ROOM
45
Astronautics
Structural radiation
shielding systems.
A computer-generated
visualisation of the Apollo
8 spacecraft in orbit
around the Moon, with
Earth rising over the
horizon. The thin metal
shell of the Apollo
capsules protected the
astronauts from most
types of radiation on their
way to the Moon.
determined by their energy levels. With sufficient
energy photons become ionising, converting
atoms or molecules into ions by stripping them of
one or more electrons, breaking molecular bonds
and damaging cells.
At the lowest end of photonic radiation energy
are radio waves, microwaves and visible light.
These are referred to as non-ionising and they
aren’t normally harmful to us.
Ultraviolet (UV) light is more powerful than
visible light and it is the weakest form of photonic
radiation that can cause us harm. It doesn’t
penetrate more than a few millimetres through
the skin but, with high exposure, can cause
skin cancer because, when it ionises atoms in a
covalently bonded molecule, it breaks those bonds
and damages the molecule. In the case of cancer,
the damaged molecule is DNA.
The most harmful energy waves are X-rays,
which most of us are familiar with, and gamma rays.
X-rays can be stopped by the atmosphere, by
heavy metals such as lead, or by sufficient volumes
of water. Gamma rays can also be halted by the
atmosphere, by water and, for the most powerful
photons, by several metres of lead. Because the
wavelength of a particle is inversely proportional
to its frequency the short wavelength gamma rays
have the highest frequency and energy.
Passive shielding
The Apollo spacecraft were incredibly well
designed for their time, though some parts of
the outer skin were as thin as 0.03 cm, no more
than a modern soda can. This thin metal shell was
enough to partially protect the astronauts from
most of these types of radiation on their way to
the Moon, although enough X-rays and gamma
rays got through to give them a dose of between
1.8 and 11.4 millisieverts.
The average annual dose of radiation exposure
on Earth is around 3 millisieverts, making the
Apollo 11 crew’s dose of 11.4 millisieverts about
three years’ worth in nine days. This seems
high, but they suffered no immediate ill effects,
although there is some question of increased heart
disease and cataracts in these men.
A thicker metal shield is more effective at
stopping photonic radiation, especially when
materials that have a high concentration of
hydrogen atoms, such as water or plastic is
layered with the metal as is the case with modern
spacecraft. This raises an odd issue, however.
When these photons impact the metal, or to a
lesser degree water, they come in contact with
the electron clouds around other atoms and can
produce a type of secondary radiation called
Bremsstrahlung or ‘braking’ radiation. Layers of
metal and water or metal and plastic should be
able to absorb most of this if it is thick enough.
Particulate radiation
The second type of radiation we have to worry
about is particulate radiation, which will usually be
in the form of free high-speed electrons, protons
(the simplest hydrogen nucleus) or more massive
atomic nuclei.
These nuclei are usually hydrogen or helium and
their isotopes (as produced by our Sun) but they
can be much heavier nuclei, iron or even uranium
if they come from more energetic events such as
supernovae and black holes. High-speed atomic
particles produced by supernovae and other
energetic deep-space events are referred to as
galactic radiation and, while solar radiation comes
from the direction of the Sun, galactic radiation
comes from all directions in space.
These particles have a tremendous amount of
energy and can cause serious damage to human cells,
penetrating into metal shielding and sending out
Space
travellers
moving out
of the shelter
of Earth’s
atmosphere
and magnetic
field will have
to contend
with high
levels of solar
and galactic
radiation
NASA
NASA
