radiation on mars_ final

Running head: RADIATION ON MARS AND THE RISKS AND MITIGATION 1

Radiation on Mars and

the Risks and Mitigation for Future Astronauts

Heidi D. Beemer

Embry Riddle Aeronautical University

Author Note

This paper was prepared for Space Habitation & Life Support Systems ACSI 513, taught

by Professor Kevin Allen.

RADIATION ON MARS AND THE RISKS AND MITIGATION 2

Abstract

The National Aeronautical and Space Administration (NASA) laid out their plan before congress

in 2014 of sending American men and women to Mars by 2030. This plan includes a 180 day

flight to the red planet, a 500 day stay on the surface and a return trip of an additional 180 days.

This plan is known as NASA’s design reference mission. There are many problems that need to

be overcome to make this mission successful. Among them will be to thoroughly investigate the

radiation exposure astronauts will receive on the surface of Mars. This paper indentifies the types

and amount of space radiation that astronauts will be exposed to while on Mars using data

collected by the Radiation Assessment Detector (RAD) on board the Mars Science Laboratory

(MSL). It also identifies ways of mitigating the radiation exposure through natural methods of

shielding to ensure the safety of our astronauts as they return home.


Radiation on Mars and the Risks and Mitigation for Future Astronauts

Introduction

Mars is the next step in human exploration of our Solar System. After leaving the Moon

for the last time in 1972, mankind has been waiting for the moment when humans would once

again take the first steps on an unknown surface. In order to demonstrate mankind’s eagerness

for exploration we must continue pushing the boundaries of the unknown. Before this can occur,

however, there are still many unknowns that must be investigated to ensure the safety of the

brave men and women that will embark on this life altering endeavor. Among these topics of

inquiry are the physiological effects on the human body to include the effects of microgravity on

the musco-skeletal systems, cardio-vascular systems, nerosensory systems, and immune system.

There are also environmental concerns, as well as physco-sociological issues that need to be

thoroughly studied prior to sending astronauts to Mars (Clément, 2005). One of the most well

known concerns about an expedition to Mars is the long term effects of space radiation on the

human body. Scientist and flight surgeons understand that these risks are present, but are

continuously searching for more data to truly understand how space radiation will affect future

deep space missions, in particular, how will the surface radiation on Mars affect a 500 sol

expedition on the red planet.

Understanding Radiation

Radiation has always been a scary subject for both scientists and the public alike. When

most people think of radiation today they think of the graphic images of casualties of the atomic

bomb on Hiroshima or those that died trying to combat the Fukushima or Chernobyl reactors.

Although ionizing radiation can be very deadly in large levels, not all radiation is harmful. In

fact, radiation from the sun is what keeps us all alive on Earth and is an important part of life as

we know it. Unfortunately, ionizing radiation on Mars is a known risk that must be investigated


prior to exposing future astronauts. Today, when the public and academics similarly discuss the

feasibility of planetary missions, radiation is always among the show stopping talking points

(Clément, 2005). It becomes important to investigate and understand the levels of exposure that

astronauts will encounter during deep space travel not only during transit from Earth to other

planets, but also while living and working on the surface of Mars.

Radiation is the term used to describe energy in transit. This energy comes in the form of

high-speed particles and electromagnetic waves. It is around us every day in the form of

sunshine, radio waves and microwaves. This form of energy is known as non-ionizing radiation

due to the fact that it lacks the sufficient energy needed to remove electrons from the orbits of

atoms. The more dangerous radiation is known as ionizing radiation. This form does have

sufficient energy to remove electrons, causing particles to become charged. This can result in

atoms becoming unstable if the ionization has sufficient energy to remove electrons found within

inner orbits leaving the atom very chemically reactive (Lee, 2014).

The radiation that is associated with long duration space travel, and is concerning to

astronauts, falls into three categories: trapped radiation, galactic cosmic radiation, or GCR, and

solar particle events, or SPE. All three of these types are forms of ionizing radiation and can be

harmful to humans in large doses. These high-energy, charged particles are located in varying

places within the Solar System (Lee, 2014).

