How big is the universe?

We often get asked about the nature of the cosmos by schoolchildren. We like to start simple, with something they are familiar with and then work from there.

When discussing how big the universe is, we talk about the following:

• Beyond our town/city there are other towns/cities.
• Beyond our country there are other countries.
• We all live on planet Earth, which is a huge ball of rock.
• If you started walking in a straight line, and kept walking, never stopping, you would eventually cover a distance of 40,000km and you would arrive back at the same spot! Our Earth is a sphere, as are all the other planets in our solar system.
• If you look up, you see the sky. Our atmosphere extends upwards to a height of around 100 km, after which you are pretty much in space.
• What if you were to take a journey to the moon? Well the distance to the moon is almost 10 times the distance you would cover walking around the Earth. In a space rocket it would take around three days to get there safely (including going into orbit).
• If we kept going, then we might reach other planets. In the future people will go to Mars — it will take them about eight months to get there, covering a distance of more than 60 million km. One of the astronauts might be you!
• Eventually we might leave the solar system, a whopping big distance of 8 trillion km (8 million million km). Travelling at the launch speed of a rocket, it would take us around 33,000 years to leave the solar system entirely. That means going way past the final planet, Neptune, and outside the big bubble of icy comets surrounding us.
• The next star, Proxima Centauri, is over 4 light-years away. The light from this star takes 4 years to reach us. We can’t travel at the speed of light, so it would take us at least 70,000 years to get there in our space capsule.
• To reach the edge of the Milky Way galaxy would take 1.3 billion years — wow!
• To reach the nearest big galaxy to us, Andromeda, it would take — wait for it — around 45 billion years. That’s 10 times the age of the Earth. A text message would take 2.5 million years to reach Andromeda.
• Finally, to reach the current ‘edge’ of the universe (although there is no real edge), it would take us 800 trillion years. That’s an 8 with 14 zeros after it…

Who would like to attempt that journey?