Stellar Horizons is a "build your own space program" game in which you lead one of five Earth factions to explore and develop our solar system. Designed by a real-life space engineer with a PhD in long-duration spaceflight from MIT, Stellar Horizons is intended to be a plausible representation of the first steps of humanity towards the stars between 2030 and 2169, with each turn representing a year of time. You control your faction's space program, outposts, and fleets spanning across the solar system, although you also have some influence over your faction's politics back home on Earth as space development becomes more important.
Movement is based on real physics. You move from orbit to orbit, or conduct long range transfers to move between planetary systems like Earth, Mars, or Jupiter. As you send out robotic explorers and crewed vehicles to explore the solar system, they bring back valuable data to further your scientific research. Technologies are intended to represent plausible extrapolations of existing development during the next 150 years: there are no transporters or warp drive, but you will be able to develop rockets powered by fusion and even anti-matter. In the engineering and biology domains, you'll eventually be able to construct space elevators and put your crews into safe hibernation for long journeys.
You'll design your own ships using over fifty different component types based on your level of technology on the tech tree. These range from tiny probes intended to merely take photographs as you fly past Jupiter, to giant destroyers, cruisers, and battleships which ply the space lanes with peaceful or hostile intentions. The policies you choose determine your focus (robotic, crewed, economic, diplomatic, settlement, or military), allowing you to draw new ship and base components you'll use to design and construct your ships and bases. These range from ship components like engine rooms, medical bays, airlocks, and command centers, to weapons and defenses like lasers, rail guns, and stealth systems, to base facilities like mining stations, research labs, or spaceports.
You'll be given a budget to spend on Earth and can also build bases to harvest minerals, organics, and fuel to expands your economy. You can also forge diplomatic relationships with players and non-player factions alike, and trade resources at their bases to earn extra cash. Each of the five factions has different strengths, weaknesses, abilities, and available ships based on their unique geopolitical situation. Military conflict is certainly possible but not always encouraged. Stellar Horizons features more diplomatic, technological, and economic competition than outright combat, although neglecting warships entirely leaves you vulnerable to pirates, trade embargoes, and sneak attacks by rogue players and factions.
Stellar Horizons includes short co-operative and competitive scenarios lasting an hour or more, and campaigns lasting a day up to about a full weekend for experienced players. Up to seven players can play at a time and the game is most fun with at least two, but there are also one-player scenarios, and the campaigns are highly suited for solo play. There are three ways to win the campaign: be the first to develop an Interstellar Colonizer, terraform a world, or achieve dominance in space over your rivals.