What is a satellite?

NASA's fleet of satellites
Credit: NASA

A satellite is a moon, planet, or machine that orbits a planet or star. An orbit is when one object in the Universe (for example a planet or a star) goes round another one without touching it. Earth is a natural satellite of the Sun. The Moon is a natural satellite of the Earth.

How are satellites useful?

Can I see satellites from Earth?

On this page, when we use the word satellite, we mean artificial satellites. These are human-made machines.

Satellites all have two main parts:

  • An antenna that sends and receives information
  • A power source – this can be a solar panel (that collects energy from the Sun) or a battery

Satellites can have cameras and scientific instruments on them.

The smallest satellite is only 10 cm wide. The largest one is the size of a football pitch.

There are thousands of satellites orbiting the Earth today. Most are between 2,000 km and 36,000 km above the Earth's surface and travel at speeds of up to 8 km per second.

The Hubble Space Telescope
Credit: NASA

Some satellites are space telescopes. These telescopes can observe stars, galaxies, and other objects in space without needing to look through the Earth’s atmosphere. Humans have also put satellites into orbit around the Moon and the other planets in our Solar System. Space stations, like the International Space Station (ISS), are also satellites. 

Most of the satellites orbiting the Earth focus their attention on our planet. They are useful because they can monitor large areas at a time from high above Earth. This lets them collect more data, more quickly than ground-based instruments.

They orbit the Earth along two main paths:

  • Geostationary orbits
  • Polar orbits

 Have a go with our interactive demo to investigate how satellites stay in orbit around Earth. 

What is a geostationary orbit?

A satellite in a geostationary orbit travels from west to east over the Earth's equator. It orbits the Earth in the same direction and at the same speed that the Earth turns. This means that from Earth, it looks like it is staying still. It’s always above the same location. The satellite can keep a constant watch over the area of the Earth in its view.

What is a polar orbit?

A satellite in a polar orbit travels in a north-south direction from pole to pole. As the Earth spins underneath it, the satellites can scan the entire globe, one strip at a time.

Replica of the Sputnik 1 space probe
Credit: NASA

Can satellites crash?

Yes, it is possible. But collisions are rare because when a satellite is launched, it is placed into an orbit designed to avoid other satellites. Space agencies keep track of satellites because orbits can change over time. However, the chances of a crash increase as more and more satellites are launched into space.

In February 2009, two communications satellites - one American and one Russian - collided in space. This is believed to be the first time two satellites have collided accidentally.

What was the first satellite?

The first satellite was Sputnik. It was launched in 1957 by the Soviet Union. It orbited for three weeks before its batteries died.