Re: A Funny Thing Happened on The Way to The Moon

John LaCroix wrote:

And as for tin foil protecting from radiation, cosmic radiation no but
it will stop alpha and beta particles - if I recall correctly from
collage physics. Will correct me if I'm wrong please...

You're right, with two minor caveats. One is that cosmic
rays also consist primarily of alpha and beta particles;
the important difference between cosmic rays and the van
Allen radiation is that cosmic rays have a much higher
probability distribution of energy but a much lower flux.
The second caveat is that shielding is not all or nothing:
fairly thin layers of shielding can reduce radiation but
don't eliminate it entirely.

Tashi/MT has been talking about radiation as though he
doesn't understand the difference between the van Allen
radiation, the radiation produced by solar flares, and
extrasolar cosmic rays.

Aluminum shielding is fairly effective against the van
Allen radiation; were that not so, the electronics on
geosynchronous satellites would fail. According to
Wikipedia, 3mm of aluminum shielding reduces van Allen
radiation to about 2500 rem (25 Sv) per year [1]. The
Apollo astronauts went through the van Allen belts in
a matter of hours, and had considerably more than 3mm
of aluminum shielding, so their exposure to van Allen
radiation was comparable to the natural background
radiation they'd have received by spending a year in
Albuquerque, New Mexico [2]. Considering the other
risks of the Apollo missions, living in Albuquerque
for a year is no big deal.

Dr Blakely and Gene Krantz, whom Tashi has been citing,
weren't concerned about van Allen radiation. They were
worried about the more energetic alpha and beta particles
that come from solar flares and extrasolar cosmic rays.
As Dr Blakely observed, aluminum does a poor job of
shielding against these more energetic particles. The
flux density of high-energy particles is low, however,
so the probability of the astronauts (or their ship)
being hit by lethally energetic particles was also low.
The Apollo astronauts just accepted that risk.

They won that gamble, but subsequent research has shown
that the risk from solar flares and extrasolar cosmic
rays is higher than was known at the time. That is one
of several reasons no astronauts have returned to the
moon for several decades now, and is one of the more
important technical problems that will have to be solved
before manned missions to Mars can proceed.

To put all of this in perspective: http://xkcd.com/258/

Will


[1] http://en.wikipedia.org/wiki/Van_Allen_radiation_belt
[2] http://adsabs.harvard.edu/abs/1992EnGeo..19...11B
.

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