Eridanus Secundus
From Halopedia, the Halo wiki
Eridanus Secundus is an asteroid in the Eridanus Star System.
It became the stronghold of the Eridanus Rebels after their insurrection was put down during Operation: TREBUCHET. It was the center of all activity for the rebels and was serviced by freighters from the nearby Eridanus II. The Asteroid was outfitted with Titanium-A battleplate and other alloys that would deflect both UNSC radar and Covenant sensors. It was effectively a hollowed-out rock that was fitted on the inside with improvised corridors and facilities from salvaged vessels. In the center of the asteroid was a city, complete with skyscrapers and other large buildings with streets. It was a great accomplishment for the rebels.
In addition, several facilities were in place around the asteroid that caused outsiders to believe the rebels had nuclear weapons around the facility. A hundred improvised civilian craft served to accommodate the navy surrounding the base.
The Spartans visited this place twice. The first time, in 2525, they captured the rebel leader, Colonel Robert Watts, as their first mission as an active duty unit. They also placed a Panic Button on the 'green' setting inside the asteroid.
The second time, in 2552, the Spartans and Admiral Whitcomb visited the facility aboard the Gettysburg-Ascendant Justice for repairs before returning to Earth. The Covenant followed them and located the facility. This asteroid along with all the rebels were presumed destroyed by the Covenant.