From Halopedia, the Halo wiki
Eiro is the lone natural satellite of a planet in the Ectanus 45 system and a human colony world of the Unified Earth Government, located along the edge of the Inner Colonies. Eiro is tidally locked to the planet, though a narrow, gray-blue "twilight ring" along the moon's equator supports a small human settlement, with one communications satellite, two transport ships, and very little defensive capabilities. The temperature within the twilight ring could be as high as 75°F (23.9°C), while the dark side of the moon just outside of the twilight ring could be as low as -50°F (-45.5°C).
Though the planet the moon orbits is unsettled, Eiro has a small population. During the war, Halcyon-class light cruiser UNSC Roman Blue crashed onto the surface of the moon, approximately fifty-six kilometers away from Eiro's lone settlement on the dark side of the moon. In January 2557, Rion Forge and the crew of Ace of Spades traveled to Eiro to salvage the remains of Roman Blue, though they were forced to retreat when they were attacked by an unknown opponent while searching the decrepit warship.
List of appearances