales of Martians, Venusians, Jovians, Mercurians (see Mercury), and the occasional species from Neptune or the moons of Jupiter and Saturn (see Outer Planets). Aliens from the Saturnian moon Dione invade Earth in "The World in the Balance" by Marius (16 April 1927 Argosy All-Story); Ralph Robin's "The Pygmies of Phobos" (April 1936 Amazing) features two intelligent Martian species (one humanoid, the other intriguingly protozoan) waging genocidal war on each other; slender but fully human Martians interbreed with Earthlings in Manly Wade Wellman's "When Planets Clashed" (Spring 1931 Wonder Stories Quarterly); and "The Men Without Shadows" (October 1933 Amazing) by Stanton A Coblentz depicts gigantic cloud-like Saturnians. The parameters which had governed the centuries-long debate over Life on Other Worlds continued to shape the imagination space of sf, though the rationales for such limitations had largely vanished from the scientific discourse. A story like John Russell Fearn's "Wings Across the Cosmos" (June 1938 Thrilling Wonder) as by Polton Cross is most rare for the period in its portrayal of a small, super-dense, vaguely turtle-like alien from a world circling a distant unnamed star, as well as its evocation of the pelagic emptiness of deep space and its embrace of an Exogamy that doesn't involve an ersatz alien princess.

A significant advance in the representation of aliens was achieved by Stanley G Weinbaum, whose "A Martian Odyssey" (July 1934 Wonder Stories) made a deep impression on readers with its array of strange lifeforms, especially the ostrich-like Tweel, who proves not only more clever than the human protagonist, but also unfailingly loyal. Though Weinbaum described only a handful of creatures, his Martian ecosystem was something the Pulps had never seen before, chiefly because it stood on its own, apparently imagined as an exercise in itself, free for the most part of the satirical, melodramatic, or formulaic drives which typically shaped the depictions of aliens. Weinbaum's success in conveying the alienness of Tweel's habits of thought likewise far exceeded anything that had yet been accomplished. Weinbaum followed this up with other accounts of relatively complex alien biospheres (see Ecology), including most notably "The Lotus Eaters" (April 1935 Astounding), which features among other lifeforms a hyperintelligent vegetable doomed to extinction by its inability to defend itself against ravaging, less intelligent vermin. Another popular story which directly challenged the prevailing anthropocentrism and vulgarized Darwinian assumptions was Raymond Z Gallun's "Old Faithful" (December 1934 Astounding), in which humans and a Martian set aside their extreme biological differences and acknowledge intellectual kinship. This spirit was echoed in "Liquid Life" (October 1936 Thrilling Wonder) by Ralph Milne Farley, which proposed that a man was bound to keep his word of honour, even to a filterable virus.

Some of the more interesting and adventurous alien stories written in the 1930s ran afoul of editorial Taboos: The Creator (March/April 1935 Marvel Tales; 1946 chap) by Clifford D Simak, which suggested that our world and others might be the creation of a godlike alien – the first of the author's many sf considerations of pseudo-theological themes (see Gods and Demons; Religion) – was considered dangerously close to blasphemy and ended up in the semiprofessional Marvel Tales, which also began serialization of P Schuyler Miller's "The Titan" (1934-1935), whose description of a Martian ruling class sustained by vampiric cannibalism was seen as too erotic, and which eventually appeared in complete form as the title story of The Titan (1952 coll). The influence of these taboos in limiting the potential the alien being offered writers of this period, and thereby in stunting the evolution of alien roles within sf, should not be overlooked.

Despite the Wellsian precedents, aliens were much less widely featured in the UK Scientific Romances. The giant lizards in Alun Llewellyn's The Strange Invaders (1934) are deeply alien. Eden Phillpotts used aliens as "objective observers" to examine and criticize the human world in Saurus (1938) and Address Unknown (1949), but the latter novel explicitly challenges the validity of any such criticism. Olaf Stapledon's Star Maker (1937) built humans and aliens into a cosmic scheme akin to that envisaged by Rosny and Flammarion, and described a variety of nonhuman aliens of impressive strangeness with a resolute lack of anthropocentrism, including the symbiotically-linked "ichthyoids" and "arachnoids" who bring the talents of their two species together in an epitome of harmonious coexistence. Stapledon also employed the alien as a standard of comparison in one of his most bitter attacks on contemporary humanity, in The Flames (1947), which imagines an ancient race of solar beings entrapped in rock by the formation of the planets, only returned to activity when humans heat stone to high temperatures. Stapledon's work would exercise a powerful influence on many major sf writers and their conceptions of aliens and humans in a vast cosmos, including Brian W Aldiss, Arthur C Clarke and Stephen Baxter, but his interest in alien beings was unusual in his place and time; in general early UK writers devoted little attention to aliens.

The popularity of the alien-menace story did not begin to wane until shortly before the outbreak of World War Two, and has never been in danger of dying out. This should hardly come as a surprise; not only does the alien-menace formula offer unparalleled fodder for melodrama, it's also the simplest and most straightforward expression of the fear and horror felt by many since the idea of alien beings became the subject of serious speculation. The rank xenophobia displayed by many early examples of this story type eventually became unfashionable in the more reputable magazines, but monstrous aliens maintained their popularity in less sophisticated outlets, and found fresh life in film and television. The Cinema lagged behind written sf in this respect, as it often has, producing a host of cheap Monster Movies during the 1950s and 1960s such as Night of the Blood Beast (1958), The Eye Creatures (1965; vt Attack of the Eye Creatures), and the infamously inept Robot Monster (1953). But it wasn't all bad: worthy sf films of the period involving aliens include The War of the Worlds (1952), The Thing (1951; vt The Thing from Another World), and The Day the Earth Stood Still (1951).

Alien Monsters did not disappear from the flagship magazines, but became more sophisticated in their nature and in the potency of the fears they played on. John W Campbell Jr's "Who Goes There?" (August 1938 Astounding as by Don A Stuart) presented one of the earliest examples of the alien assimilator, a Shapeshifter which does not merely kill its victims but takes on their shapes and identities and thus conquers through stealth and deception rather than physical force. The profound Paranoia generated by Campbell's scenario could be more chilling than a straightforward Wellsian invasion. A E van Vogt's "Black Destroyer" (July 1939 Astounding) and "Discord in Scarlet" (December 1939 Astounding) likewise give us deadly aliens who use deception rather than simple force, and van Vogt reveals much more of his monsters' inner lives and motivations than did his more formulaic predecessors. Neither the large cat-like Coeurl of "Black Destroyer" nor the four-armed, four-legged Ixtl of "Discord in Scarlet" become precisely sympathetic, but their encounters with humans are more two-sided than in most earlier tales, and the outcomes (as in "Who Goes There?") turn on problem-solving more than firepower.

The emphasis shifted during the World War Two years toward the problems of establishing fruitful Communication with alien races. In this period human/alien relationships were often and perhaps inevitably represented as complex, delicate and uneasy. In van Vogt's "Co-operate – or Else!" (April 1942 Astounding) a man and a bizarre alien are castaways in a harsh alien environment during an interstellar war, and must join forces in order to survive, while in "The Cave" (January 1943 Astounding) by P Schuyler Miller a man takes refuge from a Martian sandstorm among a gathering of various native species, and his failure to understand the cooperative ethical code shared by the nonhumans leads to his demise. In "First Contact" (May 1945 Astounding) by Murray Leinster, two spaceships meet in the void (see First Contact), and each crew instinctively adopts a Paranoid position, determined to yield no information and make no move which could possibly give the other a political or military advantage, though they ultimately overcome the stalemate rather ingeniously. Another Leinster story, "The Ethical Equations" (June 1945 Astounding), asserts that a "correct" decision regarding mankind's first actions on contact with aliens will be very difficult to achieve, but that priority should be given to the attempt to establish a friendly relationship in hope of reciprocation; still another, "Propagandist" (August 1947 Astounding), reproduces the framework of mistrust depicted in "First Contact" but resolves it through the mediation of the human ship's dog: humans and aliens judge each other unthreatening by observing each side's treatment of the loyal animal, and together they defeat a third race of implacably hostile aliens. By contrast, "Arena" (June 1944 Astounding) by Fredric Brown bleakly assumes that the meeting of human and alien might still be a test of their ability to destroy one another. (Significantly, a 1967 adaptation of "Arena" for the television series Star Trek changed the ending of the story to bring it into line with later attitudes.)

A few writers in the 1940s began to consider that the Communication problem might actually be insoluble, that alien minds and motives might be too strange for human beings to understand and for any meaningful contact to occur. The title aliens of Fredric Brown's "The Waveries" (January 1945 Astounding) consist of electromagnetic energy; they are drawn to Earth by the leakage of broadcast radio signals, but they do not appear to understand the content, nor do they attempt to communicate. Humans eventually conclude that they'll never be able to tell if the waveries are sentient, since there's no common ground, no mutual reference point, from which to forge any kind of connection. In "Goldfish Bowl" by Robert A Heinlein (March 1942 Astounding), two men trapped in a featureless room by a mysterious alien intelligence wonder if there's any way to unmistakably indicate their intelligence to an alien mind. They fear that aliens might be too profoundly different, that there may be no mutual wellspring of experience from which to begin, and they speculate that their captor may have caught them purely by accident and might not have any interest in them at all. Similarly pessimistic meditations would become commonplace in subsequent decades.

