The International Space Station Orbits Earth Every 90 Minutes

The incredible velocity of the International Space Station is the key to its continuous journey around our planet. To stay in orbit, the ISS must achieve a precise balance between its forward momentum and the constant pull of Earth's gravity. This phenomenon creates a state of perpetual freefall, where the station is constantly falling toward Earth but moves so rapidly that it continuously misses, following the curve of the planet instead. This principle was famously illustrated by Isaac Newton's thought experiment of a cannonball fired from a mountaintop with enough force to travel all the way around the world. The station's high speed of roughly 17,500 mph (about 28,000 km/h) is the exact requirement to maintain this delicate orbital balance at its altitude of approximately 250 miles (400 kilometers).

This constant, high-speed lap around the globe results in the crew witnessing about 16 sunrises and sunsets each day. Though it flies far above the densest part of the atmosphere, the station still experiences a small amount of atmospheric drag in the thin layer known as the thermosphere. This slight resistance causes the ISS to gradually lose altitude over time, a process known as orbital decay. To counteract this, the space station must periodically fire its thrusters in maneuvers called "reboosts." These engine burns push the station back up to its desired altitude, ensuring it remains a functioning laboratory and home for astronauts high above the Earth.