Pegasi Delta was a small world in the 51 Pegasi-B System. It was a hot red world with orange dusty skies and hot temperatures. It had seas that were unusually rich in Deuterium and Tritium, usable in the reactors of ships.
During the Human-Covenant War, the Covenant forces constructed a massive refinery on Pegasi Delta. With its close proximity to UNSC space, Pegasi Delta became a massive foreward resupply and refueling line for the Covenant ships making incursions in UNSC space.
In 2545, the UNSC sent in 300 Spartan-III supersoldiers from Beta Comapny to destroy the refinery in Operation: TORPEDO. They sucessfully destroyed the refinery, cripling the Covenant supply lines, but with a 99.3% casualty rate; 298 of the 300 Spartans of Beta Company were killed.