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How far is Mars?

Space and time · level 2, lesson 5 of 6

A question to start

Some years Mars shines brightly in the night sky. Other years it is hard to find. Why?

The idea

The Earth and Mars both go round the Sun, on different paths and at different speeds. So the distance between them keeps changing.

  • When both are on the same side of the Sun, Mars is closest: as near as about 55 million km. Then it looks bright.
  • When they are on opposite sides, Mars is farthest: up to about 401 million km. Then it looks faint.

They come closest about every 26 months, a little over two years. That is when rockets set off for Mars.

Distance from the Sun Mars Earth
Average 228 million km 150 million km
closest: same sidefarthest: opposite sides

Worked examples

1. Closest and farthest. \(401 \div 55 \approx 7\). At its farthest, Mars is about seven times as far away as at its closest.

2. Driving there. At 100 km an hour, even at its closest, 55 million km would take \(550{,}000\) hours: about 63 years.

3. Next close approach. Mars was close in January 2025. About 26 months later brings it close again, in early 2027.

Try it

  1. Why does the distance to Mars keep changing?
  2. About how close can Mars come?
  3. About how far away can it be?
  4. Why do rockets to Mars set off about every two years?
Answers
  1. The Earth and Mars go round the Sun on different paths at different speeds.
  2. About 55 million km.
  3. About 401 million km.
  4. That is how often Earth and Mars come close, so the trip is shortest.

Common mistake

"Mars is always the same distance away." It can be seven times farther at some times than at others.

Where it leads