The first form of space radiation is trapped radiation. Majority of radiation coming from

the Sun is deflected by the Earth’s magnetic field. This protective shell around our planet is

formed by the Earth’s molten iron core which creates protective electric currents. Although most

radiation is sent around the Earth much like water flows around the bow of a ship, some of the

radiation is trapped in-between the magnetic lines in an area known as the Van Allen radiation


belt. This radiation will be a threat to astronauts leaving Earth and heading towards Mars,

however, Marstronauts will leave this type of radiation in the rear view mirror as they travel

towards the red planet (Lee, 2014).

The next type of radiation is known as galactic cosmic radiation, or GCR. These particles

originated from outside the Solar System and can range in composition and size. They generally

move close to the speed of light and can produce intense ionization as they pass through objects,

including spacesuits (Lee, 2014). According to G. Hassler et al., these particles vary in size and

include, “protons (85-90%), helium ions (~10-13%), electrons (~1%) and heavier nuclei( ~1%)”

(2014, p. 1). This type of low-dose, ionizing radiation is what would be expected to be found on

the surface of Mars after millions of years of the surface being bombarded with GCR traveling

through the Solar System. GCR’s high-energy particles can penetrate several meters into the

Martian regolith making them very hard to shield against (Hassler et al., 2014).

The last concerning type of space radiation is the solar particle event, SPE. Commonly

referred to as solar radiation, these high-energy particles are ejected from the sun during solar

flares, shocks, coronal mass ejections (CMEs) or other solar events. These events cannot be

predicted or forecasted making them even more dangerous to space travelers (Clément, 2005).

These events last anywhere from minutes to hours and can even last for days. The energy levels

of these events vary in magnitude and are heavily composed of protons. SEPs are also broken

down into “hard” and “soft” spectrum events depending on the amount of energy present.”Soft”

spectrum events contain mostly protons and helium ions and have an energy level less than 150

MeV/nuc. “Hard” spectrum events contain ions that have been accelerated beyond 150 MeV/nuc

(Hassler et al., 2014).


Medical Effects and Federal Radiation Regulations

Radiation can dangerously affect the human body when charged electrons or protons

come into contact with tissue causing the ionization of water or oxygen present in the body.

These charged particles will produce a reactive product called a free radical, which when

released throughout the body can cause severe cellular damage when these free radicals interact

with genetic material such as Deoxyribonucleic acid, or DNA and Ribonucleic acid, or RNA,

causing a decrease in mitotic division rate among the cells (Clément, 2005).

The effects of radiation are both seen in the long term, Stochastic Risk and in the short

term, Acute Radiation Syndrome. The long term effects result in various types of cancers where

as the short term symptoms include nausea, decreased blood cell count, and in extreme cases, can

even result in death. These symptoms vary according to the amount of radiation in which the

person is exposed. Table 1 demonstrates the levels of radiation exposure and the acute issue that

arise with increased exposure (Clément, 2005).

Table 1

Symptoms and time course of Acute Radiation Syndrome

Note. Adapted from Fundamentals of space medicine, by G. Clément, 2005, p. 271.

1 Rem is the Roentgen equivalent in Man or the equivalent dose used to measure radiation in the

United States and is comparable to 0.01 Sieverts (SV) in the International System of Units.

Dose (Rem)1

Probable Medical Effects

10-50 No Effects except minor blood changes.

50-100 5-1-% subjects experience nausea or vomiting; fatigue for 1-2 days; slight reduction in white

blood cells.

100-200 25-50% nausea and vomiting, with some other symptoms; 50% reduction in white blood cells

200-350 75-100% nausea, vomiting, fever, with anorexia, diarrhea, and minor bleeding; 75% reduction in

al blood elements. 5-50% subjects die.

350-550 100% nausea, vomiting, fever, bleeding diarrhea, and emaciation. Death of 50-90% in 6 weeks.

Survivors require 6 months of convalescence.

550-750 100% nausea and vomiting in 4 hours. 80-100% die

750-1,000 Severe nausea and vomiting for 3 days. Death within 2.5 weeks.