Aliens from beyond the solar system began to appear as often as the homegrown sort during the 1940s. The encounter in Leinster's "First Contact" takes place in the Crab Nebula; Methuselah's Children (July-September 1941 Astounding; exp 1958) by Heinlein depicts an interstellar voyage that meets with a variety of beings on extrasolar planets; van Vogt's "The Monster" (August 1948 Astounding; vt "Resurrection" in The Other Side of the Moon anth 1956 ed August Derleth) brings an advance party of colonizing aliens from a distant star to a barren far-future Earth; and "Rescue Party" (May 1946 Astounding) by Arthur C Clarke features an ancient Galactic Federation uniting countless alien species (see Galactic Empires). Local aliens remained quite common – Venus is home to a radon-based lifeform in Simak's "Tools" (July 1942 Astounding), the heavy gravity of Jupiter traps its hostile natives in "Not Final!" (October 1941 Astounding) by Isaac Asimov, and Martians of various sorts appear in Lester del Rey's "Dark Mission" (July 1940 Astounding), Anthony Boucher's "Expedition" (August 1943 Thrilling Wonder), and many other stories – but by the late 1940s and early 1950s, as new research revealed the inhospitable conditions that prevailed on the other solar planets, aliens from those nearby worlds were increasingly relegated to the lesser magazines, juvenile fiction, and stories adopting a comic or humorous tone. Asimov, in The End of Eternity (1955), could refer casually to "the dead worlds of a solar system in which only Earth was livable" with confidence that this had become the dominant view among serious writers. Other star systems have been the source of the overwhelming majority of alien beings in sf ever since, though the solar planets continue to house the occasional alien being down to the present day.

Attempts to present more credibly nonhuman aliens – peculiar beings adapted to extraordinary environments – became gradually more sophisticated in the post-World War II period as well, particularly in the work of Hal Clement, whose Mission of Gravity (April-July 1953 Astounding; cut 1954) describes the intelligent centipede-like inhabitants of the fast-spinning disc-shaped planet Mesklin, on which the force of gravity varies from three times Earth normal at the equator to nearly 700 times Earth normal at the poles. In contrast, Cycle of Fire (1957) takes place on a world with a Great Year of two seasons lasting 40 terrestrial years each, which has forced drastic adaptations on the natives. Clement crafts these alien worlds with an attention to scientific detail unmatched by any of his predecessors, and he shares a good deal of that background with his reader along the way, but his imagination often falters when it comes to embedding his speculations in engaging stories (a problem that would often face future writers working in the same tradition). And for all the interesting physical alienness of his creations, his Mesklinites and other aliens still think very much like humans. Few writers adopted this intensive world-building approach at the time, but some portrayed distinctly nonhuman aliens in their own ways, such as the Spaceship-sized starfaring organism of Damon Knight's "Cabin Boy" (September 1951 Galaxy); sentient in itself, its parts (somewhat like organs) possess their own intelligence, at times acting against the wishes of the whole. Robert Sheckley cooks up something similar in "Specialist" (May 1953 Galaxy), in which interstellar Ships are composed of independently intelligent beings dedicated to various functions (an Engine, Walls, a Thinker, an Eye, etc) who cooperate voluntarily and almost instinctively; they are incredulous when they encounter the reflexive individualism of humans.

Sheckley's tale makes no pretence to the kind of rigorous scientific speculation practiced by Clement; for he was one of the many writers of the period who saw an opportunity for a more playful approach to the long-established topos of the alien, an approach made marketable by the arrival of new magazines such as Galaxy and F&SF, which staked out different philosophical and stylistic territory from Campbell's Astounding. Theodore Sturgeon's deceptively whimsical "The Hurkle Is a Happy Beast" (Fall 1949 The Magazine of Fantasy) packs a bite at the end, but its playful style and invented terms – "fardled", "fupped", etc. – make it read at times as a cross between Lewis Carroll and Dr Seuss. Poul Anderson and Gordon R Dickson – who would become stalwarts of sober-minded sf – kicked off their popular series of lighthearted Hoka stories with "The Sheriff of Canyon Gulch" (May 1951 Other Worlds), a tale in which the teddy-bear-like alien Hokas use their inborn linguistic abilities and talent for mimicry to create their own version of a Wild West town, based entirely on a few films and magazine stories, calling each other "Tex" and their reptilian enemies "Injuns". Damon Knight's classic "To Serve Man" (November 1950 Galaxy) delivers a blackly comic twist on the Trojan Horse Invasion plot, and "The Silly Season" (Fall 1950 F&SF) by C M Kornbluth gives us alien invaders who pave the way with a series of boy-who-cried-wolf episodes eagerly disseminated by a sensationalist press. Knight's "Catch That Martian!" (March 1952 Galaxy) jokes about how visitors from places much less distant can have a hard time in New York, so pity the poor alien. The most memorable use of comedic Martians came in Fredric Brown's Martians, Go Home (1955), in which a horde of stereotypical Martians – Little Green Men – come not to conquer but to pester humanity with an unending stream of insult, criticism, and wisecracks.

John W Campbell's editorial favouring of human chauvinism had slanted the presentation of aliens in sf for at least a decade, and that influence persisted well into the 1950s and 1960s. Stories such as Clarke's "Rescue Party", van Vogt's "The Monster", Heinlein's Starman Jones (1953), and L Ron Hubbard's Return to Tomorrow (1954) reassure us that humanity is more than a match for any extraterrestrial competitor, trickier and pluckier, nobler and somehow destined for intergalactic greatness. But as the hegemonic position of Campbell's Astounding waned in the post-World War II years, many writers resumed the time-honoured practice of using aliens as contrasting exemplars to expose and dramatize human follies or as stand-ins for oppressed and mistreated human groups. "All the Way Back" (July 1952 Astounding) by Michael Shaara appears as a sort of pivot point: Shaara affirms humanity's exceptional status, but that uniqueness lies in the unusual cruelty and ruthlessness of the species – a characteristic which has led to the race's quarantine and near-total destruction.That militarism is attacked in "Minister Without Portfolio" by Mildred Clingerman (February 1952 F&SF), Robert F Young's "The Other Kids" (March 1956 Fantastic Universe), and "Albatross" by Mack Reynolds (April 1955 Imagination), in which a fighter jet shoots down an alien craft, destroying the emissary of the Galactic Union bearing a message which offers solutions to the world's troubles. "The Waitabits" (July 1955 Astounding) by Erik Frank Russell and Simak's "'You'll Never Go Home Again!'" (July 1951 Fantastic Adventures; vt "Beachhead" in Beachheads in Space, anth 1952, ed August Derleth) impugn not only the militaristic approach but the exploitative and xenophobic urges that underlie it in stories of alien worlds which resist human conquest without recourse to weapons or violence of any sort.

Sexual prejudices are questioned in Theodore Sturgeon's "The World Well Lost" (June 1953 Universe), wherein a secretly homosexual human spacer saves a pair of aliens who've been condemned by their people for their same-sex liaison. (This was a particularly radical position to espouse; the British mathematician and codebreaking war hero Alan Turing had been tried and convicted on homosexuality charges in the UK just the year before.) Racism (see Race in SF) and sexism come under fire in "Dumb Martian" by John Wyndham (July 1952 Galaxy; vt "Out of This World" in Space Movies II, anth 1996, ed Peter Haining), in which a man bound for lonely asteroid work buys a female "Mart" as a servant and companion who's not nearly as dumb as he thinks; Leigh Brackett's "All the Colors of the Rainbow" (November 1957 Venture) is a tale of green-skinned humanoid aliens humiliated and brutalized by rural white townsfolk on Earth; Heinlein – a vigorous opponent of racial prejudice – likewise uses the "Venerians" (Venusians) of his Space Cadet (1948) as proxies for mistreated minority groups, and in Double Star (1956) he translates the era's casual racism to a colonized Mars, where his bigoted hero sheds his prejudice so completely as to be proclaimed an honorary Martian. The politics of human Imperialism and colonialism (see Colonization of Other Worlds) receive a stinging critique in "The Helping Hand" (May 1950 Astounding) by Poul Anderson, which depicts the corrosive effects on native society and culture of the Marshall Plan-style aid provided by the interstellar human Commonwealth. A similar case for local self-determination appears in Asimov's "Blind Alley" (March 1945 Astounding), in which an alien population languishes, failing even to reproduce, when relocated to a reservation on a different world. Lester del Rey had been among the earliest critics of rapacious Imperialism in "The Wings of Night" (March 1942 Astounding), the story of a human trader who saves the last survivor of ancient lunar civilization from enslavement at the hands of his avaricious partner; Simak's "Tools" (cited above) imagines a ruthless mining company which transforms radon-based Venusian lifeforms into robotic control mechanisms for its excavating equipment. Critiques of imperialistic exploitation – particularly as practiced by profit-mad corporations – continued in Invaders From Earth (1958 dos) by Robert Silverberg and Little Fuzzy (1962) by H Beam Piper, whose plot turns on a human court's willingness to recognize the alien "fuzzies" as sentient.

