Segmentum Solar

Segmentum Solar is one of the Imperium's five Segmentum Majoris, the core cube whose fortress is Mars and whose principal world is Terra, the densest Human region since the Dark Age of Technology.

Sector Solar, Armageddon, Ullanor, Fenris on this chart, Necromunda, and the approaches of Beta-Garmon all sit here. Battlefleet Solar is the Navy's heaviest stick because this is the Throneworld's own half-galaxy.

Segmentum Solar still matters as the Imperium's centre of gravity. Unofficial fan summaries keep a null parent and refuse to confuse this row with sector-solar, the nested cube around Sol.