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. 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. 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. Career mode expands upon science mode's progession 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 let to 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.
Breaking Ground Breaking Ground, released in 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. 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 ==