What is orbit?

Diagram of the planets and other heavenly bodies.

Orbit is a word we hear quite often. Every time the Space Shuttle lifts off the launch pad, we hear it. Every time the Space Shuttle meets up with the International Space Station (ISS), we hear it. And every time a rocket launches a payload, we hear the word "orbit." It's a widely used term, but do you know what an orbit really is?

An orbit is a regular, repeating path that one object in space takes around another one. An object in an orbit is called a satellite. A satellite can be natural, like the Earth or the Moon. It can also be man-made, like the Space Shuttle or the ISS.

In our solar system, the Earth and the eight other planets orbit the Sun. Most of the objects orbiting the Sun move along or close to an imaginary flat surface. This imaginary surface is called the ecliptic plane. Many planets also have moons. These moons orbit around them.

Orbits are elliptical in shape, this means they are similar to an oval. For the planets, the orbits are almost round. The orbits of comets have a different shape. They are highly eccentric or "squashed." Satellites that orbit the Earth are not always the same distance from the Earth. Sometimes they are closer, and at other times they are farther away. The closest point a satellite comes to the Earth is called its perigee. The farthest point is the apogee. The time it takes a satellite to make one full orbit is called its period. The inclination is the angle the orbital plane makes when compared with the Earth's equator.

The space station Skylab orbiting the Earth.

An object in motion will stay in motion unless something pushes or pulls on it. This is Isaac Newton's First Law of Motion. Without gravity, an Earth-orbiting satellite would go off into space along a straight line. With gravity, it is pulled back toward the Earth. There is a constant tug-of-war between the satellites tendency to move in a straight line, or momentum, and the tug of gravity pulling it back.

An object's momentum and the force of gravity have to be balanced for an orbit to happen. If the forward momentum of one object is too great, it will speed past the other one and not enter into orbit. If momentum is too small, the object will be pulled into the other one and crash. When these forces are balanced, the object is always falling into the planet, but because it's moving sideways fast enough, it never hits the planet.

Escape velocity is the speed an object must go to break free from a planet's gravity and enter into orbit. Escape velocity depends on the mass of the planet. Each planet has a different escape velocity. The object's distance from the planet's center is also important. The escape velocity from the Earth is about 11.3 kilometers (7 miles) per second.

Orbital velocity is the speed needed to stay in orbit. At an altitude of 242 kilometers (150 miles), this is about 17,000 miles per hour. This is just a little less than full escape velocity.

The Space Shuttle in

Low-Earth Orbit.

Low-Earth Orbit (LEO) is restricted to the first 100 to 200 miles of space. LEO is the easiest orbit to get to and stay in. This is where the Shuttle and ISS conduct their operations. One complete orbit in LEO takes about 90 minutes.

Satellites that seem to be attached to some location on Earth are in Geosynchronous Earth Orbit (GEO). These satellites orbit about 23,000 miles above the equator and complete one revolution around the Earth precisely every 24 hours. Satellites headed for GEO first go to an elliptical orbit with an apogee about 23,000 miles. Firing the rocket engines at apogee then makes the orbit round. Geosynchronous orbits are also called geostationary.

Any satellite with an orbital path going over or near the poles maintains a polar orbit. Polar orbits are usually in low-Earth orbit. They remain in place while the Earth passes under. This means that eventually, the entire Earth's surface passes under a satellite in polar orbit.

When a meteorite enters our atmosphere and becomes a "shooting star," it is no longer in an orbit. Some space probes, like Voyager, have reached escape velocity and broken away from the pull of the Sun's gravity. These probes are leaving the solar system. They are not in orbit around a planet or the Sun.

Courtesy of NASA's Human Exploration and Development of Space Enterprise Published by NASAexplores: October 25, 2001