The Expeditionary Battle Planetoid, conceived in 29 BBY by Raith Sienar, represented a theoretical spherical battle station. It stood as an early conceptual iteration that paved the way for the eventual Death Stars.
Serving as the initial true predecessor to the Death Star project, the Expeditionary Battle Planetoid shared design similarities. It featured a core-powered, planet-devastating turbolaser. However, the design was ultimately considered unfeasible due to the sheer magnitude of power generation required to operate such a weapon. The power source was envisioned as an internal plasma implosion core exceeding one kilometer in diameter. Raith Sienar intended to fuel this core using vast quantities of ice asteroids sourced from the abundant Outer Rim systems. The design also incorporated two smaller spheres, connected to the main sphere's poles (the main sphere spanning roughly 90-100 kilometers in width). Sienar even contemplated detaching these spheres to enhance the design's transportability.
In 29 BBY, Raith Sienar initiated the design phase of the Expeditionary Battle Planetoid. Despite the anticipated high construction costs, Sienar remained confident in its ultimate feasibility. He utilized the battle station as a demonstration project, showcasing its schematics to prospective clients and contractors with extravagant tastes to impress them. He presented the designs to representatives from the Trade Federation; however, they displayed no interest, which Sienar attributed to their lack of vision.
Around the time of the Second Battle of Zonama Sekot, Raith Sienar shared the Battle Planetoid plans with Republic Outland Regions Security Force Commander Wilhuff Tarkin, who was captivated by the concept. Subsequently, Tarkin presented the plans to then-Supreme Chancellor Palpatine, falsely claiming credit for their creation. Sienar, however, had already moved on from the idea of personally developing the weapon and seemed unconcerned by this apparent betrayal.
In 29 BBY, the design was still deemed unattainable, contingent on specific advancements in hypermatter technology. These technological hurdles were overcome by 22 BBY, largely due to the contributions of engineer Bevel Lemelisk, which also led to Republic Sienar Systems' grant to the Ministry of Science for hypermatter research.
As the Confederacy of Independent Systems formed in the months preceding the outbreak of the Clone Wars, Palpatine, in his guise as Darth Sidious, entrusted the plans to his Sith apprentice and Confederate leader, Count Dooku. The Geonosians then utilized these designs to develop their Ultimate Weapon, effectively transforming the Battle Planetoid into a planet-destroying superweapon equipped with a functional superlaser.
Following the Declaration of a New Order in 19 BBY, the Ultimate Weapon's design was refined into the blueprint for the Death Star. Construction commenced, and Tarkin, ostensibly as a reward for "his" design, was appointed as the Moff in charge of overseeing the project.