How Does the International Space Station Stay in Orbit? Explained
The International Space Station (ISS) remains one of the most incredible feats of human engineering, floating high above Earth for decades. But how does the ISS stay in orbit without falling to the ground? The answer lies in a deep understanding of physics, from Sir Isaac Newton’s law of gravity to modern orbital mechanics.
Summary
- The ISS orbits at about 402 kilometers (250 miles) above Earth.
- It travels at a speed of 7.6 kilometers per second (4.7 miles per second).
- This speed balances Earth’s gravitational pull, preventing the ISS from falling to the surface.
- The ISS’s orbit decays slightly every day due to atmospheric drag, requiring periodic boosts to maintain its altitude.
- When the ISS’s usefulness ends, it will be deliberately deorbited in 2031 to fall into a remote area of the Pacific Ocean.
How Does the International Space Station Stay in Orbit?
The secret behind how the International Space Station remains in orbit can be traced all the way back to the genius of Sir Isaac Newton, the father of gravitational theory. The ISS is gravitationally accelerated along a curved path around the Earth, preventing it from falling into the atmosphere and burning up.
Newton’s Cannonball Thought Experiment
To understand this, let’s start with a simple analogy. Imagine a cannonball fired horizontally from a high mountain. Newton theorized that, as the cannonball travels, its path curves downward due to gravity. However, if fired at a high enough velocity, the cannonball’s curve would match the curvature of Earth itself, never hitting the ground. Instead, it would continue to fall in tandem with the Earth’s curvature, never reaching the surface.
In simpler terms, the ISS follows a similar principle. The ISS is constantly falling toward Earth but moves forward fast enough to keep “missing” the Earth, maintaining a stable orbit.
The Role of Centripetal Force and Orbital Velocity
In the case of the ISS, its orbital velocity (the speed at which it moves forward) is perfectly balanced with the centripetal force required to keep it in orbit. This centripetal force acts toward the center of the Earth, continually pulling the ISS in the same direction. But due to the forward motion of the ISS, it never falls to Earth—it remains in a constant state of freefall.
Height and Velocity: The Perfect Combination
The ISS orbits at an altitude of 402 kilometers (250 miles) above Earth. At this height, the station travels at a speed of 7.6 km/s. This velocity prevents the ISS from falling into Earth’s atmosphere. If the ISS were at a higher altitude, it would need less speed to maintain orbit. Conversely, if the ISS were closer to Earth, it would need to travel faster to maintain its orbit.
The Thin Atmosphere at the ISS’s Orbit
Even though the ISS is well above Earth’s surface, it still remains within Earth’s atmosphere. It orbits within a thin region of the thermosphere, where there are still some molecules that create drag, slowing the ISS down over time. As a result, the ISS loses about 100 meters of altitude per day, and its speed decreases by approximately 5 centimeters per second.
To compensate for this drag, the ISS periodically fires its thrusters to boost its altitude and maintain its intended orbit. If this adjustment didn’t occur, the ISS would eventually fall into a lower orbit, where atmospheric resistance would further slow it down until it eventually burns up in the atmosphere.
How the ISS Will Meet Its End
Despite its remarkable stability, the ISS will eventually be deorbited. The station’s construction began in 1998, and the oldest parts are now over a quarter-century old. Once it reaches the end of its useful life, the ISS will be deliberately brought down in 2031.
Instead of allowing the ISS to burn up uncontrollably, a space tug will latch onto the ISS and gradually reduce its orbit. The ISS will then be directed to a remote part of the Pacific Ocean, where it will safely reenter the atmosphere and break up, with any surviving debris sinking to the ocean floor. This area, known as the Spacecraft Cemetery, is an isolated region where space debris can safely be discarded without threatening populated areas.
Facts About the ISS
- The ISS travels at 28,000 kilometers per hour (17,500 miles per hour), circling Earth roughly every 90 minutes.
- Astronauts aboard the ISS experience microgravity, often referred to as zero gravity, which affects their bodies and daily activities.
- The ISS is manned by international teams of astronauts from NASA, ESA, Roscosmos, and other space agencies.
- The ISS is about the size of a football field—it measures 109 meters (358 feet) in length.
- The station has been continuously inhabited by humans since November 2, 2000, marking over two decades of human presence in space.
The ISS stays in orbit due to a perfect combination of physics principles, particularly those discovered by Sir Isaac Newton. The station is constantly falling toward Earth, but its orbital velocity keeps it in a delicate balance, never falling to the surface. Thanks to periodic adjustments and careful engineering, the ISS has been able to remain in orbit for over two decades, contributing greatly to scientific research and international cooperation in space.
