Room 17 of Paraparaumu Beach School asks :-

How far is it to the planets?

Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responded.

It depends on the planet and where it is in its orbit.

Of all the planets Venus comes closest to us at 40 million km. Mars, at closest, is 56 million km away. When they are on the far side of the sun from us then Venus is around 260 million km away, Mars 380 million km. We are 150 million km from the sun.

Spacecraft travelling to the planets can't take the short, direct line when the planet is closest as this would require too much rocket fuel. They have to follow much longer tracks taking many months.

The gas-giant planets are at vastly greater distances. Jupiter is fives times our distance from the sun; Saturn 10 times, Uranus 20 times and Neptune 30 times. It took a spacecraft 12 years to reach Neptune, even using the `sling- shot' effect of the other big planets.

Another way to get an idea of these distances is to consider how long light takes to travel there. Light from the moon gets here in one second. (It took the Apollo spacecraft four days to get there.) Light from the sun reaches us in eight minutes. Sunlight reaches Jupiter in 40 minutes and Neptune in 4 hours. Pluto and the other icy asteroids are further away again. Some comets travel out to a light-year from us, sunlight takes one year to reach them.

Light takes four years to reach Alpha Centauri, the nearest star. Using current technology, a spacecraft would take around 100 000 years to cover the same distance. And the stars known to have planets are even further way.