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Is it a bird? Is it a plane? Or is it one of humankind's biggest scientific achievements?

Yes, you guessed it! It's the International Space Station (ISS) - a man-made satellite that orbits the Earth.

The 20 November 2018 marks a very special date in its calendar, as it has now officially been orbiting the Earth for 20 years.

What is the International Space Station?

The ISS is a satellite that flies around the world every 90 minutes.

That means it flies at 5 miles per second.

It's not just up there to watch the Earth turn. The astronauts on board conduct lots of scientific experiments to find out more about life in space.

On 20 November 1998, a module called Zarya was launched from a Russian base in the country of Kazakhstan, which formed the first part of the station.

After Zarya launched, it was joined by an American module called Unity.

Zarya, Unity and many more modules now orbit the Earth as part of the station.

It often has new parts added to it, constructed by a number of countries from around the world, including Russia, the US and Italy.

All the parts are attached as the satellite orbits the Earth.

What is it like on the ISS?

Many famous astronauts from lots of different countries have lived on board this massive vessel, including Chris Hadfield, Peggy Whitson and British astronaut Tim Peake.

Life can be hard for the astronauts when they're living in zero gravity, but they have found ways to adapt to living in space for months or sometimes even years at a time.

What is the future of the International Space Station?

It's hard to say exactly what the future of the ISS is, but Nasa scientists believe that it will be time for the ship to return to Earth in the year 2028.

In the meantime, the ISS still has lots yet to discover and will continue orbiting the Earth until its time to come home.