Orbital mechanics without the maths
Published 19 October 2026 · All guides
In short: a satellite is falling all the time and moving sideways fast enough to keep missing the Earth. Higher orbits are slower. The Earth turns underneath, so the path over the ground shifts every lap. The tilt of the orbit decides how far north and south it reaches. And changing an orbit costs propellant, which a satellite carries only a little of. Tilting is far dearer than climbing.
An orbit is falling and missing
Throw a ball and it curves to the ground. Throw it faster and it lands further away. At about 7.7 km/s, a little above the air, the ground curves away beneath it as fast as it falls, and it never lands. That is an orbit. Nothing holds a satellite up; it is in free fall, which is why astronauts float.
Higher is slower
Gravity weakens with distance, so a higher orbit needs less speed and has further to go. Both make it take longer to go round:
- The International Space Station, about 420 km up, laps the Earth every 93 minutes or so, at about 7.7 km/s.
- GPS satellites, about 20,200 km up, take just under 12 hours.
- Geostationary satellites, 35,786 km up, take one turn of the Earth (23 hours 56 minutes) at about 3.1 km/s, so they seem to stand still in the sky.
One consequence feels backwards: to catch up with something ahead of you on the same orbit, you slow down. Braking drops you to a lower, faster orbit, you gain on the target, and then you speed back up.
The ground track moves west
The orbit stays fixed in space while the Earth turns underneath it. In the 93 minutes the ISS takes to go round, the Earth turns about 23 degrees, so each lap crosses the equator roughly 2,600 km further west than the last. Over a day the track sweeps across most of the globe. This is why a low satellite passes over any given place only at certain times, and why those times change from day to day.
Inclination: how far north and south
An orbit's tilt against the equator is its inclination, and it sets the most northerly and southerly latitude the satellite ever flies over. The ISS is inclined at 51.6 degrees, so it passes over London (51.5°N) only at the very top of its path and never over Oslo. An orbit inclined near 90 degrees is a polar orbit, and over a day it passes over every part of the Earth.
The Earth's bulge turns orbits
The Earth is slightly fatter at the equator than through the poles. That extra mass tugs on every orbit, slowly swinging its tilted plane around the Earth's axis. Mission planners use this rather than fight it: pick the right height and tilt and the plane turns once a year, keeping pace with the Sun. That is a sun-synchronous orbit, and it is how many imaging satellites see every place at the same time of day.
Changing orbit costs propellant
Engineers count the cost of a manoeuvre in change of speed, called delta-v. A satellite carries a fixed amount; once it is spent, it can no longer change its orbit until someone refuels it, which until recently meant never.
Some changes are cheap and some are brutal:
- Climbing from 400 km to 500 km costs about 56 m/s: one push to stretch the orbit, a second at the top to round it off.
- Tilting the orbit by a single degree at that height costs about 133 m/s, because the whole of the satellite's sideways speed has to be turned. Ten degrees costs over 1.3 km/s, more than most satellites carry in their lifetime.
That is why a satellite is launched straight into the tilt it needs, and why satellites almost never change which latitudes they cover. It is also why spy satellites are built in families, each on a different orbit, rather than one being moved around.
Low orbits slowly fall
A few hundred kilometres up there is still a trace of atmosphere. It drags on a satellite, lowering it a little every orbit, faster when the Sun is active and heats the upper air. The ISS has its orbit raised several times a year. Without propellant to push back, a satellite in a low orbit eventually re-enters and burns up.
Where Celestrike comes in
All of this is the game in Celestrike rather than background. Satellites follow real orbits, including the slow turn the Earth's bulge gives them, so passes arrive when the orbit says they do. Propellant is finite, and changing an orbit spends it. Celestrike is a free spy-satellite strategy game for iPhone and Android, played on real orbits — on the App Store and Google Play.