ESS 7 Lectures 22 and 23 November 26 and December 1 2008 Humans in Space Exploration Initiative Moon in 2015 Stepping Stone to Mars What are some of the Dangers in Exploring the Moon and Mars Radiation Doses and Risks 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 Damages DNA Radiation 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 Citizen Sources 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 cSV Flux 60MeV ions GCRs and 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 steepened enough by Mars orbit to create SEPs Neutron monitor 60MeV 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 minimagnetosphere 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 2007 Gargat L et al arXiv 0802 0107 2008 Building a Mini magnetosphere in the lab space lab BSW 10nT 0 01T Bmag 0 1T 0 5T nsw 5 cm 3 1012 cm 3 Vsw 450km s 400km s Tsw 20eV 5eV MCA 4 6 0 9 Mcs 7 3 12 9 0 4 0 005 rL 469km 20 8cm c pi 102km 22 8cm Can Laboratory Mini magnetosphere be Scaled to Spacecraft size MHD theory Pressure balance at the magnetopause 16 p B 2 2 0 0 KB 2 rmp 2nm v 2 where B is the magnetic field intensity n is the i 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 2 p nm v i magnetopause dyn How Good is the Simple Model It isn t clear that the MHD model is good since the Larmor radius of energetic ions is comparable to the mini magnetosphere Used a very sophisticated hybrid simulation to model the lab results and test the MHD model Comparison with MHD Model MHD model is the solid line Symbols give the results from the simulation B versus distance Excellent agreement at low B At B 0 2T the simulation gave rmp 26 7 2 5 mm compared with experiment rmp 28 5 mm The MHD works well Simulation and MHD plasma values For density rmp n 1 6 For velocity rmp v 1 3 means larger changes occur for velocity changes For a magnetic field as large as the present simulation the MHD results say the magnetic field should stand off the solar wind at a distance of n nsw 1 6 76 The stand off distance would be a few meters Stopping a 1MeV Proton While our magnetic field could stand off the solar wind it would take more to stop a 1MeV proton Plasma injection can change the fall off to 1 r with 3 Assume the shielding field can be made to fall off as 1 r For efficient deflection we need the Larmor radius to be about 1 5 the distance to the spacecraft A magnetic field of 0 72T would be required This could be accomplished with a 1m current loop and a magnetic moment M 7 2X106 Am2 Forecasting Space Weather Like with terrestrial weather some problems could be lessoned by …