Battlefleet Solar is the colossal fleet of Imperial Navy vessels charged with the the protection of Terra and the vitally important Segmentum Solar. The Imperial Navy orbital shipyards for the Segmentum are located on Luna, and the Imperial Navy headquarters in the Segmentum is based within Mars' Ring of Iron. Spacecraft and other large specialised constructs are fabricated in these orbital factories that spin around the equatorial belt of the Red Planet. Warships of the Battlefleet Solar are based in these huge floating docks, and other spacecraft from all over the Imperium often visit what are the largest man-made objects in the entire galaxy.
Following the end of "Old Night" (the Age of Strife), the Emperor of Mankind finally revealed Himself to the people of Terra. In secret, He had been planning for this moment in history for millennia, ever since the Age of Strife had fractured what remained of the ancient human federation which had once stretched across a part of the galaxy. The Emperor moved to create the military organisations He would need to begin the reunification of Mankind, and He used the raw materials at hand after millennia of savage conflict between the techno-barbarian nation-states of Old Earth. When the Emperor unified Terra the Lords of Mars scrutinised the unfolding events with envious eyes, and some among them recognised a kindred spirit, for the Emperor was to them a man of science who valued the machine and technological advancement, while others saw only a deadly new power to be feared. Some Tech-priests even began to equate the Emperor with their own Machine God in fulfillment of ancient prophecies and debate raged on Mars as whether or not to treat the Emperor as saviour or enemy.
The Emperor was not ignorant of such facts, for He too had watched the Red Planet from afar and made his plans accordingly. The Emperor sought a negotiated alliance with the Mechanicum Parliament. In return for supplying materials for his armies and building a mighty war fleet for his Great Crusade into the stars, the Emperor promised to protect the Tech-priests and respect the sovereignty of their Forge Worlds scattered across the galaxy, affording them a level of independence unequalled within the Imperium. Furthermore, the Emperor gave to the service of the Mechanicum six of the Houses of the Navigators to replace their long dead thrall-Navigators, so that their ships might once again travel safely through the Warp. Given such an incentive Mars assented to an alliance with the nascent Imperium in the Treaty of Mars, and the powerful Fabricator-General was given a seat on the War Council of the Great Crusade. And so, Mars lent its arts to the construction of the massive Warp-capable Battleships that could transport the Emperor's Space Marine Legions across the galaxy.
The Great Crusade and the Horus Heresy
During the Great Crusade to reunite all the scattered colony worlds of Mankind beneath the rule of the Imperium of Man, both the Imperial Guard and Imperial Navy were originally a single service: the Imperial Army. Under this form of organisation, each Imperial Cruiser would have a single Imperial Army regiment assigned to it. Imperial Army regimental commanding officers held command over both their regiment and the warship assigned to them, making a single warship a tactically flexible combined arms unit and minimising the damage to the Imperium in the event of the loss of a starship, its crew and its assigned troops in the Warp. During the Horus Heresy, however, it appeared that some Traitor Army regiments used the power of the starships at their disposal in order to forge interstellar empires for themselves in the fires of anarchy that swept the galaxy during the seven bloody standard years of the Heresy. This tendency to make use of the power of an Imperial starship combined with that of an Imperial Army regiment to establish tyrannies on many worlds led to the eventual reorganisation of the Imperial Army into the Imperial Guard and the Imperial Navy to deliberately foster a bureaucratic and inter-service rivalry between the two. The Emperor of Mankind Himself issued a decree before His internment within the Golden Throne that starships could no longer be commanded by the officers of the newborn Imperial Guard but only by the members of their own service.
The fires of the Horus Heresy gave birth to the modern Imperial Navy and the Battlefleet Solar as it exists in the late 41st Millennium. The Imperium is divided into five fleet zones known as the Segmentae Majoris. Every starship of the Imperial Navy is assigned to one of these Segmentae, and falls under the command of the respective Lord High Admiral who commands all the Imperial Navy assets of that Segmentum. There are five Lord High Admirals, one controlling the assembled Battlefleet of each Segmentae. Although all 5 High Admirals are ranked equally, command of the Segmentum Solar is considered the oldest and therefore the most prestigious and senior posting. It is not uncommon for one of these Lord High Admirals to sit on the Senatorum Imperialis as a ruling High Lord of Terra.
The Segmentum Solar is the central hub of the Imperium of Man and the most ancient and densely settled region of human space. Centred on Terra, where the Emperor of Mankind sits upon his Golden Throne, the central Segmentum is protected on four sides by the other Segmentae Majoris. The Battlefleet Solar of the Imperial Navy is the largest of the Segmentae fleets and is based in the orbital fleet docks of the so-called Ring of Iron that surrounds the Adeptus Mechanicus' primary Forge World of Mars. Since that time, this venerable battlefleet has continued to protect this vitally important Segmentum, and has on occasion provided warships to the far reaches of the galaxy to protect the Imperium from the various enemies of Mankind. Only during the very largest of wars does the Imperium bring Sector or Segmentum battlefleets together and dispatch them en masse to a warzone. Most recently, the Battlefleet Solar sent warships in support of the Imperial defence of the strategically important Cadian Gate from the predations of the Forces of Chaos during Abaddon the Despoiler's 13th Black Crusade in 999.M41.
- Codex Imperialis (2nd Edition), pg. 42
- Codex: Imperial Guard (5th Edition), pg. 6
- Codex: Imperial Guard (3rd Edition, 2nd Codex), pp. 4-5
- Imperial Armour The Horus Heresy: Betrayal - Book One, pp. 17-18
- Rogue Trader: Core Rulebook (RPG), pp. 314-315
- White Dwarf 140 (US), "Space Fleet: Additional Background, Rules, Data Cards and Counters", pp. 46-75
- White Dwarf 139 (US), "Space Fleet: Rules Expansion", pp. 8-36