Alien

Aliens, known as zetans by pre-War conspiracy theorists, are an extraterrestrial yellowish-green skinned humanoid race from an unknown planet. The only thing known about them is that they use their ability to travel space to probe other planets and species for information.

Background
It is not known from what system or what planet they come from. They have orbited and observed the Earth with two known motherships (Zeta and an unnamed one) at least since the 1600s in Earth's solar system, but humans had little real awareness of them and less evidence of their existence. Before the 1960s, only "crackpot" groups such as Quaere Verum believed in them, thinking that aliens existed but that evidence of their existence was covered up by a government conspiracy.

However, after the mysterious disappearances (abducted by aliens in Zeta in fact) of the USSA space capsule Clarabella 7 and the pilot, astronaut Colonel Hartigan, during the 1960s Space Race, the aliens became recognized by national leaders. It is said that alien technology was the basis of many of the Enclave's more exotic and impressive weaponry and robotics. Toy manufacturers such as Wilson Automatoys weren't immune to these suspicions. The pre-War United States government was said to keep its alien specimens and technology mainly in Area 51 (such as a flying saucer that was later recaptured by aliens, but crashed in California probably during a flight sometime after the Great War, instantly killing the two alien pilots).

The Brotherhood of Steel around 2161 and the Shi around 2241 have encountered the bodies of aliens or their spaceships. By extension, the Brotherhood of Steel in the Capital Wasteland have 72x alien power cells in the Citadel's armory and the Brotherhood Outcasts also have some alien power cells at Fort Independence. Also before 2277, an alien body was discovered in Fort Bannister, probably by the Enclave, carrying what may have been recovered (like an alien blaster and some alien power cells) in the armory of the mobile base crawler at Adams Air Force Base.

Some advanced human technologies were created by using alien technology, such as the AI project number 59234, created in the 2050s, which manages the entire Sierra Army Depot Skynet, the device blocking telepathic and psychic commands, the psychic nullifier and probably the advanced accelerated organic tissue regeneration processes gel mostly used for brain bots (robobrain, humanoid brain bot, think tank) and the bio med gel developed at the Boulder Dome.

Jack Cabot also suggests the presence of "ancient aliens" as a precursor to original human civilizations such as Assyria, that guided humanity to develop. His findings are enormously compounded by an “artifact” discovered by his father, Lorenzo Cabot, which granted him incredible abilities of telekinesis, “immortality” which could be replicated by extracting his blood and transforming it into mysterious serum, and incredible strength. Lorenzo's artifact itself utilized an ancient Egyptian scarab to focus the ordinary Gamma Gun into a powerful kinetic blast. In Jack's terminal notes, he speaks of a location his father may have relayed to him, Ubar, which is a legendary “Atlantis of the Sands” believed to have been located and lost somewhere in Arabia.

It is also possible that the zetans may have had some influence on the Great War, evidenced by a recording aboard Mothership Zeta. In this recording, a soldier (or other military official) of the United States is heard as he is attempting to resist the zetans' attempts to probe his mind for information, one subject in particular, the United States' nuclear launch codes. This is purely based on speculation, as it is unknown whether or not the zetans were successful in their attempts, or if they used the codes even if successful. Although we may never know for sure, the idea that the zetans used the nuclear launch codes to "kick off" the Great War is possible.

Biology
 Aliens are diminutive, hairless, yellowish-green-skinned humanoids with short, wiry limbs and green blood. Their heads are disproportionately large, with round, heavy-lidded, solid black eyeballs with no discernible iris or pupil; a thin, toothless mouth with green tongue, and two pairs of small holes where ears and nostrils would be. Aliens have no opposable thumbs, each hand instead having three long, tentacle-like fingers with large suction cups at each fingertip that allow them to manipulate objects. They breathe a similar atmosphere to humans, and (at least those on Mothership Zeta) subsist mainly on a diet of strange alien worms and squid.

Description
They seem to have a strict society focused on work and progress, with no apparent concern for hobbies and personal pursuits. Their architecture is spartan and purely functional: for instance, their bed (called a stasis chamber) is a sterile metal capsule with no mattress, blankets, sheets or other pieces of bedclothes typically found in a normal human bed, and no discernible means of entertainment can be seen aboard Zeta. However, the aliens, in their own right, appear to have some degree of emotion, since alien workers display obvious fear when confronted with armed assailants. In another case, the aliens are shown to have an unusual fascination with Giddyup Buttercup, as demonstrated in a captive recorded log onboard Zeta, of Sally being taken to the recording room and noticing a Giddyup Buttercup. She notes that upon her calling attention to it, the aliens become very excited, though she has no idea why they act this way.

They view other species as laboratory animals, worthy of preservation and storage at best and vivisection and experimentation at worst. They speak in their own language of high-pitched screeches and wails. As such, communication between humans and aliens is supposedly impossible. However, linguistics specialist Holly Barrisford did make an attempt to communicate with the aliens during her abduction. Unfortunately, barring recording some brief biographical information, the aliens simply aren't interested in discussion.

They are incredibly advanced in their various technological fields, capable of spaceflight and technological feats equal to, and often beyond, that of even the most advanced human pre-War technology. They also possess technologies such as long-term cryostasis, powerful rayguns, genetic manipulation, teleportation and holograms. However, they are totally dependent on technology. Because of this, they are physically fragile, lacking in raw strength and ineffective in melee-based combat. Very few possess especially protective armor and clothing.

