NASA Technical Reports Server (NTRS) 19930007678: Human safety in the lunar environment

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by  NASA Technical Reports Server (NTRS)

ORIGINALCOHTAS ST 98- 1G8 7
OLORILLUSTR E!Q I
/l_,, o Human Safety in the Lunar
Environment
I"
Robert H. Lewis
Any attempt to establish a Compared to the Earth, the Moon
continuously staffed base or is geologically inactive. Volcanism
permanent settlement on the and internally generated seismic
Moon must safely meet the activity are almost nonexistent.
challenges posed by the Furthermore, water and
Moon's surface environment. atmospheric processes are
This environment is drastically unknown on the Moon. Other
different from the Earth's, and than igneous differentiation,
radiation and meteoroids are which occurred early in lunar
significant hazards to human history, the main geological
safety. These dangers may be process that has acted on the
mitigated through the use of Moon is impact cratering.
underground habitats, the piling
up of lunar material as shielding, The Moon was heavily bombarded
and the use of teleoperated by meteoroids throughout much of
devices for surface operations. its early existence. Evidence from
the Apollo expeditions suggests
that the bombardment decreased
The Lunar Environment significantly about 3.8 billion years
ago. This early bombardment and
The Moon is less dense than the subsequent impacts during the past
Earth and considerably smaller. Its 3.8 billion years have pulverized
density indicates that the Moon's the lunar surface into dust and
bulk composition is also somewhat small fragments of rock, a layer
different from Earth's, although it is referred to as the lunar "regolith."
still aterrestrial (rocky) body. The The majority of the Moon's surface
Moon's surface gravity is only is made up of heavily cratered
one-sixth the Earth's. And, with terrain, rich in the mineral
its consequently lower escape plagioclase feldspar and known
velocity, the Moon cannot maintain as the lunar "highlands." The
a significant atmosphere. Thus, uncompacted, upper portion of
the surface is directly exposed to the highlands' regolith is 10 to
the vacuum of space. Lacking an 20 meters deep in most places. A
atmospheric buffer, the Moon has smaller portion of the lunar surface,
asurface temperature that varies mostly on the Earth-facing side,
over several hundred degrees consists of basaltic lava flows and
Celsius during the course of a is known as the lunar "maria." The
lunar day/night cycle. A complete maria are geologically younger than
lunar day, one full rotation about the highlands and thus have been
its axis, requires approximately cratered far less than the highlands
27-1/3 terrestrial days. have. The depth of uncompacted
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regolith in the maria is roughly 4 to The solar wind is an isotropically
5 meters. distributed, neutral plasma
travelling at an average velocity of
The bulk density of lunar regolith 400 km/sec, tn Earth/Moon space,
increases with depth. Its upper it has an average density of about
surface is believed to have 45- 10 particles per cubic centimeter
percent porosity (Taylor 1982, (Taylor 1982, p. 155). This plasma
p. 119). The porous upper is composed of a relatively constant
20 cm of the regolith results from flux of charged particles, mainly
repeated meteoroid impacts, which electrons and protons, plus ions of
stir up the exposed surface and various elements.
occasionally form large craters.
These meteoroids represent A solar flare is similar in composition
potential hazards to both manned to the solar wind, but its individual
and unmanned activities. The particles possess higher energies.
meteoroid hazard on the lunar A solar flare may be considered
surface may be greater than a transient perturbation in the
that in free space (Mansfield 1971, solar wind. Exact timing of the
p. 1-4-14). In addition to the free- occurrence of a flare is difficult to
space flux of meteoroids, there predict, but the frequency of flares
is also ejecta from the impacts. may be related to the 11-year solar
Some fragments of ejecta could cycle. Most flares can be observed
have larger masses and slower at the Sun's surface some time
velocities than the free-space before a large increase in the solar
population of meteoroids. wind's higher energy particles is
detected in the vicinity of the
The Moon's surface is exposed to Moon. Not all solar flares yield
three types of hazardous ionizing particles that reach the Earth/Moon
radiation. The first two, the solar vicinity, but, of those which do, this
wind and solar flares, are produced flux reaches a peak within hours
by the Sun. The third type has its and then decreases over several
origin outside the solar system and days to the previous solar wind
is known as galactic cosmic rays. level.
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Galacticcosmicraysareapparently The Earth's magnetic field and
isotropically produced outside the atmosphere provide significant
solar system. The average cosmic protection, lacking on the Moon.
ray flux has been almost constant The cosmic ray flux per square
over the past 50 million years centimeter of lunar surface per
(Taylor 1982, p. 159). Cosmic rays year (during minimum solar activity)
are made up of very high energy contains 1.29 x 108 protons plus
particles consisting mostly of 1.24 x 107 helium nuclei plus
protons and electrons, plus some 1.39 x 106 heavier ions for a total
heavy nuclei (iron, for example), of 1.4279 x 108 particles per cm 2
positrons, and gamma rays. per year.* Fortunately, as the
Both the Earth and the Moon energy of the radiation increases
are exposed to these cosmic rays, from solar wind to cosmic rays, the
but the Moon's surface receives a frequency of encountering that
higher intensity of cosmic rays radiation decreases.
than does the Earth's surface.
