ROOM
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Space Environment
landing craft were part-shielded to afford some
protection to the crew.
The dosage of radiation a human being can tolerate
depends on how quickly it is delivered. One Sv over
a short period of time will cause acute radiation
sickness and 10 Sv is often fatal. Emergency workers
at Japan’s Fukashima nuclear plant disaster in 2011
received up to 678 mSv, yet none of them suffered
acute radiation sickness. The effects of radiation are
somewhat cumulative, however, and long periods of
continued exposure to higher than normal levels of
radiation cause increased likelihood of cancer.
Studies of radiation exposure in large
populations like Hiroshima, Nagasaki, Chernobyl
and Fukashima have allowed projections to be
made about relatively safe levels of radiation. NASA
considers an increase in lifetime cancer risk of
three percent to be reasonable.
Evidence from these events has also shown that
the younger the person is at the age of exposure
the greater the risk over the lifetime of the person,
and that women suffer greater effects from
radiation than men. As a result, NASA has created
a graduated standard for maximum radiation
exposure for astronauts, dependant on age and
sex for one year and lifetime limits.
The European Union (EU) and Russia have set a more
conservative standard based on their own research, a
1000mSv career limit across all ages and sexes.
NASA career radiation exposure limits.
AGE
MALE
FEMALE
25
1500 mSv
1000 mSv
35
2500 mSv
1750 mSv
45
3250 mSv
2500 mSv
55
4000 mSv
3000 mSv
The first colonists tasked with building habitats
on the Moon or Mars will need to survive on the
surface with little protection, at least until the first
shielded structures have been completed. These
structures will likely have to be buried under about
800 cm of regolith to offer sufficient long-term
protection. How long this will take without heavy
equipment has not yet been accurately determined
though we can still broadly estimate the ‘safe’ time
builders can spend working on such a project.
A seven-day journey to the Moon would give
a dose of about 11 mSv. Yearly exposure without
shielding should average 438 mSv. This would give
the Russian and European astronauts 2.58 years
to work on the surface in inflatable or thin metal
habitats before they reach their career limits,
which would seem to be a reasonable amount of
time to accomplish habitat construction.
Conversely, a six-month journey toMars will give a
dose of about 324 mSv and, assuming an equal dose on
the way home, would leave only 352 mSv of unshielded
surface work before the 1000mSv limit was reached.
This would translate to about 1.44 years at 245 mSv
per year exposure. Again this seems like a reasonable
amount of time to get the job done, depending on the
size and sophistication of the habitats.
NASA limits are more generous and would allow
a minimum of 1.44 years for a 25-year-old female
to a maximum of 13.68 years for a 55-year-old
male. It seems clear that, even using a shovel and
the most stringent standards, it should be possible
to build sufficiently shielded habitats on the Moon
or Mars without the builders exceeding their
nation’s career limits for radiation exposure.
Of course, not every one of the same age and
sex has the same risk. Some people are genetically
more resistant while others are more vulnerable.
So age, sex, medical history and individual
susceptibility to radiation will also have to be taken
into account when selecting the first colonists.
Depending on the speed of travel, the Moon
has the advantage of being only a few days from
Earth. This allows for an easy return for someone
who has reached their exposure limit or some
other emergency. Mars is much farther away and,
while some colonists will choose to return, those
travelling there to establish the colony should
be willing to stay and make their lives there. The
question of who to send as the first colonists to
Mars or the Moon is therefore very important to
the success of these endeavours.
About the author
L. Joseph Parker has a doctorate of medicine from the Mayo Clinic with
honours in Neuroscience, and a Master’s of Science in Space Studies from
the American Public University. He was a Captain in the US Air Force, serving
first as a Minuteman II ICBMDeputy Commander at Malmstrom AFB in
Montana, and also with the US Strategic Air Command and later with US
Space Command. He went to medical school becoming an Air Force doctor
and now practices emergency medicine and has a private clinic.
Orion is NASA’s first
spacecraft for long-
duration deep space
exploration. It will
transport humans to and
from interplanetary
destinations such as
asteroids, the Moon and
eventually Mars.
Lockheed Martin
The first
colonists
tasked with
building the
habitats on the
Moon or Mars
will need to
survive on the
surface with
little protection
