Re: Space Travel by Humans is Possible

On Jan 23, 8:26 pm, John Schilling <schil...@xxxxxxxxxxxxx> wrote:
On Mon, 21 Jan 2008 13:28:05 -0800 (PST), Einar <eina...@xxxxxxxxx> wrote:
On Jan 21, 1:20 pm, John Schilling <schil...@xxxxxxxxxxxxx> wrote:
On Sun, 20 Jan 2008 13:07:14 -0800 (PST), Einar <eina...@xxxxxxxxx> wrote:
On Jan 20, 1:54 am, John Schilling <schil...@xxxxxxxxxxxxx> wrote:
Based on the known radiation hazards associated with, e.g. three-year
manned Mars exploration trips, there will be plenty of well-informed
volunteers. And their total radiation exposure will be within the
relevant federal regulatory limits, at least for US astronauts making
a single such trip.
Now, obviously, the free-neutrons-make-baby-Jesus-cry crowd will be
complaining that "they" are callously risking the lives and health
of the astronauts, who must have been nefariously pressured/decieved
or they'd never have volunteered, but so what?
According to what I have heard, exposure to cosmic rays lasting a year
will cause significant brain damage.
What you've heard is highly speculative and probably wrong. We will
of course double-check to make sure; that work is ongoing now.
I stand fully by what I said.
The free-neutrons-make-baby-Jesus-cry people always do, and they rarely
have trouble finding dupes to stand with them. Standing fully by what
one says, however, is rarely correlated with saying true things.
Citing your sources is a somewhat more persuasive approach, especially
if those sources are peer-reviewed scientific journals. I think I know
which ones you have heard third-hand reference to, and where they have
been misinterpreted along the way. You might find in instructive to
look into that yourself.
Hmm, well - - apparently it still exists in theyr arquive. The journal
entry referenced can be seen at the bottom. You tell me if this is the
article you had heard about:
It's new to me, but it's not a primary reference and it looks to be
derived from the same sort of stuff I've seen before.
http://www.newscientist.com/article.ns?id=dn7753
Note that New Scientist is not in fact a peer-reviewed scientific journal.
It may be a decent place to start looking for an answer, but it isn't
where you will actually find an answer. Well, not one you can trust.
"Cosmic rays may prevent long-haul space travel

[or not; New Scientist daisy-chains worst-case estimates to get 10%
mortality, mean value more like 2%]

The way you put it, things don´t appear as dire as I had been lead to
believe, however I´ll make further internet searches.
There is though one thing, I find it quite mysterious how slow
particles can be more dangerous than cosmic rays. Sure, there is more
of the slow ones. But high energy cosmic rays are more or less
constantly bombarding our upperathmosphere, creating particle showers.
What I don´t understand is how it can be reasonably claimed that
cosmic rays burn themselves right through a brain without doing much
damage. On the contrary I would expect them to impact some particle
within the brain, creating an instant highenergy particle shower
inside the brain...a somewhat scary though.
What is the explanation that something like that isn´t supposedly
happening? While the brain is not the densest part of our body, it´s
still a lot denser than the upperathmosphere, containing on the
average higher mass particles. So it sounds passing strange for cosmic
rays not to be creating particle showers within the brain.

Well, the brain is a lot denser than the upper atmosphere, it's also
a lot lot lot smaller. The average cosmic ray has to pass through
about six kilometers of air (sea-level equivalent) for its particle
shower to develop peak intensity. Brain tissue is roughly a thousand
times denser than air, so a mere six meter thickness of brain would
suffice to produce a really nasty particle shower.

Fortunately, while I've seen some bug-eyed monsters in cheesy sci-fi
movies with six-meter brains, humans are not so lavishly endowed.

More seriously, note that this makes it a really *bad* idea to shield
your spaceship with six meters of water or plastic, or half a meter
of steel, or the like. More reasonable thicknesses of shielding or
structure, including the human skull, simply don't put enough mass
in front of the cosmic ray to generate particle showers very often.

As for how a slow particle can be more dangerous than a fast one,
consider a simplified thought experiment where the human body is a
cloud of stationary atoms, themselves composed of nuclei and free
electrons. At the energy levels we are talking about, chemical and
even electronic bond strengths are insignificant. And nuclei are
so tiny that the odds of a direct hit even when passing through
the full thickness of a human body is small.