"Countingonlyparticleswithavelocitygreaterthan10MeV pernucleon. Informationfrom
D.StuartNachtwey,MedicalSciences Division,LyndonB.JohnsonSpace Center,Houston.
274
Lunar Public Works should be able to obtain these elements concrete. [These manufacturing processes
by heating the soil. Once people have are also discussed in the accompanying
Thedry, barren Moon mightnot seem like a provided them with lunar water, carbon Materials volume.] The unprocessed soil
promising land for settlement. But, with the dioxide, oxygen, and nitrogen, plants itself can serve as shielding against the
eyes of a chemist, a pioneer settler may see should be able to extract nutrients directly diurnal temperature fluctuations and,
lunar conditions as advantages and lunar from the lunar soil. more importantly, against the hazards of
soil as a bountiful resource. radiation unscreened by an atmosphere
and undeflected by a magnetic field, as
Lunar Filling Station "_ discussed by Rob Lewis in this paper.
Lunar Water Works ,_
x ll/
U I I The lunar covered wagon will be a chemical f-'_1
The first concern of a lunar pioneer must be
water. There may or may not be water, as rocket, its horsepower hydrogen and Lunar Lightand Power _(_.'-
trapped ice, at the lunar poles, but there oxygen. Hauling these propellants from
certainly is an abundance of its chemical Earth will be expensive. Itmay prove The Sun shines on the Moon plentifully and
components, oxygen and hydrogen. cheaper to provide them from the lunar soil. predictably, but only half the time. Storing
Oxygen isthe most abundant chemical Forty tonnes of hydrogen, a reasonable solar energy over the 2-week-long lunar
element (45% by weight) in'the lunar soils, estimate of the amount needed for all nightseems difficult and may have to be
from which it may be extracted by various transportation from low Earth orbit for a done in the form of hydrogen, metals, and
processes. In contrast, the concentration year, could be obtained from just 0.3 kin2 of oxygen whose extraction was powered by
of hydrogen in lunar soil is very Iowl but the soilmined to a depth of 1m. Alternatively, energy from the Sun. Thus, initially, lunar
total quantity available is nevertheless lunar transport vehicles might burn a metal power is likely to come from an imported
great. The lunar surface has been bathed such as iron, aluminum, or silicon, even nuclear power plant. But electrical power
for billions of years in the solar wind, a flux though these are less efficient rocket derived from the Sun is alikely lunar product
of ionized atoms from the exterior of the fuels than hydrogen. All three are major and may even be the first major export to
Sun. These ions embed themselves in constituents of lunar soils, in chemical the Earth from the Moon (once the souvenir
the surface of grains of lunar topsoil. combination with oxygen, from which they market has been satisfied). Eventually,
Furthermore, meteorites, unimpeded by an can be extracted. In fact, each is a the solar cells will probably be derived
atmosphere, continually plow under the old byproduct of one or more processes for from lunar silicon, a byproduct of oxygen
solar-wind-rich grains and expose new producing oxygen. [Several techniques extraction, or from lunar ilmenite, recently
grains. In this way, large amounts of for extracting oxygen from lunar soils are shown to be photovoltaic. Conversion need
hydrogen have become buried in the soil, proposed in the Materials volume of this not be efficient if a local material, simply
enough to produce (if combined with lunar Space Resources report./ obtained, is used as the photovoltaic. More
oxygen) about 1million gallons (3.8 million futuristically, lunar hefium-3 has been
fiters) of water per square mile (2.6 km2) of proposed for use as a fusion fuel superior to
soil to a depth of 2 yards (1.8 m). This Lunar Lumberyard tritium in that it is not radioactive, does not
hydrogen can be extracted by heating the have to be made Fnnuclear fission reactors,
soil to about 700°C. Supplying the Lunar Better than burning the iron, aluminum, and and yields a proton instead of a more
Water Works is a matter of technology and sificon produced as byproducts of oxygen destructive neutron when it fuses with
economics, but not a matter of availability of extraction from lunar soils might be to use deuterium.
them to construct lunar shelters. Iron and
oxygen and hydrogen on the Moon. Lunar soilcontains In abundance the
aluminum can be fabricated into beams.
materials required for fife support,
Theboards of space construction may well
transportation, construction, and power.
