Look up at the night sky, and you might spot a satellite moving steadily overhead. It can seem as though the spacecraft is floating above Earth, held in place by some invisible force that prevents it from falling.

The reality is more surprising. A satellite is constantly falling toward Earth, but its sideways motion carries it forward as the planet's surface curves away beneath it. This combination of gravity and motion is what makes an orbit possible.

The Falling Trick That Keeps Going

Imagine throwing a baseball horizontally. Gravity pulls it downward as it travels forward, causing it to follow a curved path until it reaches the ground. Throw it faster, and it travels farther before landing.

Now imagine a much faster object moving above Earth without significant air resistance. At the right speed, its path curves around the planet instead of intersecting the surface. The object remains in continuous free fall, with gravity providing the inward acceleration needed to keep it in orbit.

For a circular orbit near the International Space Station's altitude, roughly 400 kilometers (250 miles) above Earth, the required speed is about 28,000 kilometers per hour (17,500 miles per hour). This is fast enough for the spacecraft to complete an orbit in approximately 90 minutes.

The astronauts aboard the station experience apparent weightlessness because they and the spacecraft are falling together. Gravity has not disappeared. At the station's altitude, Earth's gravitational acceleration is still nearly 90 percent of its value at the surface.

This is why astronauts float inside the station rather than standing on its floor. The spacecraft and everything inside it are accelerating together under gravity, leaving little supporting force to create the familiar sensation of weight.

How Altitude Determines Orbital Speed

For a satellite traveling in a circular orbit, altitude and speed are closely connected. Lower circular orbits require higher speeds, while higher circular orbits require lower speeds. The relationship comes from the strength of Earth's gravity and the distance between the satellite and the planet's center.

The International Space Station travels at approximately 17,500 miles per hour at an altitude near 250 miles. A GPS satellite operates much farther away, at roughly 12,550 miles above Earth, and travels at about 8,700 miles per hour. Its orbital period is close to 12 hours.

Farther out, a satellite in a circular orbit approximately 22,236 miles above the equator takes one sidereal day, about 23 hours and 56 minutes, to complete an orbit. This matches Earth's rotation relative to distant stars.

When such an orbit is circular, equatorial, and follows Earth's rotation, the satellite is geostationary. It appears to remain above the same location on the equator, making it useful for communications and continuous weather monitoring.

These speed relationships apply specifically to circular orbits. Satellites can also follow elliptical paths, moving faster when closer to Earth and slower when farther away. Their orbital speeds and distances therefore change throughout each revolution.

Why Satellites Do Not Simply Drift Away

A satellite in orbit has forward motion, but Earth's gravity continually changes its direction. Without gravity, it would continue along a straight path at constant velocity. With gravity, its trajectory bends around the planet.

Changing that trajectory requires a change in velocity. When a spacecraft fires its engine in the direction of travel, it initially speeds up and gains orbital energy. If the burn occurs in a circular orbit, it generally raises the opposite side of the orbit, producing an elliptical path.

A second engine burn at the appropriate point can establish a new circular orbit at a higher altitude. Although the spacecraft gained speed during the first maneuver, its final speed in the higher circular orbit is lower than it was in the original orbit.

A burn opposite the direction of travel can lower the opposite side of the orbit. Carefully planned maneuvers can then place the spacecraft in a lower orbit, where the circular orbital speed would be greater.

Satellites in low Earth orbit face another challenge: atmospheric drag. Even hundreds of kilometers above the surface, traces of Earth's atmosphere create resistance that removes orbital energy.

As a satellite loses energy, its orbit gradually decays. Spacecraft such as the International Space Station require periodic reboost maneuvers to maintain their operating altitude. Without them, atmospheric drag would eventually lead to reentry.

The Balance Behind Orbital Motion

A simple experiment can help illustrate the forces involved. Imagine a small weight attached to a string and moving in a horizontal circle. The string pulls inward, continually changing the direction of the weight's motion. If the string breaks, the weight initially moves along a tangent to its circular path, rather than continuing around the circle.

Gravity plays the inward-force role for an orbiting satellite. Unlike a string, however, gravity acts over enormous distances and changes in strength as the spacecraft moves farther from Earth.

A circular orbit requires a particular speed at a given distance. But satellites do not immediately fall to Earth if they move slightly slower, nor do they automatically escape if they move slightly faster. Small changes generally produce elliptical orbits rather than immediate collisions or escapes.

To leave Earth on an unbound trajectory, a spacecraft needs sufficient orbital energy. The ideal escape speed at Earth's surface is approximately 11.2 kilometers per second, or 25,000 miles per hour, ignoring atmospheric effects and Earth's rotation.

That does not mean every spacecraft traveling beyond low Earth orbit must reach this speed. The Apollo missions, for example, used translunar injection burns to enter trajectories toward the Moon without needing to escape the Earth-Moon system entirely.

Orbital motion is therefore not a fragile balance between two opposing forces. It is the predictable result of gravity acting on an object already in motion, with its trajectory determined by its energy, direction of motion, and the surrounding gravitational field.

Satellites remain in orbit not because gravity stops pulling them downward, but because their forward motion allows them to keep falling around Earth. The speed required depends on the orbit, while changes in velocity can raise, lower, or reshape a spacecraft's path. Some satellites remain in stable orbits for very long periods, but those closer to Earth gradually lose altitude because of atmospheric drag. The next time you use satellite navigation or watch a spacecraft pass overhead, remember that it is not suspended motionless in space. It is moving rapidly along a path continuously shaped by Earth's gravity.