Note that this is somewhat related to this question but here I am specifically asking about the suitability of using existing shielding for Mars travel.

This is about this article and video titled "Scientists overlooked a major problem with going to Mars — and they fear it could be a suicide mission" here at Business Insider.

The article says that:

Going to Mars may be more dangerous than we thought. The major problem is high-energy space radiation. Scientists know that cosmic rays can damage DNA. They had just overlooked how bad it could get.

A team re-examined how damaged DNA can cause cancer. They then estimated levels of radiation exposure in space and on Mars. Their results are devastating.

The risk of cancer on Mars is twice as high as previously thought.

It comes down to how damaged DNA spreads throughout the body. A detailed study in mice reveals a sinister side to radiation. Damaged DNA doesn't just keep to itself.

It sends signals to nearby healthy cells, which triggers the healthy cells to mutate, which could cause more cancer.

The protection provided by Earth's magnetic field, referred to in the article, is through the Van Allen belts (held in place by Earth's magnetic field) that trap cosmic rays and high energy particles from the Sun.

Considering that the Moon and Mars are both completely outside of the Van Allen belts (as opposed to ISS which is below the belts), we must have had some (relatively lightweight) radiation shielding when traveling to the moon.

Thus I was thinking that this shielding could for example be sent to Mars in advance (e.g. on some kind of supply ship manned by robots).

Admittedly this would require that the astronauts on Mars would always have to stay inside structures built with such shielding (except for relatively brief moments as was done during the moon landing). This does not sound very unusual to me however.

So the question I would like to ask:

Why is radiation a problem when traveling to or living on Mars when we can use the same shielding as has been used before? I.e. what makes the existing shield technologies unsuitable?

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    $\begingroup$ One major difference is that a trip to the Moon took some days. A trip to Mars would take months. $\endgroup$
    – chirlu
    Jun 21 '17 at 17:22
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    $\begingroup$ The radiation shielding we used to go to the Moon was basically "watch the Sun and hope we can spot a solar flare in time to cancel the launch". $\endgroup$
    – Mark
    Jun 21 '17 at 22:35
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    $\begingroup$ Another difference is we just didn't care as much back then. $\endgroup$
    – imallett
    Jun 23 '17 at 0:34

Radiation exposure is a cumulative risk. The more radiation you receive, the more likely you are to develop cancers.

The Apollo missions took no more than two weeks to complete; the astronauts flying those missions accepted that dose of radiation with the health risks that come with it.

A manned Mars mission will take, at minimum, months of travel. For the most fuel-efficient mission plans, the total time including the stay on Mars is about 32 months. So we're considering about 50-100 times the amount of radiation exposure.

Moreover, a solar flare occurring during the trip could be immediately debilitating or lethal to the crew. Flares of that kind are infrequent, so the risk was accepted for Apollo, but again, with the longer travel window of a Mars mission, the chances of encountering such a flare are much higher.

  • $\begingroup$ universetoday.com/107093/… gives some details. tl;dr: Full trip gives 1 Sv or 5% chance of fatal cancer while NASA guidelines is to keep the risk below 3%. $\endgroup$
    – JollyJoker
    Jun 22 '17 at 9:26
  • $\begingroup$ The combined detriment from stochastic effects (excess cancer and heritable effects) is indeed at around 5 % per Sv. Actually, the current value for cancer that we use in radiation protection is 4.1 %/Sv for adults [ICRP 2007]. The previously used value was 4.8 % [ICRP 1990]. $\endgroup$
    – user10840
    Jun 24 '17 at 11:07
  • $\begingroup$ @Loong The effect per Sv is only observed for exposure over a very small time. $\endgroup$
    – curiousguy
    Jun 15 '18 at 16:27

In addition to what Russell Borogove says about cumulative risk you're operating under a false assumption--that there was shielding on the Apollo capsules.

Not only did the Apollo capsules not have shielding but shielding was considered undesirable. There are two main radiation threats in space: cosmic rays and solar flares.

Their "defense" against solar flares was to launch when they weren't expected. Had a flare nailed an Apollo capsule we would have lost the crew. The boosters simply weren't powerful enough to lift the necessary mass.

Cosmic rays are at a much lower dose but they are very, very high energy particles and shielding against them is difficult. Doing a poor job of it is actually worse than not doing anything. The thing is they are coming in hot enough they go right through you, most of the energy remains in the particle. Put an inadequate shield in the way and that one particle knocks others loose, which knock others loose, you get a whole shower of much lower energy particles and your body actually does absorb the energy that gets through the shield.

Shielding against them is far harder than shielding against solar flares. Against solar flares we can at least arrange things so all the extra mass they are carrying (food, fuel etc.) is between them and the sun. A shield that can stop cosmic rays, though, is simply impractical to lift on any chemical rocket, period.

  • $\begingroup$ You were possibly thinking about Bremsstrahlung? In that case, I think your description of a "shower of particles" is wrong. $\endgroup$
    – DevSolar
    Jun 22 '17 at 13:35
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    $\begingroup$ @DevSolar For high-energy particles entering dense matter, Bremsstrahlung is joined by pair production, and both together create particle showers. I think that is what Loren was thinking of. A known problem in radiation shielding (thin shielding might be worse than no shielding at all). $\endgroup$
    – Dubu
    Jun 22 '17 at 14:59
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    $\begingroup$ No, he means not bremsstrahlung nor pair production, both being processes where matter is needed, but not influenced by the particles. High energy cosmic rays are mainly protons, which smash nuclei of any matter into pieces. The energy / momentum is shared with that pieces, resulting in a shower of particles with still lots of energy. $\endgroup$
    – sweber
    Jun 22 '17 at 22:10
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    $\begingroup$ @Pavel: The Wiki article on the Van Allen Belt makes it sound like the magnetic field does the actual job. The particles just end up captured in the belt (I guess they are not numerous enough to block much by themselves, otherwise we would have trouble seeing through them). $\endgroup$
    – Michael
    Jun 23 '17 at 7:06
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    $\begingroup$ @Pavel I'd rather put it this way: the Van Allen Belts are a side-effect of the particle-trapping properties of a magnetic field. $\endgroup$ Jun 23 '17 at 14:21

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