Orbital Velocity Calculator
An orbit is a balance between forward speed and gravitational pull. This calculator gives the velocity and period required at any altitude.
Formula
v = √(GM ÷ r) · T = 2π√(r³ ÷ GM)
- M = mass of the body being orbited
- r = orbital radius from the centre
- Lower orbits are faster, not slower
Low Earth orbit at 400 km altitude: about 7.67 km/s, circling in 92 minutes
The Formulas
Orbital velocity: v = √(GM ÷ r)
Orbital period: T = 2πr ÷ v
Where GM for Earth is 3.986 × 1014 m³/s² and r is measured from Earth's centre.
The ISS orbits at about 400 km altitude, so r = 6,771 km. That gives a velocity of 7.67 km/s and a period of 92.4 minutes — roughly 16 sunrises a day.
Lower Orbits Move Faster
Velocity falls as r rises, which produces a counter-intuitive result: to catch a spacecraft ahead of you, you must slow down. Dropping to a lower orbit increases your speed and shortens your period, letting you gain ground before climbing back.
Geostationary orbit sits at 35,786 km, where the period is exactly 24 hours and a satellite appears fixed above one point on the equator — which is why satellite dishes never need to move.
Orbit Is Sideways, Not Up
Reaching orbit is far more about horizontal speed than altitude. Getting to 400 km is comparatively easy; staying there requires roughly 7.7 km/s sideways, and that velocity accounts for most of the energy a launch consumes.
An orbit is really a permanent fall: the spacecraft falls towards Earth continuously while moving sideways fast enough that the surface curves away beneath it at the same rate.
Quick Reference
| Orbit | Altitude | Speed | Period |
|---|---|---|---|
| Low Earth orbit | 400 km | 7.67 km/s | 92 minutes |
| Medium (GPS) | 20,200 km | 3.87 km/s | 11h 58m |
| Geostationary | 35,786 km | 3.07 km/s | 23h 56m |
| The Moon | 384,400 km | 1.02 km/s | 27.3 days |
Frequently Asked Questions
- How fast does the ISS travel?
- About 7.67 km/s, or roughly 27,600 km/h, completing an orbit every 92 minutes at 400 km altitude.
- Why do lower orbits move faster?
- Gravitational pull is stronger closer in, so a higher speed is needed to balance it. Velocity falls as the square root of orbital radius rises.
- What is geostationary orbit?
- An orbit at 35,786 km where the period is exactly 24 hours, so the satellite stays above one point on the equator and dishes can point permanently at it.
- Why is reaching orbit about speed rather than height?
- Altitude alone would leave you falling straight back. Staying in orbit needs roughly 7.7 km/s of sideways velocity, which is where most launch energy goes.