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• Distances in space are really, really big. The Voyager 1 spacecraft is heading out of our Solar System at 62,000 km per hour, but even at that speed, it would take it 77,000 years to reach the nearest star. It would take over a billion years to cross the Milky Way galaxy. And to get to the next galaxy? We need a bigger calculator!

Rights: Public domain - U.S. Government

Space Shuttle and Earth

Two astronauts outside the Space Shuttle in orbit 200km above the Earth.

## Relating to what we know

Professor Denis Sullivan, an astrophysicist at Victoria University of Wellington, thinks it is important that we get a ‘feel’ for these big distances, even if we can’t visualise them. He finds that it helps to relate big measurements to smaller ones that we already know.

• You know what a kilometre is – you can walk that distance easily.
• You know what 100 km is – that’s how far a car could travel in an hour at the speed limit on a main road.
• Travel more than 100 km above the surface of the Earth, and you’re in space, at the edge of the atmosphere.
• When the Space Shuttle goes into space, it orbits about 700 km above the surface of the Earth – that’s less than the length of the South Island of New Zealand.
• The Moon is 400,000 km away. That’s the same distance as 10 times round the Earth.
Rights: National Aeronautics and Space Administration (NASA)

International Space Station over New Zealand

Astronauts Robert L Curbeam (left) and Christer Fuglesang working on the construction of the integrated truss structure on the International Space Station as it passes over Cook Strait, the divide between the North Island and South Island of New Zealand.

Numbers start getting really big when we look at distances from Earth to other planets in our Solar System. It’s 78 million kilometres (78,000,000 km) from Earth to Mars, and it’s 4350 million kilometres (4,350,000,000 km) to Neptune.

## Small-scale Solar System

To get an idea of distances inside our Solar System, it helps to reduce everything in size. The Sun is 1,391,980 km across – let’s shrink that to 1 km. We can now shrink the planets, and the distances between them, by the same amount.

The table and map show what we’d get if the Sun was a ball 1 kilometre across in the centre of Wellington.

Diameter of planet

Distance from Sun

In orbit over

Mercury 4 m

40 km

Kapiti and Cook Strait

Venus 9 m

80 km

Masterton and Marlborough Sounds

Earth 10 m

100 km

Nelson and Levin

Mars 5 m

160 km

Wanganui and Kaikoura

Jupiter 100 m

560 km

Great Barrier Island and Dunedin

Saturn 90 m

1030 km

Half way to Australia

Uranus 30 m

2070 km

Eastern edge of Australia

Neptune 30 m

3240 km

Rights: The University of Waikato Te Whare Wānanga o Waikato

Scaled-down Solar System

This map shows our Solar System if it was reduced in size so that the Sun was only 1km across in the centre of Wellington.

Now let’s head to the nearest star outside the Solar System, called Proxima Centauri. This star is 40 trillion kilometres away. Even if we shrank that by the same amount as we shrank our Solar System to fit across New Zealand and Australia, Proxima Centauri would still be over 30 million kilometres away!

## Light years

We need a different unit of measurement, because at these distances, kilometres are too small to be much use. One unit for measuring very big distances is the light year. This is the distance that light travels in a year, so 1 light year is about 9,500,000,000,000 km. Using this unit, we say that the star Proxima Centauri is about 4.2 light years away.

Even using this unit, numbers in space get pretty big.

Distance from Earth

Sirius, brightest star in sky

8.6 light years

Acrux, brightest star in Southern Cross

321 light years

Rigel, brightest star in Orion

777 light years

Centre of our Milky Way galaxy

27,700 light years

Andromeda, nearest large galaxy

2,900,000 light years

Perseus cluster of 500 galaxies

190,000,000 light years

Furthest galaxies seen in the universe

15,000,000,000 light years

If we were still using kilometres, the distance to the furthest galaxies seen would be 145,000,000,000,000,000,000,000 km.

## Activity idea

In the activity, Shrink the Solar System, students create a scale model of the Solar System to help them understand distances in space.