The Rough Guide To The Moon And Mars - CentAUR

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The rough guide to the Moon and MarsArticlePublished VersionLockwood, M. and Hapgood, M. (2007) The rough guide to theMoon and Mars. Astronomy and Geophysics, 48 (6). 6.11 6.17.ISSN 1366 8781 doi: https://doi.org/10.1111/j.1468 4004.2007.48611.x Available athttp://centaur.reading.ac.uk/38537/It is advisable to refer to the publisher’s version if you intend to cite from thework. See Guidance on citing .Published version at: http://dx.doi.org/10.1111/j.1468 4004.2007.48611.xTo link to this article DOI: http://dx.doi.org/10.1111/j.1468 4004.2007.48611.xPublisher: Wiley Blackwell Publishing Ltd.All outputs in CentAUR are protected by Intellectual Property Rights law,including copyright law. Copyright and IPR is retained by the creators or othercopyright holders. Terms and conditions for use of this material are defined inthe End User Agreement .www.reading.ac.uk/centaurCentAURCentral Archive at the University of ReadingReading’s research outputs online

ROUGH GUIDESTHE ROUGH GUIDE toThe Moon& Mars

Lockwood, Hapgood: Human space travelThe Rough Guide tothe Moon and MarsMike Lockwood and Mike Hapgoodrough out essential physics for theastronaut intending to arrive alive.Several space agencies around the world,notably in the US and China, are planning to return humans to the Moon, witha view to being the first also to visit Mars. Thereasons are a complex mix of national prestige,economic spin-offs, technological capabilityand inspiration, in particular of potential youngscientists and technologists. It is impossible toquantify the benefits that have accrued to theUS in the decades following the Apollo lunarmissions – but to doubt that they were of fundamental importance is to fail to understand thetechnological drivers of modern economies.Two people who well understood both theaspirational and inspirational importance ofmanned space travel were John F Kennedyand his brilliant speechwriter Ted Sorensen. Inhis speech at Rice University on 12 September1962, Kennedy delivered the famous words:“We choose to go to the Moon. We choose togo to the Moon in this decade and do the otherthings, not because they are easy, but becausethey are hard.” The end of that speech acknowledged the hazards known at the time: “Manyyears ago the great British explorer GeorgeMallory, who was to die on Mount Everest,was asked why did he want to climb it. He said,‘Because it is there.’ Well, space is there, andwe’re going to climb it, and the Moon and theplanets are there, and new hopes for knowledgeand peace are there. And, therefore, as we setsail we ask God’s blessing on the most hazardous and dangerous and greatest adventure onwhich man has ever embarked.”Since that speech, surely among the most significant in the history of humankind, we haveachieved a much better understanding of boththe inspirational power and the tremendoushazards of space travel. We know of primitive bacteria that might be able to tolerate theenergetic particles that exist outside the twinprotective shields of Earth’s atmosphere andmagnetic field: Deino coccus radiodurans canreadily withstand radiation doses 500 timeslarger than a fatal dose for a human (LevinZaidman et al. 2003). We know of no similarlyrobust complex lifeforms.A&G December 2007 Vol. 48 AbstractSpace is a dangerous place forhumans, once we step beyond theprotection of Earth’s atmosphereand magnetic field. Galactic cosmicrays and bursts of charged particlesfrom the Sun damaging to healthhappen with alarming frequency – theApollo astronauts were very lucky.Understanding the physics of radiationfrom distinct sources in space will beuseful to help future space voyagers planjourneys in greater safety, and produceeffective shields for these unavoidableevents on journeys