The player administers a space program operated by Kerbals, a species of small green humanoids, who have constructed a spaceport called the Kerbal Space Center (KSC) on their home planet, Kerbin. From this space center players can create rockets, aircraft, spaceplanes, rovers, and other craft from a provided set of components. Once built, the craft can be launched by players from the KSC launch pad or runway in an attempt to complete player-set or game-directed missions while avoiding partial or catastrophic failure (such as lack of fuel or structural failure). Players control their spacecraft in three dimensions with little assistance other than a Stability Assist System (SAS) to keep their rocket oriented.[9] Provided it maintains sufficient thrust and fuel, a spacecraft can enter orbit or even travel to other celestial bodies. To visualize vehicle trajectory, the player must switch into map mode; this displays the orbit or trajectory of the player vehicle, as well as the position and trajectory of other spacecraft and planetary bodies.[9] These planets and other vehicles can be targeted 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 to plan out trajectory changes in advance, which helps in accurately planning burns. Missions (either player-set or assigned "contracts") involve goals such as reaching a certain altitude, escaping the atmosphere, reaching a stable orbit,[10] landing on a certain planetary body, rescuing stranded Kerbals, capturing asteroids, and creating space stations and surface bases.[11] Players may also set challenges for each other on the game's forums, such as visiting all five moons of Jool (the in-game analog for Jupiter), or use mods to test each other's spacecraft in air combat tournaments. Players can control in-game astronauts, known as Kerbals, who can perform extravehicular activities (EVA). While on EVA, Kerbals may use their EVA suit propellant system to maneuver in space and around craft and space stations, similar to the use of NASA's Manned Maneuvering Unit. Actions that can be performed while on EVA include repairing landing legs, wheels, and deploying or repacking parachutes. Kerbals can also collect material from science experiments, allowing them to store data inside the ship's capsule. During an EVA on any solid planet or moon, a Kerbal can place a flag or take a surface sample. 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 install mods which implement destinations, weapons, additional rocket parts, and goals, such as attempting challenges in a real-scale solar system. Mods can also add informational displays showing craft and orbital statistics such as delta-v and orbital inclination, while a few can near-fully automate flight. Some mods have been added into the game, due to popularity. For example, resource mining, to get ore for refining into fuel, has been implemented from a popular mod.[12] As of version 1.9.1, the major celestial bodies in the game in order of their proximity to the parent star, the Sun, are Moho, Eve, Kerbin, Duna, Dres, Jool, and Eeloo (respectively analogs of Mercury, Venus, Earth, Mars, Ceres, Jupiter, and Pluto). Community modifications are able to expand this planetary system, even being able to add exoplanets or other solar systems. Moons in the system include the captured asteroid around Eve, Gilly; the two moons of Kerbin, Mun and Minmus; The singular moon of Duna, Ike; and the five moons of Jool. The innermost is an ocean moon dotted with small islands, Laythe, the only moon to have an atmosphere or liquid water; an ice moon, Vall; a moon the size of Kerbin, Tylo; a captured asteroid, Bop; and the outermost moon of Jool, Pol. Eeloo has no natural satellites.
The player starts a new game by choosing one of three game modes: sandbox, science, and career mode.[13] In sandbox mode, players may attempt to construct a suitable vehicle for any desired project without penalties for failure and entirely user-assigned missions. Many players have constructed unrealistic spacecraft in this mode, such as impractically large, complicated, or expensive rockets. This mode is also frequently used to create replicas of real-life vehicles.[14] In science mode, the initial selection of parts is limited. More complex parts can be unlocked in the Research and Development building by advancing "science"[13] with various experiments on Kerbin and elsewhere throughout the solar system. This mode was designed to ease new players into the game and prevent them from getting overwhelmed. Career mode extends science mode by adding funds, reputation, and contracts. To build and launch new rockets, the players must complete contracts, earning funds to pay for the necessary parts and fuel. Reputation affects how many contracts are given to the player; less reputation leads to fewer, lower-quality contracts. Declining a contract will reduce the likelihood that a contract of the same type will appear later while also decreasing reputation. Players must upgrade buildings in the space center to unlock new features such as improved tracking, higher spacecraft mass limit, larger part count limit, and increased available contracts.[15]
There are three currencies in Kerbal Space Program: Science, Credits or Funds, and Reputation. Although credits are the in-game analogs of money, Science and Reputation are referred to as currency as well. In Sandbox mode, no currencies are available, as everything is free. In Science mode, only science is available. All three currencies are available in Career mode. Science is gained by performing experiments, crew reports or EVA reports. The experiments can only be turned into Science through either transmissions via antennae,[13] usage of specialized parts, or recovery of a craft on Kerbin. Recovery of an experiment is worth more science than transmitting it. Each time an experiment is turned into science, it is worth less science the next time it is recovered, until it is worth no Science at all. Science can be spent to unlock new parts. Funds, or credits, can be obtained in two ways: One; completing contracts by various agencies, and Two; by exploring new celestial bodies (in which a record-keeping society rewards the player). The Kerbin World Record-Keeping Society does have contracts; however, the rewards for exploring celestial bodies are not contracts. Contracts are accepted through the Mission Control building in KSC, and are completed during flight. Reputation is gained through successful missions, and lost through failed ones. It is essentially how safe Kerbals believe it is to fly on spacecraft. Generally, the better the space program's reputation, the better are the contracts received.
While the game is not a perfect simulation of reality, it has been praised for its accurate orbital mechanics;[16] all objects in the game except the celestial bodies are simulated using Newtonian dynamics. For instance, rocket thrust is applied to a vehicle's frame based on the placement of force-generating elements, and joints between parts have limited strength, allowing vehicles to be torn apart by excessive or misdirected forces. The game simulates trajectories and 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. According to the developers, implementing full n-body physics would require the entire physics engine to be rewritten. The in-game astronauts, Kerbals, are physically simulated. Hitting an object with their feet will cause them to tumble. 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. Aerobraking, however, has become a much more difficult method of velocity reduction since the full 1.0 release due to improved aerodynamics and optional heating during atmospheric entry. 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 600 kilometres (370 mi), approximately 1⁄10 that of Earth's. To compensate for the gravitational consequences of this size difference, Kerbin's density is over 10 times that of Earth's. The planets themselves are also significantly closer together than the planets in the real-life solar system. However, some mods port the real-world solar system into the game with accurate scaling, environments, and additional parts to make up for the extra power requirements.
There are two downloadable content (DLC) expansions: Making History and Breaking Ground.
The Making History, released March 2018, adds additional elements to the game, some of which are historic parts from the Apollo program. These include a lunar lander, basic rocket parts of the Saturn V and more. A level editor allows the players to create their own scenarios. New launch sites, including an island, are added.[17]
Breaking Ground, released May 2019, adds robotic parts, which can be used to build helicopters, propeller airplanes, suspension systems, and robots. The parts include pistons, hinges and rotors. New suits were added as well. A major addition are surface features and science. The player can find certain rocks on surfaces of planets and analyse 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.[18]