radiation biophysics and human spaceflight

Radiation Biophysics and Human Spaceflight

Dr. John M. JuristAdjunct 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

Radiation Biophysics and Human Spaceflight 2

Radiation Exposure

Sources of exposure:

• 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


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


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


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


Radiation Exposure

2 types of ionizing radiation:

• Electromagnetic (photons – gamma rays or X-rays)

• ParticulateRadiation Biophysics and Human Spaceflight 7

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


Radiation Exposure

Particulate:• Neutral particles – such as neutrons• Charged particles:

• Alpha particles (ionized helium nuclei)• Protons (ionized hydrogen nuclei)• Beta particles (electrons) or their anti-

counterparts• Other ionized nuclei


Radiation Exposure

Particulate Cosmic Ray Spectrum:


Composite energy spectrum for particulate cosmic rays

[Microcosm, SME 2011]

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)


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 radiationRadiation Biophysics and Human Spaceflight 12

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]

Radiation Exposure

Units and their evolution


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 airRadiation Biophysics and Human Spaceflight 15

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


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


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


Radiation Exposure

Energy/Mass Bioeffect

100 rad times Q(RBE) 100 Rem1 Gray (Gy) times Q 1 Sievert (Sv)

Relation between modern SI units and older units – 1 Gy = 100 Rads


Radiation Exposure

Radiation Species RBE or Q

Alpha particles (helium nuclei) 20Beta particles (electrons) 1High energy protons (hydrogen nuclei) 10X-rays (photons) 1Gamma rays (photons) 1Neutrons (unknown energy) – conservative 10


Radiation Exposure

RBE or Q for Neutrons

0

2

4

6

8

10

12

1.00E-08 1.00E-06 1.00E-04 1.00E-02 1.00E+00 1.00E+02 1.00E+04

Energy (MeV)

Q


Radiation Exposure

Short term acute whole body exposure (Rems):

10-50 Minor blood changes50-100 5-10% nausea (1 day), blood, survivable100-200 1/4-1/2 nausea (1 day), blood, GI, survivable200-350 Most nausea (1 day), blood, GI, 5-50% die350-550 450 LD50 Most nausea, blood, GI, 50-90% die

550-750 Nausea (hours), blood, GI, almost all die750-1,000 Nausea (hours), blood, GI, fatal (2-4 weeks)1,000-2,000 Nausea (hours), fatal (2 weeks)4,500 Incapacitation (hrs), fatal (1 week)


Radiation Exposure

Radiation biological effects and biophysics


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


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


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


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


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


Radiation Exposure

Chronic exposure examples


Radiation Exposure

• 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


Radiation Exposure

Dose vs Altitude(nonTerrestrial)

1

10

100

1,000

1 10 100 1,000 10,000

Altitude (thousands of feet)

Do

se

(m

icro

Sie

ve

rts

/da

y)


Radiation Exposure

Estimated HTO suborbital flight:

• Upper limit 0.0053 mSv per flight• Compare: Polar airline flight 0.10-0.23 mSv per day


Radiation Exposure

Orbital and beyond:

• 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


Radiation Exposure

Approximate

Dose (mSv/day) Source Average RBE

0.146 Galactic cosmic 2.75

0.077 Van Allen belt 1.83

trapped radiation in LEO


Radiation Exposure

Shielding factors


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


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


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


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


Radiation Exposure

Protective Modality Aluminum Equivalent (g/cm2)

Atmosphere of Earth 1,020

Atmosphere of Mars 22

International Space Station 10

Space Shuttle (STS) 10

Apollo space suit 0.1


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


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


Radiation Exposure

Career limits for radiation workers (1994):

Blood-Forming Organs

Limit at Lens Skin Male Female

Age 25 4.0 Sv 6.0 Sv 1.5 Sv 1.0 SvAge 35 4.0 Sv 6.0 Sv 2.5 Sv 1.75 SvAge 45 4.0 Sv 6.0 Sv 3.2 Sv 2.5 Sv Age 55 4.0 Sv 6.0 Sv 4.0 Sv 3.0 Sv

Historical trend to reduce limits: Compare 1994 to 2000 limitsRadiation Biophysics and Human Spaceflight 43

Radiation Exposure

Radiation carcinogenesis:

0.5/106/mSv/year Breast

0.4/106/mSv/year Thyroid

0.3/106/mSv/year Lung

7-17/106/mSv/year All cancers

100 mSv/105 800 deaths added to 20,000 w/o radiation (4% increment/10

rads)Radiation Biophysics and Human Spaceflight 44

Radiation Exposure

Is radiation a show-stopper for a trip to Mars?


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)Radiation Biophysics and Human Spaceflight 46

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 tripRadiation Biophysics and Human Spaceflight 47

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)


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


Trip segments:

•Transit Van Allen belts (departure) depends on trajectory

•Free space baseline (departure) 1.8 mSv/day times 253 days

•Solar flares (CMEs) and galactic pulses probabilistic (see slide 13)

•Mars stay (ignore for free return)

• Habitat & Rover ~ 0.4 mSv/day *

• Suit 0.6-15 mSv/day **

•Free space baseline (return) 1.8 mSv/day times 253 days

•Solar flares (CMEs) and galactic pulses probabilistic (see slide 13)

•Transit Van Allen belts (return) depends on trajectory* Estimated average between solar maximum and minimum

** Depends on suit construction, posture, and particulate mix

Radiation Exposure


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


Radiation Exposure

The conundrums:

• 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


Radiation Exposure

Orbital (and beyond) human factors status


NewSpace awaremess is dismal:

• 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

Radiation Exposure


Why? Culture shock (engineering vs. biomedical):

• 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

Radiation Exposure


Medical issues related to living in space and going to Mars:

• 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?

Radiation Exposure


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


radiation biophysics and human spaceflight

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