Table of Contents
Earth Orbits
An orbit is the gravitationally curved trajectory of an object around a point in space. For circular orbits around Earth, the orbital velocity decreases with altitude because gravity weakens with distance. The International Space Station at 400 km altitude orbits at about 7.67 km/s (27,600 km/h), completing an orbit every 92 minutes. Higher orbits are slower but take longer to complete.
Orbital mechanics, founded on Kepler's laws and Newton's law of gravitation, govern all satellite operations, space station maintenance, planetary exploration, and space debris tracking. Understanding orbital parameters is essential for satellite communications, Earth observation, GPS navigation, and planning interplanetary missions from Earth orbit.
Orbital Mechanics
Where v is orbital velocity, G is the gravitational constant, M is Earth's mass (GM = 3.986 x 10^14 m³/s²), r is the orbital radius (Earth radius + altitude), and T is the orbital period.
Notable Orbits
| Orbit | Altitude | Velocity | Period |
|---|---|---|---|
| ISS (LEO) | 400 km | 7.67 km/s | 92.6 min |
| GPS (MEO) | 20,200 km | 3.87 km/s | 11.97 hrs |
| GEO | 35,786 km | 3.07 km/s | 23.93 hrs |
| Moon | 384,400 km | 1.02 km/s | 27.3 days |
Orbit Types
- LEO (160-2000 km): Earth observation, ISS, Starlink
- MEO (2000-35786 km): GPS, navigation satellites
- GEO (35,786 km): Communications, weather satellites
- HEO: Highly elliptical orbits for high-latitude coverage
FAQ
What is geostationary orbit?
Geostationary orbit (GEO) is a circular equatorial orbit at exactly 35,786 km altitude where the orbital period equals Earth's rotation period (23 hours 56 minutes). A satellite in GEO appears stationary from the ground, making it ideal for communications and weather observation. There is only one GEO altitude, and all GEO slots are in the equatorial plane.
Why do satellites not fall down?
Satellites are constantly falling toward Earth due to gravity, but they have sufficient horizontal velocity that as they fall, the Earth's surface curves away beneath them at the same rate. The result is a continuous free-fall that never reaches the ground. If a satellite lost its velocity, it would indeed fall straight down.
What is the minimum orbital altitude?
The practical minimum is about 160 km, below which atmospheric drag causes rapid orbital decay. Even at 400 km (ISS altitude), there is enough residual atmosphere to require periodic reboosts. The ISS loses about 2 km of altitude per month and must fire its thrusters regularly to maintain orbit. Below 160 km, any satellite would reenter within days.