David M. Klaus, Ph.D.Assistant Professor, Aerospace Engineering Associate Director, BioServe Space Technologies

ASEN 5335 Guest LectureRadiation Effects on Astronauts

April 10, 2003

VacuumTemperature Extremes ~ -120 to +110°C in LEO

MicrometeoroidsWeightlessness

Radiation - Electromagnetic and Particulate

Challenges to Staying Alive in Space

R a d ia tio n D an g ers to A s tro n a u ts

B etw e en A p o llo 1 6 an d 1 7 ,o n e o f th e la rg es t so la r p ro to nev e n ts ev e r re co rd e d a rr iv eda t E a rth . T h e rad ia tio n le v e lsan a s tro n au t in s id e a sa te llitew o u ld e x p e rie n ce d u r in g th isev e n t w ere s im u la ted . E v e nin s id e a sp a ce c ra ft, a s tro n a u tsw o u ld h a v e ab so rb ed le th a ld o ses o f rad ia tio n w ith in 1 0 h rsa f te r th e s ta rt o f th e e v en t(4 0 0 0 m S v ).

•Electromagnetic (EM) Radiation

Characterized by and C, high flux, low energy

- waves or streams of massless particles traveling in a wave-like fashion and each carrying energy (photons)

Decay at 1/x2 from the source

Primary source is the sun (solar wind)

Radio, light, X-rays, IR, extreme UV

Gamma () rays are most abundant and originate outside the solar system

•Particulate

Characterized by mass and 2, low in flux, high energy

HZE particles (cosmic sources) High Mass and Energy (Z = atomic number, E = energy)

SEPs – Solar Energetic Particles, also SPEs Solar Particle Events

GCRs – Galactic Cosmic Radiation (neutrons, protons & nuclei)median velocity is ~0.95 C

Electrons, protons, neutrons and nucleiRanges from helium to uranium, with peak in abundance of iron - helium nuclei (solar and galactic)

- electrons (sun and van Allen belts)

ionizing radiation – dislodged electron, capable of producing charged atoms (ions) as passes through matter

UV is non-ionizing

 Trapped Belt Radiation

Van Allen Belts (inner 1-3x Earth Radii, outer 4.5-10x)

- Verified by Explorer I (31 Jan 58)

SAA – cusp in VA Belts -        

90% of exposure in LEO occurs in the SAA region

Solar Flares-         EM waves reach Earth in ~8.5 min-         Magnetic cloud (particulates) reaches Earth 2-3 days later-         Solar Max moderates GCRs but increases SEPs

Units

RAD = Radiation Absorbed Dose (amount of energy absorbed in the body by radiation)

RBE – Relative Biological Effectiveness (varies dependent on type of radiation)

Roentgen – basic unit for measuring amount of radiation exposure

REM (Roentgen Equivalent, Man - measure of biological effect)REM = [dose, RAD] x RBE = ~1.3 RAD

SI: Sievert (Sv) = 100 REM

1 RAD = absorption of 100 ergs/gm = 0.01 Gray (Gy) or 10 mGy (SI)

1 Gy = absorption of 1 J/kgmGy = 0.1 RAD

Biological Risks• Primary biological risk from space

radiation exposure is cancer• When radiation is absorbed in

biological material, the energy is deposited along the tracks of radiation.

• Neutrons and heavy ions produce much denser pattern of ionization causing more biological effects per unit of absorbed radiation dose.

• Secondary concerns such as cataracts are beginning to receive more administrative attention

During violent solar events,the Sun can accelerate

electrons and protons toalmost the speed of lightwhich gives them huge

amounts of energy. Protonsand electrons at these

high energies can be verydangerous to living cells

Lethal Dosages of Radiation

1 sievert=1000 millisievert=100 REM

Two major factors in the determination of radiation damages: (1) total dose over the life of a material, (2) dose rate, the rate at which energy is deposited.

Different materials have different susceptibilities to damage:

Biological EffectsTwo general categories: somatic (exposed individual) and genetic (hereditary effects)Different types of radiation produce different amounts of damageHZE and low energy protons > electrons and high energy protonsHigher rate of energy loss per length of track Linear Energy Transfer (LET)Tissue effects = thermal, chemical, cellular and geneticSensitivity proportional to complexity e.g. eyes > skin > boneSymptoms: nausea, vomiting, illness, death

~RADs required for inactivation/death        Molecules 107

        Viruses 105

        Bacteria 104-106

        Mammalian cells 100-104

        Mammals 320-540

How Much is Too Much ??

LD50 for humans = 320-540 RAD

Sickness can occur at 25-30 RAD

1 sievert (Sv)=100 REM

“ When the intensity of relativistic electrons is greatest, a single ill-timedEVA could deliver a radiation dose big enough to push an astronaut over the short-term limit for skin and eyes. “

Recommendation 3c: A project should be initiated to develop a protocol for identifying the conditions that produce highly relativistic electron events based on the demonstrated good correlation between changes in solar wind conditions and the onset of such events.

NRC Report (2000)

Ave exposure in the US: ~40 mREM / year (soil, rocks, wood, etc.)

