Skip to content

A whole mission

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

A question to start

A dose of about 1,000 mSv can cause radiation sickness. A whole Mars mission adds up to about that much. So would the crew come home sick?

The idea

Add the parts. Suppose a mission takes 180 days to get there, stays 500 days, and takes 180 days to come back:

Part of the mission Days Dose per day Dose
The two trips 360 1.8 mSv 648 mSv
On the surface 500 0.67 mSv 335 mSv

The total is \(648 + 335 = 983\), about 980 mSv over 860 days. NASA put it the same way: on the order of 1,000 mSv for a whole mission.

Mars missionat homethe two trips: 648 mSvon Mars: 335 mSvabout 4 mSv, too small to see

The bars are to scale.

That would not make the crew sick. Radiation sickness comes from a big dose all at once, in hours or days. Spread over more than two years, the same dose does not; the cells have time to repair. The worry is the long-term risk: a dose of 1,000 mSv is linked to about a 5% higher risk of dying of cancer later in life. In 2013, NASA's career limit for astronauts in Earth orbit was a 3% higher risk.

Dose over about 2.4 years Mars mission Earth (Canada)
Natural radiation about 980 mSv about 4 mSv

Worked examples

1. At home. 860 days at home in Canada: \(860 \div 365 \times 1.8 \approx 4.2\) mSv.

2. The risk. 5% for each 1,000 mSv, so 980 mSv gives about \(0.98 \times 5 \approx 4.9\%\) extra risk. That is above NASA's 2013 limit of 3%.

3. A shorter mission. Suppose 120 days each way and 30 days on the surface: \(240 \times 1.8 + 30 \times 0.67 = 432 + 20 = 452\) mSv.

Try it

  1. Why would a 1,000 mSv mission not cause radiation sickness?
  2. What is the main radiation risk of a Mars mission?
  3. Find the total dose for 200 days each way and 400 days on the surface.
  4. What extra cancer risk goes with a dose of 600 mSv?
Answers
  1. It is spread over more than two years, not given all at once.
  2. A higher risk of cancer later in life.
  3. \(400 \times 1.8 + 400 \times 0.67 = 720 + 268 = 988\) mSv.
  4. \(0.6 \times 5 = 3\%\).

Common mistake

Reading the risk as a certainty. A 5% higher risk does not mean 5% of the crew will surely get cancer from the trip. It is an average risk over many people, and a lifetime one.

Where it leads