The horrors of the Second World War, and in particular the appalling specter of the atomic bomb, introduced grave doubts into sf about the character of the human species and its fitness for planetary – let alone galactic – dominance. The late 1940s and 1950s saw a sharp turn toward representations of aliens superior to humans not only in technology but in wisdom and morality. Simak's "Immigrant" (March 1954 Astounding) pricks the bubble of human vanity with remarkable acuity, presenting a human species not merely childlike in comparison to the super-powerful aliens of the planet Kimon, but stubbornly reluctant to admit the fact. In The Star Beast (1954), Heinlein neatly undermines humanity's grandiose self-image when an alien brought to Earth as a pet turns out to be the child of a previously unknown, vastly superior species. Lummox, as she's been known, has always thought of herself as raising her human companion rather than the other way around; the destruction of presumptuous Earth is narrowly averted via her maternal affection. Poul Anderson's "The Martyr" (March 1960 F&SF) offers a telling reassessment of the human spirit by a writer more typically inclined to celebrate it: determined to learn the secret of the vast psychic powers wielded by the alien Cibarrans, a human cabal abducts a number of them and subjects them to Torture; the humans are shamed by the stoic endurance of their captives and by the discovery that the aliens have withheld the information only to spare humanity the knowledge of a devastating truth about itself. (Once again, humans are exceptional, but in a decidedly undesirable way.) Other writers revived a theme popular in late nineteenth-century stories of aliens, in which humanity's warlike behavior is a sure sign of its immaturity and dangerousness. In "Loophole" by Arthur C Clarke (April 1946 Astounding) and "The Sky Was Full of Ships" by Theodore Sturgeon (June 1947 Thrilling Wonder; vt "The Cave of History" in Encounters with Aliens, anth 1968, ed George Earley), aliens view the development of nuclear weapons by humans as a call for intervention or interdiction, while in any number of other stories aliens stand in judgment of humanity's fitness for membership in the galactic community. In Heinlein's Have Spacesuit – Will Travel (August-October 1958 F&SF; 1958), for instance, two human girls face an intergalactic tribunal inclined to reject the species; a friendly alien appeals for more time, comparing humans to children deserving of some indulgence and an opportunity to grow up. Each of these notions – interdiction and collective judgment – became notable Clichés.

Aspects of the emerging Cold War mentality revealed themselves in the rise of the "paranoid" invasion plot (see Paranoia), in which alien invaders assume the appearance of humans or take control of human minds rather than relying on overt military attack. Leinster's "The Man in the Iron Cap" (November 1947 Startling Stories; exp as The Brain-Stealers 1954) is an early example, but by far the most famous and influential were Heinlein's The Puppet Masters (September-November 1951 Galaxy; 1951; text restored 1990) and Jack Finney's The Body Snatchers (November 26-December 24 1954 Collier's; 1955; rev as Invasion of the Body Snatchers 1978). Heinlein's story presents slug-like parasitic aliens which attach themselves to their victims' backs (see Parasitism and Symbiosis), while Finney imagines space-borne seeds which generate copies of sleeping humans to take the place of the originals. Heinlein made the Cold War resonances of his scenario explicit, openly comparing the alien mind-control to the effects of Communist indoctrination. Many have read a similar subtext onto Finney's story – particularly in its film version – but Finney's text itself attempts a critique of the relentless pattern of exploitation underlying human civilization, to which his invaders compare their actions.

Reflections on the politics and propaganda of the era also arise in stories such as Chad Oliver's "Blood's a Rover" (May 1952 Astounding) and "The Liberation of Earth" by William Tenn (May 1953 Future). Oliver describes an interstellar human effort to speed the technological development of compatible alien worlds (see Uplift) in order to build an alliance against an attack by a hostile alien empire – a none-too-subtle recasting of Cold War bloc formation. Tenn recounts the reduction of Earth to a barely habitable wasteland after it becomes a minor strategic outpost in a war between two superpowerful alien civilizations, each of whom promises the Earthlings that they're being saved – "liberated" – from the domination of the other.

The 1950s also saw the coalescence of a popular mythology surrounding UFOs which indelibly influenced images of and ideas about aliens in the public imagination. A spate of reported sightings by airplane pilots and others in the late 1940s popularized the notion that extraterrestrials were actively visiting and studying Earth in their "flying saucers" and spawned an enthusiastic subculture that developed a suggestive fabric of legend and its own theories about alien physiognomy, capabilities, and intentions. Sf writers occasionally adopted the imagery and conventions of UFO reports – most notably the flying saucer, which appears as extraterrestrial transport in Heinlein's The Puppet Masters and dozens of other works – but in general sf writers recognized the UFO enthusiasm as something different from and even antipathetic to the animating spirit of their work. They could hardly ignore it altogether, but they usually handled it with wry humour – as in Avram Davidson's "The Grantha Sighting" (April 1958 F&SF), in which a lonely farm couple, under questioning by a team of amateur ufologists, replace the pedestrian details of their actual alien encounter with a sensational tale that better fits the expectations of their eager audience – or the kind of biting critique found in Theodore Sturgeon's "Fear Is a Business" (August 1956 F&SF), which imagines the meeting between a writer of bestselling fraudulent UFO books and a real alien whose mission of global salvation is blocked by the fears and misconceptions stoked by the author's writings. Though often confused in the popular imagination, sf adopted a dismissive, skeptical and even hostile stance toward UFO culture early on, which it has maintained with few exceptions to the present.

One of the most remarkable redeployments of alien beings in the sf of the 1950s and 1960s was an upwelling of pseudo-theological themes (see Religion). Ideas about the inhabitants of other worlds had been governed by theological notions from the seventeenth through the nineteenth centuries – interplanetary romances of the nineteenth century often featured spirits or angels – and the traditional linkage had been revived outside the sf magazines by C S Lewis in his Christian allegories Out of the Silent Planet (1938) and Perelandra (1943; vt Voyage to Venus 1953). Within sf itself, however, the religious reading of aliens had previously been echoed only in a few Shaggy God Stories (see also Adam and Eve); Nelson S Bond's "Another World Begins" (November 1942 Blue Book; vt "The Cunning of the Beast" in Strange Ports of Call, anth 1948, ed August Derleth) in a classic example. In sf of the 1950s, though, aliens began to appear in metaphorically and even literally transcendental roles. Aliens are enlightened tutors in "Dear Devil" (May 1950 Other Worlds) by Eric Frank Russell and "Guardian Angel" (April 1950 Famous Fantastic Mysteries; rev Winter 1950 New Worlds; exp 1953 as Childhood's End) by Arthur C Clarke, in each case wearing diabolical physical form ironically to emphasize their angelic role. Russell's is a tentacled Martian poet who helps humanity recover from nuclear Holocaust, while Clarke's are benevolent invaders who guide humanity into a golden age, despite their literal resemblance to devils. Edgar Pangborn's "Angel's Egg" (June 1951 Galaxy) is less coy; it conveys a palpable sense of despair at human failings – it is not too much to say sinfulness – and a frank yearning for spiritual salvation. Still, the aliens of Russell, Clarke, and Pangborn remain closer to angels than to gods, superior to humans in wisdom and ability but not in essence. Raymond F Jones's The Alien (1951), on the other hand, is ambitious to become a kind of deity, and the alien in Philip José Farmer's "Father" (July 1955 F&SF) really is one. In Clifford D Simak's Time and Again (October-December 1950 Galaxy as "Time Quarry"; 1951; vt First He Died 1953) every living creature, Androids included, has an immortal alien "commensal", an sf substitute for the soul. James Blish's classic A Case of Conscience (September 1953 If; exp 1958) echoes the sorts of questions disputed in the seventeenth and eighteenth centuries: alien beings of impeccable ethics but without knowledge of God challenge the theology of a Jesuit, who fears they are creations of the Devil. Other churchmen achieve spiritual enlightenment by means of contact with aliens in "The Fire Balloons" (April 1951 Imagination as "In this Sign"; vt in The Martian Chronicles, coll 1953 UK ed) by Ray Bradbury, "Unhuman Sacrifice" (November 1958 Astounding) by Katherine MacLean, and "Prometheus" (March 1961 F&SF) by Philip José Farmer. In Lester del Rey's "For I Am a Jealous People!" (in Star Short Novels, anth 1954, ed Frederik Pohl) alien invaders of Earth turn out to have made a new covenant with God, who is no longer on our side. del Rey develops his conceit as more than an ironic gimmick, devoting ample space to a consideration of the nature of a divine covenant and the suitability of humanity to enter into one.

Of course, aliens continued to appear and evolve in less pointed contexts during the 1950s as well, as the problems of First Contact and Communication receded in favour of stories presenting human-alien interactions as commonplace and even unremarkable. Gordon R Dickson's "Lulungomeena" (in Looking Forward, anth 1953, ed Milton Lesser) employs a member of an unfailingly honest alien species to resolve a bet between two men on a remote border station, and in his "Black Charlie" (April 1954 Galaxy) the primitive sculptures of an otter-like alien provoke a touching meditation on the artistic impulse. James White kicked off his long-running Sector General series with "Sector General" (November 1957 New Worlds) and "Tableau" (May 1958 New Worlds), featuring a colourful array of credibly-developed nonhuman aliens as doctors and patients on a hospital space station. By the end of the decade, the uses of and approaches to aliens had achieved a range and diversity which prefigured many of the branchings and divergences that sf would undergo in the 1960s. "The Big Front Yard" (October 1958 Astounding) by Clifford D Simak offers a radically revised vision of Invasion and First Contact: a team of rat-like aliens reconfigure a small town handyman's house into a dimension-twisting portal that links Earth to an ever-expanding network of worlds. It's done without fanfare, ceremony, or explanation, and the apparent motivation is trade. Humanity has been plunged into a galactic community in which it will be anything but dominant, but the prevailing mood is one of quiet relief rather than fear; the atmosphere and concerns of Leinster's "First Contact" have been left far behind. Contrarily, Heinlein's Starship Troopers (October-November 1959 F&SF as "Starship Soldier"; 1959) upped the ante of xenophobia and militarism in a tale of ruthless interspecies warfare designed to resonate in the Cold War political atmosphere. Meanwhile, Kurt Vonnegut demonstrated how deeply aliens (and other sf tropes) had seeped into the cultural carpeting in the dizzying farce of The Sirens of Titan (1959), which reveals, among other things, that the entirety of human history has been shaped by the distant alien Tralfamadorians in order to produce a replacement part for their stranded robot explorer. Vonnegut draws on the by now rich library of sf concepts but deploys them without concern for their traditional effects or meanings, cutting them loose from their moorings and helping pave the way for the experimentation of the coming New Wave.

