Most explanations of how do satellites work start with gravity and stop there, which leaves the interesting parts out. A satellite is not a machine that floats; it is a machine that is permanently falling and permanently missing the ground. Get that one idea straight and the engineering — the orbits, the power budget, the radios, the fuel — starts to make sense.
The Short Answer
A satellite stays up because it moves sideways fast enough that as it falls toward Earth, the Earth curves away beneath it. At an altitude of about 250 miles (400 km), that speed is roughly 17,000 mph — about 4.7 miles per second. One orbit takes around 90 minutes. Nothing holds it up except its own momentum.
Once it is there, a satellite does four things: keeps itself pointed the right way, generates power, stays warm enough to function, and sends data home. Everything else on board is the payload — a camera, a radio relay, a radar.
Why They Do Not Fall Down
Newton’s cannonball thought experiment is still the clearest version. Fire a cannon horizontally from a mountaintop and the ball travels some distance before hitting the ground. Fire it faster and it goes farther. Fire it at about 17,000 mph and the curve of its fall matches the curve of the Earth — it never lands. That is orbit.
Gravity at 250 miles up is still about 90% as strong as it is on the surface. Astronauts are not weightless because gravity is absent; they are weightless because they and their spacecraft are falling together.
The Three Orbits That Matter
| Orbit | Altitude | Period | Used for |
|---|---|---|---|
| LEO (low Earth orbit) | 100-1,200 miles | 90 min – 2 hr | ISS, imaging, Starlink |
| MEO (medium) | ~12,500 miles | 12 hours | GPS, Galileo navigation |
| GEO (geostationary) | 22,236 miles | 24 hours | Weather, TV broadcast |
A geostationary satellite orbits at exactly Earth’s rotation rate, so it appears to hang motionless over one spot on the equator — which is why satellite dishes never have to move. GPS satellites sit in MEO deliberately: high enough that a receiver can see several at once, low enough that signals arrive strong enough to use.
Power: The Budget That Runs Everything
Ask an engineer how do satellites work as an engineering problem and the answer usually starts with the power budget, because everything else is downstream of the watts you can generate. Almost every satellite runs on solar panels plus rechargeable batteries, and the arithmetic is unforgiving. A satellite in LEO spends roughly 35-40 minutes of every 90-minute orbit in Earth’s shadow. It runs entirely on batteries during that eclipse, then has to recharge while in sunlight.
Solar arrays on a modern communications satellite generate somewhere between 5 and 20 kilowatts — roughly what a few houses use. Deep-space missions too far from the Sun for panels use a radioisotope thermoelectric generator, which converts heat from decaying plutonium-238 into a few hundred watts. Not much power, but it lasts for decades.
Staying Warm, Cool, and Pointed
Vacuum is an insulator, so the real thermal problem is usually getting rid of heat, not keeping it. Sunlit surfaces can hit 250°F while shaded ones drop to -250°F. Radiators, reflective blankets, and heaters do the work.
Pointing is handled by reaction wheels — spinning flywheels that let a satellite rotate by spinning something else the other way — plus magnetorquers and small thrusters. Reaction wheels eventually saturate, which is why satellites carry propellant to dump that stored momentum. Running out of propellant ends the mission even when the panels and electronics are still fine.
Talking to the Ground
Satellites communicate on radio frequencies allocated by international treaty. The pattern is simple: a ground station sends commands on an uplink, the satellite replies with data on a downlink. Bandwidth is the constraint. A high-resolution imaging satellite collects far more data than it can send, so it stores images and waits for a pass over a ground station.
This is also why latency works out as it does. Signals travel at light speed, so a round trip through a satellite 340 miles up adds only a few milliseconds — much better than the quarter-second or so you get from geostationary satellites at 22,000 miles.
How Satellites Die
Two things end a mission: running out of propellant for station-keeping, or component failure from radiation and thermal cycling. Below roughly 300 miles, atmospheric drag is strong enough to pull a satellite down within months. Above it, dead satellites can linger for decades or centuries.
That is why operators now plan for disposal: either a controlled re-entry over open ocean, or a boost into a higher graveyard orbit where the dead satellite will not bother anyone. The rules are imperfectly enforced, and the accumulated leftovers have become a serious problem in their own right.
Keep Exploring
- What is space debris? — what happens to satellites after they stop working
- How does GPS work? — the constellation you use daily without thinking about it
- What is gravity? — the force that keeps satellites falling forever
