The player controls the flow of space program operated by Kerbals, a race of small green humanoids, who have constructed a fully furnished and functional spaceport (known as the Kerbal Space Center, or KSC) on their homeworld Kerbin, which is modelled after Earth. Despite being shown as funny cartoon-like little green men, sometimes lacking common sense, they have shown themselves to be able to construct complex spacecraft parts and perform experiments.
Gameplay has three distinct phases that can be divided into "construction", "flight" and "exploration". Players can create rockets, aircraft, spaceplanes, rovers, or other craft from a provided set of components. Once built, players launch their craft from the in-game space center's launch pad or runway, attempting to complete player-set or game-directed missions while avoiding various opportunities for partial or catastrophic failure such as lack of fuel or structural failure.
Missions (either player-set or proposed in the form of "contracts" with set parameters to achieve) involve goals such as reaching a certain altitude, escaping the atmosphere, reaching a stable orbit, landing on a moon or planet, capturing asteroids, and creating space stations and surface bases. Players also set challenges for each other on the game forums, such as visiting all five moons of Jool (the in-game analog for Jupiter) or using mods to test each other's craft in air combat tournaments.
Once a spacecraft is built, it is placed on the launchpad or the runway and is ready for launch. Players control a spacecraft in all three dimensions with little assistance other than a stability system to keep their rocket pointed at a player-specified direction or to keep a constant attitude. If the rocket has enough thrust and fuel, it is possible to reach orbit or even travel to other celestial bodies. In flight, to visualize the player's trajectory, the player must switch into map mode. The map shows the orbit or trajectory of the current craft as well as the current position and trajectory of other spacecraft and planetary bodies. Planetary bodies and other spacecraft can be targeted in order to view information needed for rendezvous and docking such as ascending and descending nodes, target direction, and relative velocity to the target. While in map mode, players can also access 'maneuver nodes' which can be used to plan out trajectory changes in advance.
Historical spacecraft can be recreated and their accomplishments mimicked, such as the Apollo program, the Mars Science Laboratory rover, or the International Space Station. Players may also install mods which can implement destinations, weapons, rocket parts, and goals for the game, such as mining for resources or incrementally deploying an interplanetary communication network. Mods can also consist of informational displays showing craft and orbital statistics such as delta-V and orbital inclination. Squad has often implemented popular mods into the stock game, such as the "Karbonite" resource mining system made by user RoverDude.
The game can be played in three different game modes that are selected when the player starts a new game: "Sandbox" mode, "Science" mode, and "Career" mode. In sandbox mode, players are free to attempt any mission for which they can construct a suitable vehicle, with no punishments for failure and entirely user-assigned missions. Many players have constructed unrealistic spacecraft in this mode, such as unrealistically huge, complicated, and/or extremely expensive rockets. Many players have also created replicas of real-life rockets and aircraft as well as replica and non-replica automobiles, trains and boats.
In Science mode, the initial selection of parts is limited, with the more advanced parts unlocked via "Science", in the Research and Development building, gained by performing various scientific experiments on Kerbin and throughout the solar system. Science mode was partially designed to ease new players into the game and prevent them getting overwhelmed. Science gained on a mission needs to be received by the space port. This can be done by transmissions via antennas in the game, or recovery of a craft by landing back on Kerbin.
Career mode is an extension upon science mode, adding funds, reputation, and contracts. In order to build and launch new rockets, the players must complete contracts to earn funds to pay for the new parts. Reputation affects how many contracts are given to the player (less reputation leads to fewer and lower-quality contracts). Declining a contract will reduce the likelihood that a contract of the same type will appear.
While the game is not a perfect simulation of reality, it has been praised for its representation of orbital mechanics. Every object in the game except the celestial bodies themselves are under the control of a Newtonian dynamics simulation. Rocket thrust is applied accurately to a vehicle's frame based on the positions in which the force-generating elements are mounted. The strength of the joints connecting parts together is finite and vehicles can be torn apart by excessive or inappropriately directed forces.
The game simulates trajectories and orbits using patched conic approximation instead of a full n-body simulation, and thus does not support Lagrange points, perturbations, Lissajous orbits, halo orbits and tidal forces. According to the developers, full n-body physics would require the entire physics engine to be rewritten.
The in-game astronauts, known as "Kerbals", have physics calculations applied to them when they are on extra vehicular activities. For example, hitting an object with only the Kerbal's feet will send them into a tumble. While on EVA, Kerbals may use their EVA suit propellant system (analogous to the NASA Manned Maneuvering Unit) to maneuver in space and around crafts and space stations. Actions that can be performed while on EVA include repairing landing legs, wheels, and solar panels. Kerbals can also collect material from science experiments, allowing them to bring back scientific data without having to bring back heavy or bulky parts or risking destruction of the data and experiment during atmospheric entry.
