Radiation Biophysics and
Human Spaceflight
Dr. John M. Jurist
Adjunct Professor of Space Studies, Odegard School of Aerospace Sciences,
and
Adjunct Professor of Biophysics and Aviation, Rocky Mountain College
Note: This material was used in formal biophysics, human factors, and
aerospace medicine and physiology classes as well as in various seminars at
the above institutions, and is not to be reused without attribution
Radiation Exposure
Sources and characteristics
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Radiation Exposure
Sources of exposure:
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On board fluid level sensors
Cosmic photons (includes gamma bursts)
Cosmic particulate radiation
Solar photons
Solar particulate radiation (includes flares)
Trapped particulate radiation belts (Van Allen)
Terrestrial background
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Radiation Exposure
Primary issues for external sources:
• Radiation exposures vary widely with location relative
to Earth or other bodies
• Radiation of solar origin varies with multi-year solar
cycle – periodic solar storms cause intense radiation
bursts
• Radiation of extrasolar or galactic origin consists of
relatively steady background component punctuated by
intense random bursts of particulate and photon
radiation
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Radiation Exposure
Primary issues for external sources – cont:
• Location of occupants within spacecraft and
construction of spacecraft influence quality and
magnitude of radiation to which occupants are exposed
• Biological response to radiation exposure varies widely
with nature of radiation and exposure rate
• Cumulative effect of multiple exposures not purely
additive – depends at least partly on intervals between
exposures
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Radiation Exposure
Primary issues for external sources – cont:
• Biological response variable and depends on gender,
age and health
• Biological effects of low doses are poorly
characterized since responses are confounded by other
unknown variables
• No agreement on threshold dose below which exposure
is harmless or whether response is dose-related down
to infinitesimally small exposures
• High exposure responses depend on availability and
quality of medical care for managing complications
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Radiation Exposure
2 types of ionizing radiation:
• Electromagnetic (photons –
gamma rays or X-rays)
• Particulate
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Radiation Exposure
Photons:
• X-rays are created in extranuclear events such as
decelerating or accelerating charged particles and
gamma rays are created in nuclear events
• Otherwise, they are same – packets of short
wavelength electromagnetic radiation
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Radiation Exposure
Particulate:
• Neutral particles – such as neutrons
• Charged particles:
• Alpha particles (ionized helium nuclei)
• Protons (ionized hydrogen nuclei)
• Beta particles (electrons) or their anticounterparts
• Other ionized nuclei
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Radiation Exposure
Particulate Cosmic Ray Spectrum:
Composite energy spectrum
for particulate cosmic rays
[Microcosm, SME 2011]
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Radiation Exposure
High end particulate event was so-called Oh My God
particle October 15, 1991 over Dugway Proving
Ground (Utah) (repeatedly confirmed since then):
• Particle had a kinetic energy of about 3*1020 eV or
50 Joules (60 MPH baseball for comparison)
• Believed to be proton traveling within 1 part in 1024
of the speed of light (3*108 m/sec)
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Radiation Exposure
• Solar photons in X-ray region occur as part of
blackbody emission spectrum of hot solar gases
• Photons are short wavelength tail of regular solar
spectrum that humans see in visible light
• Solar flares can also emit X-rays at flux that can exceed
1 mw/m2 at Earth's orbit
• Solar particulate emissions include the solar wind
(mostly conductive plasma) and coronal mass ejection
events
• Emissions associated with ionizing radiation
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Radiation Exposure
Typical time history of a solar flare or
coronal mass ejection event for X-rays
and high and low energy protons as
observed on Earth
The vertical axis is in relative units of
intensity with 100 corresponding to
baseline values
[Microcosm, SME 2011]
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Radiation Exposure
Units and their evolution
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Radiation Exposure
• Units used in radiobiology of historic interest since
recent rationalization to SI units