to Mars or beyond.We need to understand the mechanismsresponsible for the acceleration of harmfulparticles and of their propagation through theheliosphere in order to develop systems and procedures to minimize the health risks of spacetravel. But it is also important to understandhow a parent star and the atmosphere and magnetic field of a planet control the surface particleenvironment, in order to identify planets thatcould support advanced lifeforms. It is oftenassumed that the atmosphere is such an efficientshield for the surface that the magnetic fieldcontribution is insignificant (e.g. Waddington1967). Indeed, for the present-day Earth, cosmicsurface radiation is limited to a very low flux ofmuons; heliospheric and geomagnetic modulations become factors for cosmic radiation dosesonly on high-altitude aircraft (Shea and Smart2001). However, it is thought that the magneticfield has at least a partial role in preventing theloss of the atmo sphere of an Earth-like planet(e.g. Dehant et al. 2007) and hence the atmo spheric shield would also be weaker withoutthe magnetic shield (Grießmeier et al. 2005).Furthermore, because oxygen has a strongmodifying influence on radiogenic mutationrates, changes in the oxygen abundance duringthe history of the Earth means that radiogenicmutation rates in organisms have been up to 2.5times greater than at present for most of the history of life (Karam et al. 2001). Such factors givecredence to the idea that recent observations ofcycles in Earth’s fossil diversity over the past542 Myr with period 62 3 Myr are induced bycosmic rays (Medvedev and Melott 2007). Studies of the importance of Earth’s atmosphericand magnetic GCR shields for sustaining anddeveloping life are likely to play an importantpart in answering the question that science isincreasingly reclaiming from metaphysics: “Arewe alone?”The radiation hazard in spaceThe cover of Douglas Adams’ wonderful Hitchhiker’s Guide to the Galaxy carries (in large andfriendly letters) the words “Don’t Panic”. Whilethis is undoubtedly useful advice under almostall circumstances, it may not be specific enoughfor the space traveller. He or she will need toknow all about the radiation health hazard ininterplanetary space. The two main concerns areGCRs and SEPs. GCRs are galactic cosmic rays:particles ranging from protons to iron and heavier ions, moving at almost the speed of light having been accelerated, for example, at the shockfronts generated by supernovae explosions. SEPstands for solar energetic particle, some of whichcome directly from solar flares, but most are generated in the heliosphere at shock fronts ahead ofCMEs (coronal mass ejections, e.g. Reames et al.1996, Gopalswamy et al. 2 nd CIRs (co-rotating interaction regions, e.g. Mason et al. 1997).It is already clear that the Rough Guide willneed a good glossary of TLAs – Three-LetterAcronyms – for in the Hitchhiker’s Guide, SEPstands for Somebody Else’s Problem. To thoseof us who remain safely in Earth’s biosphere,SEPs are indeed both invisible and somebodyelse’s problem, but to the space traveller they,like GCRs, pose a real danger.The risks associated with GCRs and SEPsare quite different. For GCRs, chronic exposure time is important, whereas SEPs are acutebursts. Most of the biological dose (see “Radiation doses and risks” p6.12) for GCRs comesfrom the heavy ion component of the mass spectrum, not the protons. Heavy ions can generatea large track of damage in biological materialand also generate many damaging secondaryneutrons and ions in surrounding material (Terato and Ide 2004, Antonelli et al. 2004). On thesurface of the Moon, secondary neutrons (thelunar neutron albedo) increase the effective biological dose by 1.5% for SEPs and by between14 and 24% for GCRs, at solar maximum andminimum respectively. The dose from GCRs is6.11

