英语作文journettothewest

Ourphysiologicaljourneyintospaceactuallybeginsseveraldaysbeforelaunch.Toavoidlastminuteexposuretocoldsandflu,youandyourcrewmateshavebeeninpartialisolationatKennedySpaceCenter.Duringisolation,youhavegraduallyadjustedyourcircadianrhythmtomatchthesleep-restcyclerequiredforthemission.

Now,arookieastronaut,youhavealreadyboardedtheSpaceShuttleOrbiterand,dressedinanorangelaunchandentrysuit,lieinareclinedpositionstrappedintooneofthemiddeckseats.Thechangeinyourorientationwithrespecttogravityproducesaheadwardfluidshift.Duringlaunch,youexperienceasmuchas3Gsofacceleration,butthereclinedpositionpreservesbloodflowtoyourbrainandyouremainconsciousandalertthroughoutthenineminutesittakestoreachorbit.

Onentryintoweightlessness,thelackofahydrostaticgradientcausesacontinuationofthisheadwardfluidshift,leadingtopuffinessofyourfaceandafeelingoffullnessinyourhead.Yourbodyperceivesthisfluidshiftasexcessivefluidvolume.Togetridoftheexcessfluid,sodiumexcretionisincreasedleadingtoincreasedurineflow.Inaddition,stimulationofthehypothalmicreceptorsreducesthirstandsoyoudrinklesswater.

TransitioningfromEarth'sgravitytoweightlessnessdisruptsyourbody'sspatialorientationandposturecontrolsystems.Youexperiencedisorientationandmotionsicknessduringthefirstfewdaysinspace.Youalsofindthatthelackofregular24hourcyclesoflightanddarkdisruptsyourcircadianrhythm.Combinedwithabusyworkschedule,crampedquartersandtheotherphysiologicalstresses,whichdisruptsyoursleeppatternsandyouloseabout2hoursofsleepadayforashortdurationspacemission.

Yourmission,however,islong-durationandinvolvesathreemonthstayontheInternationalSpaceStation.Afteramonthofweightlessness,additionalchangesinphysiologicalsystemsbegintobecomeapparentasyouarefullyadaptedtothemicrogravityenvironment.Younowgetaroundusingmostlyyourarmsandyoudon'tneedtouseyourlegstosupporttheweightofyourbody.Thereducedworkloadonweight-bearingmusclesleadstomusclewasting(oratrophy)withanassociatedstrengthlossthatcouldbe30%to40%.Thesmallforcesusedtogetaroundrequirereducedgrossmuscleactivityandincreasedfinemuscleactivityleadingtoaconversionofmusclefibersfromslowtwitchtypetofasttwitchtype.Measurementsofastronautintravehicularactivity(IVA)revealsaverageforcemagnitudesof9N(2lb)withmaximumlodsof60N(13.5lb).Yourheartnolongerhastopumpbloodagainstthepullofgravityandthereducedloadleadstoatrophyofyourheartmuscle.Lackofweightbearingalsoreducesstresslevelsinyourbonesleadingtobonebreakdownandreleaseofcalcium.Lossofbonemineraldensityatarateof1­2%permonthincriticalareassignificantlyreducesthestrengthofyourbones.

Throughsomecombinationoffactors,yourimmunesystemgetsweaker,andyourealizethatyouarenowmorepronetoinfectionandperhapsevenatriskforreactivationoflatentviruses.

TheseprofoundchangesinyourbodyinresponsetotheweightlessenvironmentcanposeserioushealthrisksuponreturntoEarth.Formostphysiologicalsystems,yourbodyreadaptstothe1-genvironment,althoughoftenataslowerratethanonentryintoweightlessness.Youmightexperienceorthostaticintolerancebroughtonbythehydrostaticgradientandareducedfluidvolume.Yourmusclestrengthwillprobablyreturntonormalwithin4-8weeks.Yourbonemassshouldalsorecoverwithinapproximately8monthsorayear,butmayneverrecoverit'soriginalstrengthforthesameamountofmass.Youmayalsoencounterdifficultieswithposturalequilibriumduetoweightlessnessinducedchangestoyourneurovestibularsystem.

Researchneedstobeperformedtoassessastronautperformanceforlong-durationspacemissionswithparticularemphasisondevelopingcountermeasurestospaceflight-induceddeconditioning.Pre-,inflight,andpostflightinvestigationsofastronautsexposedtolong-durationspaceflightaboardtheInternationalSpaceStationwillhelpanswerthequestionofwhatarethebasicmechanismsunderlyingadapationtomicrogravity.Thisinformationhasimplicationsforfuturecountermeasureinterventions.Iflong-durationspaceflightleadstogeneraldeteriorationinastroanutperformancetheninflightcountermeasuresthatenhanceperformancecanbeenvisioned.Somecombinationofexercise,medication,andperhaps,artificialgravity,willbeprescribedascountermeasuresforlong-durationspacemissions,especiallyforfuturemissionstothemoonorMars.