Orbits Explained
Why satellites don’t fall down, how fast they go, and how rockets change orbits, in plain language.
What is an orbit?
- Kids
Imagine throwing a ball from a very tall mountain. Throw it faster and it lands farther away. Throw it fast enough and the ground curves away beneath it as fast as it falls, so it keeps falling around the Earth forever. That is an orbit. Isaac Newton drew this "cannonball" idea more than 300 years ago.
Check yourself: Is the space station floating, or falling?
Falling! It falls all the time, but it moves sideways so fast that it keeps missing the Earth.
How fast is that?
- Kids
At the height of the International Space Station (about 400 km up), you need about 7.7 km every second. That is fast enough to go once around the whole Earth in about 93 minutes, so astronauts see about 16 sunrises a day.
Check yourself: About how many minutes does one trip around the Earth take for the space station?
About 93 minutes, roughly an hour and a half.
Higher means slower
- Teens
Gravity gets weaker with distance, so a higher orbit needs less speed: v = √(μ/r). Weather and TV satellites in geostationary orbit, 35 786 km up, move at only 3.07 km/s and take exactly one sidereal day (23.93 h) to go around, so they seem to hover over one spot.
Check yourself: If you quadruple r, what happens to circular speed?
It halves, because v depends on 1/√r.
Speeding up to go higher
- Teens
To climb to a higher orbit, a spacecraft fires its engine forward. That stretches its circle into an ellipse whose far end touches the new height. Half an orbit later it fires again to make the new orbit round. This two-burn trick is the Hohmann transfer. From a 300 km orbit to geostationary it takes about 3.9 km/s of speed change and about 5.3 hours.
Check yourself: After the first burn, is the spacecraft faster or slower when it reaches the top of the ellipse?
Slower: it trades speed for height as it climbs, then needs the second burn.
Kinds of orbits
- Community college
Low Earth orbit (LEO, roughly 160–2000 km) holds the space station and most Earth-watching satellites. Medium Earth orbit (around 20 000 km) holds navigation satellites such as GPS. Geostationary orbit sits over the equator at 35 786 km. Sun-synchronous orbits are tilted slightly past the poles (about 98.2° at 700 km) so Earth’s bulge turns the orbit once a year and the satellite passes over each place at the same local time, ideal for comparing images.
Check yourself: Why do Earth-imaging satellites like sun-synchronous orbits?
The lighting is the same on every pass, so changes on the ground (fires, floods, forests) are easy to compare.
Energy: the one equation to remember
- Community college
Vis-viva, v² = μ(2/r − 1/a), gives the speed anywhere on any orbit from the distance r and the orbit size a. With a = r it gives circular speed; with a → ∞ it gives escape speed, √2 times faster (11.2 km/s from Earth’s surface, ignoring air).
Check yourself: Using vis-viva, where on an ellipse is a spacecraft fastest?
At the closest point (periapsis), where r is smallest.
Ready for the equations? Continue with the step-by-step math. · Try it in the Orbit Lab