Monday, August 09, 2010

Deep Fried Astronauts

Back in 1967 the Bellcomm put together a study for the then Manned now Marshall Spaceflight Center. The mission was a one year human flyby of Venus. The study included some innovative stuff, like using the Earth departure stage tanks as a living module after venting any remaining fuel into space, but it also contains a fair bit of misinformation about radiation exposure, advocating that no attempt be made to shield against galactic cosmic radiation. This is to be expected.

What isn't expected is that this is still the general consensus today, even though a more recent computational study has provided some interesting numbers for various shielding materials.

Shield Material (5g/cm^2)Annual radiation dose (mSv*)
Aluminum542
Polyethylene450
Iron581
* Quality factor recommended in ICRP-60 is assumed.

This looks pretty good when the astronaut lifetime radiation limits are considered.

Astronaut ageCareer effective dose limits
(mSv, average life loss)
MalesFemales
25520, 15.7370, 15.9
30620, 15.4470, 15.7
35720, 15.0550, 15.3
40800, 14.2620, 14.7

Polyethylene shielding works better than aluminum which works better than iron because it has more carbon atoms. Adding shielding to the spacecraft and only sending crew of the appropriate age can drastically reduce the amount of life you take from astronauts on one-year long missions. Of course, most Mars missions designs are much longer duration than this and so more heavy radiation mitigation is needed.

Before discussing those options, let's think about how heavy this "light shielding" actually is. A Saturn S-IVB provides a heck of a lot of space compared to an ISS module, and has proven sufficient for a one year journey by SkyLab. It was 17.8m long by 6.6m diameter. With flat ends, this is a surface area of 4,374,982 cm^2, and at 5g/cm^2 the polyethylene shielding would weigh 21,874 kg.

On a trip to Mars one does not only have to consider the fuel required to burn in LEO to get to escape velocity, one must also consider the fuel required to do course corrections to get to and maintain Mars transit and perform Mars orbit injection. One way to reduce this fuel is to carry a large heat shield and do aerobraking. All this fuel is mass, typically with a lot of carbon in it, and so can also be used to shield the crew.

It seems reasonable at this point to wonder exactly how much radiation protection you and I get here on the surface of the Earth. The answer "enough" is sufficient so perhaps a better question is, how? Go outside and look up, what do you see? Air. How much? The answer is 1,030g/cm^2. As such, the 22t of shielding we added is providing just 0.49% as much protection.

If we want to provide sufficient radiation protection for long duration spaceflight, it seems obvious that we need to make the spacecraft smaller. Shielding just a smaller part of the spacecraft would be pointless as the crew is required to stay in there for the majority of the trip anyway.

Let's consider a 8m long cylinder of 4m diameter. Internally, it would be about 80 m^3 of pressurized volume. The surface area is 1,256,637.06 cm^2. Here's the masses required for various levels of radiation protection.

Radiation protection (vs sea level)Mass (kg)
1%12,943
5%64,717
25%323,584
50%647,168
100%1,294,336

As you can see, the life expediency of the crew can be improved by almost 3.5 times by reducing the surface area of the living volume by just under half. An interesting rule of thumb: 7% of Earth normal radiation shielding takes one year of lifespan off men for every year spent on the mission.

Making the spacecraft even more cramped and arranging the storable propellant and supplies as additional shielding would permit the creation of 5% of Earth normal radiation shielding, meaning the crew could go on the long multiyear excursions required to explore Mars, but no conceivable technology today can practically provide passive shielding for 100% radiation protection.

7 comments:

  1. Victor Moraes7:30 PM

    Friend Trent.

    I have an intuition with calcium. OpenNasa.com already commented on and gave an explanation. You have a table that identifies the level of protection of calcium? The eggs mature alligators sun scorching. The astronaustas suffer great decalcification. This softening of the bones of astronaustas can be caused because the calcium "opposition force" against radiation? Or is it purely a softener by reason of lack of gravity? I do believe that the solid calcium in plaque, may be used for "filtering" or "shield".

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  2. Bone loss is caused by long term exposure to zero-g, the other major problem faced by astronauts.

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  3. Victor Moraes8:35 PM

    I agree. But see how calcium appears in X-ray pictures Note that the Falat of garvidade yes, causes a softening, but notice how it is accelerated by a factor "in the picture X-ray." When we are on earth, perhaps there is a softening by calcium be structured without reforms (such as whether to build slabs of pressed calcium). On land not to make a perfect simulation of the conjunction of factors of weightlessness and radiation. Only in space. And we must analyze without dogmas, or short answers to unresolved problems. Indeed, if the problem is not resolved, it is because there is still no response. Dear friend Trent. Maybe an alloy using calcium ...

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  4. Victor Moraes8:38 PM

    Correcting the translation .. forgive me .. "I agree But see how calcium appears in X-ray pictures Note that the lack of gravity but causes a softening, but notice how it is accelerated by a factor" in the picture X-ray "...

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  5. Victor, yes indeed, I believe it was at first believed to be the cause but they've done ground-based experiments to rule out that conflation. There's actually a lot understood about calcium degradation now, which has led to a number of people who have called for genetic manipulation to mitigate it. Of course, genetic engineering on astronauts isn't exactly medically acceptable just yet.

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  6. Victor Moraes9:07 PM

    For the lack of seriousness we will change the style of space travel, from a trip to cast a journey accelerated / decelerated. Calm down friend, still podermoa take a coke on Mars. Let us join forces and wills that Supari barriers. I trust in your ability and especially in its influence, understanding something, forward who is interested, and can. We are first and foremost enthusiasts of evolution, and that's good. Remember, acceleration, deceleration. We'll have 1G.

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  7. Victor Moraes9:17 PM

    Yes indeed. I will try, perhaps, find something to talk about the calcium level of protection against radiation, and quality, a kind of radiation "blocked" by calcium. I believe that at least protect the waves of X-ray Now, other types of cosmic rays, might protect them if the elements sensitive to them. Preferably keep our nature and the world around us is that they adapt. Good night! ;)

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