1,000-2,000 Nausea and vomiting within 1 hour. 100% subjects will die within less than 2 weeks.

4,500 Incapacitation within hours. 100% subjects will die within 1 week.


Severe issues also arise from long-term exposure of smaller amounts of radiation over

time. As free radicals or the radioactive particles continue to interact with human DNA, cells

undergo uncontrolled cell division causing mutations that will lead to cancer. DNA

fragmentation eventually leads to cell death. DNA replication may also be damaged which would

result in the DNA not being able to repair itself which also leads to cancer (Clément, 2005).

In the United States, the governing body for the labor force exposed to ionizing radiation

during their employment is regulated by the Occupational Safety and Health Administration, or

OSHA. OSHA’s regulations are broken down into types of radiation exposure, and are generally

represented in the form of dose (Rem) per calendar quarter, or three months (Ionizing Radiation,

1994). According to the Presidential Executive Order 12196, all federal US agencies must

comply with OSHA regulations relating to ionizing radiation exposure. Astronauts are

employees of the National Aeronautical and Space Administration, or NASA, and as a US

Government organization, they are required to follow OSHA standards and regulations. OSHA,

however, does not have existing doctrine for space flight, and NASA has judged OSHA’s

Ionizing Radiation 29 CFR 1910.96 as not fitting for space operation. Instead, they adopted and

implemented the National Council on Radiation Protection and Measurements (NCRP) form

Report 98 from July, 1989 called, Guidance on Radiation Received in Space Activities (Johnson,

1997). Since 1989, NASA has revisited the radiation exposure guidance. Today, the

administration still relies on the guidance from the NCRP, but, it now publishes its

recommendations into its own publication: NASA Space Flight Human System Standard Volume

1: Crew Health. The results of this guidance are seen in Tables 3 and 4. The numbers are

calculated to stay within a maximum 3% lifetime excess risk of cancer mortality and they are

built behind the principal of ALARA, or, “As low as reasonably achievable” (NASA, 2007).


Table 3

Organ Specific Exposure Limits in mGyfor US Astronauts

Exposure Lens Skin Heart CNS

30 Days 1000 1500 250 500

Annual 2000 3000 500 1000

Career 4000 4000 1000 1500

Note. Adapted from NASA Space Flight Human System Standard, 2007, p. 67.

Table 4

Current Year Exposure Limits by Age and Sex in REM for US Astronauts

Sex Age

25 30 35 40 45 50 55

Male 52 62 72 80 95 115 147

Female 37 47 55 62 75 92 112

Note. Adapted from NASA Space Flight Human System Standard, 2007, p. 66.

Radiation in Transit to Mars

The largest doses of ionized radiation will accumulate during the trip from Earth to Mars.

Depending on the current distance between the two planets and the speed of the rocket, humans

can spend anywhere from 150-300 days in space being exposed to both GCR and SPE radiation

(Cain, 2013). It is important to understand the amount of radiation traveling astronauts will be

exposed to, as this will be part of the accumulated annual and career dose that will affect the

overall health of the astronauts during their mission to Mars. According to Cary Zeitlin and the

Mars Science Laboratory’s (MSL) science team, when scaling the measurements made by the

Radiation Assessment Detector (RAD) aboard the MSL capsule during its 253-day trip to be

comparable to the 360 day, roundtrip made by NASA to send humans to Mars, the total exposure

of galactic cosmic radiation on an unshielded capsule is 662 ± 108 mSv (Zeitlin et al., 2013).


Radiation on the Surface of Mars

Radiation received by the surface of Mars is much higher than what is seen on Earth.

This is due to Mars’ missing magnetic field and its very thin atmosphere (Hassler et al., 2014).

As discussed before, Earth’s magnetic field, created by the Earth’s molten core, protects the

surface from bombardment of radioactive particles. Mars was thought to have had a molten core

in the first billion years after its formation. It is hypothesized that the smaller size of Mars

allowed the core to cool much more rapidly, leading to the creation of a solid core and leaving

Mars without a magnetic field. Over time, solar wind “blew” away Mars’ atmosphere which left

the thin layer of protection present today (Luhmann & Russell, 1997).

