Kerbal Space Program is a 2015 space flight simulation video game developed by Mexican studio Squad for Linux, macOS, Windows, PlayStation 4, PlayStation 5, Xbox Series X/S and Xbox One. In the game, players direct the space program of a species of green humanoid aliens known as Kerbals. The game features a pseudorealistic orbital physics engine, allowing for various real-life orbital maneuvers such as Hohmann transfer orbits and orbital rendezvous.
The first public version was released digitally on Squad's Kerbal Space Program storefront on 24 June 2011, and joined Steam's early access program on 20 March 2013. The game was released out of beta on 27 April 2015. Kerbal Space Program has support for user-created mods that add new features, such as interstellar travel, improved physics, new parts, and multiplayer. Popular mods have received support and inclusion in the game by Squad. The game has garnered commendation from spaceflight industry figures such as NASA, ESA, science communicator Scott Manley, former ULA CEO Tory Bruno, SpaceX CEO Elon Musk, and Rocket Lab CEO Peter Beck.
In May 2017, Squad announced that the game had been purchased by video game company Take-Two Interactive, who would help support Squad in keeping the console versions up-to-date alongside the personal computer versions. An Enhanced Edition was released for Xbox One and PlayStation 4 in January 2018, and for PlayStation 5 and Xbox Series X/S in September 2021 by Private Division, a publishing subsidiary of Take-Two Interactive. Two expansions for the game have been released as downloadable content: Making History in March 2018 and Breaking Ground in May 2019. A sequel, Kerbal Space Program 2, was released in early access on 24 February 2023, although the developer has since shut down and no substantive updates have been released since June 2024.
Gameplay
The player operates a space program operated by Kerbals, a species of small green humanoids, who have constructed a spaceport on their home planet, Kerbin. From the space center players can build various vehicles such as rockets, aircraft, spaceplanes, and rovers from a provided set of components. Constructed craft can be launched from the space center's launch pad or runway to accomplish various tasks while avoiding partial or catastrophic failure (such as lack of fuel or structural failure). Players control flight with little assistance other than a Stability Assist System (SAS) to keep their vehicle oriented. Provided it maintains sufficient thrust and fuel, a spacecraft can enter orbit around Kerbin, or travel to other celestial bodies. To visualize vehicle trajectories, the player is provided with a 'map' that displays the vehicle's trajectory as well as that of celestial bodies and other spacecraft, as well as their orbital parameters. Spacecraft maneuver 'nodes' can be plotted, timed, and executed, and objects can be selected and 'targeted' to facilitate flybys, rendezvous, and docking.
Missions (either player-set or assigned "contracts") involve goals such as reaching a certain altitude, escaping the atmosphere, reaching a stable orbit, landing on a certain planetary body, rescuing stranded astronauts, capturing asteroids, and creating space stations and surface bases. Players may also set challenges for each other on the game's forums, such as landing on all five moons of the gas giant Jool. In addition, the game has many user-created mods, dealing with gameplay and visuals.
Players can control in-game Kerbal astronauts known as kerbonauts, who can exit spacecraft on extravehicular activities (EVAs). While on EVA, Kerbals may use their EVA suit to maneuver in space, similar to the use of NASA's Manned Maneuvering Unit. Kerbals on EVA can collect and cache science experiments, plant flags on the surfaces of planets and moons, and repair spacecraft.
Historical space missions can be implemented and flown in the game, such as an Apollo Moon landing, the Curiosity rover, the International Space Station, and, more recently, the Artemis II manned moon mission. Certain parts in the game are based on real-life hardware, such as the Probodobodyne Stayputnik (an analogue of Sputnik 1), the Mk1-3 Command Pod (Apollo command module), and the KS-25 'Vector' (RS-25 engine). Various community-developed mods can add features such as additional parts, informational displays detailing orbital characteristics, and autopilot functionalities. Some mods have been incorporated into the core game, due to popularity. For example, resource mining, or extracting ore for refinement into rocket fuel, has been implemented into the main game from a popular mod.
Solar System The game's planetary system, called the Kerbolar system, is loosely based on the real-world Solar System, consisting of the planets Moho, Eve, Kerbin, Duna, Dres, Jool, and Eeloo, which are respectively analogues of Mercury, Venus, Earth, Mars, Ceres, Jupiter, and Pluto. Eve is orbited by Gilly, a small captured asteroid; Kerbin is orbited by two moons, the larger Mun and the smaller, more distant Minmus; and Duna is orbited by Ike, a large rocky moon, both of which are tidally locked to each other. Jool possesses 5 moons: Laythe, an ocean world with a breathable atmosphere; Vall, an icy moon; Tylo, a rocky moon the size of Kerbin; and Bop and Pol, two small irregular moons. Various community mods implement extended or alternative planetary systems, including an exact-size real-world Solar System and multiple extra systems. Because the game does not utilize n-body simulation, the planets and moons do not exert any gravitational effect on each other. Porting the system into a game with n-body simulation, such as Universe Sandbox, will result in multiple collisions between bodies and a generally unstable system.
