The International Space Station (ISS) is the largest structure ever placed into orbit by humans, serving as a permanent laboratory and home for international crews. Understanding how quickly this outpost travels and how long it takes to complete a single lap around the globe provides perspective on orbital mechanics and the physics governing its path.
The Speed of the ISS and Its Orbital Period
The International Space Station completes a full orbit around the Earth in approximately 93 minutes, which is just over an hour and a half. To achieve this rapid cycle, the station must travel at immense speed, circling the planet about 15.5 times every 24 hours. This velocity is necessary to continuously “fall” around the Earth instead of falling into it.
The required speed to maintain this orbit is approximately 17,100 to 17,500 miles per hour, or about 27,600 to 28,000 kilometers per hour. This pace means the astronauts on board experience a sunrise and a sunset with every orbit. Consequently, the crew members witness roughly 16 sunrises and 16 sunsets every day. This constant cycling between day and night is managed by the crew adhering to a fixed time zone, typically Coordinated Universal Time (UTC), to maintain a consistent sleep schedule.
Defining Low Earth Orbit
The ISS operates within a region of space known as Low Earth Orbit (LEO), which is the zone below 2,000 kilometers (1,200 miles) from the Earth’s surface. The station maintains an average altitude of about 400 kilometers (250 miles). This relatively close proximity to Earth allows for easier resupply missions and places the station beneath the more intense radiation belts.
Operating in LEO presents a challenge because the area is not a complete vacuum. Even at 400 kilometers, trace amounts of the Earth’s atmosphere generate a persistent atmospheric drag. This drag causes the ISS to gradually slow down and lose altitude, with its orbit decaying by about 2 kilometers (1.2 miles) per month.
To counteract this natural decay, the ISS requires periodic reboosts. Thrusters on Russian cargo vehicles or the station’s service module are fired to push the structure back into a stable orbit. These maneuvers ensure the station remains at an altitude that minimizes drag while still allowing for a manageable orbital period.
The Relationship Between Orbital Altitude and Time
The 90-minute orbital period of the ISS is a direct consequence of the physics dictating the relationship between orbital altitude and speed. Objects orbiting closer to the Earth are subject to a stronger gravitational pull. To avoid being pulled back to the ground, they must travel significantly faster than objects in higher orbits.
This principle is derived from the law of gravitation and orbital mechanics, which shows a precise mathematical relationship between a satellite’s distance from the central mass and the time it takes to complete an orbit. Simply put, the lower the altitude, the faster the required velocity and the shorter the orbital period. Conversely, a satellite placed in a much higher orbit, such as a geosynchronous orbit over 35,000 kilometers away, travels much slower and takes a full 24 hours to complete one revolution.
The 400-kilometer altitude of the ISS is the specific point where the necessary balance is struck between the Earth’s gravitational pull and the centrifugal force generated by the 17,500 mph velocity. This precise combination is what locks the station into its quick, 90-minute cycle around the globe. The time it takes to orbit is mathematically determined by the station’s distance from the planet’s center of mass.