The Soviet Union launched the first artificial satellite, Sputnik 1, in 1957. Since then, the United States and about 40 other countries have developed, launched, and operated satellites. Today, it is estimated that about 3,000 useful satellites and 6,000 pieces of space junk are orbiting the Earth.

An artificial satellite is a manufactured object that continuously orbits Earth or some other body in space. Most artificial satellites orbit Earth. People use them to study the universe, help forecast the weather, transfer telephone calls over the oceans, assist in the navigation of ships and aircraft, monitor crops growth and other resources, and support military activities.

Artificial satellites also have orbited the moon, the sun, asteroids, and the planets Venus, Mars, Jupiter and others. Such satellites mainly gather information about the planets and bodies they orbit.

Piloted spacecraft in orbit, such as space capsules, space shuttle orbiters, and space stations, are also considered artifi orbiting pieces of "space junk,” such as burned-out rocket boosters and empty fuel tanks that have not fallen to Earth.

Artificial satellites differ from natural satellites, natural objects that orbit a planet. Earth’s moon is a natural satellite.

Satellite orbits have a variety of shapes. Some are circular, while others are highly elliptical (egg-shaped). Orbits also vary in altitude. Some circular orbits, for example, are just above the atmosphere at an altitude of about 250 kilometers, while others are more than 32,200 kilometers above Earth. The greater the altitude, the longer the orbital period (the time it takes a satellite to complete one circle around the Earth).

A satellite remains in orbit because of a balance between the satellite’s velocity (speed at which it would travel in a straight line) and the gravitational force between the satellite and Earth.

To help understand the balance between gravity and velocity, consider what happens when a small weight is attached to a string and swung in a circle. If the string were to break, the weight would fly off in a straight line.

However, the string acts like gravity, keeping the weight in its orbit. The weight and string can also show the relationship between a satellite’s altitude and its orbital period. A long string is like a high altitude. The weight takes a relatively long time to complete one circle. A short string is like a low altitude. The weight has a relatively short orbital period.

A high altitude, (geosynchronous orbit) lies above the equator at an altitude of about 35,900 kilometers. A satellite in this orbit travels around Earth’s axis in exactly the same time, and in the same direction, as Earth rotates about its axis. Thus, as seen from Earth, the satellite always appears at the same place in the sky overhead. To boost a satellite into this orbit requires a large, powerful launch vehicle.

A medium altitude orbit has an altitude of about 20,000 kilometers and an orbital period of 12 hours. The orbit is outside Earth’s atmosphere and is thus very stable. Radio signals sent from a satellite at medium altitude can be received over a large area of Earth’s surface. The stability and wide coverage of the orbit make it ideal for navigation satellites.

Because the satellite flies over all latitudes, its instruments can gather information on almost the entire surface of Earth. An example in this type of orbit is that of the TERRA Earth Observing System’s NOAA-H satellite. This satellite, studies how natural cycles and human activities affect Earth’s climate. The altitude of its orbit is 705 kilometers, and the orbital period is 99 minutes.

A low altitude orbit is just above Earth’s atmosphere, where there is almost no air to cause drag on the spacecraft and reduce its speed. Less energy is required to launch a satellite into this type of orbit than into any other orbit.

Satellites that point toward deep space and provide scientific information generally operate in this type of orbit. The Hubble Space Telescope, for example, operates at an altitude of about 610 kilometers, with an orbital period of 97 minutes.

Ends