The evolution of alien roles in Eastern European sf seems to have been very different. The alien-menace story typical of early US-UK sf is absent from contemporary Russian sf, and the ideological calculation behind this absence is made clear by Ivan Yefremov in "Cor Serpentis" (1959; trans 1961 as title story of The Heart of the Serpent, anth 1961 Moscow, ed anon), which is explicitly represented as a reply to Leinster's "First Contact" (see First Contact). Yefremov argues that, by the time humans are sufficiently advanced to build interstellar ships, their society will have matured beyond the suspicious militaristic attitudes of Leinster's crew, and will be able to assume that aliens are similarly grown up. UK-US sf has rarely been that confident, though some writers of the period entertained a kindred optimism: The Overlords of Clarke's Childhood's End prove entirely benevolent, and in Damon Knight's "Rule Golden" (May 1954 Science Fiction Adventures; 1991 dos), the galactic confederation of alien species is likewise thoroughly peaceful. Their emissary argues persuasively against the logic of interstellar conquest, but it is characteristic of US sf that, despite many demonstrations of the aliens' kindly intentions, Knight's human protagonist fears a trick until the very end. Ideological replies to earlier work in the spirit of "Cor Serpentis" are certainly not unknown in US sf. Ted White's By Furies Possessed (1970), in which mankind finds a useful symbiotic relationship with rather ugly aliens, is a reply to The Puppet Masters by Heinlein; Joe Haldeman's The Forever War (June 1972-January 1975 Analog; fixup 1974) and Harry Harrison's Bill, the Galactic Hero (December 1964 Galaxy as "The Starsloggers"; exp August-October 1965 New Worlds; 1965) similarly respond to the xenophobic tendencies of Heinlein's Starship Troopers; and Barry B Longyear's Enemy Mine (September 1979 Asimov's; 1989 chap dos) can be seen as either a reprise of van Vogt's "Co-operate – or Else!" or a reply to Brown's above-cited "Arena".

Eastern European sf may have rejected the ideological assumptions behind the standard Invasion scenario, but that optimism did not extend to other questions surrounding contact with aliens. Stanisław Lem repeatedly emphasized the hopelessness of establishing meaningful Communication with alien beings, from his earliest sf work, Człowiek z Marsa ["The Man from Mars"] (serial 1946 Nowy Świat Przygód ["New Adventure World"]; 1994; partial trans Peter Swirski 2009), through Eden (1959; trans 1989), Solaris (1961; trans 1970; new trans 2011), and Niezwyciężony (1964; trans 1973 as The Invincible), to his penultimate novel, Fiasko (1986; trans 1987 Michael Kandel as Fiasco). Solaris, perhaps Lem's best-known work, details the fruitless and psychologically traumatic efforts of a human research crew to communicate with the sentient ocean-like organism covering the surface of a planet; in Fiasko, a human expedition sent to make contact with an alien civilization ultimately destroys the alien world in an act of ironic misunderstanding. The prospects are not much better in "Piknik na obochine" (1972 Avrora; trans 1977 Antonnia W Bouis as Roadside Picnic; new trans 2012 Olena Bormashenko) by Arkady and Boris Strugatski. Earth is visited by aliens who make no overt contact (no one seems even to have seen them) but leave behind several apparent landing sites strewn with variously useful, dangerous, and incomprehensible debris. Earth scientists cannot divine the principles behind any of the alien Technology, most of the world goes on as if the event had never occurred, and the best explanation is the analogy of the title: the aliens stopped on Earth like picnickers, unaware of or uninterested in humanity, and the debris left behind is the equivalent of trash and the occasional lost trinket.

In the 1960s and 1970s, the rise of the New Wave split the field into hostile factions, as some writers sought to reinvigorate what they saw as a moribund literature by importing techniques from mainstream fiction and shifting the emphasis to the human experience of sf scenarios, the inner lives of characters. Interest in aliens dropped off a bit among writers of the New Wave persuasion; they tended to adopt a stance like that of the Eastern Europeans, positing severe limits to the possibilities of Communication and understanding, challenging human self-importance, and employing aliens more for their metaphorical than their literal significance. Thomas M Disch offers an unflinching take on alien indifference in The Genocides (1965), as extraterrestrials completely reconfigure Earth's ecosystem (see Ecology) and treat humans as little more than pests. In his Mankind Under the Leash (1966 dos), human beings are the pets of alien masters, and most of them like it that way. Brian Aldiss's The Dark Light Years (1964) skewers human vanity and prejudice as humanity makes First Contact with an alien species that wallows in its own excrement. In novels such as The Game-Players of Titan (1963), Galactic Pot-Healer (1969), and Our Friends from Frolix-8 (1970), Philip K Dick handles stories of aliens with a sense of the absurd akin to Vonnegut's and a suggestion of cosmic futility as deep as Lem's. Enigmatic aliens establish huge five-pointed crystalline bases on a nuke-ravaged Earth in John Brunner's The Day of the Star Cities (1965; vt Age of Miracles 1973); as in Roadside Picnic, there is no attempt at communication, and bits of unfathomable alien tech litter the ground around each structure, giving rise to an active but risky black market. "Vaster Than Empires and More Slow" (in New Dimensions 1, anth 1971, ed Robert Silverberg) by Ursula K Le Guin describes the encounter of an interstellar expedition with a planet-spanning organism whose hostile response is driven by its horror at the very notion of other intelligent creatures. Communication appears at first to be more successful in Terry Carr's "The Dance of the Changer and the Three" (in The Farthest Reaches, anth 1968, ed Joseph Elder), but true understanding of the alien Loarra proves impossible – they inexplicably attack and destroy the human mining colony they had previously approved, and their explanations are nonsensical to humans. The illusion of mutual understanding only disguises the unbridgeable gulf.

Robert Silverberg's "Passengers" (in Orbit 4, anth 1968, ed Damon Knight) takes the by-now familiar notion of parasitic mind-controlling aliens (see Parasitism and Symbiosis) and employs them in a story not of planetary conquest but of inexplicable perversity, as the aliens seize control of people apparently at random, "riding" them for several days, piloting them into drunken binges, wild sexual escapades, and other excesses of which their hosts will have little or no memory; the aliens perform symbolic and metaphorical functions as a mysterious irresistible force shaping human experience. The emphasis on the metaphorical emerges even more clearly in Silverberg's "The Reality Trip" (May-June 1970 If), in which an alien observer hidden in a human-shaped shell finds himself falling in love, against all his training and inclination, with a persistent woman from down the hall; and in "Schwartz Between the Galaxies" (in Stellar 1, anth 1974, ed Judy-Lynn del Rey) the aliens become almost entirely figurative, delusions in the mind of a future anthropologist hungry for encounters with otherness. The predominant mood is one of disillusionment and desperation, an inconsolable sense of isolation in the face of dead solar planets and unreachable stars, the promises of earlier sf betrayed. Gene Wolfe, meanwhile, characteristically embraces the dual nature of the alien figure in The Fifth Head of Cerberus (fixup 1972); his shapeshifting indigenes have the literal tangibility of traditional sf, but Wolfe's fragmented and elliptical narrative employs them to magnify the questions of the mutability of identity which lurk at the center of his triptych.

The gradual decay of editorial Taboos from the 1950s onward permitted more adventurous and explicit exploration of sexual and psychological themes (see Psychology), and aliens proved especially useful to writers interested in Sex, Feminism, and challenging entrenched notions of Gender roles. This work was begun by Philip José Farmer, in such stories as The Lovers (August 1952 Startling; exp 1961), "Open to Me, My Sister" (May 1960 F&SF) and "Mother" (April 1953 Thrilling Wonder), and was carried forward by others such as Harlan Ellison in "How's the Nightlife on Cissalda?" (in Chrysalis, anth 1977, ed Roy Torgeson) and Gardner Dozois, whose Strangers (in New Dimensions IV, anth 1974, ed Robert Silverberg; exp 1978) is a more sophisticated reprise of The Lovers. James Tiptree Jr, produced some of her most potent and mordant reflections on sex and gender relations in a handful of influential alien stories of the 1970s. "And I Awoke and Found Me Here on the Cold Hill's Side" (March 1972 F&SF) displays human fear and loathing of the alien curiously alloyed with self-destructive erotic fascination. A spider-like alien struggles with the violence of its species' reproductive cycle in "Love Is the Plan, the Plan Is Death" (in The Alien Condition, anth 1973, ed Stephen Goldin), and in "The Screwfly Solution" (June 1977 Analog as "Raccoona Sheldon") aliens clear the Earth for colonization by seeding a plague that exaggerates latent inclinations, causing infected men to kill women. Tiptree's best-known story, "The Women Men Don't See" (December 1973 F&SF), depicts the intolerable isolation of women constantly dismissed by and misunderstood by men; two of them plead with aliens to take them away, though their fate may be no better than that of animals in a zoo. Lisa Tuttle's "Wives" (December 1979 F&SF) is a similarly scathing attack on traditional gender roles. Ursula K Le Guin's Hainish series narrows the range of physical alienness – the inhabitants of most planets are derived from common humanoid stock, adapted as necessary to their specific environments – but the consequent emphasis on cultural variance adds piquancy to her exploration of the social construction of gender in The Left Hand of Darkness (1969), and to the critique of war and colonialism in The Word for World is Forest (in Again Dangerous Visions, anth 1972, ed Harlan Ellison; 1976 chap).

