
Mars trip to use astronaut poo as radiation shield - theoutlander
http://www.newscientist.com/article/dn23230-mars-trip-to-use-astronaut-poo-as-radiation-shield.html
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lifeisstillgood
I suppose it is a silly question but how long can water be exposed to
radiation before no longer being drinkable?

(that is the shielding _before_ it is turned into the other kind of shielding)

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InclinedPlane
Indefinitely. The kind of radiation that causes breeding of hazardous isotopes
in local materials is neutron radiation, which is comparatively rare in
interplanetary space because free neutrons have a half-life of only 15
minutes. The biggest source of danger in space is high energy proton
radiation. Neutrons can, and are, produced in secondary reactions from
particle radiation but the flux isn't a sufficient concern to worry about
"activation" of materials like water.

Exposure to ionizing radiation can cause formation of free radicals as well as
peroxides which could be a concern but the particle fluxes are too low to be a
serious concern. At the levels of radiation where it would be a concern the
crew would have been exposed to lethal levels of radiation many times over,
even with meters worth of water shielding.

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quaunaut
That headline title.

But then again, why is this needed? Won't they be getting plenty of radiation
soon as they're outside the o-zone layer? Does our normal method of radiation
shielding decay over time, or something?

(Total layman here, curious)

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danielweber
There are two kinds of radiation in space: cosmic rays, and solar flares.

The water and food, and "the stuff water and food turns into over the course
of a trip" as the joke goes, are very useful as a "storm shelter" against a
solar flare. If one comes up and the crew is not shielded like this against
the solar flare, they will die within a day or two.

Cosmic rays are very hard to effectively shield against. They don't stop for
much, including people, but there are so many of them that you are bound to
take a dose. Right now the best way to deal with it, for temporary trips in
space, is probably just to deal with it: accept the 1% or 2% rise in cancer
risk, and make up for it with superior health care once they are back on
Earth.

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lifeisstillgood
Dead within a day or two of a solar flare !?

Honestly I had no idea it was so active up there.

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danielweber
_If_ you get a solar flare, you're going to die quickly.

Solar flare particles are very eager to find something to interact with. But
flip side of that it only takes a few inches (IIRC) of water, or a watery
substance, to absorb them.

Cosmic rays, on the other hand, are likely to pass right through the entire
spacecraft without interacting with anything. The problem is that there are so
many of them that, by chance, some will decide to stop inside your body and
dance on your DNA. It takes something like a planet to really completely block
them.

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InclinedPlane
Even a single meter of water is enough to protect against the bulk of the
damage from all common radiation in interplanetary space except perhaps the
most powerful and infrequent of solar flares.

I don't know where you get the idea that it takes an entire planet to protect
you from solar flares or cosmic rays, that's very wrong, solar flares are not
that penetrating. In fact, on Earth almost all particle radiation from solar
flares (and from cosmic rays) is stopped by the upper atmosphere.

Edit: added additional clarification.

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danielweber
A simple atmosphere will protect you from flares. I think even Mars's very
thin atmosphere would be enough to save you.

I brought up a planet only for the fact that you would need something really
really big to block all cosmic rays. I did not mean to imply it was necessary.
People on Earth are hit from cosmic rays from "above" as part of our
background radiation.

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InclinedPlane
Atmospheres help but only somewhat. The radiation exposure on Mars' surface is
still much, much higher than on the surface of Earth, enough to warrant taking
extra precautions to lessen exposure (such as placing regolith filled sand
bags on top of habitats).

Also, to be precise people aren't hit by primary cosmic rays on Earth but by
secondary cosmic rays, the particle debris from high energy protons and nuclei
colliding with atoms in the upper atmosphere.

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DocG
Another silly question, we don't use magnetic fields for deflecting radiation
because we can build only weak magnets?

(just idea, electricity comes from solar panels/"nuke plants" and is probably
only non ending resource in space.)

A burst from capacitors for powerful electromagnet, could be future?

edit: found answer from comments:
<http://news.ycombinator.com/item?id=5315143>

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lighthazard
So why can't they have manually powered televisions to pick up TV and watch
something on the craft? Since Earth has been broadcasting for so long,
shouldn't there be TV broadcasts at the range of Mars that they could pick up
and watch? If it's powered by some sort of exercise machine, it'd also assist
with the muscle atrophy, no?

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51Cards
Brings new meaning to "I'll be on the poop deck"? (come on, it couldn't be
resisted)

More seriously, it's an interesting idea and it solves a couple issues...
storage, overall travel weight, and shielding. Interesting.

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Pinatubo
This would be a great Start Trek episode.

"Shields are down to 10%, Captain!"

"All hands report to the poop deck!"

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theoutlander
That's epic!

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philhippus
Water and nutrients from the poop may need to be recycled, certainly for deep
space missions. Using it as a rad-shield would help to keep it sterile while
in storage.

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theoutlander
They certainly can't go into space unshielded, so human waste & water will
only be the secondary/tertiary shields.

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stuqqq
Hooooorrrray! I can finally be useful in the space!