The Martian atmosphere is chemically and physically very different from Earth’s and

provides the Martian surface with very little shielding from cosmic radiation. The composition of

the Martian atmosphere is predominantly CO2 and N2 at 0.08 and 0.004 atm, respectively, as well

as a combination of the two to form CN, a molecule that is capable of holding radiant material

two times more than the ordinary molecules of air (James, 1964, p. 470).

The radiation that does penetrate the atmosphere of Mars includes GCRs and SEPs. Both

types of radiation, if energetic enough, are capable of breaking through the Martian atmosphere

and down into the regolith. When this occurs, secondary particles, in the form of neutrons and γ-

rays can be created, becoming a part of the ionizing radiation environment. “Soft” spectrum

events do not carry enough energy to penetrate the Martian regolith; however, “hard” spectrum

events can break through the surface. Secondary neutrons produced by SEPs in the atmosphere

of Mars are capable of reaching the surface of Mars as well (Hassler et al., 2014).

The Mars Science Laboratory’s Radiation Assessment Detector (RAD) continuously

makes measurements of the surface and subsurface of Mars. Hassler et al. analyzed


measurements taken during the solar maximum starting on 7 August 2012 through 1 June 2013,

collecting data on both GCR as well as “hard” and “soft” SEP events on the Martian surface. The

RAD device took a variety of radiation measurements, to include: time series of absorbed dose

rate, the average absorbed dose rate, average dose equivalent rate, and Linear Energy Transfer

(LET) for the duration of ~300 sols, or Martian days, which compared to earth is 24 hours and 39

minutes.

Although radiation levels will vary during different phases of the solar cycle, GCR is

higher during solar minimums and SEP events are more frequent during solar maximums, the

data collected by the rover platform is a very good representation of the amount of daily

radiation astronauts expect to be exposed to on the surface of Mars and according to Hassler, is

“directly relevant to planning for future human missions” (Hassler et al., 2014, p. 1). During the

~300 sol period, RAD observed GCR dose rates between 180 and 225 microgray (µGy) per day.

It also observed one “hard” SEP event on sol 242 and several “soft” SEP events caused by

interplanetary coronal mass ejections on sols 50, 97, 208 and 259 (Figure. 1). It was concluded

that Mars’ surface dose rate due to GCR as recorded at Gale crater is 0.210±0.040 mGy/day. The

SEP dose on the Martian surface was recorded as 0.025 mGy/event as expressed in Table 5.

Figure 1. Time series of radiation dose rate measured by RAD. (Hassler et al., 2014, p. 2).


Table 5

Mars Radiation Environment Summary During 2012-2013 Solar Maximum

GCR dose rate

(mGy/Day)

GCR dose-equivalent rate

(rem/day)

SEP dose

(mGy/event)

SEP dose equivalent

(rem/Event)

Mars Surface 0.210 0.064 0.025 0.0025

Note. Adapted from Mars Surface Radiation Environment Measured with the Mars Science

Laboratory’s Curiosity Rover, D. M. Hassler et al., 2014 on p. 4.

According to the data provided, astronauts sent to Mars for a 500 day, NASA design

reference mission, would be exposed to a total mission dose equivalent of 32 ± 5 Rem. From this

data one can conclude, for both male and female astronauts, the radiation exposure is within the

limits allotted for astronauts in low earth orbit and is still within the range of having no health

effects from the radiation for the duration while on the surface of mars.

Table 6

Year Totals for a 500 day Stay on Mars


Laboratory’s Curiosity, Rover, D. M. Hassler et al., 2014 on p. 3.