Writers in the classic mode extended their presentations of aliens in the 1960s and 1970s chiefly by developing alien biologies, cultures and civilizations with greater complexity and a thicker veneer of scientific plausibility. Larry Niven's Tales of Known Space is the most significant example to emerge from the 1960s, a series of sophisticated space adventures involving the ferocious feline Kzinti, the cowardly and supremely manipulative Puppeteers, and a catalogue of other alien species engaged in impressively grandiose feats of physical and social engineering within a Future History context of steadily accreting elaboration. Where ferociousness or deviousness might have been sufficient characterization of alien foils in earlier Space Opera adventure tales, Niven provides his with motives and interests grounded in plausible histories of physical and cultural evolution. The first stories in the sequence appeared in the mid-1960s – "The Coldest Place" (December 1964 If), "The Warriors" (February 1966 If), "Neutron Star" (August 1966 If), etc. – and it continued in Niven's landmark Macrostructure novel Ringworld (1974) and a flurry of other works. Protector (1973; exp of "The Adults", June 1967 Galaxy) details the peculiar life-cycle of the Pak, another recurring Known Space species who have appeared as recently as Destroyer of Worlds (2009) by Niven and Edward M Lerner. Other notable classic mode work of the period includes Gordon R Dickson's The Alien Way (1965), which delves into the biology, psychology, and history of the alien Ruml, and depends upon the human hero's insights into the alien culture for the peaceful prevention of interspecies war, and The Watch Below (1966) by James White, in which parallel stories of the survival of human and alien groups in extremis preface (and provide the cure for) an Invasion driven by ingrained militarism and a failure of Communication.

These writers, along with Poul Anderson, Frederik Pohl, Frank Herbert, and a host of others, insisted on the continuing viability and intellectual fertility of traditional sf modes – including traditional deployments of aliens – and they embraced them without irony or a hint of shame. In Niven's World of Ptavvs (1966; exp from March 1965 Worlds of Tomorrow), for instance, all life on Earth descends from the yeast farmed here by aliens a billion years ago – a scenario rich with potential for Satire in the hands of Vonnegut or Disch (or, later, Douglas Adams), played by Niven without the slightest wink. The staunch defenders of classic sf reinvigorated flagging traditions with a combination of dash, audacity, and muscular plotting reinforced by greater sophistication and a grounding of informed scientific speculation, and their brand of reformed space adventure produced a string of worthy alien stories throughout the 1970s, among them Anderson's "The Queen of Air and Darkness" (April 1971 F&SF), Fire Time (1974), and The Avatar (1978); The Mote in God's Eye (1974) by Niven and Jerry Pournelle; Niven's "Assimilating Our Culture, That's What They're Doing!" (November-December 1978 Destinies); Arthur C Clarke's A Meeting with Medusa (December 1971 Playboy; 1988 chap dos); Frederik Pohl's Gateway (1977) and Jem (1979); and Frank Herbert's Consentiency novels Whipping Star (1970) and The Dosadi Experiment (1977). Faith in the universality of reason, and hence in the fundamental similarity of all intelligent beings, is strongly evident in many of these accounts of physically exotic aliens – even those of Isaac Asimov's The Gods Themselves (March/April-May-June 1972 Galaxy; 1972), who dwell in a parallel universe with different physical laws, nevertheless pose few challenges to establishing fruitful conversation. Not every writer in the classic mode embraced this faith – Arthur C Clarke flouted it in much of his late 1960s and 1970s work, from the mysterious monoliths of 2001: A Space Odyssey (1968) to Rendezvous with Rama (1973), in which a titanic alien starship (see World Ship) skims through the solar system, indifferent to humanity and baffling to the humans who explore it before it slingshots back into deep space. Nevertheless, the frequent affirmations of alien intelligibility form a kind of steady rebuttal to the corrosive doubts voiced by the New Wave.

Stories dealing soberly and thoughtfully with problems arising out of cultural and biological differences between humans and aliens began to appear with great frequency in the 1970s and remained numerous into the 1980s and 1990s. The subject has been a constant and continuing theme in the work of several writers, notably Jack Vance, Poul Anderson, David J Lake, Michael Bishop and C J Cherryh. Cherryh's novels – including her Faded Sun trilogy (1978-1979), Serpent's Reach (1980), the Chanur series (1982-1986) and Cuckoo's Egg (1985) – present an elaborate series of accounts of problematic human-alien relationships and often involve the integration of human characters into alien cultures. In the Faded Sun books, a man must become a member of an alien society by adopting the harsh rules of its warrior caste; in Serpent's Reach, a woman takes refuge among the hive-minded alien Majat (see Hive Minds); and in Cuckoo's Egg, a Cloned human child is raised by aliens as an emissary to humanity. Other examples of humans raised by aliens can be found in Robert A Heinlein's Stranger in a Strange Land (1961; text restored 1991) – whose enigmatic Martians are however kept almost entirely offstage – Philip E High's The Prodigal Sun (1964) and Frederik Pohl's Homegoing (1989).

Cherryh's ongoing Foreigner series [1994-current] centres on a lone human who lives among the alien Atevi and serves as translator and diplomat in relations with the human colony on the planet. The novels set in Alan Dean Foster's Humanx Commonwealth series rarely delve as deeply into intercultural and interspecies relations as Cherryh's, but Foster echoes the spirit of Cherryh's work in his depiction of human-alien connections of varying degrees of intimacy, from the personal tie between recurrent protagonist Flinx and his alien companion Pip to the individual lifetime bonds that link humans and the alien "furcot" in Midworld (1975). The relationship between humanity and the insectoid Thranx, which gives the series its name, is closer to symbiosis than political alliance (see Parasitism and Symbiosis); Nor Crystal Tears (1982), which recounts the First Contact between the two species, offers a humanely optimistic account of the navigation of mutual human-alien fear and mistrust. Neither Foster nor Cherryh invoke religious notions or imagery in their accounts of human-alien relations – while the experience can be transformative, it is never transcendental – but other writers freely deploy spiritual allusions and resonances, sometimes quite fevered, in tales of literal salvation provided to humans via the adoption of alien ways, such as Robert Silverberg's Downward to the Earth (1970) and George R R Martin's "A Song for Lya" (June 1974 Analog). As countless Europeans and Americans sought religious transport in the traditions of the exotic East, sf located it in the more radically unfamiliar cultures of extraterrestrials.

Serious scientific interest in intelligent Extraterrestrial life underwent a revival in the 1960s and 1970s, with major implications for the imagining of aliens in sf. Even as new observations of the solar planets – especially those of Mars, Venus, and Mercury by the Mariner (USA) and Venera (USSR) probes, and the Viking Mars lander – rendered the hope of finding complex lifeforms there untenable, radio astronomers began to look for transmissions from alien civilizations circling other stars (see SETI). Carl Sagan became the best-known proponent of this effort, and of the belief in mutual intelligibility and the beneficial effects of alien contact for humankind. Sf writers took little notice of this development at first, perhaps because it offered few of the obvious dramatic possibilities provided by existing approaches – an exception was A for Andromeda (1962) by Fred Hoyle and John Elliot, Hoyle himself being a professional astronomer – but by the late 1960s radio contact featured in a number of interesting works, including James Gunn's "The Listeners" (September 1968 Galaxy), The Cassiopeia Affair (1968) by Chloe Zerwick and Harrison Brown, and Robert Silverberg's Tower of Glass (1970). Stanisław Lem turned his skepticism on the project in His Master's Voice (1968; trans 1983), but Sagan's optimism infused most sf treatments of the 1970s and 1980s, frequently endowing successful contact with quasi-transcendental significance. Gunn's original story focused on the psychological strain imposed by a project which might not produce results in less than a century, if at all, but the novel-length version of The Listeners (fixup 1972) includes the reception of a translatable signal with largely positive consequences, while Ben Bova's Voyagers (1981), Jeffrey A Carver's The Infinity Link (1984), Sagan's own Contact (1985), and Fred Fichman's SETI (1990) adopt a similar and often extravagant optimism. The Hercules Text (1986) by Jack McDevitt may not be quite so overheated, but it reiterates the belief that communications from distant aliens will offer revolutionary advancements in science, technology, and philosophy which might help solve the increasingly intractable troubles of human beings.