Game modes Three game modes are available: sandbox, science, and career. Sandbox mode imposes no limitations, with unlimited parts available for any player-directed project. Many players have used Sandbox mode to implement replicas of historical real-life vehicles, as well as create impractically large or complex spacecraft.
Science mode (sometimes called "scientific sandbox") presents a limited selection of parts at the beginning of the game. Players must travel to and perform experiments at various sites around Kerbin, outer space, and on other celestial bodies to gain 'science', which can be used to unlock additional parts. This mode was designed to ease new players into the game and prevent them from getting overwhelmed. Aside from the unlocking of parts, this mode is functionally identical to sandbox mode.
Career mode expands upon science mode's progression by adding funds, reputation, and contracts. Parts and fuel must be purchased with funds. Completing contracts on time will pay out funds and increase reputation, while unsatisfactory outcomes (such as missing deadlines or killing astronauts) result in penalties to funding and reputation. Greater reputation results in more difficult and prestigious contracts. Players must spend funds to purchase parts and upgrade buildings to unlock new features such as larger rocket build size and improved tracking.
Physics
While the game's physics engine is not a perfect simulation of reality, it has been praised for its largely accurate orbital mechanics; all objects in the game except the celestial bodies are simulated using Newtonian dynamics. Rocket thrust (and torque) is applied to a vehicle's frame based on the placement of engines, and joints between parts have limited strength too much force will break a vehicle apart. The stock, unmodded game simulates orbits using patched conic approximation instead of a full n-body simulation; thus, it does not support Lagrange points, perturbations, Lissajous orbits, halo orbits or tidal forces.
The in-game astronauts, Kerbals, are physically simulated. Forceful collisions with objects or terrain will cause Kerbals to tumble until they can regain their balance, and sufficiently forceful impacts will result in the Kerbals' death.
Some celestial bodies have atmospheres of varying heights and densities, affecting the impact of drag on wings and parachutes. The simulations are accurate enough that real-world techniques such as aerobraking are viable methods of navigating the solar system. Flight through an atmosphere at excessive speeds results in aerodynamic heating; high enough temperatures will cause components to fail or explode, necessitating the use of heat shields or more careful flight profiles. In-game atmospheres thin out into space but have finite heights, unlike real atmospheres.
Kerbal Space Program alters the scale of its solar system for gameplay purposes. For example, Kerbin (the game's analog of Earth) has a radius of only , approximately that of Earth's. To compensate for the gravitational consequences of this size difference, the densities of the planets are unrealistically high, with Kerbin's density being over 10 times that of Earth's.
Expansions There are two downloadable content (DLC) expansions: Making History and Breaking Ground.
Making History The Making History expansion, released in March 2018, adds additional elements to the game, some of which are historic parts from the Space Race. These parts include the Apollo Lunar Module, Soviet R7 fuel tanks, and more. A level editor allows the players to create their own scenarios in which they must complete various tasks to succeed each level. New Space Race-inspired suits and pre-built missions were also added.
Breaking Ground Breaking Ground, released in May 2019, adds robotic parts, which can be used to build helicopters, propeller airplanes, suspension systems, custom landing gear, and robots. The parts include pistons, hinges, grip pads, and rotors. A new retro-futuristic spacesuit was added as well. A major addition are surface features and new science experiments. The player can find certain rocks on surfaces of planets and analyze them using robotic arms. Science experiments such as active seismometers and weather stations can be deployed by the Kerbals and can be used to gather extra science points in science and career mode. Development
Pre-development Director Felipe Falanghe was hired by Squad in April 2010. At the time, the company did not develop software. According to Falanghe, the name "Kerbal" came from the names he gave small tin figurines he installed in modified fireworks as a teenager. In October 2010, development on Kerbal Space Program was authorized by co-founder Adrian Goya but deferred until Falanghe had completed his projects. Kerbal Space Program was first compiled on 17 January 2011. The game's first public release, version 0.7.3, was on 24 June 2011. The game entered beta on 15 December 2014, with version 0.90, and was released out of beta on 27 April 2015.
Alpha
Version 0.7.3 was the first public release of Kerbal Space Program, and was released on 24 June 2011. It was downloaded over 5,000 times. Compared to future versions of this game, 0.7.3 was quite rudimentary. There was no stability assist mode, Kerbin did not rotate and the Sun was simply a directional light source. There were no fuel flow mechanics, no control surfaces, and no other celestial bodies. Later versions added additional planets and moons, as well as the ability to load and save collections of parts, known as "subassemblies". Tutorials were also added at this stage.
On 20 March 2013, Steam's early access program was started, with Kerbal Space Program being one of twelve games already accessible from its beginning.
Version 0.24, titled First Contract and released on 17 July 2014, added the contracts and reputation system to the game's career mode; however, players were still able to play career mode without these features in the new science mode. The final alpha release, 0.25, extended career mode and reworked aircraft components.
Beta