An orbit is a regular, repeating path that one object takes around another object or center of gravity. Orbiting objects, which are called satellites, include planets, moons, asteroids, and artificial devices.

Objects orbit each other because of gravity. Gravity is the force that exists between any two objects with mass. Every object, from the smallest subatomic particle to the largest star, has mass. The more massive the object, the larger its gravitational pull. Gravitational pull is the amount of force one object exerts on another object.

The sun is the most massive object in our solar system. All other objects in the solar system are subject to the gravitational pull of the sun.

Many satellites orbit on orbital planes. An orbital plane is a flat, disk-shaped space that connects the center of the object being orbited with the center of orbiting objects. Because all planets in our solar system share a similar orbital plane, planets don't collide.

All the planets in our solar system line up with each other on the same general orbital plane. However, sometimes orbital paths of other objects in the solar system intersect, and the objects can collide. Comet Tempel-Tuttle, for instance, passes through Earth's orbit. The debris from the tail of this comet passes through Earth's atmosphere as meteors, or falling stars, at a specific time every year. The debris from the comet's orbit is called the Leonid meteor shower.

The time it takes for an object to orbit around another object is called its orbital period. Earth's orbital period around the sun is complete in slighly over 365 days. The farther away a planet is from the sun, the longer its orbital period. The planet Neptune, for example, takes almost 165 years to orbit the sun.

Each orbit has its own eccentricity. Eccentricity is the amount an orbit's path differs from a perfect circle. A perfect circle has an eccentricity of zero. Earth's eccentricity is 0.017. Mercury has the largest eccentricity of all the planets in the solar system, at 0.206.

Types of Orbits
Moons orbit planets, while planets orbit the sun. Our entire solar system orbits the black hole at the center of our galaxy, the Milky Way. There are three major types of orbits: galactocentric orbits, heliocentric orbits, and geocentric orbits.

Galactocentric orbits circle the center of a galaxy. Our solar system orbits the Milky Way.

Heliocentric orbits go around stars. All the planets in our solar system, along with all the asteroids in the Asteroid Belt and all comets, follow this kind of orbit. Each planet's orbit is regular: They follow certain paths and take a certain amount of time to make one complete orbit. The planet Mercury completes its short heliocentric orbit every 88 days. Comet Kohoutek may take 100,000 years to complete its heliocentric orbit.

A geocentric orbit is one that goes around Earth. Our moon follows a geocentric orbit, and so do most artificial satellites. The moon is Earth's only natural satellite. It takes about 27 days for the moon to complete its orbital period around Earth. There are three major types of geocentric orbits: low-Earth orbit (LEO), medium-Earth orbit (MEO), and geostationary orbit.

Low-Earth orbit exists between 160 kilometers (100 miles) and 2,000 kilometers (1,240 miles) above Earth's surface. Most artificial satellites with human crews are in low-Earth orbit. The orbital period for objects in LEO is about 90 minutes.

Medium-Earth orbit exists between 2,000 kilometers (1,243 miles) and 36,000 kilometers (23,000 miles) above the Earth's surface. Satellites in MEO are at greater risk for damage, because they are exposed to powerful radiation from the sun. Satellites in MEO include global positioning system (GPS) and communication satellites. MEO satellites can orbit Earth in about two hours.

Satellites in geostationary orbit circle Earth directly above the Equator. These satellites have geosynchronous orbits, or move at the same rotation of Earth. Therefore, the orbital period of geosynchronous satellites is 24 hours.

Geostationary satellites are useful because they appear as a fixed point in the sky. Antennae pointed toward the geostationary satellite will have a clear signal unless objects in the atmosphere (such as storm clouds) between Earth and the satellite interfere. Most weather satellites are geostationary and provide images of Earth's atmosphere.

Satellite Orbits
Artificial satellites are sent to orbit Earth to collect information we can only assemble from above the atmosphere. The first artificial satellite, Sputnik, was launched by the Soviet Union in 1957. Today, thousands of satellites orbit Earth. Weather satellites provide images of weather patterns for meteorologists to study. Communication satellites connect cellphone users and GPS receivers. Military satellites track movement of weapons and troops from different countries.

Sometimes, artificial satellites have people on them. The most famous artificial satellite is the International Space Station (ISS). Astronauts from all over the world stay on the ISS for months at a time as it orbits Earth. Astronomers and stargazers can see the ISS and other satellites as they orbit through telescopes and even powerful binoculars.

Not all artificial satellites orbit Earth. Some orbit other planets. The Cassini-Huygens mission, for instance, is studying the planet Saturn. The project has a spacecraft, Cassini, which orbited Saturn.

Putting satellites into orbit is complex and costly. Few governments can afford large space programs. Artificial satellites from the United States are sent into orbit by the National Aeronautics and Space Administration, or NASA. The European Space Agency (ESA) launches satellites from countries in the European Union. The Russian Federal Space Agency (Roscosmos), the Japanese Space Agency (JSA), and the Iranian Space Agency (ISA) are some of the governments that have successfully put satellites into orbit.

Satellites are put into orbit from spaceports, which are carefully constructed for that purpose. The Baikonur Cosmodrome in Kazakhstan and the Kennedy Space Center in the U.S. state of Florida are both well-known spaceports.