Part of: 
NASA's fleet of satellites
Credit: NASA

A satellite is an object which orbits another. An orbit is the curved path an object in space takes around another. Earth is a natural satellite of the Sun because it orbits the Sun. The Moon is a natural satellite of the Earth because it orbits the Earth. On this page, when we use the word satellite, we mean human-made machines

The first satellite was Sputnik, launched by Russia in 1957. It was in space for 3 weeks before its batteries died. Today, thousands of satellites orbit the Earth. Most are 2,000 km to 36,000 km above the Earth. They travel at speeds of up to 8 km per second. The size of a satellite depends on what it is used for. The smallest kind of satellite is only 10 cm wide. These cube sats send information or gather data. The largest satellite (the ISS) is the size of a football pitch. It is big enough for 6 or 7 crew to live and work inside. You sometimes see satellites in the sky at night.

Satellites contain two main parts. They have an antenna which sends and collects information from Earth. They also have a part that powers the equipment on board. This is either a solar panel (that collects energy from the Sun) or a battery. Many also contain cameras and science equipment. 

Replica of the Sputnik 1 space probe
Credit: NASA

Satellites are useful in lots of ways. They let us send information around the world and help us navigate. Most of the craft which orbit the Earth study and solve problems for our own planet. They send and collect information over large areas, which makes them very efficient. But space stations, like the ISS, and space telescopes are also satellites. Humans have also made satellites which orbit the Moon and other planets. They send data and images back to Earth using radio waves. So satellites let us study our Solar System and the Universe, as well as Earth.

Satellites orbit the Earth along two main paths. We call these paths geostationary orbits and polar orbits.

A satellite in a geostationary orbit looks like it stays in the same place above the Earth. This lets it keep a watch on the area of the planet under it. The craft does this by moving at the same rate and in the same direction as the Earth. It makes 1 orbit of the Earth every 23 hours, 56 minutes and 4 seconds and travels west to east. A satellite must be 35,786 km above the Earth's equator to do this.

The Hubble Space Telescope
Credit: NASA

A satellite in a polar orbit travels in a north-south direction. This means it travels over the North and South poles of the Earth. As the Earth spins, the craft scans the entire globe, 1 strip at a time. Some of them pass over the same part of Earth at the same time each day. They can take a photo of a location each time they pass. Scientists use the images to compare how a place changes over long amounts of time. 

Today, there are thousands of craft orbiting the Earth, but collisions are rare. This is because space agencies plan the orbits of their satellites to avoid other craft. But space agencies must keep track of their machines because orbits can change over time. In February 2009, two satellites - one American and one Russian - collided in space. This is thought to be the first time two craft collided in orbit.