LV-426

LV-426, also known as Acheron, is the name of the fictitious moon (frequently, but erroneously, referred to as a planet) where the xenomorph was first encountered by humans in the movie Alien. ."

Description
LV-426 is a moon in orbit around a gas giant in the Zeta II Reticuli system, 39 light years from Earth, which lies beyond the "Outer Rim." LV-426 has .86 Earth gravity and is geologically active, judging from its surface composition (igneous rock, "lava base") and steam vents on its surface. Prior to terraforming, the atmosphere consisted mostly of nitrogen and methane, giving it a strong resemblance to that of Saturn's moon, Titan. The moon was cold enough that carbon dioxide froze putting its temperature somewhere between -182° and -57°C (-296° and -70°F).

The source materials differ widely on the diameter of this world: figures of 1,200 m, 1,200 km and 12,201 km are given in different sources. Out of these, 12,201 km (from the Aliens: Colonial Marines Technical Manual) is the only one that is possible given the surface gravity. Upon questioning, it was stated as being a rock, with no indigenous life.

In Film
In the year 2122, the commercial starship Nostromo, on a return trip to Earth, intercepts an extraterrestrial transmission originating from the moon's surface. The transmission is found to be an acoustical beacon repeating at twelve-second intervals, emanating from a derelict alien spacecraft (the Derelict).

During the 57 years that elapse between the end of Alien and the events of Aliens, the Weyland-Yutani Corporation are responsible for establishing a colony on LV-426. They made the world habitable with the aid of an enormous, fusion-powered atmosphere processor and established a terraforming colony called "Hadley's Hope." The colony is destroyed when the atmosphere processor suffers critical damage during military operations, causing a catastrophic explosion which destroyed the colony.

Expanded Universe
According to the computer game Aliens versus Predator, the derelict ship survived the explosion and the Company established a new colony, atmosphere processor, and research facility dedicated to studying the derelict ship and its contents. All were destroyed ten years later during a second xenomorph infestation, when an unnamed Marine disabled the atmosphere processor's cooling systems, causing another explosion.