The International Space Station (ISS) is the largest structure ever placed in orbit by humans, serving as a permanent habitat and a unique microgravity laboratory. Although it appears to float effortlessly in space, the station is constantly battling Earth’s gravity, which is still quite strong at its operational height. Keeping this massive vehicle from crashing back to Earth involves a precise manipulation of physics, maintaining a delicate balance between forward speed and altitude.
The Physics of Orbital Motion
Orbiting is fundamentally a state of continuous freefall, where the spacecraft is constantly falling toward the Earth but never hits the ground. This concept can be understood through a thought experiment proposed by Isaac Newton, often called Newton’s cannonball. He imagined firing a cannonball from a mountaintop: if fired fast enough, the projectile would be traveling forward at such a speed that its curved path of descent exactly matches the curvature of the Earth beneath it.
This balance is a combination of two forces: gravity, which pulls the object toward the Earth, and inertia, the object’s tendency to continue moving in a straight line. For the ISS to achieve orbit, its forward motion must be fast enough to constantly “miss” the Earth. The resulting path is not a straight line, but a stable curve around the planet.
The Critical Balance of Speed and Height
The specific conditions for the ISS to maintain its orbit are dictated by a precise combination of velocity and altitude. The station travels in what is known as Low Earth Orbit (LEO), typically at an altitude between 370 and 460 kilometers (about 230 to 286 miles) above the surface. At this height, the gravitational pull is still significant, only slightly weaker than it is on the ground.
To maintain orbit at this distance, the ISS must achieve an incredible forward velocity of approximately 7.7 kilometers per second, which translates to about 27,700 kilometers per hour. Achieving this high orbital speed is necessary to counteract the force of gravity at that altitude. If the station’s speed were to drop, the gravitational pull would overcome the forward momentum, causing the ISS to spiral downward.
The current LEO altitude provides a balance for accessibility while minimizing the effects of the atmosphere. The station completes one full orbit of the Earth in about 90 minutes.
How the ISS Corrects for Orbital Decay
Even at its altitude, the ISS is not entirely in a vacuum, as it flies through the extremely thin upper layer of the Earth’s atmosphere, called the thermosphere. The sparse air molecules in this region create a small but constant amount of atmospheric friction, or drag, against the station’s massive structure. This drag acts like a slow brake, gradually stealing momentum from the station and causing it to lose speed and consequently, altitude, a process known as orbital decay.
If left uncorrected, this decay would cause the ISS to lose about 100 meters of altitude each day, leading to its eventual uncontrolled re-entry into the atmosphere. To counteract this constant slowdown, the station undergoes periodic maneuvers called “re-boosts.” These maneuvers involve firing thrusters to inject energy back into the orbit and raise the station’s altitude.
Re-boosts are typically performed using the engines of visiting Russian Progress cargo spacecraft while they are docked to the station, though the Zvezda module’s engines can also be used. The frequency of these boosts varies based on solar activity, which influences the density of the upper atmosphere, but they are generally required every few weeks to months. By briefly firing these engines, the station is pushed back up to its desired orbital height, restoring the precise velocity needed to continue its perpetual fall around the Earth.