Sigma Octanus is a planetary system in FLEETCOM Sector Three, with at least four planets orbiting a binary star. The system is home to the human colony world of Sigma Octanus IV, which had a population of over 17 million people prior to the Battle of Sigma Octanus IV.
During the Battle of Sigma Octanus IV, Captain Jacob Keyes performed the famous Keyes Loop in the system. In addition, John-117 and the Blue Team found a strange Forerunner artifact in Côte d'Azur Museum of Natural History on Sigma Octanus IV while the Covenant were in the process of retrieving information from it via infrared lasers. The system remained under UNSC control after the battle which ended in a rare UNSC victory.
- While there is in fact no actual star with the designation "Sigma Octanus", there is one named "Sigma Octantis", a slight spelling change, located 270 light years from Sol. However, it is impossible to be the same system for a number of reasons. The real-world Sigma Octantis is classified as a F0 III-type giant instead of a binary star. In addition, it would make Sigma Octanus the second furthest known UEG colony, significantly further than the Outer Colony system 23 Librae which was considered to be "behind enemy lines" by 2535. Furthermore, the UNSC Iroquois traveled from the system to Reach in less than a month. Traveling over 250 light years in such timeframe would be an impossibility with human slipspace velocities.
- Halo: The Fall of Reach (First appearance)