• Evolution of units followed logical but potentially
confusing progression
• Early unit of radiation exposure was Roentgen (R)
• Defined as the amount of ~100 kilo-electron volt (kV)
energy X-rays that would produce 1 ESU of ionization
per cm3 of dry air at 1 atm and 0oC
• This is ~258 microColumbs (μC) per kg of dry air
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Radiation Exposure
• Dry air and tissue ionize differently when exposed to
same amount of radiation
• Radiation absorbed dose (rad) was defined as amount
of 100 kV X-rays resulting in 100 ergs of absorbed
energy per gram of material
• For dry air under standard conditions, 1.0 R is
equivalent to 0.876 rad
• Equivalency is defined as f factor and varies with
material
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Radiation Exposure
• Biological systems differ in response to different types
of radiation
• Relative Biological Effectiveness (RBE) was defined as
ratio of standard X-ray dose to actual dose of radiation
to which system was exposed and which would produce
same biological response
• 50 rads of some type of radiation with an RBE of 2.0
would produce same biological damage as 100 rads of
standard X-rays
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Radiation Exposure
• Roentgen Equivalent Man (rem) was defined as
product of RBE and dose in rads
• Rem is indication of biological damage with acute
radiation exposure
• When units were rationalized, RBE was defined as Q,
the quality factor
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Radiation Exposure
Energy/Mass
100 rad
1 Gray (Gy)
Bioeffect
times Q(RBE)
times Q
100 Rem
1 Sievert (Sv)
Relation between modern SI units and older units
– 1 Gy = 100 Rads
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Radiation Exposure
Radiation Species
Alpha particles (helium nuclei)
Beta particles (electrons)
High energy protons (hydrogen nuclei)
X-rays (photons)
Gamma rays (photons)
Neutrons (unknown energy) – conservative
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RBE or Q
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1
10
1
1
10
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Radiation Exposure
RBE or Q for Neutrons
12
10
Q
8
6
4
2
0
1.00E-08
1.00E-06
1.00E-04
1.00E-02
1.00E+00
1.00E+02
1.00E+04
Energy (MeV)
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Radiation Exposure
Short term acute whole body exposure (Rems):
10-50
50-100
100-200
200-350
350-550
550-750
750-1,000
1,000-2,000
4,500
Minor blood changes
5-10% nausea (1 day), blood, survivable
1/4-1/2 nausea (1 day), blood, GI, survivable
Most nausea (1 day), blood, GI, 5-50% die
450 LD50 Most nausea, blood, GI, 50-90% die
Nausea (hours), blood, GI, almost all die
Nausea (hours), blood, GI, fatal (2-4 weeks)
Nausea (hours), fatal (2 weeks)
Incapacitation (hrs), fatal (1 week)
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Radiation Exposure
Radiation biological effects and biophysics
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Radiation Exposure
Low to moderate acute (prompt) doses damage rapidly
dividing cells in blood-forming tissues, skin, and
gastrointestinal tract:
• Decreased white cell blood counts (impaired ability to
combat infection)
• Erythema (reddening), blistering, or destruction of skin
depending on dose
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Radiation Exposure
Low to moderate acute (prompt) doses:
• Destruction of gastrointestinal system lining (nausea,
diarrhea, and secondary infection)
• Long term effects from damage to genetic and other
cellular systems
• Carcinogenesis although mechanisms and immune
system role unclear
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Radiation Exposure
Treatment and effects:
• Extrinsic radiation exposure treatment involves
medical management of complications as appropriate
• Success depends in part on quality and quantity of
available medical care
• Isolation makes treatment during deep spaceflight
problematic
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Radiation Exposure
Treatment and effects:
• Subsequent radiation injuries not treated with maximum
effectiveness as result of isolation
• Radiation effects studied in humans exposed to
radiation accidentally, for medical diagnostic or
therapeutic procedures, and as result of nuclear
weapons
• In addition, many animal experiments have been
conducted
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Radiation Exposure
Chronic low dose effects:
• Example: Typical total dose for treating breast cancer is
3,000 to 3,500 rads split into 30 daily doses
• If administered as single dose, it would destroy the skin
overlying the breast and most likely be fatal
• Skin effects of split dose administration usually limited
to erythema or reddening and easily managed
• Not clear if repair mechanisms result in threshold dose
