A survey of Radiation Hazards & Shields for Space Craft & Habitats By Philip Erner pe4828@albany.edu Presented at Institute for Nuclear Theory s Summer School on Nuclear & Particle Astrophysics, University of Washington, Seattle, 10 July 2009
Acknowledgements INT Summer School on Nuclear & Particle Astrophysics organizers and participants U.-Albany physics research advisers Drs. Jesse Ernst, Kevin Knuth and Eric Woods
GCR is the greatest expected threat to long-term human presence beyond earth s magnetosphere -Semkova
Units Dose = energy deposited per gram of material traversed Acute exposure: Absolute dose rad = 100 erg/g gray (abbrev. Gy; photo above left) = 1 J/kg = 100 rad Chronic exposure rem = effect of 100 rad of x-rays sievert (abbrev. Sv; photo above right) = 100 rem Equivalent dose = Dose x generic weight factor for each body tissue Summed over all radiation types > integrated over the whole body OR Effective dose equivalent = dose x specific weight factor for each body tissue Integrated over each organ > sum of all organs
Abbreviations Orbits LEO: low-earth orbit, 200-1000 km Geo: Geostationary orbit, earth rotational period, about 38,000 km EVA: extra-vehicular activity Particle types/events SPE: solar energetic particle event CME: coronal mass ejection GCR: galactic cosmic radiation ERB: Earth s trapped radiation (Van Allen) belts Other particle properties HZE: high charge and energy LIS: local interstellar spectrum Radiation effects LET: linear energy transfer
A simple summary Particle type/event Locations Max flux, (m 2 sr sec) -1 Energy, log(mev) LET or Dose, Gy Quality Factor Optimal Shield Protons ERB, CME, GCR 0-3,, Neutrons Electrons ERB Inner: 1 Outer: 10 0-1 Heavy Nuclei GCR 1-12, 3 max Gamma photons Interstellar flight EVA Solar flare 150 mm Al
Types of radiation ERB - Trapped protons & electrons - Depends on inclination and altitude near Earth -2 bands -worst for Earth escape GCR -protons & heavier nuclei SPEs - Flares & CME - High flux of particles & photons Albedo - Secondaries from GCR & atmosphere - Low threat
Rules of thumb Penetration Z 2 Radiation dose (velocity) 3 Maintenance: redundancy better than EVA repair Prevention: chronic harder than acute Solar cycle is ~11 years. At solar minimum, GCR is enhanced. At solar maximum, CMEs are acute hazards.
National Council on Radiation Protection & Measurements (NCRP) Recommendation for relative biological effectiveness numbers Particle Type RBE 1 to 5 MeV neutrons 6.0 5 to 50 MeV neutrons 3.5 Heavy ions 2.5 Protons > 2 MeV 1.5 Reference: Tripathi
Dose experiments LIULIN (several generations): radiation in atmosphere and space Human Phantom Cosmic Ray Experiments Advanced Composition Explorer (ACE), 1997-2024? - L1 Earth-Sun Alpha Magnetic Spectrometer (AMS) -ISS -2 nd generation launch 2010 July 29 Geostationary Operational Environmental Satellite (GOES) - Weather Interplanetary Monitoring Platform (IMP), 1960s-2000s - Various missions - Several Earth radii
Principal Problems Leaving the Terrestrial shield: outside Earth s magnetosphere, charged particles are not deflected; unknown effects of human exposure, long-term or acute, to cosmic radiation beyond Earth s magnetosphere Unpredictable Solar Particle Events: Late warning; characteristics variable Weight: Heavy shield cannot be launched from Earth
What s safe? NCRP to NASA: < 3% lifetime cancer increase Wolfson: 4 Sv in a relatively short period kills half of the people exposed. Conservative: 5 rem/life, 0.1 rem/year Perhaps 200 rem/several years Permissive: 0.5 rem/year Perhaps 500 rem/life
Hazards to Humans & Domestic Animals (& Space Craft) Laika, 1st Earth-orbiting passenger
LEO ISS from Space Shuttle Atlantis, 2007 -Background: 1 rem/day -Van Allen Belts: South Atlantic Anomaly -Solar flare: 100s-1000s rem / several hours -Inclination: stronger field; avoid SAA -Altitude: closer to Van Allen
Van Allen Belts
Trans-lunar passengers FORE! Alan Shepard Outside magnetosphere Lunar albedo Through Van Allen belt One week
Aluminum: NASA standard space craft shield Secondary radiation Differential shield of the space craft
Options for Space craft Shield Electric-magnetic -1 GV - + attracts - -neutrons Material: Light weight, total and by Z -Liquid hydrogen / Water -Polyethylene
She turned me into a newt! What s so bad about neutrons?
Bundled Bricks, Folded Fibers: Reinforced Polyethylene - Blocks / fragments more radiation - Half the weight of aluminum - Bonus: Deflects micrometeorites Study by Raj Kaul, Marshall Center
Dosimeter: radiation detector Active: heavy, powered, time included Passive: lightweight; unpowered; no time
Record events 1859, Carrington 1956 February 1972 August, between Apollo 16 and Apollo 17 1989 September-October, aboard Mir 2000 July 14, Bastille Day CME 2001
Zubrin s Interplanetary Proposals Mars Direct - Launch: conjunction Jovian moons - Callisto OK; Ganymede with heavy shielding - Io, Europa impossible
Interstellar Overdose 10 x speed = 1000 x rads! Gamma Photons, Lethal Protons at 0.01c: 50 kev/particle = 1 rad/s in top mm - skin-safe at 0.1c: 5 MeV/particle = 10000 rad/s - spacesuit/plastic OK
SUMMARY Particle -Type -Location -Energy Hazard -Equipment -Biology Shield -Material -Reliability
References Benton, E. R., and Benton, E. V. Space radiation dosimetry in low-earth orbit and beyond. Nuclear Instruments and Methods In Physics Research B. 184 (2001), p. 255-294. Gaisser, T. K., and Stanev, T., eds. Cosmic Rays, PDG,(2008), ch. 24. Hannah, E. C. Radiation Protection for Space Colonies. JBIS,(1977), p. 310-313. Kline, R. L. Habitat Requirement, Design and Options. In: Human Factors of Outer Space Production, AAAS Symposia (1980), p. 79-96. Mauldin, John H. Prospects for Interstellar Travel. Science and Technology, vol.80 (1992). MSC, Environmental Factors Involved and the Choice of Lunar Landing Sites, NASA Project AWP No. 1100, (22 Nov. 1963), p. 6-7. Pinsky, L., NASA s interest in Cosmic Ray as a radiation hazard to a human presence in space. In: Il Nuovo Cimento, Vol. 120B, N.6-8, (2005), p. 909-914. Semkova, J., et. al. Experiment for Radiation Dose Measurements in a Human Phantom IEEE (2005). In: RAST conference proceedings. Townsend, L. W. Implications of the Space Radiation Environment for Human Exploration in Deep Space. In: Radiation Protection Dosimetry, Vol. 115, No. 1-4, (2005), p. 44-50. Tripathi A., Nealy, J., Mars Radiation Risk Assessment & Shielding Design, IEEE. In: IEEEAC paper #1291, V4, (23 Nov. 2007). Wolfson, R. Energy, Environment & Climate. W.W. Norton (2008), p. 203-207. Zubrin, Robert. The Case for Mars. (1996) p. 114-121. Zubrin, Robert. Entering Space. New York : Penguin Putnam (1999), p. 166-168. http://www.nasa.gov/vision/space/travelinginspace/radiation_shielding.html Edit: 14 Jan. 2004. Access: June 2009.