According to the same research, during an initial and a return trip to and from Mars,

totaling 360 days, unshielded astronauts will receive a total dose equivalent of 66.2 ± 10.8 Rem

giving a total dose rate of 98 ± 15.8 Rem for a 860 day journey (Table 6); which meets the yearly

allotted dose for both males and females under the age of older than 35, but is uncomfortably

Year 1

(180 day transit, 180 days on Mars) Year 2

(320 days on Mars, 40 days transit) Year 3

(140 days transit to Earth)

180 Transit

(1.84± 0.30

mSv/day)

331 ± 54 mSv 320 days on Mars

(0.64±0.12) 205 ± 38 mSv

140 Transit (1.84±

0.30 mSv/day) 258 ± 42 mSv

180 days on

Mars (0.64±0.12) 169 ± 22 mSv

40 Transit (1.84± 0.30

mSv/day) 73.6 ± 12 mSv 220 days on Earth negligible

Total mSv 500 ± 76

mSv/year Total mSv 279 ± 50 mSv/year Total mSv 258 ± 42 mSv/year

Total Rem 50.0 ± 7.6

Rem/year Total Rem 27.9 ± 5.0 Rem/year Total Rem

25.8 ± 4.2

Rem/year


close. This justifies the importance of using shielding methods both during the cruise to and from

Mars and while living and working on the surface (Hassler et al., 2014, p. 3).

Mitigating surface radiation exposure

When discussing radiation safety, there are three ways to eliminate or reduce radiation

exposure: time, distance and shielding. One should minimize the time that radioactive materials

are handled; maximize the distance from the material, and shield as much as possible to reduce

exposure (Ionizing Radiation, 2014). On Mars, there is no escaping the radiation environment

that the astronauts will be exposed too, therefore there is no way of reducing time or distance

from the radioactive source. Instead the primary means of radiation reduction is shielding.

Natural Shielding

Due to the extreme cost of launching excess weight into space, it is beneficial to use the

natural shielding materials present on Mars, such as Martian regolith, to shield the habitat in lieu

of bringing additional shielding materials from earth. Most of the plans for a manned mission to

Mars include some form of regolith shielding. Mars One has included this in their plan for

permanent settlement of Mars by 2025 as seen in Figure 2. Another type of natural shielding is

to use terrain features to block the habitat from Radiation exposure. (Simonsen, Nealy,

Townsend, & Wilson, 1991).

Figure 2. Mars One’s artistic rendering of Martian Regolith used to shield the radiation

environment on Mars retrieved from, http://www.mars-one.com/mission/roadmap, 2014.

http://www.mars-one.com/mission/roadmap


The shielding properties of the regolith vary depending on the rock, ice, or soil densities.

In table 7, Hassler (2014) extrapolates a precise estimation of the subsurface radiation dose-

equivalent rate using the surface measurements of RAD at Gale Crater with an average rock

density of 2.8 g/cm3. There are two things that can be inferred from the results of table 7. The

first is that trapped radioactive partials found in the regolith, at -10 cm in depth, is higher than

the surface dose due to the presences of secondary particles, which could prove more harmful

then beneficial to astronauts using this depth of regolith to protect their habitat. The second is

that the radiation observed below three meters is present due to the natural radiation background

on Mars and not by additional GCR radiation. This means, that there are no shielding benefits

beyond a depth of three meters.

Table 7

Subsurface Radiation Estimates Scaled to RAD Surface Measurements

Depth Below surface Effective shielding mass (g/cm

2 ) GCR dose-equivalent rate(mSv/year)

Mars Surface (RAD) 0 232

-10cm 28 295

-1 m 280 81

-2 m 560 15

-3 m 840 2.9


Laboratory’s Curiosity, Rover, D. M. Hassler et al., 2014 on p. 4.

According to Simonsen, Nealy, Townsend, & Wilson of the NASA Langley Research

Center (1991), another way of shielding habitats from the radiation environment is to place the

structure in a naturally shielded location. These locations include stationing the habitat next to a

cliff, in a valley or crater, or in a preexisting cave or lava tube. When a structure is placed next to

a cliff, the radiation will travel through the CO2 atmosphere and continue to traverse through

outcrops of rock material before it continues into the habitat (Figure. 3). The total dose will be


the combination of all the vectors surrounding the structure. The more natural rock present in the

surrounding areas will greatly reduces the amount of radiation the habitat and crew is exposed

too over time.

