Satgen 546  Radiation and Humans  by GM4IHJ   (BID SGEN546) 1999-09-11

Proponents of manned space flight have up to now , refrained from 
publishing data regarding the effects of the various forms of space 
radiation which can affect space travellers. This has began to change 
, and the facts emerging make uncomfortable reading.

Human beings first encountered life threatening radiation doses in the 
atomic bomb raids on Hiroshima and Nagasaki. There the crude 
statistics available suggested that a cumulative dose of 200 REM was 
very debilitating, 600 REM meant possible death and, 1000 REM meant 
certain death.

In modern nuclear establisments dose equivalent units are now given in 
sieverts (Sv). Where 1 REM roughly equates to 0.01 Sv. With an annual 
worker dose limit of 20mSv set as "acceptable". Although there appears 
as yet to be no presently accepted level of dose limit which gives 
assurance that ones highly susceptible reproductive capabilities will 
not be damaged.

In space there are 3 main sources of radiation:-
a.  The protons and electrons accelerated to high speed whilst trapped 
in the Earth Van Allen Radiation Belts. Belts which occupy space 
roughly 1000 to 30000 kms above the earth over all earth latitudes 
between 60S and 60N.
b.  The Solar wind, which is normally quiet, but is capable of 
producing lethal levels of radiation during solar storms.
c.  Galactic Cosmic Radiation from which earth surface dwellers are 
generally totally protected by the combination of the earths magnetic 
field and, its thick atmosphere. But for spacemen outside the 
atmosphere and magnetic field , it is a quite different story. 
Radiation levels in unprotected space are highest in the years around 
solar sunspot cycle minimum, declining in the years around sunspot 
maximum because the stronger solar wind at that period of the cycle 
provides some useful reduction in radiation levels for craft inside 
the heliosphere. Unlike van Allen or solar radiation  however, cosmic 
radiation is generally considered to be much more damaging to human 
tissue because it can consist of heavy ionized particles with much 
more kinetic energy than the lightweight protons and electrons of the 
other radiation sources.

So what can you expect if you decide to be an astronaut ? NASA sets 
different cumulative career limits for different ages and different 
sexes. Limits which vary between 1 and 4 Sv. The Russians have a 
rather similar view of radiation hazard but state it slightly 
differently as 0.665 Sv per year over no more than 3 years.
The trouble is that while actual dose rates over time may be generally 
acceptable, nature does not necessarily operate steadily. Indeed some 
individual solar flares measured over the last 30 years or so have 
easily topped 1Sv in a single incident dose. While in one incident 
fortunately when no astronauts were aloft , a monster solar flare 
produced a single dose for a hypothetical individual of over 20 Sv 
, ie enough to kill anyone unlucky enough to be caught by it, twice or 
thre times.