Orbit Basics

Orbits are the paths that objects follow through space because of gravity. Instead of moving in a straight line forever, planets, moons, satellites, and other objects travel along curved paths around larger bodies. This balance between forward motion and gravity is what keeps our Solar System stable and allows artificial satellites to remain in space for years.

An orbit occurs when an object is constantly being pulled toward a larger body by gravity while moving sideways fast enough that it never falls directly into it. The Moon orbits Earth, Earth orbits the Sun, and thousands of satellites orbit our planet using this same principle. Because space contains very little air resistance, an object that reaches the right speed can remain in orbit with very little additional energy.

How Orbits Form

Every object with mass produces gravity, which pulls nearby objects toward it. At the same time, moving objects naturally continue traveling in a straight line unless another force changes their direction. An orbit forms when gravity continuously bends that straight-line motion into a curved path.

A useful way to picture this is to imagine throwing a ball from an extremely high mountain. A slow throw falls back to Earth almost immediately. A faster throw travels farther before hitting the ground. If it could be thrown fast enough, it would keep falling toward Earth while the planet's surface curves away beneath it, allowing it to circle the planet instead of landing. This is the same basic principle that keeps the International Space Station in orbit.

• Gravity pulls an object toward the larger body.
• Inertia keeps the object moving forward.
• When these two forces are balanced, the object follows a stable orbit.

Most Orbits Are Elliptical

Although orbits are often pictured as perfect circles, most are actually ellipses, which are slightly stretched oval shapes. The object being orbited sits at one of the ellipse's two focal points rather than at its exact center. Because of this, an orbiting object moves faster when it is closer to the central body and slower when it is farther away.

Earth follows an orbit around the Sun that is almost circular, but it is still slightly elliptical. Many comets have much more elongated orbits, carrying them from the outer reaches of the Solar System toward the Sun before sending them back into deep space. As they approach the Sun, the increased heat causes their icy surfaces to release gas and dust, making them appear brighter and producing their distinctive tails.

"An orbit is a regular, repeating path that one object in space takes around another." - NASA Space Place

Why Orbits Matter

Orbits make much of modern life possible. Communication, navigation, and weather satellites all rely on carefully planned orbits to provide services on Earth. The International Space Station completes one orbit roughly every 90 minutes, allowing astronauts to live and conduct research in microgravity. On a much larger scale, the orbits of planets around the Sun help shape the rhythm of our Solar System and, together with Earth's tilted axis, contribute to the changing seasons.