Some celestial bodies have atmospheres of varying heights and densities, affecting the efficiency of wings and parachutes and causing drag during flight. The simulations are accurate enough that real-world techniques such as Hohmann transfers and aerobraking are viable methods of navigating the solar system. Aerobraking, however, has become a much more difficult method of velocity reduction since the full 1.0 release due to the addition of a better aerodynamics model and optional atmospheric entry/reentry heating. Atmospheres thin out into space, but have finite, set heights unlike real atmospheres.
Kerbal Space Program makes some changes to the scaling of its solar system for gameplay purposes. For example, Kerbin (the game's analog of Earth) is only 600 kilometres (370 mi) in diameter (approximately 1/10 that of Earth's). Kerbin's density is over 10 times that of Earth's, in order to produce similar gravitational accelerations at the surface. The planets themselves are also significantly closer together than the planets in our solar system. However, there are mods that port the real-world solar system into the game, with accurate scaling and environments, and provide additional parts to make up for the extra power requirements.
The game's first public release was on June 24, 2011 with version 0.7.3. The game entered beta on December 14, 2014, with version 0.90, and was officially released out of beta on April 27, 2015.
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 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 in progress. Kerbal Space Program was first compiled on January 17, 2011.
Version 0.7.3 was the first public release of Kerbal Space Program, and was released on June 24, 2011. It was downloaded over 5,000 times. The version lacked many features present in later versions, such as a 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.
Version 0.17, among other features, added a total of 11 celestial bodies to the in-game system, including moons around other planets and a gas giant. This marked a huge leap forward in the possibilities for missions, such as interplanetary manoeuvres that had to be plotted beforehand, space probes that utilized gravitational slingshots in order to save fuel or decrease flight time. This also marked the first edition of the tutorials and scenarios, making it easier for new players to get to know the complex controls.
Version 0.22 added "career mode" to the game. However, systems such as Contracts, Funds, and Reputation were not yet implemented, making this Career mode more like the Science mode of later updates. It only involved collecting science points from various locations, and using these science points to unlock new parts. It also featured new models for the Kerbal Space Center, and the ability to save and load collections of parts as "subassemblies".
Version 0.24, titled First Contract and released on July 17, 2014, added the contracts and reputation system to the game's Career mode. (Players were still able to play Career without these features in the new Science game mode.) Contracts reward the player with currency, named "Funds" by the developers, and reputation. Funds can be used to purchase rocket parts, and reputation results in better and more lucrative contracts.
Version 0.25, titled Economic Boom and released on October 7, 2014, was the final alpha release of Kerbal Space Program. It added a new facility: the Administration Building. The Administration Building allows the player to put into effect various "strategies" that boost intake of one resource (Funds, Science, or Reputation) while reducing the intake of the other. For example, the player can choose to hire unpaid interns for the Research and Development Department, increasing intake of science while decreasing the amount of reputation earned. Version 0.25 also added the ability to destroy KSC facilities, which can then be repaired for a fee. Version 0.25 also integrated the SpacePlane Plus mod into standard Kerbal Space Program, reworking most of the aircraft components.
Version 0.90, nicknamed Beta Than Ever, was released on December 15, 2014. This was the first (and only) beta update to Kerbal Space Program. The update featured a full rewrite of the entire editor code, and now allowed for the ability to offset parts, sometimes into empty space. The update made several other improvements to the editor, such as the ability to sort parts by company, module, and mass, as well as the ability to assign parts to custom categories. The update also updated the style of the "Mk3" plane components, which were changed to be similar to the Space Shuttle.
This version also implemented upgradeable buildings in Career Mode. When the player starts, they only have basic facilities allowing for small rockets with low mass and part count. The player can upgrade each of the facilities to increase mass and size limits, or unlock various other capabilities; for instance, a non-upgraded tracking station does not allow for placing maneuver nodes or for trajectory prediction using patched conics.
Version 1.0 was the first full release update of Kerbal Space Program. It was nicknamed We Have Liftoff! and released on April 27, 2015. 1.0 completely overhauled the flight and drag model, making it far more realistic. Before version 1.0, drag was calculated on every single part, regardless of whether that part was occluded from airflow or not. 1.0 also allowed for body lift, so that parts that were not specifically designed as wings (for instance, structural panels) could still generate lift.
