Atmosphere

An atmosphere is the nasty sticky bit around a sufficiently large planet. From an Orbiter pilot's point of view, there are basically three sorts:

• Thin enough to give little aerodynamic benefit, thick enough that you can't just ignore them. Example: Mars

• Thick enough to be useful aerodynamically; re-entry needs some care, but with sufficient preparation no fuel need be expended after the deorbit burn if a runway is available for landing. Example: Earth, Titan

• Thick enough to impair vehicle thrust if the complex flight model is enabled. Example: Venus