East coast ~20 mREM / yearRocky Mtn area ~90 mREM / year

Cosmic Rays: add ~40 mREM / year (~160 mREM high in the Rocky Mtns)

Food and water: add ~ 20-50 mREM / year

NY to Paris flight: add ~4 mREM

~ 100 mREM / year compared to ~65 – 195 mREM / typical shuttle flight

Biological effects are cumulative

Effects are acute (early) or chronic (late)tissue damageloss of fertilitylens opacificationcancer inductionheritable effects

Sunburn melanoma

Carcinogenic effects are of great concern

Proliferating cells of renewing tissue & organs are most sensitive - bone marrow, lymph, intestine and reproductive organs

Younger people and women are more susceptible to radiation damage in general- Youth have longer for potential damage to develop- Women have 2 radiation sensitive organs (breasts and ovaries) and

longer expected life span than men

PARTICLE ENERGIES OF CONCERN

EVA

• EVAs - additional radiation exposure concern

– Lower shielding

– Eye dose

– Skin dose

• 51.6 degrees, new concern for electron events

– One area where there are no currently a good model.

• 140 more EVAs are planned for ISS completion

• Legal and moral reasons require NASA limit astronaut radiation exposures

• U.S. Occupational Safety and Health Administration officially classifies astronauts as “radiation workers”

• Adherence to ALARA (As Low As Reasonably Achievable) is recognized throughout NASA’s manned space flight requirements documents– Radiation protection philosophy--any radiation exposure results in some risk

• ISS astronaut exposures will be much higher than typical ground-based radiation worker– Astronaut legal dose limits (In BFO: 50 REM/yr and 30 REM/mo) are 10 times that

allowed ground based radiation workers

• Space radiation more damaging than radiation typically encountered by ground-based workers

DOSE FACTORS

• ISS originally planned at 28.5 degrees latitude, now at 51.6

• Dose received is a factor of:

– Altitude

– Attitude

– Shielding

– Solar Cycle

– Time in orbit

RADIATION MONITORING SYSTEM

Current Research Summaries

EVA Radiation Monitoring Experiment on ISS

The purpose of EVARM is to carry out flight experiments to characterize the radiation doses experienced by astronauts during extravehicular activity (EVA). These measurements will include doses to skin, eye, and blood-forming organs and will be carried out using a relatively new type of electronic radiation dosimeter, the Metal Oxide Semiconductor Field Effect Transistor (MOSFET).

The three EVARM radiation badges, which are less than 8 centimeters long and 3 centimeters wide, are small enough to fit comfortably into pockets placed inside TCUs. The badges use tiny MOSFET chips to read radiation dose. [NASA/JSC]

http://spaceresearch.nasa.gov/research_projects/ros/evarm.html

Radiation exposure—and the subsequent physiological damage—is one of the primary hazards faced by long-duration space crews. Because neutrons do not carry a charge, they are able to deeply penetrate the body, potentially damaging blood-forming organs. In low Earth orbits, the contribution of the neutron component is estimated to be about 20 percent of the total radiation. Characterization of this radiation environment will help scientists develop safety measures to protect space crews.

http://spaceresearch.nasa.gov/research_projects/ros/bbnd.html

Bonner Ball Neutron Detector (BBND)

BBND Detector Unit opened to show interior structure. A metal counter sits inside a white sphere of polyethylene plastic. [NASA/JSC]

solar-neutron (from the Sun.), albedo-neutron (produced by collision of high energy proton and gas particle in atmosphere), and local-neutron (produced by collision of high energy proton and spacecraft material)

http://sees.tksc.nasda.go.jp/English/WhatsSEES/bbnd.html

BBND (BONNER BALL NEUTRON DETECTOR)

Dosimetric Mapping (DOSMAP)

In order to better understand the internal environment of the ISS, this experiment will map the radiation levels throughout the Station and in the immediate vicinity of each crew member. The measurements will be taken using Thermo Luminescent Dosimeters (TLDs). The resulting data will help determine the best radiation shielding locations on board the Station, providing the crew with the best possible protection during unusually high levels of radiation due to solar flares or other cosmic phenomena.

http://spaceresearch.nasa.gov/research_projects/ros/dosmap.html

Phantom Torso (TORSO)

http://spaceresearch.nasa.gov/research_projects/ros/ptorso.html

Fred the Phantom Torso is an anatomical model of a torso and head containing more than 300 radiation sensors. [NASA/JSC]

Currently, both experimental and operational methods for assessing radiation levels are limited to the measurement of surface (skin) levels. Internal level estimations can only be done by calculations using the radiation environment model and the appropriate radiation transport models. They do not account for the diffusion or redirection of particles as a result of other surfaces, namely the spacecraft walls, interior air, or human skin, and do not include the production of secondary particles within the spacecraft and the human body.

Countermeasures

Distance from local source (decays at 1/x2)

Timing of exposure (when radiation is least intense) e.g. no scheduled EVAs over SAA

Shielding (but secondary effects…) evaluation of effectiveness is complex and depends on actual composition of the impacting radiation

Pharmaceutical treatment

Shielding stops or alters the trajectory of high energy particles before the reach humans

In general, the more dense a material, the more effective it is as shielding

Protection against non-ionizing radiation is relatively simple, but ionizing sources create secondary and tertiary particles, some of which produce gamma rays

Low (equatorial) orbits are considerably less hazardous than polar orbits

CONCLUSIONS

• Of all the risks encountered by astronauts during space flight, cancer induction from radiation exposure is one of the few that persists after landing

• During construction of the ISS, there is a relatively large probability that EVAs will coincide with a radiation enhancement in the belts

• Issues for Mars Mission include: monitor and forecast, storm shelter, effects on humans, evaluation of risk, pharmaceutical intervention and/or shielding

• Liquid Hydrogen likely best shielding candidate