1.0 added shock heating and heat shields, making atmospheric entry much more dangerous. 1.0 also added airbrakes, as well as procedurally generated farings. Internal models for all parts that previously did not have them were added as well.
Resource mining was added, based on the "Karbonite" mod by Roverdude. Players could now mine resources (named "ore") to refine into fuel or monopropellant.
1.0 also brought several improvements to Kerbals, who could now have various specializations. For example, "Engineer" Kerbals are able to repair wheels and landing legs. Female Kerbals were also added to the game.
One of the most wanted systems was added into the game at the release of 1.0.5; the buoyancy was fixed, so that adds a possibility to build an actual working water vehicle.
Version 1.1, nicknamed Turbo Charged, was released on April 19, 2016, almost one year after the last major update. The game engine was upgraded from Unity 4 to Unity 5, resulting in a massive increase in performance, as well as a stable 64-bit client, removing memory constraints caused by too many mods being installed. Nearly all the game's UI elements had to be completely rewritten, and many other components (such as wheels) were also overhauled.
On March 2, 2016, Squad announced an optional public beta for the 1.1 update. The beta ran for three weeks before the full release of the update, and allowed Squad to test the large update with a massive audience. The beta was only available to those who had bought the game through Steam, and not through Squad's store, Amazon.com, or other sites such as GOG.com, due to the large amount of data from constant updates to fix bugs discovered by the community. On March 29, the beta was made available to Steam users, and ran for slightly longer than planned due to issues with the new wheel system and joints between parts. On April 28 and 30, 2016, respectively, Squad released two patches, 1.1.1 and 1.1.2. 1.1.1 was significantly more bugged than 1.1.2, as rover wheels would explode on contact with the surface, as sodium does with water. Many of the bugs in both 1.1.1 and 1.1.2 were fixed with the 1.1.3 patch, which also improved the performance.
Squad has released 1.2, nicknamed Loud And Clear, which not only aims to improve game performance and add in new features, but will also be a "minor" upgrade from Unity 5 to 5.4. The patch entered experimental testing on 6 September 2016 and it was officially released on 11 October 2016. Its main new features include communication satellites, relay systems, and KerbNet.
On January 27, 2014, it was revealed that Squad was working on an education-themed version entitled KerbalEdu in collaboration with TeacherGaming LLC, creators of MinecraftEdu. It has since been released and includes an improved user interface for easier data gathering and summary, pre-made lessons that focus on certain constructions, options to use the metric system, and a "robust pedagogy" that includes information outside of the game that ties into its content.
Squad has also made an Asteroid Mission Pack, with full support from NASA. Released on April 1, 2014, it is based on the real-life initiative to send humans out to study asteroids.
The majority of the game's music was provided by royalty-free composer Kevin MacLeod, with the rest of the soundtrack having been written by Squad in-house composer Victor Machado. The game's main theme was composed by lead designer Felipe Falanghe, and arranged by Machado.
On June 5, 2015, it was announced that Kerbal Space Program was being ported to the PlayStation 4 by Flying Tiger Entertainment. In August 2015, it was announced that Xbox One and Wii U ports were also in development by Flying Tiger Entertainment.
In the hours after its Steam early access release on March 20, 2013, Kerbal Space Program was one of the top 5 best sold games, as well as the best seller on Steam for Linux. Squad have also released many non-digital products such as clothing and plush toys. In March 2015, Squad and 3D printing service Eucl3d announced a partnership that would allow players to order physical 3D printed models of their craft.
The public alpha and beta releases were well received. Many publications and individual people have referred fondly to Kerbal Space Program and praised the game's replayability and creative aspects, including Kotaku, Rock, Paper, Shotgun, IGN, PC Gamer, GameSpy, Eurogamer, Polygon, Destructoid,
In May 2015, PC Gamer awarded Kerbal Space Program 1.0 a score of 96 out of 100, their highest review score of 2015. They praised the "perfect blend of science and slapstick" as well as the sense of accomplishment that came from reaching other planets and completing goals.
The game has notably crossed over into the scientific community with scientists and members of the space industry displaying an interest in the game – including NASA, SpaceX's Elon Musk, and ESA. Squad has added a NASA-based Asteroid Redirect Mission pack to the game, allowing players to track and capture asteroids for mining and study. Squad has also developed an official mod for the game centered around observing and tracking threatening asteroids, named Asteroid Day. The mod was developed in partnership with the B612 Foundation. Some parts from this mod (though not the core functionality) were added as part of the release of the 1.1 update.