From Halopedia, the Halo wiki
Pegasi Delta is a small world in the 51 Pegasi System, and a natural satellite of the planet 51 Pegasi B. The surface of the world is covered by red rocks and sand. Orange dust permeates the sky and the nearby star 85 Pegasi-194A is visible as a faint yellow sun. It has seas that are unusually rich in deuterium and tritium, which the Covenant used in their plasma reactors.
During the Human-Covenant War, the Covenant forces operated a massive refinery on Pegasi Delta, used to extract the deuterium and tritium from the moon's oceans in order to obtain fuel for their ships. With its close proximity to UNSC space, Pegasi Delta became a massive forward resupply and refueling line for the Covenant ships making incursions in UNSC space.
The UNSC tried many times to neutralize the refinery, including using nuclear weapons launched from slipspace. However, the plutonium linings of the weapons let off an aura of Čerenkov radiation upon reentering normal space, which therefore made their stealth coatings useless. The Covenant easily found and destroyed them. Similarly, there were too many Covenant ships near the planet's moon to send a slow, distantly-launched weapon, a regular invasion force or even Orbital Drop Shock Troopers.
In 2545, the UNSC sent in Spartan-III super-soldiers from Beta Company to destroy the refinery in Operation: TORPEDO. They successfully destroyed the refinery by overloading and destroying the reactor causing a massive explosion, crippling the Covenant supply lines, but came out with the apparent death of 298 out of the 300 Spartans of Beta Company. The explosion also destroyed seven Covenant cruisers that were docked over the factory. Because of this, Lucy-B091, one of the two surviving Spartans, was so shocked from the incident that she lost her ability to speak; she was later diagnosed with posttraumatic vocal disarticulation.
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