|Initial release||27 November 2000|
|Stable release||2016 (30 August 2016)|
|Operating system||Windows Vista and later|
Orbiter is a freeware space flight simulator program developed to simulate spaceflight using realistic Newtonian physics. The simulator was released on 27 November 2000; the latest edition, labeled "Orbiter 2016", was released on 30 August 2016, the first new version of the simulator since 2010.
Orbiter was developed by Dr. Martin Schweiger, a senior research fellow in the computer science department at University College London, who felt that space flight simulators at the time were lacking in realistic physics-based flight models, and decided to write a simulator that made learning physics concepts enjoyable. It has been used as a teaching aid in classrooms, and a community of add-on developers have created a multitude of add-ons to allow users to fly assorted real and fictional spacecraft and add new planets or planetary systems.
Orbiter is a realistic physics simulator which allows users to explore the Solar System in a number of spacecraft, both realistic, such as the Space Shuttle Atlantis; and fictional, such as the "Delta-Glider." Schweiger has included fictional spacecraft to allow for easier flights for less experienced users. The simulator is realistic enough to re-enact historical space flights, and the ability to fly fictional ships also allows the player to reach areas of the Solar System that cannot be reached by human spaceflight at the present time.
A spacecraft's engines are defined only by the amount of thrust they put out and amount of fuel they use, allowing anything from solar sails to conventional rocket engines to futuristic nuclear fission and fusion drives to be simulated. Everything between ground movement and interplanetary travel is supported, including orbital and sub-orbital flight, although only vessel-ground collisions are supported. Docking and attachment systems allow the user to simulate docking with a space station or other spacecraft, and rendezvous with and retrieval of satellites. Users can also build space stations in orbit.
The Solar System as presented in Orbiter consists of the Sun, the eight planets and their major moons. Many dwarf planets, asteroids (except Vesta), and comets not included in the simulator are available as add-ons. Although Orbiter contains a database of over 100,000 stars, these are for display purposes only and interstellar travel is currently not possible in the simulator. The simulator also includes a planetarium mode that allows ecliptic and celestial grids to be overlaid onto the star map, along with labels of the constellations and other celestial markers. The planetarium mode can also display labels indicating the location and identity of objects in the Solar System, such as planets, moons, or vessels, that appear within a certain proximity based on their type. This mode can also display labels on the celestial bodies in the Solar System at certain coordinates on their surface for indicating cities, historical markers, geological formations, and other interesting sites.
The traditional simulated control interface in Orbiter consists of two multi-function displays and a head-up display. Each features several modes of operation, with all commands given via the keyboard or mouse. The simulator also supports customized control panels and instruments, including 3-D virtual cockpits and 2-D instrument panels. These allow the player to use the mouse to interact with the panels, and allows more complex systems and instruments that are customized for each ship. The addition of a virtual cockpit also allows the player to freely look around from the perspective of the pilot. Since Orbiter 2006 Patch 1, it has supported TrackIR, which allows the simulator to track the head-movements of the player and adjust the view accordingly.
Orbiter was developed as a simulator, with accurately modeled planetary motion, gravitation effects (including non-spherical gravity), free space, atmospheric flight and orbital decay. The position of the planets in the solar system is calculated by the VSOP87 solution, while the Earth-Moon system is simulated by the ELP2000 model. Only n-body Newtonian mechanics are simulated, not taking relativistic effects into account. This means that phenomena such as time dilation due to relativistic effects are not simulated.
The default version of Orbiter has no sound, however popular add-ons called OrbiterSound and XRSound are available. Both of them provides engine noises, ambient sounds in the cabin, radio chatter and other sounds including playlists. Both of them contain options to maintain a realistic silence when the craft is viewed externally during spaceflight. There is no collision detection for objects in space, apart from defined docking ports.
Since Orbiter 2016, terrain elevation is modeled for Earth, the Moon, and Mars. Support was also added for higher definition surface textures and clouds. 
Orbiter's standard distribution includes real and fictional spacecraft and space stations:
While the source is not editable, an extensive API enables Orbiter users to contribute by creating add-ons. Many spacecraft are available for download as add-ons, ranging from the Soviet Vostok spacecraft to Apollo program. Another popular category of add-ons are modifications of the standard spacecraft of Orbiter, ranging from simple visual changes to complex simulations of the internal subsystems of these fictional crafts. Examples of these more advanced default spacecraft include the XR Series of vessels.
Add-ons are also available for new surface bases, MFD modes, extensions of the simulation menu, space stations, planets, and even other planetary systems. Since Orbiter 2006, a scenario editor is included, which can also be extended for supporting the special attributes of add-on vessels.