The yearning for alien contact infused some of the most prominent sf films of this era (during which sf films graduated to major cinematic status), most notably Stephen Spielberg's Close Encounters of the Third Kind (1977), which merged UFO lore with a powerful transcendental urge and left a permanent impression on the popular imagination. Stanley Kubrick's 2001: A Space Odyssey (1968), co-written by Arthur C Clarke, had pioneered the Exogamic mode in film with its psychedelic rendering of alien communion. It marked a revolutionary break with the predominant film paradigm of the alien as Monster or invader (see Invasion), and to a great extent contributed to a decade-long extinguishing of that Cliché. The trend reached its apogee in the early 1980s with the cloyingly adorable E.T.: The Extra-Terrestrial (1982), who experiences a Christ-like death and resurrection; Jeff Bridges as Starman (1984), who appears as a cloned version of a recently widowed woman's husband and leaves her gravid with a wonder-child; and the fountain of youth granted to a gaggle of creaking elders in Cocoon (1985). Where once the notion of the alien being was inherently fearful, the sf of the 1970s and 1980s manifested an eager determination to meet and establish significant contact with aliens, and the hope that they might save us from ourselves and the fate to which all flesh is heir. Despite continued exploitation of the melodramatic potential of alien invasions and interstellar wars, the predominant anxiety in the sf of the period was that we might prove unworthy of such communion.

In the very midst of this contact hunger, however, Ridley Scott's Alien (1979) reintroduced the alien-as-monster to sf Cinema, and its drippy, sticky, viscerally biological vision of alien menace immediately challenged the angelic alien paradigm. Scott's film inaugurated a new age of silver-screen alien terrors (and a wave of imitations) which has yet to fade. Inseminoid (1981), Galaxy of Terror (1981), The Deadly Spawn (1983), Xtro (1983) and other low-budget efforts rode Alien's coat-tails; John Carpenter's remake of The Thing (1982) replaced James Arness' shambling humanoid monster with the Campbell story's original Shapeshifting alien, inclined here toward grotesque transmutations of human flesh; Predator (1987) upgraded the 1950s-style alien Monster-Movie formula by swapping Arnold Schwarzenegger for the usual hapless teens; and James Cameron's official sequel, Aliens (1986) cemented the motif of alien peril in the popular consciousness with far more effective and disturbing imagery than had ever been served up before. It's little surprise that writers in the late 1980s and 1990s began to express deepening unease at the prospect of surrendering to alien grace. Octavia Butler's Xenogenesis trilogy (1987-1989) imagines Exogamy at its most extreme, when the only hope for the survival of the human species in any form after a nuclear Holocaust rests on sexual and genetic merger with the alien Oankali; such mingling is simply the Oankali way of life, but it presents the humans with psychological challenges not all can or wish to meet. A horror of hybridization goes back at least as far as Heinlein's Methuselah's Children, but the Xenogenesis treatment is richer, more complex and ambivalent than any earlier work on the subject. It suggests a broadening of the imagination along with a faltering confidence in the inherent virtue of humanity. In similar fashion, the immortality offered by alien visitors in The Harvest (1993) by Robert Charles Wilson requires humans to have alien cells implanted in their bodies. Most of the population accepts, but it's unclear whether or not they get what was promised, or that it's worth the price. (Upon "ascending", the newly immortal disquietingly leave behind their skins.) Individual communion can be just as fraught: The heroine of Sheri S Tepper's Grass (1989) turns her back on an unhappy marriage and a lifetime of dutifulness in favour of a relationship with one of the elusive alien "foxen", but the bond between the human hunters and their horselike alien mounts has a much darker tinge.

The skeptical spirit extended to less existentially threatening forms of alien assistance as well. In Nancy Kress's "Out of All Them Bright Stars" (March 1985 F&SF), a brief encounter with a benevolent humanoid alien leaves a kind-hearted waitress wishing it had never occurred –the meeting has managed only to highlight the various miseries of her situation, changing nothing and making them harder to bear. The reptilian aliens of James Patrick Kelly's "Think Like a Dinosaur" (June 1995 Asimov's) offer humanity a method of interstellar travel, but the use of it requires overcoming some natural squeamishness – learning to think more like the aliens – and it's far from obvious that we'd be better for it. In late twentieth-century sf, alien assistance frequently creates as much or more trouble than it solves, as in Patricia Anthony's Cradle of Splendor (1996), whose alien benefactors deliver a variety of technological wonders to the citizens of Brazil, provoking international suspicion and jealousy and eventually an invasion by the United States. Alien intervention often arrives unbidden and unannounced: Presumably it lies behind the sudden disconcerting "Change" in Robert Reed's Beyond the Veil of Stars (1994); the sky becomes oddly inverted, clear as ever during the day but reflecting an image of the other side of the planet by night, and the shift reveals holes in space-time through which humans can travel to other planets, though they are transformed into indigenous alien forms in the process. In Joan Slonczewski's The Wall Around Eden (1990), aliens preserve small communities of humans in safe zones on Earth after a nuclear war, but they forbid the use of most technology. Their motives and intentions remain obscure, leading some humans to suspect that the aliens caused the war themselves. Similar suspicions arise in Nancy Kress's After the Fall, Before the Fall, During the Fall (2012) as aliens groom a troop of humans for release into a cleaned-up post-apocalyptic world; in Kress's Nothing Human (2003), miscalculation and human resistance complicate alien attempts to engineer a human breed to survive in contaminated postwar conditions; and her Steal Across the Sky (2009) involves the arrival of aliens who have come to make amends for tampering with human genes in the distant past. Like the gods of Greco-Roman myth, Kress's aliens wield sometimes awesome powers without superior wisdom or unadulterated good will, and mortals may be blown about like fallen leaves.

Hopes of alien salvation have not entirely faded, though they have remained for the most part muted and tentative. In Sheri S Tepper's Raising the Stones (1990), the entire population of a human colony world becomes infested with a sentient alien fungus which subtly alters moods and actions, eliminating violence and reducing conflict. Tepper presents this symbiosis as fairly palatable medicine for humanity's perennial ills, though she avoids enthusiastic endorsement. In Amy Thomson's The Color of Distance (1995), a human scientist stranded on a world whose ecosystem is hostile to human biology can only survive by genetically assimilating with the amphibious natives, and the process affords her a fulfilling connection with the aliens and their culture. (Those natives offer assistance to the environmentally ravaged Earth in the sequel, Through Alien Eyes [1999], though xenophobic humanity resists.) Remnant Population (1996) by Elizabeth Moon tells the story of an elder woman who finds purpose, validation, and even a measure of power when she assumes a crucial role in an alien culture. Robert Sawyer's Factoring Humanity (1998) and Calculating God (2000) express perhaps the most fervent optimism to be found in the millennial period. In the former, radio messages from Alpha Centauri lead to a quasi-mystical encounter with humanity's "Overmind", a kind of mass consciousness; when ours meets that of the Alpha Centaurians, an epiphany occurs, rendering people kinder, more compassionate and empathetic, and ringing in a semi-utopian era. In the latter, an giant spider-like alien anthropologist convinces a human palaeontologist that God must exist, and they journey with representatives of two other alien species to a rendezvous with the deity in distant space (see Religion). Sawyer's Rollback (2007) can be read as a response to Hoyle's and Elliot's A for Andromeda – in each case an interstellar radio transmission contains instructions for the creation of an alien lifeform; Hoyle's and Elliot's aims to take over the Earth, while Sawyer's comfortably integrate themselves into a middle-class suburban Canadian family.

Sawyer's enthusiasm notwithstanding, the aliens of the late twentieth and early twenty-first centuries often appear less grandiosely powerful than in earlier periods, their intentions more difficult to gauge, their impact on humans more ambiguous. Outside of the adventure fantasies of Space Opera, we don't expect as much from aliens anymore – we no longer anticipate that our older and wiser alien cousins will descend to hand us the keys to Heaven, or even a welcome packet from the Galactic Neighborhood Association. Individual humans may find relief or revelation in extraterrestrial communion, but aliens probably can't – or won't – stop us from nuking the planet, or fouling our nest (see Pollution), or bringing about our own downfall one way or another (see End of the World). If they intervene at all it's only after the damage has been done – as in Kress's recent work, or Paul J McAuley's Jackaroo stories – and the help they offer will be ambiguous at best. We're on our own, says millennial sf, whether we're actually alone in the cosmos or not.