under which no radiation damage incurred
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Radiation Exposure
Chronic exposure examples
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Radiation Exposure
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Polar airline flight 0.10-0.23 mSv per day
2 view chest X-ray 0.06-0.25 mSv
Bone scan 0.15 mSv
Chest CT 0.3-30 mSv (typical 10 mSv)
Billings MT background 1.2 mSv per year (quiet sun)
Typical US background 2.4 mSv per year
Typical US medical average 0.6 mSv per year
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Radiation Exposure
Dose vs Altitude
(nonTerrestrial)
Dose (microSieverts/day)
1,000
100
10
1
1
10
100
1,000
10,000
Altitude (thousands of feet)
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Radiation Exposure
Estimated HTO suborbital flight:
• Upper limit 0.0053 mSv per flight
• Compare: Polar airline flight 0.10-0.23 mSv per day
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Radiation Exposure
Orbital and beyond:
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0.6-0.9 mGy/day (Skylab)
0.2-1.3 mGy/day (Apollo landing missions)
~0.06 mGy/day (STS)
0.049-1.642 mGy/day (STS-2, STS-31)
0.053 mGy/day 0.146 mSv/day galactic cosmic
0.042 mGy/day 0.077 mSv/day trapped belt
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Radiation Exposure
Approximate
Dose (mSv/day)
0.146
0.077
Source
Average RBE
Galactic cosmic
2.75
Van Allen belt
1.83
trapped radiation in LEO
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Radiation Exposure
Shielding factors
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Radiation Exposure
• Altitude effect result of shielding action of atmosphere
on incoming solar and galactic radiation
• In terms of incoming cosmic radiation, atmosphere at
sea level equivalent to ~ 5 meters water or ~ 1 meter
lead
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Radiation Exposure
Beam hardening phenomenon:
• Softer (longer wavelength) photons generally
absorbed more efficiently by matter than harder
photons
• Effective beam spectrum progressively hardens with
absorption depth although intensity decreases
• Compton scattering is also an issue – causes fogging
of conventional X-ray images
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Radiation Exposure
• In LEO, shielding effect of Earth's atmosphere lost,
but shielding by geomagnetic field retained although
dependent on orbital inclination
• Earth’s magnetic field deflects charged particles &
reduces effective doses experienced at lower latitudes
• Contributes to latitude effect on radiation background
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Radiation Exposure
Geometric shielding effect lost during space flight
beyond LEO:
• Comes from Earth subtending large part of sky as seen from
LEO
• Shields incoming galactic & solar radiation to extent that flight
beyond LEO but in vicinity of Earth's orbit results in roughly
twice cosmic radiation exposure compared to LEO
• Same shielding effect would occur near Moon or Mars, but not
in transit
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Radiation Exposure
Protective Modality
Atmosphere of Earth
Atmosphere of Mars
International Space Station
Space Shuttle (STS)
Apollo space suit
Aluminum
Equivalent (g/cm2)
1,020
22
10
10
0.1
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Radiation Exposure
Space flight:
• Moon & Mars both have essentially no magnetic fields
• Mars field on order of one 800th strength of Earth's
field
• Vertically directed cosmic radiation at surface of Mars
about 46 times as intense as on surface of Earth
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Radiation Exposure
Dose standards (1994):
For reference, 2 view chest X-ray 0.06-0.25 mSv
• Public limit 1 mSv per year
• NASA classifies astronauts as radiation workers
• Worker whole body 50 mSv or 0.05 Sv per year
• Worker organ limit 0.5 Sv per vear
• Worker organ limit 0.25 Sv per month
Historical trend to reduce limits: Compare 1994 to 2000 limits
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Radiation Exposure
Career limits for radiation workers (1994):
Limit at
Lens
Skin
Blood-Forming Organs
Male
Female
Age 25
Age 35
Age 45
Age 55
4.0 Sv
4.0 Sv
4.0 Sv
4.0 Sv
6.0 Sv
6.0 Sv
6.0 Sv
6.0 Sv
1.5 Sv
2.5 Sv
3.2 Sv
4.0 Sv
1.0 Sv
1.75 Sv
2.5 Sv
3.0 Sv
Historical trend to reduce limits: Compare 1994 to 2000 limits
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Radiation Exposure
Radiation carcinogenesis:
0.5/106/mSv/year
0.4/106/mSv/year
0.3/106/mSv/year
7-17/106/mSv/year
Breast
Thyroid
Lung
All cancers
100 mSv/105
800 deaths added to 20,000 w/o
radiation (4% increment/10 rads)
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Radiation Exposure
Is radiation a show-stopper
for a trip to Mars?
