International Space Station (ISS) Nitrogen and Oxygen Logistics; Predictions verses Actuals

The International Space Station (ISS) nitrogen is used to maintain total pressure within the cabin. Nitrogen is also required to support on-board experiments and medical equipment. Nitrogen is stored on the ISS Airlock in two tanks. The Progress also can bring up pure nitrogen, but usually just brings nitrogen up in the form of air. A proper balance must be maintained to ensure all users have the required amount of nitrogen to support their needs.

The ISS oxygen is used to maintain oxygen partial pressures within the cabin. Oxygen is also required to support medical equipment as well as emergency masks and Extra-Vehicular Activity (EVA). Oxygen has many different sources. The main metabolic oxygen source is the Elektron, a water electrolyzer. Oxygen candles (Solid Fuel Oxygen Generators or SFOGs and BOCS) can supply metabolic oxygen as an alternative to the Elektron. Additionally, oxygen is stored in two tanks on the ISS Airlock as well as brought up in Progresses. As with nitrogen, a proper balance must be maintained to ensure all users have the required amount of oxygen to support their needs

On-orbits supplies of both nitrogen and oxygen are tracked as masses available for use. Planned resupplies up to approximately a year in the future are also included. Actual nitrogen and oxygen masses are then compared against the planned masses. If required, the plan is adjusted to ensure that the nitrogen and oxygen are available to support ISS operations as well as other program priorities.