Aliens typically wear spacesuits, the color of the suit apparently indicating rank, and soldiers generally carry shock batons, disintegrators and atomizers. They prefer to attack their foes in large numbers, and tend to have guardian drones with them.

Encounters

 * In Fallout, the Vault Dweller can encounter a device based on alien technology (the psychic nullifier) and a flying saucer with two alien skeletons.
 * In Fallout 2, the Chosen One can encounter two technologies based on alien technology, bio med gel and Skynet.
 * In Fallout 3, the Lone Wanderer can encounter a downed recon craft named Theta and a dead alien.
 * In the Mothership Zeta add-on, the Lone Wanderer can be abducted by aliens at the alien crash site and taken aboard a spaceship named Mothership Zeta.


 * In Fallout: New Vegas, with the Wild Wasteland trait, the Courier can encounter an unnamed recon craft with two aliens and an alien captain nearby.


 * In Fallout 4, the Sole Survivor can encounter an alien spaceship falling from the sky shortly after reaching level 20. Companions have unique commentary upon observing the crash. The Crashed UFO is always located east of Oberland station. Following green blood stains will lead the Sole Survivor to a hostile injured alien which can be looted for the alien blaster pistol. Fallout 4 also officially identifies the aliens as zetans through various in-game sources.
 * In Fallout 76, the player character may randomly encounter aliens, the Flatwoods monster, and find alien blaster pistols. There are also a few fake aliens.

Crashed ships
Six alien ships are known to have crashed in the United States:
 * A flying saucer in a special encounter in California. (Fallout)
 * A recon craft named Theta in the Capital Wasteland. (Fallout 3)
 * Another recon craft that crashed and exploded on contact with the ground in a random encounter near the Lone Wanderer in the Capital Wasteland. (Fallout 3)
 * At the Citadel, there is information on a terminal (in the A-Ring) about another ship of unknown type codenamed "Palandine" that crashed in a heavily wooded area which the government was not able to reach. (Fallout 3)


 * A recon craft with three aliens near Horowitz farmstead in the Mojave Wasteland. (Fallout: New Vegas)


 * An event in which a UFO crashes to the ground south of the Beantown Brewery in the Commonwealth. (Fallout 4)

Robots and computers

 * Experimental weapons drone - experimental security robot armed with a drone cannon Ex-B.
 * Guardian drone - security robot armed with a drone cannon.
 * Support drone - maintenance robot.

Vehicles

 * Flying saucer - small ship with a saucer-shaped body used for fast operations that houses two pilots. The only known flying saucer is the one that crashed in California.
 * Mothership - large ship dedicated to long-term space travel and the study of other species. It's generally located in space. It houses a large crew of aliens and is commanded by a captain. The overall shape of the ship resembles a colossal saucer with another small disc-shaped section attached to the bottom. Known motherships are Zeta and the unnamed mothership which arrives after Zeta's bridge is captured by the Lone Wanderer.
 * Recon craft - small ship with a disc-shaped rear body used for stealth operations that houses only one pilot. Known recon craft are Theta, another which exploded and sprayed in the Firelance event, and a possible unnamed craft in the Mojave Wasteland.

Medical

 * Abominations - biological weapons created from humans with alien DNA.
 * Alien biogel - ingestible curative gelatin-like liquid which accelerates organic tissue's regeneration processes.
 * Healing archway - care generator restoring health, removing all radiation, clearing any drug addiction and restoring crippled limbs.

Food

 * Alien squid food - preserved squid-like creature.
 * Alien worm food - preserved worm-like creature.

Other devices

 * Alien epoxy - advanced liquid repairing agent for weapons.
 * Alien homing beacon - teleportation beacon sent by a mothership to return quickly.
 * Drone control device - remote control for alien drones.
 * Force field generator - portable forcefield used for protective cover or blocking a passage.
 * Inertia suppression field - personal protection shield technology.

Miscellaneous

 * See alien items.

Appearances
In Fallout and Fallout 2, they only appear in the form of skeletons. The crashed recon craft Theta appears in Fallout 3 with the first dead alien, and the aliens are the primary antagonists of the Fallout 3 add-on Mothership Zeta. They also make an appearance in Fallout: New Vegas if the Wild Wasteland trait is active. If this is the case, then they are spotted in the northernmost part of the map near Brooks tumbleweed ranch. In Fallout 4 they appear during an event in which a UFO crashes to the ground south of the Beantown Brewery in the Commonwealth. In Fallout Shelter they appear during the quest Humans Like Us, where they use optical camouflage to look like male human beings.

Behind the scenes
The aliens resemble Greys, beings commonly described in alien abduction accounts. These aliens have been commonly reported since the 1950s. Later, the Greys became more mainstream when science fiction began depicting them. These kinds of aliens have come to represent the "stereotypical" alien. The name zetan may refer to the star system Zeta Reticuli, a real-world star system seen in a number of science fiction stories, such as the Alien franchise. It was also rumored to be the origin of the aliens who allegedly abducted Barney and Betty Hill in 1961, which was the first widely publicized report of an alien abduction. 