How Satellites Stay in Orbit: Falling Sideways Around Earth

A satellite does not float above Earth. At 400 km it is still pulled hard toward the planet, but it moves sideways so fast that the surface curves away beneath it at the same rate it drops. That balance between falling and moving sideways is the whole secret of staying in orbit, and it is why a satellite that slows down or flies too low eventually comes back down.

Gravity never switches off

Gravity does not stop at the top of the atmosphere. It weakens with distance, following the inverse square law, so the force on a body falls to one quarter when the distance doubles. At the surface, Earth's gravity gives about 9.8 m/s² of downward acceleration. At 400 km, the distance from Earth's center is about 6,780 km instead of 6,370 km, so gravity is roughly 88 percent of its surface value, or about 8.7 m/s².

That means the International Space Station is not beyond gravity. It is falling the whole time. Astronauts inside feel weightless because they and the station fall together, with nothing pushing them against the floor.

Newton's cannon

Isaac Newton described the idea with a thought experiment. Imagine a cannon on a very tall mountain firing a ball horizontally. With a small charge, the ball lands some distance away. With a bigger charge, it lands farther. If the charge were large enough, the ball would fall toward the ground while the curved surface dropped away beneath it, so it would never land. It would keep circling the planet.

Real satellites are that thought experiment. Rockets do the hard work of lifting a payload above most of the air, then give it a sideways push. Once the sideways speed is right, gravity bends the straight-line path into a closed curve around Earth.

The speed that makes an orbit

For a circular orbit, the required speed depends only on the mass of the planet and the distance from its center. The formula is v = √(GM/r), where GM is Earth's gravitational parameter, about 398,600 km³/s², and r is the orbit radius measured from Earth's center. A satellite at 400 km altitude needs about 7.7 km/s. That is roughly 28,000 km/h, more than 30 times the cruise speed of a passenger jet.

The orbital period, the time for one lap, follows from the circumference divided by the speed. At 400 km, one lap takes about 92 minutes, so the ISS sees about 16 sunrises and sunsets each day. Higher satellites move more slowly and take longer, while lower satellites move faster and take less time.

Altitude sets speed and period

The table below uses circular orbits and rounded figures. The pattern matters more than the exact decimals: the higher the orbit, the slower the satellite moves and the longer each lap takes.

Altitude (km)	Orbital speed (km/s)	Period (min)
200	7.8	88
400	7.7	93
800	7.5	101
20,200 (GPS)	3.9	718
35,786 (GEO)	3.1	1,436

The geostationary value matches one sidereal day, about 23 hours 56 minutes. A satellite at that altitude, over the equator, moves in step with the planet's rotation, so it hangs over one spot. That special case is covered in more depth in the orbit comparison article.

Escape, not orbit, is the high-speed limit

Speed also has an upper limit. If a satellite is moving faster than the local orbital speed but not fast enough to escape, its path becomes an ellipse. The closest point, perigee, is where it moves fastest, and the farthest point, apogee, is where it moves slowest. Escape velocity is √2 times the circular speed at the same distance, so at 400 km it is about 10.9 km/s. At the surface it is about 11.2 km/s, the figure engineers quote for leaving Earth entirely.

A rocket that undershoots the orbital speed will not circle the planet. It will follow a suborbital arc and fall back. A rocket that overshoots will leave an elliptical path or escape completely. Launch providers aim for a precise sideways velocity at a precise altitude, and small errors in that number decide whether the satellite is useful or lost.

Drag is the slow leak

Above the atmosphere, orbits would last for thousands of years. Real satellites in low orbit are not that lucky. At 200 to 400 km there is still a thin layer of air. The density is billions of times lower than at sea level, but a satellite moving at 7.7 km/s hits enough molecules over each lap to lose energy steadily.

Drag reduces speed, and a slower satellite drops to a lower orbit. Lower orbits are denser, so drag grows, and the descent accelerates. Satellites near 200 km can re-enter within weeks, while satellites at 500 km may last years. Solar activity matters too. During strong solar storms the upper atmosphere heats and expands, which increases drag and can shorten satellite lifetimes sharply. This effect is one reason operators watch solar weather closely.

Reboosts and station keeping

To stay up, satellites periodically fire thrusters. The ISS receives reboosts from visiting cargo vehicles or its own engines to offset drag. Geostationary satellites face a different problem. Sun and Moon gravity nudge them out of position, so they use small thruster burns to hold their slot. Typical station keeping for a geostationary satellite costs roughly 50 m/s of velocity change per year.

Earth itself is not a perfect sphere. It bulges at the equator, and that bulge slowly rotates the orbital plane around the planet's axis. Engineers use this effect on purpose. A sun-synchronous orbit, typically near 700 to 800 km and tilted about 98 degrees from the equator, precesses at just the rate needed to keep the same local solar time over a region. Earth observation satellites rely on this consistent lighting.

Sunlight, shadow and power

Orbit also decides how a satellite gets power. Most satellites carry solar panels, and in low Earth orbit the spacecraft passes through Earth's shadow on each lap. At 400 km, roughly a third of each 92-minute orbit is spent in eclipse. Batteries carry the spacecraft through the dark part, then solar arrays recharge them in sunlight. Designers size the batteries and panels around that cycle, which repeats about 5,700 times per year.

Heat follows the same rhythm. A satellite swings between sunlight and shadow many times a day, so thermal blankets, radiators and heaters keep electronics within safe limits. Engineers therefore treat orbit height as a design input for power, thermal control and radiation exposure, not only for coverage.

Key takeaways

A satellite stays in orbit because its sideways speed matches the rate at which the planet's surface curves away beneath a falling body. At 400 km that speed is about 7.7 km/s, and it sets a lap time near 92 minutes.

Altitude controls speed and period together. Lower orbits are faster and shorter, while higher orbits are slower and longer, and geostationary orbit is the special height where the period equals one sidereal day.

Drag is the constant enemy of low orbit. Without reboosts, satellites lose altitude and eventually re-enter, so station keeping is a regular operational cost rather than a rare emergency.