ESS 7 Lectures 22 and 23 November 26 and December 1, 2008 Humans in Space Slide Number 2What are some of the Dangers in Exploring the Moon and Mars?Slide Number 4Radiation Damages DNARadiation Doses and RisksSlide Number 7Sources of Human RiskGalactic Cosmic RaysSolar Energetic Particles (SEPs)Effects of SEPsGCRs and SEPsHow Dangerous are SEPs?Probability of encountering SEP versus days beyond the EarthHow much shielding do you need?Historic SEP EventsIs it Possible to Shield a Spacecraft from SEPs?Building a Mini-magnetosphere in the labCan Laboratory Mini-magnetosphere be Scaled to Spacecraft size?How Good is the Simple Model?Comparison with MHD ModelSimulation and MHD plasma valuesStopping a 1MeV ProtonForecasting Space WeatherAn Example of a Space Weather Forecast Model A Coronal Mass Ejection ModelWhite Light Coronagraph Image of CMEPropagation from the Sun to the EarthComparison of ENLIL Simulation and ACE Observations at L1.Comparison of Magnetosphere for Simulations Based on CME model and ACE dataCME Model Driven Simulation and ACE Driven Simulation Gave Different ResultsSlide Number 32ESS 7ESS 7 Lectures 22 and 23Lectures 22 and 23 November 26 and December 1, 2008November 26 and December 1, 2008 Humans in SpaceHumans in Space• Moon in 2015• Stepping Stone to MarsExploration InitiativeWhat are some of the Dangers in Exploring the Moon and Mars?• When high energy particles encounter atoms or molecules within the human body, ionization may occur.– Ionization can occur when the particle is stopped by an atom or molecule. The resulting radiation can ionize nearby atoms or molecules.– Bremstrahlung (radiation released by a “near” miss) can also ionize atoms or molecules.• A rad is the amount of ionizing radiation corresponding to 0.01 Joule absorbed by one kilogram of material.– The rad unit is independent of the type of radiation.– ~100 rads will cause radiation sickness (1Gray (Gy) = 100 rads).– 1 Gy has a high probability of killing a cell by producing a lesion in its DNA.– 1 rad received from x-rays is less harmful than 1 rad from high energy protons.Radiation Doses and RisksRadiation Damages DNARadiation Doses and Risks• The relative biological effectiveness (RBE) of radiation is normalized to 200 keV x-rays. – The biological damage is measured in rem (rem=dose(rad)X RBE).– The SI unit of equivalent dose is the Sievert – rem=0.01Sv = 1cSv.– Electrons, protons, neutrons and alpha particles are the most damaging because they penetrate deeply into human tissue.– 1cSv is three years dose on the surface of the Earth.– A chest x-ray gives 0.01cSV and a CAT scan gives 4cSV.– Values are frequently given as the dose behind 1 gm cm-2 which is roughly the protection of a thick space suit.– Current limits for astronauts are 0.5Sv per year – 3% excess cancer mortality risk.Average Annual Radiation Dose for Average U. S. CitizenSources of Human Risk• Astronauts must worry about a number of sources.– Galactic cosmic rays– Secondary neutrons from heavy galactic ions– Solar energetic particle events (SEPs)– Relativistic electron events (REE)– Passages through the south Atlantic anomaly– Radiation belts.Galactic Cosmic Rays• GCRs are atomic nuclei – 85% protons, 14% alpha particles and 1% heavy nuclei.• At solar minimum the dose behind 1gm cm-2 50cSv/yr• At solar maximum 18cSv/yr• Doses <20cSv/yr pose no acute health hazard.• On a 600 day trip to Mars at solar minimum would use up the lifetime dose of a male and twice the dose of a female (30cSv for men and 15cSv for women).• A trip to Pluto would essentially kill all of the cells in the body.Solar Energetic Particles (SEPs)• There are two types of SEP events– Impulsive and gradual– Fluxes of energetic ions are much higher and longer lived in gradual events. They pose a health hazard. – Gradual SEPS are associated with the shock front ahead of CMEs. (>60MeV black, >10MeV mauve, >4Mev blue,>2MEV orange, >1MeV red) The shock is marked with orange bar.Effects of SEPs• SEP events during Apollo era • Flux of >60MeV ions and skin dose.• Color bars give estimates of the seriousness of radiation.• If astronauts had been at the moon during the August 1972 storms the dose would have been fatal.Skin dose cSVFlux >60MeV ionsGCRs and SEPsNeutron monitor >60MeV SEPs• SEPS and GCRs tend to be anticorrelated.• The CMEs that create SEPs also cause decreases in cosmic rays called Forbush decreases. • CIRs do not create SEPs at Earth but have steepenedenough by Mars orbit to create SEPs.How Dangerous are SEPs?• Fraction of time since 1968 that daily mean flux (>60MeV protons) exceeds horizontal value.• Since daily values they are for a 1 day mission.Probability of encountering SEP versus days beyond the Earth• Based on “space age” statistics• Probability of exceeding annual safety limit is ~100%• Probability of at least one fatal (10cSv) is 10%• Probability of a 2cSv event (35% fatality rate) is 30%How much shielding do you need?• (top) >60MeV flux from SEPs during the August 1972 storm• (bottom) cumulative skin dose behind various shields.• Even with 250 gm cm-2 astronauts would exceed make lifetime limit.Historic SEP Events• (top) Frequency of SEP events in number per solar cycle.• (bottom) >30MeV fluence based on nitrate abundance in ice cores.• Nitrates are formed by ionization by SEPs and precipitated in snow• We are currently in a period with relatively few SEP events. • In 440 years there were 32 events that would have exceeded the fatal skin dose (10Sv) in near-Earth space (one every 13.75 years).Is it Possible to Shield a Spacecraft from SEPs?• The greatest risks are outside of the magnetosphere.• Is a mini- magnetosphere a possible way to protect astronauts?• How strong would B have to be?Bamford, R., R. Bingham and M. Hapgood, A&G, 48,l 6.18, 2007Gargaté, L. et al., arXiv:0802.0107, 2008Building a Mini-magnetosphere in the labspace labBSW10nT 0.01TBmag0.1T 0.5Tnsw5 cm-31012 cm-3Vsw450km/s 400km/sTsw20eV 5eVMCA4.6 0.9Mcs7.3 12.9β0.4 0.005rL469km 20.8cmc/ωpi102km 22.8cmCan Laboratory Mini-magnetosphere be Scaled to Spacecraft size?• MHD theory– Pressure balance at the magnetopause– where B is the magnetic field intensity, n is the density, v is the flow velocity of the solar wind– K is a free parameter that accounts for deviation of B from its dipolar value and deviation from specular reflection at the