The International Space Station (ISS) serves primarily as a multinational laboratory for scientific research in the microgravity environment of low Earth orbit. This continuously crewed structure is a platform where astronauts and cosmonauts conduct experiments that benefit life on Earth and prepare for future deep-space missions. To maintain its path, the ISS must travel at an astounding velocity, approximately 17,500 miles per hour, or about 28,000 kilometers per hour.
Defining the ISS Orbital Velocity
The velocity of the International Space Station is precisely controlled to keep it from either falling back to Earth or escaping into deeper space. This specific measurement of speed is known as orbital velocity, which is the speed an object needs to continuously fall around a celestial body without hitting its surface.
The speed translates to roughly 4.8 miles every second. In metric terms, the ISS is moving at about 28,000 kilometers per hour, which is approximately 7.7 kilometers per second. This pace is necessary to achieve a stable trajectory at the station’s altitude. The velocity far exceeds the speed of any commercial jetliner, highlighting the difference between atmospheric flight and orbital mechanics.
The Result: Orbit Time and Daily Cycles
The speed required for orbit results in a highly compressed time frame for the station to circle the planet. The International Space Station completes one full lap around the Earth in about 90 minutes. This means that in a standard 24-hour period, the ISS makes approximately 16 complete orbits.
The consequence of this rapid circuit is a highly unusual experience for the crew. As the ISS circles the planet at an altitude of about 250 miles (400 kilometers), the astronauts witness a cycle of daylight and darkness with every 90-minute rotation. They experience roughly 16 sunrises and 16 sunsets every day, a constant and stunning shift from light to shadow. This rapid, repeated cycle creates unique challenges for the crew, who must adhere to a standardized time zone to maintain a regular sleep schedule.
The Physics of Staying in Orbit
The reason the International Space Station must move so quickly comes down to a balance between two natural forces. The primary force at play is Earth’s gravitational pull, which constantly attempts to draw the ISS downward toward the planet’s surface. The station is also subject to the principle of inertia, which dictates that an object in motion will remain in motion in a straight line unless acted upon by an external force.
To achieve a stable orbit, the ISS must be moving forward at a velocity that exactly balances the downward pull of gravity. The station’s forward motion is so great that as gravity pulls it toward Earth, the planet’s surface curves away at the same rate. This combination of forces causes the station to be in a perpetual state of freefall, constantly falling around the Earth rather than falling into it.
If the ISS were traveling any slower than its orbital velocity, the pull of gravity would overcome its forward momentum, causing its path to curve too steeply and eventually leading it to spiral back into the atmosphere. Conversely, if the station were traveling significantly faster, its inertia would overpower the Earth’s gravitational force, causing it to break free from orbit and fly out into space.