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Radiation on the way to Mars

Radiation and the atom · level 5, lesson 6 of 6

A question to start

When the Curiosity rover flew to Mars, it carried a radiation detector switched on for the whole trip, to find out what a crew would meet. What did it find?

The idea

Between the planets there is no air and no magnetic field to help. A crew meets two kinds of radiation:

  • Solar particles. Outbursts on the Sun send out showers of particles, mostly protons. They come in bursts lasting hours or days. They are not too fast, so thick enough walls can block most of them.
  • Galactic cosmic rays. These arrive all the time from every direction, at nearly the speed of light. They include heavy nuclei, which do the most damage. They are so fast that walls barely slow them; worse, when they hit a wall they break up into showers of other particles, called secondary radiation.

Curiosity's detector found that over its 253-day cruise, solar particles gave only about 5% of the radiation. The rest came from galactic cosmic rays. The team said even an aluminium hull 30 cm thick would not change the dose much.

So mission plans include a small, well-shielded storm shelter to hide in during a solar outburst. The steady cosmic rays are harder: the main defences are a shorter trip and better shielding materials (level 7).

solar particles:stopped by wallscosmic rays:pass through

Worked examples

1. The speed of the cruise. Curiosity flew 560 million km in 253 days: \(560{,}000{,}000 \div 253 \approx 2.2\) million km a day.

2. The share. If solar particles gave 5% of the radiation, cosmic rays gave \(100 - 5 = 95\%\).

3. A thick hull. Aluminium has a density of 2.7 g/cm³. A wall 30 cm thick has \(2.7 \times 30 = 81\) g of metal over each square centimetre: 810 kg over each square metre of wall. That is a great deal to launch, for little gain.

Try it

  1. What are the two kinds of radiation a crew meets in space?
  2. Which kind can the walls of a spacecraft block?
  3. What is secondary radiation?
  4. Why does a mission plan a storm shelter, not thicker walls everywhere?
Answers
  1. Solar particles and galactic cosmic rays.
  2. Solar particles.
  3. Showers of particles made when cosmic rays hit atoms in a wall.
  4. A small shelter can be shielded heavily for solar outbursts; thicker walls everywhere would be very heavy and would barely slow cosmic rays.

Common mistake

"Thicker walls always mean less radiation." For cosmic rays, a thin metal wall can make the radiation inside barely smaller, or even add secondary particles. What the wall is made of matters, as level 7 shows.

Where it leads