Why can't we use the same radiation shielding in Mars that we used when going to the moon?

Question

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?

Answer 1

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.

Answer 2

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.