Orbital velocity is the speed required to achieve orbit around a celestial body, such as a planet or a star. This requires traveling at a sustained speed that:

• Aligns with the celestial body’s rotational velocity
• Is fast enough to counteract the force of gravity pulling the orbiting object toward the body’s surface

An airplane can travel in the sky but it does not travel at a velocity fast enough to sustain orbit around the earth. This means that once the airplane’s engines are turned off, the plane will slow down and be pulled back down to earth, via the force of gravity. By contrast, a satellite (such as the one that powers your phone’s GPS or the one that transmits a DirecTV signal) does not need to expend fuel to maintain its orbit around the earth. This is because such satellites travel at a velocity that overrides the force of gravity.

Take note, however, that orbital velocities vary depending on the rotating object’s distance from the celestial body that it orbits. As a general rule, objects can enter orbit at lower velocities when they are farther away from the surface of a planet or star. When they are closer to the surface, it takes greater velocity to counteract the force of gravity. As such, another reason that an airplane does not achieve orbit is that it flies much closer to the earth’s surface than a communications satellite does.

Orbital velocity is made possible by the curved surface of a planet, star, or other celestial body. An orbiting object tends to move in a straight line, whereas the body it is orbiting curves. As such the constant curvature of the orbited body prevents the orbiting object from falling all the way to the surface, provided that the orbiting object maintains the proper speed.

In space, it is easier to maintain a constant speed than it is on Earth, due to the principle of inertia. One of Sir Isaac Newton’s laws of inertia states that an object in motion tends to stay in motion unless acted on by an outside force. Within the earth’s atmosphere, a flying object encounters many air molecules, which cumulatively slow the speed of that object as it flies through the sky. As you journey beyond the earth’s atmosphere into higher altitudes, the air becomes more vacuous, with fewer molecules to counteract the forward velocity of an orbiting satellite.

Rocket scientists use the principle of orbital velocity to chart the course of space flight. This involves both getting a rocket into the sky, establishing an orbit, changing said orbit, or even breaking free of the orbit to either return to Earth or to chart a new course into space.

• Where a rocket is launched from comes down to physics. As a general rule, rockets launch from as near as possible to the equator, in order to take advantage of the velocity of Earth’s rotation, which is highest at the equator—about 1,000 miles an hour. The more orbital velocity a rocket gets from Earth, the less fuel it requires to reach orbital speed, which increases its efficiency. Not all rockets can take advantage of Earth’s spin—some are designed to send payloads such as satellites into north-to-south orbit, around the poles.
• Orbital mechanics is a term for the mathematics by which a spaceship changes orbit. For objects that are in orbit, the closer they are to the object they are orbiting, the faster they will travel around it. This applies to any object orbiting another—Earth orbiting the sun, the moon orbiting Earth, or a spaceship orbiting a planet. In orbital mechanics, the concept of speeding up and slowing down are complex and counterintuitive. In orbit, firing your engines frontwards moves you forward into a higher orbit, which actually means you slow down, because objects in a higher orbit move more slowly. In order to go faster you need to decelerate and fall into a lower orbit.
• The farther away you are from Earth, the less magnified this effect is. When you get far enough away from Earth, the relative effects of orbital mechanics are so low that you can navigate as if you are operating your spaceship in deep space.

Such orbital considerations affect the management of the International Space Station (ISS).

• Because of small bits of air around the ISS, the station gets pulled back toward Earth ever so slightly as it orbits. In order to avoid a continued spiral inward to Earth, the crew on board the ISS or Mission Control has to fire its engines every so often to move it into a higher orbit.
• To move their spaceship from a lower orbit to a higher one, the crew normally uses the classic orbital change: the Hohmann transfer. In the 1920s, German engineer Walter Hohmann, inspired by science fiction, calculated the most efficient way to move to a higher orbit. The Hohmann transfer works by firing the rocket engines once at a certain point in the lower orbit. This firing adds energy to the orbit and propels the spaceship farther from Earth, changing its orbit from a circular orbit to an oval-shaped orbit. Learn more about the Hohmann transfer here.

The Earth is in constant rotation on an axis. Such an axis runs through the planet’s North Pole and South Pole. The portion of the Earth that is farthest from this pole-centered axis is the Equator. In accordance with the principles of physics, the earth’s rotational velocity will be highest at the point furthest from the axis. For this reason, space agencies tend to launch their rockets from points that are relatively close to the equator. This allows the rockets to co-opt the Earth’s rotational velocity in their quest to achieve orbital velocity.

It takes a certain level of velocity for an object to achieve orbit around a celestial body such as Earth. It takes even greater velocity to break free of such an orbit. When astrophysicists design rockets to travel to other planets—or out of the solar system entirely—they use the rotational velocity of the Earth to speed up the rockets and launch them out and beyond orbit. The speed required to break free of an orbit is known as escape velocity.

If a spaceship in orbit fires its engine long enough, it will eventually go fast enough to fly away into deep space, escaping the planet’s gravity. That escape velocity is simply the square root of 2, or 41% faster than orbital speed.