Phobos is the innermost moon of Mars. With the radius of 11.27 km, Phobos was 7.24 times more massive than Mars' second moon, Deimos. It was named after the Greek god Phobos (which means fear). Both moons were discovered in 1877.

Phobos was home to a Weyland-Yutani research facility where they conducted experiments with Xenomorphs using large primates. However, the facility lost control of the Xenomorphs and all members within the facility were killed.