These are the minimum requirements to run ORBITER on your computer:
* 300 MHz PC or better (Pentium, Athlon, etc.)
* 128MB RAM or more
* Windows 95/98/ME/2000/XP
* 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)
There is nothing to prevent you from trying to run Orbiter on a computer with lower specs, but the results are not likely to be satisfactory.
Since ORBITER keeps evolving these specs tend to become obsolete over time. If you don't get a reasonable frame rate (say >= 20fps) using the default Orbiter.cfg on a machine which meets the specs then please drop me a note and I will correct the requirement list upward.