Why the ISS Altitude Keeps Changing

The International Space Station orbits Earth at roughly 400 to 420 kilometers above the surface. But if you watch the altitude over time, you will see it is never constant. The station slowly drifts lower, then periodically gets boosted back up. This pattern is not random - it is the result of physics and careful planning.

Atmospheric drag

Even at 400 km altitude, there are still traces of atmosphere. The air is incredibly thin - billions of times less dense than at sea level - but at orbital speed (about 27,600 km/h), even those few molecules add up. They create a small but constant drag force on the station, slowing it down and causing its orbit to decay.

The rate of decay depends on solar activity. When the Sun is more active, it heats the upper atmosphere, causing it to expand. More air molecules reach the station's altitude, increasing drag. During solar maximum, the ISS can lose several hundred meters of altitude per day. During solar minimum, the decay is much slower.

Reboost maneuvers

To counteract this natural decay, the station is periodically boosted to a higher orbit. These reboost maneuvers use thrusters on the Russian segment of the station or on visiting spacecraft (like the Progress cargo ships).

A typical reboost raises the station by a few kilometers and takes anywhere from a few minutes to half an hour, depending on the desired altitude change. Mission Control plans these maneuvers based on several factors:

• The current rate of orbital decay
• Upcoming spacecraft arrivals and departures (which require the station to be at a specific altitude)
• Debris avoidance - sometimes the station's orbit is adjusted to avoid tracked pieces of space debris
• Available propellant on the station and visiting vehicles

What you see in the chart

On the ISS Monitor altitude chart, the gradual downward slope is atmospheric drag at work. The sudden upward jumps are reboost maneuvers. If you zoom out to the 7-day view, you can often spot one or two reboosts in a typical week.

The station's target altitude is a balance between fuel efficiency (higher orbits require more fuel to reach but decay more slowly) and the needs of visiting spacecraft. Most of the time, the ISS orbits between 410 and 420 km.

It is one of those things that seems simple on the surface but involves constant attention and planning by teams on the ground. The ISS does not just float in space on its own - it needs regular maintenance of its orbit, just like everything else on board.