ROOM
45
Space Environment
The Apollo lunar
landing craft were
part-shielded to afford
some crew protection
from radiation. Apollo 14
astronauts Alan Shepard
(pictured) and Ed Mitchell,
who spent 33½ hours on
the Moon in 1971, received
the highest exposure of
any of the crews due to
active solar storms.
radiation. Microwaves have more energy and can
cause hydrogen atoms to speed up, increasing
temperature. Infrared light has more energy than
microwaves and it transmits heat in general, and
then the energy of visible light increases from red
through to violet.
When photons have enough energy to be
ultraviolet they become ionizing and can cause
damage. Ultraviolet rays cannot penetrate very far
but they are of concern as they cause plastics and
other materials to break down more quickly, and
on human skin extended exposure strips electrons
off the DNA molecules in the nucleus of skin cells,
causing mutations that can result in cancer. The
Sun emits a lot of ultraviolet light but we are
protected from most of it by Earth’s ozone layer.
X-rays are higher frequency photons which are
useful for medical imaging because of their ability
to penetrate the human body but, for the same
reason, they can cause damage if we are exposed
to too many. Earth’s atmosphere absorbs most
X-rays from space but a small proportion get
through and people who live at higher elevations
or fly often tend to receive a higher exposure than
those who stay near sea level.
The most energetic (highest frequency) photons
are gamma rays and these can penetrate from
several centimetres to several metres of lead,
depending on their energy. Almost all gamma rays
are absorbed by the atmosphere of Earth.
Our Sun produces every level of photonic radiation
- from ultra-long wavelength radio waves to gamma
rays, although the latter are converted to lower-
energy photons before they are emitted into space.
Particulate
Particulate radiation is produced when an atomic
or subatomic particle with rest mass is accelerated
to very high speeds, usually near the speed of
light. These particles are like tiny bullets and all
are ionizing.
The lightest particles are electrons, also known
as beta radiation. These have a negative charge and,
although very light, can penetrate several centimetres
of wood and cause damage to living things.
High speed protons are about 2000 times as
heavy as electrons and are positively charged
particles found within atomic nuclei. They are
produced in huge amounts by the Sun and are a
major component of solar radiation and solar wind.
A proton is simply a hydrogen nucleus. Hydrogen
can have other isotopes that have a proton and a
neutron, called deuterium, or two neutrons and a
proton, called tritium. These are much less common
but have, respectively, twice and three times the
mass of a simple hydrogen nucleus so they transfer
more energy if travelling at the same speeds.
High speed helium, or alpha particles, have two
protons and one or two neutrons. Helium-3 is not
readily found on Earth but is emitted by the sun
and is present on the Moon in the regolith.
The Sun generates a lot of this type of particle
from its fusion reactions and there is quite a bit
in the solar wind. The nuclei are all ions, their
electrons stripped away as they are accelerated,
and they have a charge.
The last level of particulate radiation to consider
is created in supernovae and other energetic cosmic
events that produce fast moving particles much
heavier than hydrogen or helium. Atoms as large
as iron atoms - the heaviest element a star can
normally make - can slam into objects in space,
transferring a huge amount of energy in the collision.
The particles interact with the electric fields of the
matter they hit and slow down, transferring energy
to whatever they hit.
This energy causes a cascade of high energy
photons, free electrons and sometimes neutrons
to be generated. These particles go on to produce
damage themselves in a process known as
secondary radiation. Particulate radiation that
comes from outside the Solar System rather than
the Sun is called galactic radiation.
Earth’s magnetic field has a strong effect on all
these ions because they have an electric charge.
The field redirects them around Earth where
they orbit and lose energy until they enter the
atmosphere at the poles and in the process create
the Northern and Southern Lights. Almost all of
the particles that get through are absorbed by
Earth’s atmosphere before they reach the ground.
One of the
greatest
threats to
future humans
living away
from Earth
is that of
excessive
radiation
exposure
NASA
