LV-426 "Acheron"

"Day on Acheron was dim twilight, night was darker than the farthest reaches of interstellar space, because not even stars shone through its dense atmosphere to soften the barren surface with twinkling light." ―Aliens novelization, chapter 9

Acheron, formerly known as LV-426, was one of three moons orbiting the gas giant Calpamos in the Zeta2 Reticuli system, 39 light years away from Earth. The moon was given its common name by the early human colonists who settled there. The main colonist base, Hadley's Hope, housed 158 people.

Terrain and Climate

The original surface before terraforming was lava base rock, with some ongoing volcanic activity.
Before atmospheric processing began, its atmosphere was reported by USCSS Nostromo crew member Ash to be primordial and inert, containing nitrogen, high concentration of CO2 crystals, methane and trace elements. Dust and turbulence created a murky atmosphere with little surface light and poor visibility. Originally the atmosphere at the surface was deep cold.
Acheron has an equatorial diameter of just 1,200 km. Despite its small size, the moon maintains a surface gravity equivalent to 0.86 of that on Earth, indicating it is largely composed of incredibly dense materials. Its rotational period is about two hours. Acheron's crust is made up of aluminum silicates, although there is evidence of magnesium silicate intrusion, manifested on the surface as basalt, rhyolite and microgranite lava flows.
Despite this evidence of past volcanic activity, as well as ongoing volcanic processes in the moon's core, the surface currently exhibits no discernible volcanism or tectonic activity. The moon's geological structure has led to the conclusion it is a recently captured body, entering Calpamos' orbit within the last 40 million years or so. Indeed, the rings around Calpamos itself are thought to be the remnants of a previous satellite that was broken apart by the gravitational shift induced by Acheron's arrival.

Flora and Fauna

Acheron has no indigenous life forms.

History

LV-426 was first detected by Weyland-Yutani Corporation astronomers on May 14, 2039. The moon was subsequently cataloged by a French deep-space probe. It was assumed the moon was a low density Galilean ice ball with an ice crust over a silicate core due to Calpamos' (then named only IV) ring system.

Nostromo Incident

The first comprehensive survey of LV-426 was not carried out until the 2130s, after USCSS Nostromo set down on the moon in 2122. Nostromo was purportedly responding to a mysterious signal emanating from LV-426, per Weyland-Yutani protocol. In reality, the signal had previously been detected and decoded by Weyland-Yutani, and Nostromo had been pre-selected to investigate, with the synthetic Ash surreptitiously placed aboard to ensure the safety of any lifeforms that were discovered.
Though Nostromo was damaged while landing on LV-426, a three-man exploration team -- consisting of Captain Dallas, First Officer Kane and navigator Lambert -- set out on a brief exploration of the moon's surface and discovered a derelict spacecraft that was the source of the signal. While exploring the derelict's interior, the crew came across the fossilized remains of an Mala'kak pilot and thousands of Xenomorph Eggs stored within the ship's cargo hull. Kane was subsequently impregnated by a Facehugger and was taken back to Nostromo for treatment. The ship departed soon afterwards, but the Xenomorph implanted in Kane's chest later birthed in space, leading to Nostromo's destruction.
Following the loss of Nostromo, the incident was covered up by the Weyland-Yutani personnel involved.

Sevastapol Incident

It would be fifteen years before another ship visited LV-426, when Nostromo's flight recorder was discovered by the crew of USCSS Anesidora, captained by Henry Marlow. Hoping to use the recorder's flight data to locate and salvage the ship, they stumbled upon the derelict's beacon and landed on the moon's surface, eventually discovering the ship itself before realizing Nostromo had found it first. Hoping it hadn't yet been claimed and adamant on ensuring nobody else would stumble across the derelict, Marlow found and deactivated the beacon. Soon after, the crew were forced to return to their ship when Marlow's wife, Catherine Foster, was attacked and impregnated by a Facehugger, much as Kane had been; this would lead to the Xenomorph outbreak on Sevastopol Station soon after.
With the beacon deactivated and all of both crews -- save for Ellen Ripley of Nostromo -- dead from both outbreaks, the moon faded back into obscurity; the derelict itself was not discovered during the moon's survey.

Terraforming and Colonization

LV-426 was later selected for terraforming and human colonization. This process began with the establishment of the colony of Hadley's Hope in 2157, with the construction of a nearby Atmosphere Processing Plant commencing two years later. The early inhabitants of Hadley's Hope christened the moon Acheron, after one of the rivers that wound through the netherworld (translated as "River of Pain") in ancient Greek mythology.
Despite this human habitation, the colonists remained unaware of the derelict due to a combination of the moon's thick, debris-laden atmosphere shielding it from both visual and satellite detection and the Ilium mountain range blocking it from the colony's ground scanners. It wasn't until twenty years later that Russ and Anne Jorden, Weyland-Yutani prospectors and residents of the colony, investigated the derelict on company orders after its location was deduced from the testimony of Ripley, who had just been rescued from deep space. Russ was subsequently impregnated by a Facehugger during his investigation of the ship, resulting in a full-scale outbreak in the colony. A Queen established a Hive in the Atmosphere Processing Plant, and the Xenomorphs killed or impregnated nearly all of the colonists.
A short while after the outbreak, a unit of Colonial Marines was dispatched aboard USS Sulaco to investigate the loss of contact with the colony, accompanied by Ripley. After several battles with the Xenomorphs that resulted in the deaths of most of the Marines, the creatures' Hive was destroyed when the Atmosphere Processing Plant's reactor exploded. Only Ripley, Rebecca, Corporal Dwayne Hicks and the synthetic Bishop escaped Acheron. Unknown to them, the Hadley's Hope Queen had stowed away on board Sulaco, but was subsequently ejected into space by Ripley through an airlock.

Planetary Features

Diameter: 1,200 km
Rotation Period: 2 Earth Hours (retrograde)
Orbital Period: 5.07 Earth days
Gravity: equivalent to 0.86 of Earth standard
Atmosphere: nitrogen base, high concentration of CO2 crystals, methane and trace elements