Lunar Community Farm __ be made of glass. Molten lunar soilcan be With proper understanding and new ideas,
cast into silicate sheets or spun into
lunar pioneers should be able to turn the
Thenext concern of a lunar pioneer will be fiberglass. These may have greater lunar environment to their advantage.
food. Like hydrogen, carbon and nitrogen strength than similar products on Earth
are available in large quantities from the because of the lack of water to interfere with Taken from Larry A. Haskin and Russell O.
lunar soil,although they are present in very their polymer bonds. Partially distilled in a Colson, 1990, Lunar Resources- Toward
low concentrations, having been placed solar furnace, soil residue may take on the Living Off the Lunar Land, in Proc.
there, like hydrogen, by the solar wind. All composition of a good cement, which when Ist Symp. NASA/Univ. of Arizona Space
the other nutrients necessary to life are combined with locally produced water and Engineering Research Center (in press),
likewise present in the soil. Pioneer settlers the abundance of aggregate would become ed. Terry Triffet (Tucson).
275
The Human Factor Meteoroid impacts may have
effects ranging from long-term
erosion of the surface materials of
In order to develop permanent
pressure vessels and space suits
human settlements on the Moon,
all the way to penetration and
we must understand how the local
subsequent loss of pressure and
environment influences the settlers'
injury to personnel (see fig. 9).
safety and health. The lack of
More serious impacts could result
atmosphere and the extreme
in destruction of equipment and
temperature range mandate the
loss of life.
use of sealed and thermally
insulated enclosures. These
enclosures--the colonists' first
line of defense--will range from
individual space suits to buildings.
The next line of defense must
protect the colonists from both
meteoroids and radiation.
Figure 9
Crisis at the Lunar Base
Aprojectile has penetrated the roof of
one of the lunar base modules and the air
is rapidly escaping. Three workers are
trying to get into an emergency safe room,
which can be independently pressurized
with air. Two people in an adjoining room
prepare to rescue their fellow workers.
The remains of the projectile can be seen
on the floor of the room. This projectile
is probably a lunar rock ejected by a
meteorite impact several kJIometers from
the base. A primary meteorite would likely
be completely melted or vaporized by its
high-velocity impact into the module, but a
secondary lunar projectile would likely be
going slowlyenough that some of it would
remain intact after penetrating the roof.
Detailed safety studies are necessary to
determine whether such a meteorite strike
(or hardware failure or human error) is
likely to create a loss-of-pressure
emergency that must be allowed for in
lunar base design.
Artist: Pamela Lee
276

In 1971, the Rockwell Lunar Base being penetrated within 100 days.
Synthesis Study investigated A logical next step would be to
several strategies for dealing with add more layers of material to the
the meteoroid hazard. They took tent. This, of course, increases
a probabilistic approach to the the weight of the tent and its
problem of safety and examined associated transportation costs.
several options. Rockwell was
interested in providing portable The next option that Rockwell
shielding for short-term surface considered was identical to that
activities as well as more permanent just described but with an
fixed shielding. The shielding might additional layer of material filling
be needed many times during an the gap. In theory, the tent would
expedition covering large distances. serve to fragment a meteoroid and
the underlying material would
On Earth, mobile expeditions which impede and absorb the fragments
require temporary environmental before they reached the pressure
protection that is lightweight and vessel. On the basis of their
easy to redeploy often use tents. surface meteoroid flux model,
The Rockwell study examined the the gap filter would need to have
use of a tent-like structure which a density of 16 kg/m3 to provide
could be erected over an inhabited a 0.9999 probability of no
pressure vessel. The tent could penetration within 100 days. A
be constructed of a lightweight design of this type may prove to
material such as aluminum foil or be practical as portable meteoroid
nylon. The Rockwell investigators shielding for short-term surface
anticipated that such a structure activities.
would act as an extra outer layer
of protection against meteoroid However, these measures would
impact. For their calculations, the be completely inadequate for any
tent had an area of 46 m2 and the long-duration habitat (for a stay
insulated wall of the pressure vessel of over 100 days), so the addition
had a density of approximately of shielding material seemed
8 kg/m3. A small gap between the desirable. If lunar regolith were
tent and pressure wall was initially used as a gap filler, significant
considered. This arrangement protection could be added without
could provide a 0.9999 probability increasing transport costs from
of no penetrations in 100 days if Earth. Rockwell concluded that
only a free-space meteoroid flux a gap of approximately 15.2 cm
was assumed. However, assuming (6 inches), filled with lunar regolith,
also the secondary ejecta hazard, would reduce the penetration risk
they found that the tent system to less than one chance in 10 000
had only a 0.1 probability of not over a 2- to 5-year stay.
