What an Orbit Really Is: Falling Around the Earth
A satellite is not floating above the planet. It is falling towards it all the time and moving sideways fast enough that it keeps missing.
Falling and missing
An orbit is the curved path an object follows around another under gravity: a planet around a star, the Moon around the Earth, a satellite around either. The useful mental picture is Newton's cannon. Fire a cannonball horizontally from a very tall mountain and it curves down to the ground. Fire it faster and it lands further away. Fire it fast enough, and the ground curves away beneath it as quickly as it falls. It never lands. It is in orbit.
That is why astronauts in orbit feel weightless. Gravity is still acting on them strongly; they and their spacecraft are simply falling together, so nothing presses them against the floor.
Kepler's three laws
In the early seventeenth century Johannes Kepler described planetary motion in three rules, now called Kepler's laws of planetary motion:
- Orbits are ellipses, with the central body at one focus, not at the centre.
- A line from the central body to the orbiting object sweeps out equal areas in equal times, so the object moves fastest when it is closest.
- The square of the orbital period is proportional to the cube of the orbit's size. Bigger orbits take disproportionately longer.
A circle is just an ellipse with both foci in the same place, so circular orbits obey the same rules.
High and low
The third law explains a pattern you can see in everyday satellite services. Spacecraft in low Earth orbit are close to the planet and go round quickly, passing over different parts of the ground on each orbit. Further out, the period gets longer. At one particular altitude the period matches the rotation of the Earth itself. A geostationary orbit is a circular orbit about 35,786 km above the equator, moving in the direction of the Earth's rotation, so a satellite there appears to hang over the same spot. That is why fixed satellite dishes can point at one place in the sky.
Speed is the price of altitude
A counter-intuitive result of orbital mechanics is that to catch up with something ahead of you in the same orbit, you briefly slow down. Slowing drops you into a lower, shorter orbit, where you go round faster; then you speed up again to rise back. Pilots of spacecraft do not think in terms of "pointing at the target", but in terms of changing the shape of the orbit.
Where probability comes in
Orbits themselves are highly predictable. What is uncertain is everything around them: whether an engine fires exactly as planned, whether debris passes close by, whether weather allows a launch at all. The rest of this blog is about that uncertainty, starting with the narrow slots of time called launch windows.
REV 2026-11-09 / Tobias Wren