From Halopedia, the Halo wiki
Luna, often referred to as the Moon, is the only natural satellite of Earth. Luna is home to a large human population and the renowned Luna OCS Academy. Luna houses a number of skyhook defense complexes suspended in its low orbit.
By 2492, a governing body of Luna was known as Luna Confederated States, the State of Mare Nubium being one of the said states. The Academy at Mare Nubium, known more generally as the Luna OCS Academy, was located here.
Despite its proximity to Earth, Luna was home to some degree of Insurrectionist activity; shortly after 2530, the parents of Michael Crespo died attempting to detonate a bomb attached to a government building on Luna.
During the Battle of Earth in late 2552, Luna's skyhook defense complexes came under attack by the Covenant and were defended by ONI Security personnel. Other Covenant forces attacked Luna's cities to ensure that the UNSC would not be able to launch a sneak attack from their Lunar military bases. Both Luna and Mars got hit particularly hard compared to other colonies in the Sol system, although they both fared better than Earth. The human presence on the moon survived the Covenant assault relatively intact.
Following the Human-Covenant War, Luna was home to the Office of Naval Intelligence vessel UNSC Rubicon, which was deployed to Installation 00 in December of 2553 to investigate the disappearance of automated probes sent eight months prior and to determine the source of an unknown signal.
Luna has a barren surface and is atmospherically devoid. It's gravity is low, which has prevented large scale terraforming. Likely due to all this, the habitats on Luna are sealed from vacuum and interconnected by tubes. They are referred to as canopy settlements.
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