From Halopedia, the Halo wiki
Basis is the largest of the twelve satellites orbiting the gas giant Threshold in the Soell system of the Milky Way galaxy. Basis is Silvery-gray in color. Halo Installation 04 was positioned in the L1 Lagrange point between Basis and Threshold. Debris left over in the wake of Installation 04's destruction often rained down on Basis, creating small craters on the moon's surface.
Circa 97,445 BCE, the L1 Lagrange point of Basis and its parent body, Threshold, was used by the remnants of the Forerunner ecumene as the location of Installation 04, or Alpha Halo, one of the seven ring-shaped superweapons that made up the Halo Array.
After Installation 04's destruction, a group of Covenant defectors built a makeshift camp on Basis, amid the wreckage of the ring. During a skirmish over Threshold following the Battle of Installation 04, Cortana maneuvered John-117's GA-TL1 Longsword into the gravity well of the moon to slingshot the fighter around Basis to avoid Covenant attacks.
After the war, Spartan-IVs were sent on reconnaissance to Basis in order to survey the debris of Alpha Halo scattered across the moon's surface. The mission marked the first use of the VECTOR-class Mjolnir armor.
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