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Radiation Exposure
Trip segments for dosimetry:
• Transit Van Allen belts (departure)
• Free space baseline (departure)
• Solar flares (CMEs) and galactic
pulses
• Mars stay (ignore for free return)
• Habitat & Rover
• Suit
• Free space baseline (return)
• Solar flares (CMEs) and galactic
pulses
• Transit Van Allen belts (return)
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Radiation Exposure
• Minimum energy transfer roughly 9 months each way
• Assume STS-like free space galactic radiation exposure
of 0.146 mSv/day (doubled due to loss of Earth’s
geometric shielding)
• 270 days times 0.146 mSv/day = 39.4 mSv for 1 way
• News Flash – New data 1.8 mSv/day (12 times above
estimate) – Zeitlin et al., Science 340:1084-1084, 2013
• MSL data: 253 days times 1.8 mSv/day = 455.4 mSv for
1 way, 911 mSv for 506 day free return round trip
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Radiation Exposure
• 2000 standards more stringent than prior standards –
Committee on the Evaluation of Radiation Shielding
for Space Exploration: Managing Space Radiation
Risk in the New Era of Space Exploration, National
Research Council, National Academies Press,
Washington, DC, 2008
• MSL data: 253 days times 1.8 mSv/day = 455.4 mSv
for 1 way, 911 mSv for 506 day free return round trip
• New Standards for 3% Added Risk of Exposure
Induced Death (REID, 95% confidence, assumes
baseline solar and galactic activity)
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Radiation Exposure
Career Limit for 1 yr mission and 3% REID (mSv)
Age (yrs)
Male
Female
30
620
470
35
720
550
40
800
620
45
950
750
50
1150
920
55
1470
1120
MSL data: 253 days times 1.8 mSv/day = 455.4 mSv for 1 way, 911
mSv for 506 day free return round trip with no CMEs
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Radiation Exposure
Trip segments:
• Transit Van Allen belts (departure)
• Free space baseline (departure)
• Solar flares (CMEs) and galactic pulses
• Mars stay (ignore for free return)
• Habitat & Rover
• Suit
• Free space baseline (return)
• Solar flares (CMEs) and galactic pulses
• Transit Van Allen belts (return)
depends on trajectory
1.8 mSv/day times 253 days
probabilistic (see slide 13)
~ 0.4 mSv/day *
0.6-15 mSv/day **
1.8 mSv/day times 253 days
probabilistic (see slide 13)
depends on trajectory
* Estimated average between solar maximum and minimum
** Depends on suit construction, posture, and particulate mix
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Radiation Exposure
• Is it legal? Compare to career limits (3 Sv old, 1.12 Sv
new for 55 yr old female)
• Issues of informed consent and regulatory waiver
• Boost cancer death risk 1.7% for baseline trip to Mars
with old dosage data – multiply old doses by 12 for new
risk calculation
• Boost cancer death risk 25% for continuous 0.146
mSv/day
• Flares and Mars orbit time, surface time additional
• Radiation issues become significant
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Radiation Exposure
The conundrums:
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Are long term space missions legal?
Informed consent vs. legal limitations
Conceive and raise children on Mars?
Conceive and raise children in space habitats?
Remember planets shield by geometry
Large variability in exposure (flares)
Large variability in response
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Radiation Exposure
Orbital (and beyond)
human factors status
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Radiation Exposure
NewSpace awaremess is dismal:
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Assumption that it is accomplished and can be ignored
Lack of appreciation of risks
Wishful thinking and denial trumps reality
Aging normative population undefined
Minimal gravity level is undefined
Radiation issues become significant
Working is microgravity is hard
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Radiation Exposure
Why? Culture shock (engineering vs. biomedical):
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Engineers look for limiting parameters
Engineers design to limiting parameters
Engineers minimize variables
Human responses vary enormously
Human responses probabilistic
Human responses – many variables
Human responses poorly characterized
Never say never in medicine
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Radiation Exposure
Medical issues related to living in space and going to Mars:
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Outside assistance is impossible or very difficult
Life support degradation – toxin accumulation
Acute urinary retention – renal lithiasis
Cardiac event
Microgravity and radiation issues
Cancer (Antarctica example – isolation and treatment)
Drug shelf life (accelerated degradation with radiation)
Medical/surgical infrastructure – how much is enough?
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Opportunities
Many unknowns: What we don’t know can hurt us or provide
opportunities for play/research:
• Microgravity – musculoskeletal, cardiovascular,
reproductive, and immune systems; embryogenesis, fetal
development; aging; optimization
• Radiation – shielding (mass, electrostatic, or magnetic),
abatement (pharmaceutical, antioxidants, modification of
humans – genetic engineering)
• Radiation and microgravity synergisms
• Long term exposure to different gas mixes vs. standard air
• Other – Moon/Mars dust and urban/rural pollution effects
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