An orbit is the curved path an object takes around another object in space, held in place by gravity. Every planet circling the Sun, every moon circling a planet, and every satellite circling Earth follows an orbit. The concept is simple at its core: an object moves forward fast enough that as gravity pulls it downward, it keeps falling around the larger body rather than crashing into it.
How Gravity and Speed Create an Orbit
A common misconception is that orbiting objects have somehow escaped gravity. They haven’t. A spacecraft in orbit around Earth is very much under the influence of Earth’s gravity, which provides the constant inward pull needed to bend the spacecraft’s path into a curve. What makes an orbit work is balance: the object’s forward speed is high enough that as it falls toward the larger body, it continuously misses it. The surface curves away just as fast as the object falls toward it.
To place a spacecraft into orbit, a rocket has to do two things: lift the spacecraft above most of the atmosphere and accelerate it sideways until it’s moving so fast that its fall traces a complete loop around the planet. For the International Space Station, which orbits about 400 km (248 miles) above Earth’s surface, that speed is roughly 8 km per second, or about 17,900 mph. At geostationary altitude (35,786 km up), the required speed drops to about 3 km per second because gravity weakens with distance.
If an object doesn’t reach orbital speed, gravity wins and it falls back to the surface. If it reaches roughly 1.4 times orbital speed at the same altitude, it achieves escape velocity, meaning it has enough energy to leave the larger body’s gravitational influence entirely and fly off into space.
Why Orbits Are Ellipses, Not Circles
In the early 1600s, Johannes Kepler worked out three laws describing how objects orbit. The first, and most surprising at the time, is that orbits are not perfect circles. They are ellipses, oval-shaped paths with the larger body (like the Sun) sitting at one of two focal points rather than at the center. Some orbits are nearly circular, with only a slight elongation. Others, like those of many comets, are extremely stretched out.
Kepler’s second law explains how speed changes along an elliptical orbit. An object moves fastest when it is closest to the body it orbits (a point called perihelion for orbits around the Sun, or perigee for orbits around Earth) and slowest when it is farthest away (aphelion or apogee). The reason is intuitive: gravity pulls harder when objects are closer together, accelerating the orbiting body as it swings in and slowing it as it swings away.
The third law connects an orbit’s size to how long one trip around takes. Larger orbits take dramatically longer to complete. Earth orbits the Sun in one year, while Neptune, orbiting much farther out, takes about 165 years. This same principle applies to artificial satellites: a satellite in low Earth orbit completes a lap in about 90 minutes, while a geostationary satellite at 35,786 km takes exactly 24 hours.
Types of Earth Orbits
Satellites are placed in different orbits depending on what they need to do. The three main categories are defined by altitude.
- Low Earth orbit (LEO): Ranging from about 180 km to 2,000 km above the surface. The lower boundary exists because atmospheric drag below 180 km would quickly slow a satellite and pull it down. LEO is ideal for satellite imaging, since closeness to the ground means higher resolution photos. The International Space Station operates here. Because LEO satellites move so fast relative to the ground, a single satellite can only cover a small area at a time, so communications and navigation systems in LEO typically use constellations of dozens or hundreds of satellites working together to provide continuous global coverage.
- Medium Earth orbit (MEO): Covers altitudes between LEO and geostationary orbit. Navigation systems like Europe’s Galileo and the United States’ GPS operate in MEO, striking a balance between coverage area and signal strength.
- Geostationary orbit (GEO): At exactly 35,786 km altitude, a satellite orbits at the same rate Earth spins, so it appears to hover over one fixed spot on the equator. This makes GEO perfect for telecommunications satellites, since ground antennas can point at one fixed location in the sky and maintain a constant link. Weather satellites also use this orbit to continuously monitor the same region and track storms as they develop.
Natural Orbits Throughout the Solar System
Orbits aren’t limited to human-made satellites. The solar system is full of natural orbits at every scale. Eight planets orbit the Sun. Over 420 moons orbit those planets. Hundreds more moons orbit dwarf planets, asteroids, and other small objects in the outer solar system. Saturn has more moons than any other planet, while even tiny Pluto has five.
Some of these natural satellites are enormous, like Jupiter’s moon Ganymede, which is larger than the planet Mercury. Others are barely more than boulders, like Mars’s two small moons, Phobos and Deimos, which are irregularly shaped and only a few kilometers across. All of them follow the same basic physics: gravity provides the inward pull, and their orbital speed keeps them from falling in.
Lagrange Points: Orbiting in Place
There are five special positions in any two-body system (like the Sun and Earth) where the gravitational pulls of both bodies combine with orbital motion to let a small object essentially keep pace with the smaller body. These are called Lagrange points, labeled L1 through L5.
Three of them (L1, L2, and L3) sit along the line connecting the two large bodies and are unstable, meaning a spacecraft parked there needs occasional small thrust corrections to stay put. The other two (L4 and L5) form the tips of equilateral triangles with the two large bodies and are stable, meaning objects naturally collect there. Jupiter’s L4 and L5 points, for example, are home to thousands of asteroids called Trojans. NASA’s James Webb Space Telescope orbits at the Sun-Earth L2 point, about 1.5 million km from Earth, where it can keep its sunshield pointed at both the Sun and Earth simultaneously while observing deep space.
What Causes Orbits to Decay
Orbits aren’t always permanent. For satellites in low Earth orbit, the thin upper atmosphere still exerts measurable drag. Even though air density at those altitudes is a tiny fraction of what it is at the surface, it’s enough to gradually slow a satellite and pull it closer to Earth. Left uncorrected, this orbital decay eventually brings the satellite low enough that it burns up on reentry.
Solar activity makes this worse. When the Sun is particularly active, it pumps extra energy into Earth’s upper atmosphere, causing it to expand. Layers of denser air rise to altitudes where satellites fly, increasing drag significantly. Geomagnetic storms triggered by solar wind interactions with Earth’s magnetic field can produce sudden, large spikes in upper atmosphere density, sometimes altering satellite orbits noticeably within hours. This is why satellite operators closely track space weather forecasts and periodically fire small thrusters to boost their spacecraft back to the correct altitude.
At higher altitudes, like MEO and GEO, atmospheric drag is negligible. Satellites there can remain in orbit for centuries or longer, which is one reason space debris in those regions is a growing concern.