Lockwood, Hapgood: Human space travelRadiation doses and risksrelatively small, only around 18.5 cGy per yearbehind 1 gm cm–2 at solar minimum (when theyare largest), of which only about 7 cGy comesfrom protons (Townsend et al. 1992).Dose and dose rate are, however, importantfor assessing acute radiation sickness fromlarge SEP events. In these cases, despite thepresence of higher mass ions in SEPs, protonsare the bigger concern because their flux is sohigh and they penetrate shielding more readily.The dose received during an SEP event variesgreatly and studies have looked at the “worstcase scenario”, the biggest event that we believehas occurred in the past 400 years (discussedbelow). This analysis takes the spectral characteristics of SEP events in recent times and scalesthem according to the proton fluence derivedfrom ice-sheet measurements (Stephens et al.2005, Townsend et al. 2006). For the shielding of 1 gm cm –2 of aluminium and one of the“harder” spectral shapes observed in recentevents, this event could have given doses tothe skin and bone marrow of up to 12 Gy and0.8 Gy, respectively, in low-Earth orbit, and45 Gy and 2.8 Gy in interplanetary space. Itmay be that the largest fluence events do nothave the hardest spectra, which would reducethese estimates (Townsend et al. 2006). Usingan appropriate relative biological effectiveness,this is a typical skin dose equivalent of 67 Sv, i.e.20 000 years’ dose on Earth’s surface!Galactic cosmic raysGalactic cosmic rays interact with the magneticfields of the Sun and Earth. The open solar magnetic flux, F S , is the total magnetic flux that isdragged out of the solar atmosphere by the solarwind and permeates the heliosphere. Structure6.12 a routine chest X-ray image gives 0.01 cSv,and a CAT scan gives 4 cSv. It is important toconsider not only the total dose but also thedose rate (in Gy s –1). A number of procedureshave been developed to compute the doses,dose rates and dose equivalents in space fora given organ of the human body, and it iscommon to consider the values for the skin,ocular lens, and blood-forming organs (forexample, Townsend et al. 2003, 2006). Suchdoses are evaluated behind different levelsof shielding (e.g. Ballarini et al. 2004), theminimum being 1 gm cm –2 of aluminium,for a (thick) space suit which is the onlyprotection during extra-vehicular activity(EVA). Doses quoted for events are integralsover the duration of the event and thecorresponding integral of the particle flux iscalled the fluence.There are no completely safe levels of1: Model spectra ofgalactic rays in nearEarth interplanetaryspace, fitted toobservations for fourdifferent values ofthe open solar flux,FS. The black linegives the inferredspectrum in localinterstellar space(LIS).10differential number flux (cm–2 s–1 sr –1)It is important to clarify the measuresand units of radiation dose. Absorbed (or“physical”) dose D is the energy absorbed byunit mass of matter due to ionizing radiation.The SI unit of absorbed dose is the Gray (Gy),defined as 1 J kg–1 (but it is also sometimesexpressed in rads 0.01 Gy 1 cGy).The dose equivalent (or biological dose,H) indicates the risk of occurrence ofbiological effects due to the absorbed dose.It is defined as the absorbed dose multipliedby the relative biological effectiveness (RBE)factor (Q), which accounts for the radiationtype (i.e. the energy and mass spectra) andcharacteristics of the affected body organ:H Q D. The SI unit of equivalent doseis the Sievert (but is sometimes given inrem 0.01 Sv 1 cSv). To put these units incontext, 1 cSv is roughly three years’ dosein a typical environment on Earth’s surface,31021010101010LISFS 2 1014 WbFS 2 1014 WbFS 2 1014 WbFS 2 1014 Wb 1 210human exposure to ionizing radiations – wehave to set thresholds to unacceptable risks.The limits for astronauts inside Earth’smagnetosphere (i.e. in low Earth orbit– LEO) are currently set at 0.5 Sv per year(with a lifetime limit that depends on age andsex) and are based on a 3% excess cancermortality risk. This limit for LEO is an orderof magnitude higher than the correspondinglimit for terrestrial radiation workers, e.g. innuclear power plants, because of the shortercareer lengths for astronauts (generallyassumed to be no more than 10 years).Although there are concerns about EVA(Johnson et al. 2005), the shielding requiredto ensure radiation is held below these limitsis readily achieved for an LEO mission.Outside Earth’s magnetic protection, thesituation is very different (Townsend et al.1992, Cucinotta et al. 2005). 3102in the heliospheric field that scatters GCRs isthe crucial component of shielding (e.g. Potgieter 1998). Rouillard and Lockwood (2004)demonstrated that there was an excellent anticorrelation between F S and the cosmic-ray fluxat various energies. Figure 1 shows modelledspectra of GCRs as a function of F S, obtained asdescribed by Lockwood (2006), and shows thatit is the lower energies (below about 10 4 MeV)of the local interstellar GCR spectrum that ismost modulated by F S. Because F S varies withthe decadal solar cycle and on longer timescales(Lockwood et al. 1999, Rouillard et al. 2007),we expect corresponding variations in cosmicray fluxes in near-Earth interplanetary space.GCRs are atomic nuclei, about 85% protons,14% alpha particles and 1% heavier nuclei(Simpson 1983). The fluence distribution ofprotons is 103 times higher than that of, for103energy (MeV)104105example, Fe ions, but the energy deposition– the dose of a single particle – depends on theatomic number squared, a factor of 562 for Fe.In interplanetary space, GCR doses behindjust 1 gm cm–2 of shielding will give a total effective dose at solar minimum of about 50 cSv perannum. At solar maximum this is reduced bythe enhanced heliospheric shield to about 18 cSv.Given that, in Earth’s biosphere, we typicallyreceive 2 mSv per year from cosmic radiation,the effective GCR doses in interplanetary spaceare greater than in the biosphere by factors ofroughly 90 and 250 at sunspot maximum andminimum, respectively. With annual doses below20 cGy, GCRs pose no acute health hazard tocrews on deep space missions, but the concern isfor stochastic effects such as induced cancers andmortality or late deterministic effects (for example cataracts or damage to the central nervousA&G December 2007 Vol. 48