So, in the simplified model, a cosmic ray is a point of charge that
passes through this cloud in a straight line at a fixed speed. As
it is charged, it will push or pull the cloud particles out of their
original positions. A little bit of disturbance, just manifests as
heat, and not enough to matter. A bigger disturbance, and when we
go back and reconsider chemical bonds we may find that Particle X
has been disturbed enough to tear it loose from whatever molecule it
was supposed to be a part of. Still more, and Particle X is careening
through the body fast enough to cause still more damage on its own.

So, how much disturbance? If we were talking about direct hits, a
fast-moving cosmic ray would obviously cause a bigger disturbance
than a slow one. But direct hits are rare. On the subatomic scale,
what we mostly get are near-misses. And in a near-miss, the cosmic
ray can only disturb its own neighbors by exposing them to the
attractive or repulsive force of its own charge.

Per Coulomb's law, the magnitude of that force is independant of the
speed of the interloper, only the charge and the distance matter.
But the time available for this perturbing force to do its work,
does depend on speed. The faster the particle, the less time it
exerts a disturbing force on the targets it nearly misses. So,
less damage.

This is an oversimplified model. It completely negelects direct-hit
nuclear collisions, which while rare are not wholly absent. And it
assumes the incident particle is unaffected by the medium through
which it is passing. For most ordinary sorts of radiation, these
are really bad assumptions. For cosmic rays, they're sort of close
and we sort of know how to correct for the secondary effects.

Finally, it's important to note that, very often, when people talk
about a fast or energetic particle doing "less damage" than a slow
or low-energy one, what they really mean is less damage *per unit
energy*.

That's particularly important because there really aren't all that
many cosmic-ray particles to start with. On the order of one per
square centimeter per second. If your body were being bombarded
with ordinary one-MeV gamma rays at one per square centimeter per
second, probably not even Greenpeace would care. That would be
like shooting a blue whale with a rapid-fire BB gun.

We care about cosmic rays because each one is a thousand times
more powerful than a typical gamma ray, and we crudely expect them
to be a thousand times more dangerous. So if it turns out that
they are "only", particle for particle, fifty times more dangerous
than gamma rays, that would be twenty times *less* dangerous per
unit energy.

Historically, we've made the conservative assumption that a typical
cosmic ray, a thousand times more energetic than a gamma ray, is a
thousand times more dangerous. And we've assumed that an iron-nucleus
cosmic ray that's fifty thousand times more energetic than a gamma ray,
is a *million* times more dangerous.

This isn't just stupidity or innate conservatism; there were good
reasons for it in the limited data available before 1990 or so.
But it's probably not so, and we've updated the tables to reflect
that iron-nucleus cosmic rays that are fifty thousand times more
energetic than typical gamma rays, are probably not more than half
a million times more dangerous.

Well, some of us have. Some people are still using the old tables.
This doesn't actually make a whole lot of difference because the
disagreements are mostly over the most energetic cosmic ray particles
and those are also the rarest. Some other people, though, aren't
using any of the generally-accepted tables but mining the literature
for the most extreme and sensational claims they can find. And that
does make a difference.

As long as its possible to publish such papers without being laughed
out of the scientific community, there is obviously some uncertainty
and thus room for more work to pin down what's really going on. And
as we're not sending anyone to Mars tomorrow, whereas we are funding
radiation biologists today, we should have those answers by the time
we need them.

But the best bet is, if we send astronauts to Mars in slow, unshielded
spacecraft[1] and tell their doctors and dieticians, "be particularly
alert for the following...", odds are about 2% of them will eventually
die of cancer[2], a smaller fraction of any children they might have
will suffer birth defects, and there will be no other significant effects.

Sending them in a faster ship will make things proportionately better;
sending them in a moderately-shielded ship will make things *worse*,
and the level of shielding needed to make things significnatly better
would be positively Herculean.

[1] Unshielded except for a "storm shelter" to which the crew retreats
during major solar flares - that's a different sort of radiation with
different risks and countermeasures,

[2] Really, about 27% of them will die of cancer - the 25% who would
have died of cancer no matter what, and the 2% who will die of cancer
specifically due to cosmic-ray exposure during the trip.

--
*John Schilling * "Anything worth doing, *
*Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" *
*Chief Scientist & General Partner * -13th Rule of Acquisition *
*White Elephant Research, LLC * "There is no substitute *
*John.Schill...@xxxxxxxxxxxxxx * for success" *
*661-951-9107 or 661-275-6795 * -58th Rule of Acquisition *

Thank you, it was highly interesting.

You appear to be saying that the cancer risks are not any greater than
the risks caused by numver of individual food products that carry
cancer risks, even though most people still choose to eat them.

I think I´ll concerve the text of your letter in my computer for later
use.

Einar
.

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