Satellites can appear to float effortlessly above Earth, with nothing holding them up and no engines constantly firing to keep them in place. Yet they are not floating beyond the reach of gravity.

In fact, satellites are continuously falling toward Earth. What keeps them in orbit is their sideways speed, which allows them to orbit the planet rather than crash into it. This combination of gravity and motion is the secret behind every satellite orbit.

Falling Is the Whole Trick

The International Space Station orbits roughly 250 miles (400 kilometers) above Earth, traveling at approximately 17,500 miles per hour (28,000 kilometers per hour). At that speed, it completes a journey around the planet in about 90 minutes.

Gravity constantly pulls the station toward Earth, but its sideways motion carries it forward as the planet's surface curves away beneath it. Instead of falling straight down, the station follows a curved path around Earth.

Isaac Newton illustrated this idea with a famous thought experiment involving the use of artillery on a high mountain. If a cannonball could be launched fast enough, and air resistance were ignored, it would keep falling around Earth rather than striking the ground.

Satellites follow the same basic principle, although real orbits are affected by additional forces.

Gravity Does Not Switch Off in Space

Astronauts aboard the International Space Station appear weightless, but gravity is still acting on them. At the station's altitude, Earth's gravitational pull is about 90 percent as strong as it is at the surface.

The astronauts float because they, the station, and everything inside it are falling together. This creates a condition called microgravity, where objects experience very little apparent weight.

A similar sensation can occur briefly during free fall, such as on certain amusement park rides. The difference is that the station has enough sideways speed to keep orbiting Earth rather than reaching the ground.

Orbital Speed Depends on Altitude

A satellite's required orbital speed depends on its distance from Earth and the shape of its orbit. Satellites in low Earth orbit generally travel much faster than those farther away.

For example, the International Space Station moves at roughly 17,500 miles per hour, while satellites in much higher circular orbits travel more slowly.

If a spacecraft changes speed, its orbit changes too. Slowing down does not necessarily make it fall directly to Earth; it may instead enter an elliptical orbit with a lower point closer to the planet.

Sputnik 1, the first artificial satellite, was launched in 1957 and remained in orbit for about three months before atmospheric drag caused it to reenter the atmosphere.

Spacecraft heading toward other planets follow different trajectories. They may escape Earth's gravitational pull, but the required speed depends on where and how the spacecraft begins its journey.

Altitude Changes the Time Around Earth

The higher a satellite travels in a circular orbit, the slower its orbital speed and the longer it takes to complete one revolution.

The International Space Station circles Earth approximately every 90 minutes. By comparison, a satellite in geostationary orbit takes about 23 hours and 56 minutes to complete one orbit, matching Earth's rotation relative to the stars.

Geostationary satellites orbit roughly 22,236 miles (35,786 kilometers) above the equator. Because they move in the same direction as Earth's rotation and remain above the same point on the equator, they appear stationary to observers on the ground.

This makes them especially useful for communications and weather monitoring, since ground antennas can remain pointed toward the same location in the sky.

Atmospheric Drag Slowly Changes Orbits

Space is not completely empty, even hundreds of kilometers above Earth. Traces of the upper atmosphere create drag on satellites traveling through low Earth orbit.

Over time, this resistance reduces the satellite's orbital energy and can cause its altitude to decrease. The effect depends on the satellite's height, shape, and mass, as well as changing atmospheric conditions.

The International Space Station periodically receives boosts to maintain its orbit. Without them, atmospheric drag would eventually bring it back toward Earth.

Many satellites also use propulsion to adjust their orbits or avoid collisions. Others are designed to reenter the atmosphere after their missions end, helping reduce the amount of long-lasting space debris.

Not Every Orbit Is Circular

Although circular orbits are common, satellites can also follow elliptical paths. In an elliptical orbit, a satellite moves fastest when it is closest to Earth and slowest when it is farthest away.

Highly elliptical orbits can be useful for missions that need a spacecraft to spend more time above particular regions. Some communications satellites use these paths to provide coverage at high northern latitudes, where geostationary satellites appear low in the sky.

Gravity and sideways motion work together throughout the orbit, but the satellite's speed and distance from Earth change continuously.

Real orbits can also shift because of atmospheric drag, the gravitational influence of other bodies, and other disturbances. Satellites may need occasional adjustments to maintain their intended paths.

Satellites remain in orbit not because gravity disappears, but because they move sideways fast enough to keep falling around Earth. Their speed, altitude, and orbital shape determine how they travel, while atmospheric drag and other forces can gradually change their paths. Some circle the planet every 90 minutes, while others appear almost motionless high above the equator.