In keeping with this sensibility, superpowerful aliens in recent decades have commonly been pushed into the deep past or a different dimension, with only their mysterious ruins and artefacts left behind. These often provide humans and other species access to the stars via Faster Than Light technology or a network of wormholes (though mucking about with Forerunner tech all too often unleashes ancient alien menaces and the like). Such conceits multiplied during the 1980s and 1990s, driving works such as Frederik Pohl's Heechee series (1977-2004), Charles Sheffield's Heritage series (1990-2002), Jack McDevitt's Academy/Priscilla Hutchins series (1994-2007), and Robert Sawyer's Starplex (1996), and continued in the twenty-first century with the Probability series (2000-2002) by Nancy Kress, Robert Reed's Marrow (2000) and The Well of Stars (2004), the Revelation Space series (2000-current) by Alastair Reynolds, and Heaven (2004) by Ian Stewart and Jack Cohen, among others. (In David Brin's Uplift universe [1980-current], intelligence itself is an artefact of the ancient Progenitors, who engineered sentience into several alien species who have over the billion years since performed the service for many more.) Most if not all of these works are examples of the so-called "new" Space Opera which blossomed during the same period and has provided a venue for all sorts of human-alien interaction in the context of vast interstellar communities (see Galactic Empires). In some cases humanity develops its own star-spanning culture before encountering alien counterparts, as in Peter Hamilton's Commonwealth Saga (2004-2005) and Jack McDevitt's Alex Benedict novels (beginning in 1989 with A Talent for War), or humanity predominates in a multi-species culture, as in Kress's Probability books. In others, humans are minor players in a society dominated by more advanced aliens – for instance, the Beyond of Vernor Vinge's A Fire Upon the Deep (1992) – or is a relatively recent member of an ancient multi-species civilization. The most interesting of these last are Iain M Banks's Culture novels, beginning with Consider Phlebas (1987) and continuing into recent years with Surface Detail (2010) and The Hydrogen Sonata (2012). The Culture includes machine intelligences as well as biological and operates on an anarchic principle, incorporating new species and societies only as they voluntarily adopt the Culture's ways, in contrast to what Banks terms "hegemonizing swarms", entities bent on conquest or outright assimilation, which the members of the Culture view as barbaric. In some works, such as Foster's Humanx Commonwealth books or the Retrieval Artist series (2000-current) by Kristine Kathryn Rusch, humans and aliens have developed a common culture or engage in active trade and interspecies politics. In others, human-alien relations are confined primarily to warfare, as in Stephen R Donaldson's Gap Cycle (1990-1996) and the dizzyingly complex Future History of Stephen Baxter's Xeelee Sequence – begun in "The Xeelee Flower" (Spring 1987 Interzone) – which recounts the expansion of humanity into space, its defeat and domination by two different alien species, and its long war with the superpowerful alien race of the series title.

Stories of interspecies war and alien invasion have remained perennially popular, but they experienced a noticeable surge in the late twentieth century. L Ron Hubbard's Battlefield Earth (1982), a bloated and unabashedly pulpy epic of rebellion against alien overlords, attained bestseller status despite widespread critical dismissal. Larry Niven and Jerry Pournelle did a more creditable job in their similarly lengthy (and similarly bestselling) Footfall (1985), wherein the invaders are the intriguing pachydermic Fithp; important elements of the plot turn on the aliens' history and distinctly different habits of warfare. In Ender's Game (August 1977 Analog; exp 1985), Orson Scott Card revives the genocidal impulses of an earlier era, as humanity not only defends itself against invasion by insectoid aliens but attempts to wipe the enemy out completely. (Card followed it up with an expiatory sequel, Speaker for the Dead [1986], in which the leader of that near-genocide seeks a suitable planet on which the former enemy Hive Queen can re-establish her species.) Gordon R Dickson's Way of the Pilgrim (1987; exp of "The Cloak and the Staff", August 1980 Analog) adopts an alien-domination scenario not unlike Hubbard's, but bases the success of the resistance on a subtle manipulation of alien cultural imperatives rather than military action (futile against the overwhelmingly superior alien technology). This was an exception. The development during this period of the Military SF subgenre created a natural platform for stories of human-alien combat; early examples include the Demu trilogy (1973-1975; omni 1980) by F M Busby and Timothy Zahn's The Blackcollar (1983) and The Backlash Mission (1986). The form boomed in the 1990s with the Fifth Imperium trilogy (1991-1996; omni 2003) by David Weber, the Man-Kzin Wars shared-world anthology series (thirteen volumes since 1988), the Heritage series (1998-2000) by Ian Douglas, and Harry Turtledove's Worldwar (1994-1996) and Colonization (1999-2001) series, among others, and stories of human-alien conflict in this mode have proliferated even further in the twenty-first century; a short list of those with significant alien involvement includes John Ringo's Posleen War books (2000-current), The Saga of the Seven Suns (2002-2008) by Kevin J Anderson, the Dread Empire's Fall trilogy (2002-2005) by Walter Jon Williams, David Weber's Safehold series (2007-current), the Inheritance series (2008-2009) and the Star Carrier series (2010-2012) by Ian Douglas, and Tony Daniel's Guardian of Night (2012). Writers in this vein occasionally develop alien races with odd and interesting traits, but the focus most often remains squarely on battle and so affords little room for exploring other aspects of human-alien relations. Much of this material indulges in improbable, even embarrassing, fantasies of human exceptionalism – alien tacticians behave nonsensically, superpowerful weaponry somehow fails to overwhelm, and humanity's apparently unique grit and determination allow the species to defeat a parade of extraterrestrial opponents who have easily subdued dozens of other alien species. The genre's age-old chauvinism thrives on, at least in this specialized corner of the sf ecosystem.

One particular motif which has enjoyed remarkable popularity in the millennial period is that of rapacious and implacable aliens bent on the destruction of other life – all other life, all intelligent life, all biological life, or sometimes just humans. Many of these locust swarms consist of machine intelligence, often self-replicating war machines left over from some ancient conflict; the model for these is Fred Saberhagen's Berserker series (1963-2005), which not coincidentally experienced an upsurge in popularity (and production) beginning in the mid-1980s (see Berserkers). A few writers made use of the idea in thoughtful Hard SF novels: such a swarm is drawn to Earth by humanity's leakage of electromagnetic signals in Greg Bear's The Forge of God (1987), and a small group of rescued humans pursue the deadly machines to their origin in the sequel, Anvil of Stars (1992). Gregory Benford structures his ambitious six-volume Galactic Center Saga (1977-1996) around an epic struggle between humanity and a vast interstellar network of biology-hating machines. (Interestingly, both Bear and Benford propose that their biocidal machines account for the failure of humanity to discover signs of other advanced alien civilizations [see Fermi Paradox] – they're snuffed out as soon as they arise.) Enemies of unreasoning and unquenchable hostility have an obvious appeal for the type of Military SF discussed above, and the gigantic scales native to Space Opera make that mode another attractive venue for powerful galaxy-threatening alien forces. Recent works have presented more of these plagues in biological rather than machine form, though their genocidal rage is none the lesser for that. Weber's Fifth Imperium trilogy centers on the re-emergence of the alien Achuultani, who have swept through the galaxy several times before, annihilating all other intelligent life they encounter. The cosmic threat in Hamilton's Commonwealth Saga comes from a massive malevolent organism which has come to dominate its native planet through an extended Hobbesian war of all against all – when it learns of life on worlds beyond its own, it pursues a policy of total extermination. The Inhibitors of Reynolds's Revelation Space series, survivors of an ancient war and planners on galactic and billion-year scales, have determined to wipe out all other intelligent life until their scheme for preventing the collision of the Andromeda and Milky Way galaxies has been completed. They are not hostile, exactly, just ruthlessly pragmatic, but the upshot remains the same for humanity and other beleaguered races.

Alongside the grand cosmic sagas, writers of the late twentieth century demonstrated a notable taste for quiet tales of alien contact as well, stories in which the context remains tightly constrained (to a single person or a small cast, perhaps a small town at most) and the alien presence serves to catalyze events, provoke changes, and reveal elements of personal and social relationships within this limited setting. The impact can be profound, but it remains localized and sometimes entirely private. In some cases, such as Karen Joy Fowler's exquisitely subtle Sarah Canary (1991) and Patricia Anthony's quietly moving God's Fires (1997), the alien visitor is not even recognized as precisely what it is – Fowler offers even the reader only a few clues – but this does not prevent the alien's presence from disrupting the lives of those it encounters. (Fowler and Anthony set their novels in the historical past, but the failure of recognition can occur in a contemporary context, too, as in "If Nudity Offends You" by Elizabeth Moon [February 1988 F&SF].) In "The First Contact with the Gorgonids" (January 1992 Omni) by Ursula K Le Guin, an American couple stumble upon a crew of semi-humanoid aliens repairing their damaged ship in the Australian Outback, and the event fortuitously saves the wife from the unpleasantness of her marriage. An alien rescues an Asian boy from a life of sex slavery as an act of atonement for an unspecified offence in Bruce McAllister's "Captain China" (in Off Limits: Tales of Alien Sex, anth 1996, ed Ellen Datlow), while Neil Gaiman's "How to Talk to Girls at Parties" (in Fragile Things: Short Fictions and Wonders, coll 2006) thrusts a couple of immature London lads into a houseful of peculiar but lovely girls whose true identities as extraterrestrial visitors literalizes the alienness of the opposite sex perhaps a touch too obviously. In Robert Charles Wilson's A Hidden Place (1986), a humanoid alien lives as a boarder in a small Midwestern American town during the Depression, awaiting the arrival of a fellow alien with whom she can return home. Facing an inevitable metamorphosis, she enlists the aid of two humans, and they react in radically different ways to the revelation of her nature.

Even in stories which admit of a wider world aware of and affected by alien contact, the focus in recent decades has often remained tightly on the experience of an individual person or of a small group. In an echo of Blish's A Case of Conscience, Mary Doria Russell's The Sparrow (1996) tells the story of a First Contact mission sponsored by the Catholic Church and led by a charismatic Jesuit priest. Here the focus lies not on subtleties of theological doctrine but on the priest's personal experiences, which have left him physically, spiritually, and psychically scarred. In Blind Lake (2003) by Robert Charles Wilson, the tensions between a divorced couple working on a government project involving remote observation of a bizarre alien world reflect clashing interpretations of what the scientists are seeing. A Paradigm of Earth (2001) by Candas Jane Dorsey centers on a young woman emotionally ravaged by the loss of her job, her lover, and her parents who becomes the trainer – the surrogate parent – of one of a handful of humanoid aliens sent to Earth to learn about humanity; the experience helps her recover from her despair, while in Ted Chiang's "Story of Your Life" (in Starlight 2, anth 1998, ed Patrick Nielsen Hayden) the successful effort to comprehend the baffling language of extraterrestrial visitors leaves a woman with an altered experience of memory and time, and a weight of inescapable sorrow. Works such as these may fit comfortably into one or more well-established types of the alien story – the first contact tale, the communications puzzle, the reflection of human nature or the human condition – and may work quite well on that level, but the narrowness of focus, the relative absence of melodramatic devices, and the reliance on the emotional trajectories of their characters to power the narrative make them distinctive products of the millennial period.

