Конец вечности
Beyond the Downwhen Terminus
Foronething,itwillhelpHarlanunderstandthecontrols."
(SuddenlyHarlanturnedawayfromthewindowsandfixedhisgazeonthecontrols.Acornerofthecurtainofdespairlifted.Whatif--)
TwissellstilllecturedCooperwiththeanxiousoverprecisetoneoftheschoolteacher,andwithpartofhismindHarlanstilllistened.
Twissellsaid,"ObviouslyaseriousproblemwasthatofdetermininghowfarintothePrimitiveanobjectissentaftertheapplicationofagivenenergythrust.Themostdirectmethodwouldhavebeentosendamanintothedownwhenviathiskettleusingcarefullygraduatedthrustlevels.Todothat,however,wouldhavemeantacertainlapseoftimeineachcasewhilethemandeterminedtheCenturytoitsnearesthundredththroughastronomicalobservationorbyobtainingappropriateinformationoverthewireless.Thatwouldbeslowandalsodangeroussincethemanmightwellbediscoveredbythenativeinhabitantswithprobablycatastrophiceffectsonourproject.
"Whatwedidtheninsteadwasthis:Wesentbackaknownmassoftheradioactiveisotope,niobium-94,whichdecaysbybeta-particleemissiontothestableisotope,molybdenum-94.Theprocesshasahalflifeofalmostexactly500Centuries.Theoriginalradiationintensityofthemasswasknown.Thatintensitydecreaseswithtimeaccordingtothesimplerelationshipinvolvedinfirst-orderkinetics,and,ofcourse,theintensitycanbemeasuredwithgreatprecision.