Lockwood, Hapgood: Human space travel2: A large CME observed by the LASCO instrument on SOHO (Solar and Heliospheric Observatory)from near the ecliptic, used here as a schematic illustration of what a CME might look like whenobserved from over the solar pole. Parker spiral interplanetary field lines, perturbed by the CME,are shown in grey and the shock front at the head of the CME in orange. The white arrows denotefluxes of SEPs generated at the shock and propagating along the field lines, both away from andback toward the Sun. The boxes show SEP events observed at three different solar longitudes in:March 1982 (yellow box), August 1998 (green box), and December 1982 (red box). These data aretaken from a variety of interplanetary spacecraft and merged into the intercalibrated “OMNI2”dataset by NASA’s Goddard Space Flight Center. The longitude of the craft relative to the eventon these three days was different in each case, such that as the event moved over them they hadmotions in the event rest frame as shown schematically by the corresponding coloured arrows. Thefluxes of protons in energy ranges are given for: 60 MeV (black), 10 MeV (mauve), 4 MeV (blue), 2 MeV (orange) and 1 MeV (red). The vertical black and orange lines mark the event onset and timeof shock crossing, respectively. (Based on schematics by Cane et al. 1988 and Reames et al. 1996).system) from chronic exposure. Unfortunately,there are no data on the increased probabilityof these effects for prolonged human exposurethat can be used to estimate risks to crews. Riskestimation is mainly based on epidemiologicaldata from atomic-bomb survivors and victimsof nuclear accidents, but these are very limitedanalogues for the space radiation environment.We have data on astronauts returning fromlong-term space missions such as on MIR andthe Apollo missions, but thanks to Earth’s magnetic field and good fortune, respectively, theseare low-level doses. Accelerator experimentshave also been performed with human cells. Theinduction of chromosome aberration is studiedbecause it is thought to be the most accurateand sensitive indicator of genetic mutations, forcancer induction in particular. A special difficulty is the continuous or protracted irradiationwith low doses in space: even the experimentson human cells are necessarily carried out withhigher, shorter doses and then extrapolated.During a mission to Mars lasting 600 days atsolar minimum, there would be an estimated220 proton and 22 He GCR traversals throughthe nucleus of each cell of the human body.Allowing for the mass and energy spectra, thiswould give an effective total dose of 30 cSv. ThisA&G December 2007 Vol. 48 should be compared with estimated reasonablysafe lifetime doses for 55-year-old males andfemales of 30 cSv and 15 cSv, respectively. Inother words, even at the most favourable time,a trip to Mars would use up the lifetime radiation allowance for men and more than doublethat for women. Fujitaka (2005) estimates thata one-way trip to Pluto with maximum possibleshielding would give 70 Sv, roughly equal to acancer therapy dose over the whole body, killingall cells. We simply cannot travel beyond oursolar system until we develop viable shielding.Solar energetic particlesSEP bursts were first detected in ground-basedionization chambers during the large solarevents of February and March 1942 (Forbush1946). Because the flare was the main impulsivephenomenon on the Sun known at the time, itwas natural to associate all SEPs with flares,a confusion that was not clarified until relatively recently when it was termed “the solarflare myth” (Gosling 1993). From radio bursts,Wild et al. (1963) indicated that there were twoclasses of events that are still termed “impulsive” and “gradual”. These authors also notedthat ion acceleration at a shock front was alsoprobably involved; such acceleration is now wellunderstood (e.g. Jones and Ellison 1991).The distinction between impulsive and gradual SEPs has been clarified using the ionizationstates and mass composition (see review byReames 1999a). The fluxes of energetic ions aremuch higher and longer lived in gradual eventsand it is these that pose a health hazard. CMEsare transient events, more common at sunspotmaximum, in which of order 1013 kg of coronalmaterial is ejected into the inner heliosphere,typically moving at 350 km s –1. Following thediscovery of CMEs, it became apparent thatthe gradual SEP events were actually associatedwith the shock front ahead of the CMEs andnot any associated flare (Kahler et al. 1984).Figure 2 shows three SEP events and what wenow understand is their relationship to theCME shock front that generated them. In allthree cases, the largest fluxes are seen when thesatellite is magnetically connected to the strongest part of the shock. For the events on March1982 (yellow box), August 1998 (green box) andDecember 1982 (red box), the peak fluxes areseen, respectively, before, as and after the shockpasses the craft: in the third case the peak fluxesare seen travelling back towards the Sun.The crew of Apollo 16 returned to Earth fromthe Moon on 27 April 1972 after an 11-daymission. Just three months later, on 4 August1972, there was a large gradual SEP event. Fourmonths later, on 7 December 1972, the finalmanned lunar mission, Apollo 17, was launched.Humans have not ventured out of the protectionof Earth’s magnetic field since.Subsequent analysis of potential biologicaleffects on human crews of the August 1972event (e.g. Wilson and Denn 1976, Townsendet al. 1991, 1992, Wilson et al. 1997, Parsonsand Townsend 2000) revealed that skin doses aslarge as 15 to 20 Gy would have arisen behindshielding of 1 gm cm–2 . Even inside a spacecraft,skin doses could have been as much as 2 Gy.In addition, the crew could have received bonemarrow doses of about 1 Gy. Clearly this eventwould have had very severe consequences foreither Apollo 16 or 17 if it had happened whenastronauts were en-route to the Moon or, worsestill, during EVA on the lunar surface. Fig

1962, Kennedy delivered the famous words: “We choose to go to the Moon. We choose to go to the Moon in this decade and do the other things, not because they are easy, but because they are hard.” The end of that speech acknowl- edged the hazards known at the time: “Many years ago the great British explorer George Mallory, who was to die on Mount Everest, was asked why did he want to climb .

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