The question of the real existence – and accessibility – of extraterrestrial intelligence grew more urgent in the sf of the 1990s and 2000s. (The original anthology Is Anybody Out There? [2011], edited by Marty Halpern and Nick Gevers, devotes itself entirely to the subject.) The stars seem ever further away. The lightspeed barrier stands firm as ever. It doesn't look like we're going to be seeking out new life and new civilizations any time soon. Decades of SETI research has produced no encouraging evidence, and the shadow of the Fermi Paradox – "If they're out there, they'd already be here" – hangs like a shroud over any speculation about extraterrestrials. The jury's still sequestered, of course, but an itch of creeping doubt has made itself known, even in stories in which aliens do in fact turn out to exist. A mood of quiet but insistent loneliness pervades much of the fiction about aliens in the millennial period. Such a mood is not entirely new – Silverberg's "Schwartz Between the Galaxies" evoked something like it almost half a century ago – but the predominance of it is something that marks the most recent era of alien-related sf. An early taste appeared in Karen Joy Fowler's "Face Value" (November 1986 F&SF), in which the difficulty of understanding aliens highlights the challenge, and maybe hopelessness, of communication between humans. Mark Tiedemann's "Texture of Other Ways" (September 1999 Science Fiction Age) waxes pessimistic in a tale of a failed attempt at contact which yields only the knowledge that aliens find us as frightening and incomprehensible as we find them. "They're Made Out of Meat" (April 1991 Omni) by Terry Bisson offers a tragicomic explanation for the lack of alien Communication with humanity, and despite its humour evokes the pain of cosmic isolation as powerfully as any work of the period. Gene Wolfe develops the loneliness theme on multiple levels in "Useful Phrases" (January 1993 Tomorrow), the story of a reclusive bookshop owner who stumbles upon an alien phrasebook, while Molly Gloss's "Lambing Season" (July 2002 Asimov's) tells of a woman, happy in the solitude of her work in the desert of the American Southwest, who establishes a tenuous connection with a periodic extraterrestrial visitor and becomes the alien's final source of comfort in the wake of a fatal crash landing. In Stephen Baxter's elegiac "Last Contact" (in The Solaris Book of New Science Fiction, anth 2007, ed George Mann), radio signals from dozens of alien civilizations begin to arrive at last just as Earth scientists discover that the cosmos is suffering a wholesale breakdown which will shortly eradicate every particle of matter, while in his "Turing's Apples" (in Eclipse Two, anth 2008, ed Jonathan Strahan) an alien message encodes a self-constructing machine which turns the Moon into a kind of time capsule designed to carry information into the bleak future when the galaxies will have spread so far apart that none will be visible to another – an image of loneliness on a scale almost beyond human comprehension. The question of "Are we alone?" may be frightening however it's answered, but in millennial sf the dread of the affirmative has largely taken over. Consider Keith Brooke's Harmony (2012) a representative statement. It provides an ingenious answer to Fermi: Once upon a time, humanity inhabited a universe replete with extraterrestrial intelligence, and Earth was dominated by aliens who arrived before humans had developed an advanced technological culture, but a political conflict over humanity's status led the friendlier aliens to create a Pocket Universe into which humanity could retreat – an otherwise lifeless pocket universe: this one.

Alongside these developments and shifts in emphasis, sf's long conversation with itself proceeds. Howard Waldrop's "Night of the Cooters" (April 1987 Omni), James Morrow's "The War of the Worldviews" (in Mars Probes, anth 2002, ed Peter Crowther), and "Foreign Devils" by Walter Jon Williams (January 1996 Asimov's) riff on Wells's The War of the Worlds. (In fact, a whole anthology of such riffs was published in 1996: War of the Worlds: Global Dispatches, edited by Kevin J Anderson.) Peter Watts's "The Things" (January 2010 Clarkesworld) reimagines of John W Campbell's "Who Goes There?" from the point of view of the alien, horrified by human biology but determined to "save" us nonetheless. Writers have also continued to explore many of the approaches to aliens pioneered in earlier decades. Robert L Forward carried the Hal Clement standard for decades until his death in 2002, developing the alien creatures of Dragon's Egg (1980) and Starquake! (1985), the Rocheworld series (1982-1995), and Saturn Rukh (1997) with the nearly obsessive devotion to scientific plausibility that made Clement's Mission of Gravity a monument of Hard SF. C J Cherryh's deep anthropological angle has been taken up by younger writers such as Julie E Czerneda, in her Species Imperative novels (2004-2006), and Eleanor Arnason, whose A Woman of the Iron People (1991) and Ring of Swords (1993) also recall the work of Le Guin in their depiction of humanoid alien species with radically different views of gender relations and sexuality (see Gender; Sex). This was the sharpest and most active arena of alien-assisted social commentary during the late twentieth century; from Butler's Xenogenesis trilogy, Mary Gentle's Golden Witchbreed (1983) and Ancient Light (1987), and Joan Slonczewski's A Door into Ocean (1986) and Daughter of Elysium (1993) through Gwyneth Jones's Aleutian trilogy (1991-1997), L Timmel Duchamp's "Motherhood, Inc." (in Flying Cups & Saucers, anth 1998, ed Debbie {NOTKIN} and "The Secret Feminist Cabal"), and Le Guin's own "Solitude" (December 1994 F&SF) and "The Matter of Seggri" (Spring 1994 Crank!), aliens and alien societies continued to offer richly fertile ground for interrogating human assumptions about sex and gender. (This has grown less common in the 2000s; it remains to be seen if this is a lull or a more lasting shift.)

On the other hand, recent decades have witnessed a progressive deracination of the alien motif. Aliens were astonishingly common in the film and television of the 1980s and 1990s, reaching millions who would never encounter them in written sf, and they frequently took forms unthreatening in either physical or conceptual ways. In Mork & Mindy (1978-1982), ALF (1986-1990), and 3rd Rock from the Sun (1996-2001) they were sources of comedy, their inevitable reflections on human culture thoroughly neutered by their determination to understand and emulate it, to go native. Alien Nation (1989-1990) and its five TV-movie sequels bit slightly harder with its portrayal of aliens as immigrants, but these aliens ultimately wanted to find a place in contemporary society as well, not rock the boat. This sort of treatment began as early as the 1960s with My Favorite Martian (1963-1966) (of which ALF is a fairly obvious update), but it reached a kind of critical mass in the latter years of the century, and the thematically vacant alien became an increasingly common element in written sf, particularly in crossovers with romance and detective fiction which were simultaneously on the rise. Hence works such as Lynn S Hightower's David Silver series (1992-1995), which pairs a human cop with an alien partner, and the Alien or Kitty Katt series (2010-current) by Gini Koch, in which the human heroine falls for a dashing secret agent from Alpha Centauri. These extraterrestrials have been thoroughly denatured; their presence provides a touch of exotica in otherwise familiar scenarios, but they provoke not the slightest twinge of reflection.

Deep alien strangeness is, thankfully, in no danger of dying out. The 1990s and 2000s have produced a wealth of carefully-conceived and conceptually challenging aliens, a richness to rival any earlier period, including perhaps the best portrayals yet of gestalt minds in Greg Bear's Anvil of Stars (1992) and Vernor Vinge's A Fire Upon the Deep (1995) and Children of the Sky (2012). Bear's non-sentient wormlike Cords unite to form individual sentient Braids, while Vinge's quasi-canine Tines form into packs with a single consciousness made up of the rudimentary personalities of its members, shifting and changing as new members are added or existing ones die. Michael Swanwick offers a tantalizing glimpse of a civilization built by gigantic millipede-like aliens in "From Babel's Fall'n Glory We Fled" (February 2008 Asimov's), as well as a remarkably convincing depiction of human-alien Communication. The swampy lives of the amphibious ranids of Elizabeth Bear's Undertow (2007) come through with unusual tactility, while both the Hwarhath of Arnason's Ring of Swords and the Aleutians of Jones's eponymous trilogy, mentioned above, deliver wallops of startling otherness of a cultural, cognitive, and (in Jones's case) biological sort. The Ariekei of China Miéville's Embassytown (2011) bristle with strangeness and invite profound speculations about the nature of language and its relationship to consciousness; the meticulously imagined aliens in Blindsight (2008) by Peter Watts present similar challenges, and cast doubt on the evolutionary value of sentience with unsettling plausibility. The Hoots of Carol Emshwiller's The Mount (2002) provoke essential thoughts on the notions of master and slave, predator and prey; the Krenken of Michael Flynn's Eifelheim (2006) earn our pity and our admiration and reflect back to us some of humanity's finer qualities. We are not done with aliens, nor they with us. [RKJK/BS/DRL]

further reading

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