Orbiter is a closed source freeware space flight simulator for the Windows operating system created by Dr Martin Schweiger, a computer scientist at University College London. It is function-oriented, with the interface focused on maneuvering the spacecraft, rather than perspective-oriented, as in Celestia. Orbiter allows any user to explore the solar system on a number of spacecraft, both realistic, such as the Space Shuttle Atlantis, and fictional, such as the Delta-glider.
Orbiter contains the realism necessary to re-enact historical space flights while the ability to fly more futuristic ships allows the player to make timely travel around the solar system. A spacecraft's engines are defined only by the amount of thrust it puts out and amount of fuel used. This allows anything from solar sails, to conventional rocket engines, to futuristic nuclear and fusion drives to be simulated. Orbital, sub-orbital, and transplanetary travel are all supported. Docking and attachment systems allow the player to perform a docking with a space station or other spacecraft and also rendezvous with and retrieve a satellite.
Orbiter's solar system consists of the sun and eight planets. Pluto, asteroids, and comets are not included but can be added via 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 Orbiter.

Hardware and software requirements:
-300 MHz PC or better (Pentium, Athlon, etc.);
-128MB RAM or more;
-DirectX 7.0 or higher;
-DirectX compatible 3D graphics accelerator card with at least 16MB of video RAM (32MB or more recommended) and DXT texture compression support;
-Approximately 60MB of free disk space for the minimum; installation (additional high-resolution textures and addons will require more space);
-DirectX compatible joystick (optional).

OrbiterSound 3.0 SPECIAL FOR ORBITER2005 !
Complete and fully integrated sound environnment, include thrust sound,gpws,launch countdown,docking,aircond,wind, atc, ambience sound and few more others. Included Mp3 player, MFD to tune radio atc frequency, Sequential_editor.exe to create sequential atc pack and the SoundSDK for vessel creator to add custom sound to their model.

*ORBITER is distributed in a number of ZIP packages. Firstly, please download the basic file, then the OrbiterSound and in the end-optional high-res graphics files.
Even if you have a high-performance computer, it is recommended that you download and install the minimum system first, and make sure everything is working properly, before installing any high-resolution packages and addons.