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PRECEDING PPlGE BLANK NOT FILMED
Althoughmeteoroid impacts may Individual responses to radiation
be a serious problem on an exposure vary somewhat and there
infrequent basis, the effect of is controversy over safe limits for
ionizing radiation on human health long-term, low-level exposures.
is continuous and cumulative over Currently, the maximum permissible
an individual's lifetime. A brief whole-body dose for radiation
discussion of radiation dosimetry workers is 5 rem/year and for the
is now in order. The fundamental general public 0.5 rem/year (CRC
unit of radiation transfer is the tad; Handbook of Tables for Applied
1 rad represents the deposition of Engineering Science 1980,
100 ergs of energy in 1 gram of p. 753). Both of these doses are
mass. The characteristics of the larger than the dose of background
deposition mechanisms vary and radiation at sea level that humans
additional factors must be are normally exposed to. Just as
considered. One conversion radiation workers must accept a
factor is the quality factor, Q, greater risk than do members of the
which is conservatively based on general public, so astronauts are
the experimentally determined prepared to accept a greater risk
than radiation workers. Table 3,
relative biological effectiveness,
RBE. When Q is multiplied by the provided by Stu Nachtwey, lists
the doses and health risks that the
rad exposure, the result is a unit
Medical Sciences Division at the
of dosage corrected for the type
of radiation; this resulting dosage Johnson Space Center estimates
is measured in a unit known as an astronaut on a Mars or lunar base
the rem. mission would be exposed to during
a period of minimum solar activity.
279
TABLE3.
Approximate Radiation Doses and Health Risks for an Astronaut
on a Mars or Lunar Base Mission During Minimum Solar Activity
[From D. Stuart Nachtwey, Johnson Space Center]
Radiation source Representative Skin dose Deep organ (5 cm) Excess lifetime
shielding equivalent dose equivalent cancer incidence in
a 35-year-old male*
Chronic exposure
< 0.1%
Trapped belts 2 g/cm2AI < 2 rem < 2 rein
(one-way transit)
Free space 4 g/cm2 AI 75 rem/yr 53 rem/yr 1.2%/yr of exposure
On lunar surface 4 g/cm2 AI 38 rem/yr 27 rem/yr 0.6%/yr of exposure
On martian surface 16 g/cm2CO2 (atm.) 13.2 rem/yr 12 rem/yr 0.3%/yr of exposure
Acute exposure to
large (e.g., Aug. '72)
solar particle event
Free space 2 g/cm2AI 1900 rem 254 rem ~ 5.7%
On lunar surface 4 g/cm2AI 440 rem 80 rem 1.8%
+ shielding 15 g/cm2AI 19 rem 9 rem 0.2%
0.1%
On martian surface 16 g/cm2 CO2 (atm.) 9 rein 4.6 rem
+ shielding 60 g/cm2AI < 1rem < 1rem < 0.03%
*The excess cancer incidence for a 35-year-old female is roughly twice that for a 35-year-old male.
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The rate of irradiation per unit time Silberberg et al. (1985) have
and the age and sex of the suggested that a compacted
individual at irradiation are also layer of lunar regolith at least
important. Younger people are 2 meters thick should be placed
more sensitive to the cancer- over permanent habitats. With
inducing effects of radiation than shielding of this thickness, the
older people, and females are more colonists' yearly exposure could be
sensitive than males because of held to 5 rem per year if they spent
cancer induction to the breast and no more than 20 percent of each
thyroid. Other serious radiation Earth month on the surface. In
effects include cataracts, genetic order to provide an overall level of
damage, and death. Radiation protection of no more than 5 rem
exposure is considered cumulative per year even in the event of an
over an individual's lifetime. extreme solar flare, such as
occurred in February 1956, the
Solar flares and cosmic rays are the depth of shielding would have to
most dangerous radiation events be doubled.
that lunar pioneers will be
exposed to. The cosmic ray For the sake of completeness, it
dosage at the lunar surface is should be pointed out that some
about 30 rem/year and, over an lunar regoliths contain a naturally
11-year solar cycle, solar flare radioactive component material
particles with energies greater than known as KREEP. KREEP,
30 MeV can deliver 1000 rem probably a product of volcanism,
(Silberberg et al. 1985). Solar contains radioactive potassium,
flares deliver most of their energy uranium, and thorium. Material
periodically during only a few days containing a high concentration
out of an 11-year cycle; whereas, of KREEP should not be used for
the cosmic ray flux is constant. shielding, and care should be
taken to avoid concentrating it
Although the lunar surface radiation as shielding is prepared. The
flux is too high to spend much time concentration of KREEP in most
in, it is definitely possible to alleviate regolith material would add an
the radiation danger with shielding. amount of radioactivity no more
When colonists are removed from than that in the granite used in
continuous exposure to surface buildings here on Earth. If the
radiation, long-term settlement small contribution by KREEP to
becomes possible. As in the case radiation dose is considered
of meteoroid protection, the when exposures are calculated,
simplest solution is to use locally it should not pose any significant
available regolith for bulk shielding health problem by itself.
of habitats.
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