Life Inside the ISS: Cabin Pressure and Temperature

Step inside the International Space Station and you would find it surprisingly comfortable. The cabin is maintained at roughly sea-level pressure and room temperature, allowing the crew to work in regular clothing without any special breathing equipment. Keeping it that way, however, requires some serious engineering.

Cabin pressure

The ISS maintains an atmosphere very similar to what you breathe on Earth's surface. The cabin pressure sits around 760 mmHg (about 14.7 psi), with a mix of roughly 21% oxygen and 79% nitrogen - just like sea level on Earth.

This is different from earlier spacecraft. The Apollo capsules, for example, used a pure oxygen atmosphere at much lower pressure (about 5 psi). The ISS chose a standard atmosphere because it is safer (pure oxygen is a serious fire hazard), healthier for long stays, and makes it easier to transition between the station and visiting spacecraft.

The pressure is regulated by the Pressure Control Assembly, which can add nitrogen or oxygen from high-pressure tanks as needed. Small leaks are normal - the station loses a tiny amount of air every day through seals and airlocks. These are tracked carefully, and any increase in the leak rate triggers an investigation.

Temperature control

The ISS cabin temperature is typically maintained between 22 and 24 degrees Celsius (roughly 72 to 75 degrees Fahrenheit). The crew can adjust it somewhat based on preference, but there are limits.

Temperature control in space is more complicated than on Earth because there is no air outside to carry heat away. The station uses two systems:

• Internal thermal control - Air is circulated through the modules by fans (there is no natural convection in microgravity). Heat exchangers transfer excess heat from the cabin air to water loops that run through the walls.
• External thermal control - The water loops carry heat to ammonia loops, which in turn carry it to the large white radiator panels on the station's truss. These radiators dump heat into space by thermal radiation.

The challenge is managing the extremes. When the station is in sunlight, the Sun-facing side can reach over 120 degrees Celsius. In shadow, the temperature drops to minus 150 degrees. The thermal control system has to handle both extremes while keeping the interior comfortable.

What the data shows

On ISS Monitor, the cabin pressure and temperature readings come from sensors in the US Laboratory (Destiny module). You will notice the values are remarkably stable - that is the life support system doing its job. Small fluctuations are normal and can happen when hatches are opened between modules, during exercise (which heats the cabin), or when equipment generates extra heat.

If you ever see a significant drop in cabin pressure on the chart, it could indicate an airlock operation (pressure drops slightly when the crew is preparing for a spacewalk and the airlock is depressurized) or, in rare cases, a leak investigation.