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 was made of red rocks and sand. Orange dust permeated the sky and had a faint yellow sun, 85 Pegasi-194A. It had seas that were unusually green and rich in Deuterium and Tritium, which the Covenant used in their plasma reactors, and it was used to refuel their 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 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 Cherenkov 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 at the factory. Because of this, Lucy-B091, one of the two surviving SPARTANs, (the other being Tom-B292) was so traumatized from the incident that she lost her ability to speak; she was later diagnosed with Post-Traumatic Vocal Disarticulation.
- Pegasi Delta is the UNSC designation for the Covenant planet. It is likely that the Covenant had their own name or designation for it, unrelated to that used by the UNSC.
- In a trailer for Halo: Reach, Jorge-052 is quoted as saying "I didn't think anyone survived Pegasi, Commander", indicating that SPARTAN-B312 was a survivor of the "successful" assault. (On a smaller note, this also indicates that the survival of Tom-B292 and Lucy-B091 is highly classified.) Though this line is not spoken in the final game.