The simplest accurate description of orbital mechanics is this: a satellite in orbit is falling. It is in free fall, accelerating toward Earth due to gravity. It just happens to be moving sideways so fast that as it falls, the Earth curves away beneath it at the same rate. It falls, the Earth curves, and the satellite never actually gets any closer to the surface. This balance between gravity and velocity is what an orbit is.
Isaac Newton described the basic concept in the 17th century with a thought experiment about a cannon on a very tall mountain. Fire a cannonball horizontally: it follows a curved arc and hits the ground some distance away. Fire it faster: it goes farther before hitting. Fire it fast enough — about 7.9 kilometers per second at Earth's surface — and it's moving so fast that the curvature of its fall exactly matches the curvature of the Earth. It orbits.
Orbital Altitude and Speed
There's a counterintuitive relationship between altitude and orbital speed. Higher orbits are slower. The reason is that gravity weakens with distance, so at higher altitudes you don't need as much sideways velocity to keep from falling.
A satellite in low Earth orbit — roughly 200 to 2,000 kilometers altitude — travels at about 7 to 8 kilometers per second and completes an orbit every 90 minutes or so. The International Space Station is in low Earth orbit, about 400 kilometers up. It laps the Earth every 92 minutes.
At about 35,786 kilometers altitude, the orbital period exactly equals one day. A satellite at this altitude, orbiting directly over the equator, appears to hang motionless over a single point on Earth's surface. This is called geostationary orbit, and it's where most communications and weather satellites live. They cover a fixed area continuously but are so far away that signals take noticeable fractions of a second to travel there and back — an issue for applications requiring low latency.
GPS satellites orbit at about 20,200 kilometers, between low Earth orbit and geostationary. Their 12-hour period means they're not fixed in the sky, so the system requires enough of them distributed across multiple orbital planes to ensure coverage everywhere on Earth at all times.
Why Satellites Eventually Fall
Low Earth orbit isn't empty. There are traces of atmosphere even at 400 kilometers — thin, but enough to create drag that slows satellites slightly with every pass. As they slow, their orbits lower. As orbits lower, they encounter more drag. Without periodic boosts from onboard thrusters, they spiral downward and eventually burn up in the atmosphere.
The ISS requires regular re-boosts to maintain its altitude. Satellites without propulsion systems decay in months to years depending on altitude. Higher satellites, above roughly 600 kilometers, experience so little atmospheric drag that they can remain in orbit for centuries without correction.
Orbital Debris: A Growing Problem
There are roughly 27,000 pieces of trackable orbital debris currently in Earth orbit, plus hundreds of thousands of smaller fragments too small to track but large enough to damage or destroy satellites. A bolt traveling at 7 kilometers per second hits with more kinetic energy than a bullet.
The concern among space agencies is Kessler Syndrome — a theoretical cascade in which a collision between two objects creates a debris cloud, which causes more collisions, which creates more debris, which eventually makes certain orbital altitudes unusable. The 2009 collision between an active Iridium satellite and a defunct Russian satellite was the most significant real-world demonstration of the problem.
Managing orbital debris is increasingly a national security issue as space becomes more congested and as adversaries explore anti-satellite capabilities that would deliberately create debris fields.