A system for calibrating a sensor in a vehicle, such as a space capsule or other space borne apparatus, uses an expandable integrating sphere. A sensor in the vehicle measures the energy from an electromagnetic energy source within the integrating sphere through a calibration window. The expandable fluid impermeable integrating sphere expands when filled with a fluid, such that when filled with the fluid, its interior is viewable through the calibration window. The system includes a source of fluid to fill the integrating sphere and a fluid regulator coupled to the vehicle to determine when to supply the fluid to the integrating sphere to maintain an appropriate gas pressure level with the integrating sphere.