Figure 3. Slant path distances (S) through CO2 and Martian Regolith as presented in Martian

Regolith as Space Radiation Shielding, Simonsen, Nealy, Townsend, & Wilson, 1991, p. 8.

While transporting a habitat or relaying on inflatable technology may be a feasible

solution for an initial establishment of a Martian outpost, as a colony continues to grow and

expand there will be a need for a more permanent residence. One of the easiest locations to

construct a Martian base will be in preexisting caves or lava tubes. Having protection under

several meters of regolith, rock, and ice will give the crew the best defense from the radioactive

environment. These caves are already preexisting and would lower the cost of the overall project

since additional building materials and drilling equipment will not necessary.

Some down sides to using caves will be the unknown factors. Site selection will be

limited to those locations with cave complexes. Today, we have the technology to remotely find

cave complexes using satellite imagery, however, there is presently no way of remotely mapping

the interior of cave structures. The only way to investigate these complexes to determine if they

will be suitable for living in will be to have astronauts on the ground. Other issues, such as, lack

of natural lighting inside the cave complex may also pose health risks to the Martian colonists.

Another option is to create man-made tunnels. This solution has all the same benefits of a natural


cave; however crews will have more flexibility over where these habitats are located and will be

able to add additional openings to allow for natural light as seen in Figure 4 (Kozicka, 2008).

Figure 4. Tunnel Outpost concepts as seen in Low-Cost Solutions for Martian Base, by J.

Kozicka, 2008, p.132.

Tunneling

In order to create these tunnel structures, astronauts will first need to adapt tunneling

methods used on earth to meet the requirements on Mars as well as transport heavy machinery to

the surface. On Earth, engineers have relied on the Rock Mass Rating (RMR) System for

characterization and design purposes during the initial assessment of rock layers for mining and

tunneling. The six parameters of this test include investigation into the rock layers: uniaxial

compressive strength of rock material, rock quality designation, spacing of discontinuities,

condition of discontinuities, ground water condition, and orientation of discontinuities (Hamidi,

Shariar, Rezai, & Rostami, 2010).

Crew 99 of the Mars Desert Research Station in Hanksville, Utah (2011) conducted

Martian analog field testing to prove that Martian astronauts were capable of completing the six

parameters of the RMR Scale, during an Extravehicular Activity (EVA) outside of the habitat. A

two person team was capable of assigning ratings to three rock layers found in the San Rafael


Swell and identified locations for future tunneling projects. This research proved that using the

RMR scale to determine future tunneling sites is a feasible practice while in a space suit on Mars.

This method can be used by trained astronauts to preselect substantial tunneling sites.

Once the site for the future outpost has been selected and certified, engineers can then

proceed to excavate loose soil and drill through exposed layers. To do this, heavy machinery,

such as Tunnel Boring Machines (TBM) will need to be either brought to Mars from earth or

fabricated on Mars using 3D printers or like technology. These TBMs, once fabricated or

brought to the surface, can be used to continuously create tunnels suitable for people to live and

will be naturally protected from the Martian radiation environment (Kozicka, 2007).

Conclusion

Whether astronauts live on Mars in manufactured habitats brought from earth or they

build their own structures on the surface or underground, the crew will need to find the most

effective ways to mitigate their exposure to the radiation environment. Although it is not

beneficial to simulate the levels of radiation found on Mars while we are on earth, we can

continue to rely on analog sites and stations, such as the Mars Desert Research Station, to

develop new technologies and parameters for protecting our future explorers. We can also

continue to refine engineering methods and developing cheaper and more efficient drilling

technology to help us establish permanent settlement below the surface as quickly as possible to

ensure that the risk of the radiation exposure is as low as reasonably acceptable.

We are continuing to gather more information about the Martian radiation environment

from the robotic missions we send to Mars. All the data supports the assumption that Mars is a

hostile environment unforgiving to humans. However, with proper shielding to mitigate radiation

exposure, it is possible to create a safe environment for explorers to live and survive.


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