looking for lurkers: objects co-orbital with earth as …with a signal-to-noise ratio>100. in any...

LookingforLurkers:ObjectsCo-orbitalwithEarthasSETIObservables

JamesBenford

MicrowaveSciences,1041LosArabisLane,Lafayette,CA94549USA

jimbenford@gmail.com

Abstract

Arecentlydiscoveredgroupofnearbyco-orbitalobjectsisanattractivelocationforextraterrestrialintelligence(ETI)tolocateaprobetoobserveEarthwhilenotbeingeasilyseen.Thesenear-Earthobjectsprovideanidealwaytowatchourworldfromasecurenaturalobject.ThatprovidesresourcesanETImightneed:materials,afirmanchor,concealment.ThesehavebeenlittlestudiedbyastronomyandnotatallbySETIorplanetaryradarobservations.IdescribetheseobjectsfoundthusfarandproposebothpassiveandactiveobservationsofthemaspossiblesitesforETprobes.

1.Introduction

Alienastronomyatourpresenttechnicallevelmayhavedetectedourbiospheremanymillenniaago.Perhapsoneormoresuchaliencivilizationwasdrawninrecently,byradiosignalsemanatingfromourworld.Ormaybeithasresidedinoursolarsystemforcenturies,millenniaorlonger.Long-livedroboticlurkerscouldhavebeensenttoobserveEarthlongago.Ifproperlypowered,andcapableofself-repair(vonNeumannprobes),theycouldreportscienceandintelligencebacktotheiroriginoververylongtimescales.Long-livedaliensocietiesmaydothistogatherscienceforthelargercommunicatingsocietiesinourgalaxy.Iwillcallsuchaprobea‘Lurker’,ahiddenobservingprobewhichmayrespondtoanintentionalsignalandmaynot,dependingonunknownalienmotivations.(HereLurkersareassumedtoberobotic.)

Observing‘nearer-Earthobjects’wouldexplorethepossibilitythattherearenearby‘exotic’probesthatwecoulddiscoverorexcite.

1.LurkerHistory

Bracewellfirstadvancedthesentinelhypothesis:thatifadvancedaliencivilizationsexisttheymightplaceAImonitoringdevicesonorneartheworldsofotherevolvingspeciestotracktheirprogress.Sucharoboticsentinelmightestablishcontactwithadevelopingraceoncethatracehadreachedacertaintechnologicalthreshold,suchaslarge-scaleradiocommunicationorinterplanetaryflight[1].Aprobelocatednearbycouldbideitstimewhileourcivilizationdevelopedtechnologythatcouldfindit,and,oncecontacted,couldundertakeaconversationin

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realtime.Meanwhile,itcouldhavebeenroutinelyreportingbackonourbiosphereandcivilizationforlongeras.

In1974LunanhypothesizedthataBracewellprobewasresponsibleforlong-delayedechoesofearlyradiotransmissionswhichwereobservedinthe1920s[2].Thesedelayswerelaterfoundtobebetterexplainedaspropagationphenomenaintheearthmagnetosphere.Suchmagnetosphericductingisthebestunderstoodmechanismforlongdelayedechoes[3].PapagiannissuggestedsearchingforLurkersintheasteroidbelt[4].InDavidBrin’sscience-fictionnovelExistence,LurkersofseveraltypesandgenerationsareresidinginEarthorbitandtheasteroids,wheretheyhavebeenformillionsofyears[5].Healsoauthoredan“AnOpenLettertoAlienLurkers”[6].JohnGertzmadeacaseforprobesinsteadofbeaconsforinterstellarcommunicatingandreviewedwheretheymaybeinourSolarSystem[7].

2.Co-orbitalObjects

LookingforsuchBracewellprobesoffersanumberofadvantagesovertraditionalSETI,whichislisteningtothestars.ApromisinglocationtosearchforLurkersliesamongtheco-orbitalobjects,whichapproachEarthverycloselyannuallyatdistancesmuchshorterthananythingexceptthemoon.TheyhavethesameorbitalperiodasEarth.Theseobjectscouldbenatural,suchasasteroidswhichhavewanderedintothistypeoforbitforaperiodofcenturiestomillennia.Ortheobjectscouldbeartificialinpartorintheirentirety.Artificialconstructscouldbenoticedbyspectroscopyinthevisibleornearinfrared,unlesstheyareburied.Someobjectshaveunusualhorseshoeorbitsthatareco-orbitalwithEarth.Sometimesthesehorseshoeobjectstemporarilybecomequasi-satellitesforafewcenturies,beforereturningtotheirearlierstatus(Figure1).BothEarthandVenushavequasi-satellites,aswellasmostoftheouterplanets.VenusmayhaveTrojans[8].

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Fig.1Horseshoeorbitof2010SO16.EislocationofEarth,SistheSun,wherethereferenceframeco-rotateswiththeEarth'sorbit.Aquasi-satelliteisanobjectina1:1orbitalresonancewithaplanet.Thismeanstheobjectstaysclosetothatplanetovermanyorbitalperiods.Aquasi-satellite'speriodaroundtheSunisthesameastheplanet,buthasagreatereccentricity.Whenviewedfromtheperspectiveoftheplanet,thequasi-satellitewillappeartotravelinanoblongretrogradelooparoundtheplanet(Figure2).Theyarenottruesatellites;theyareoutsidetheHillsphere.(Anastronomicalbody'sHillsphereistheregioninwhichitdominatestheattractionofsatellites,meaningtheHillsphereapproximatesthegravitationalsphereofinfluenceonasmallbodybyamoremassivebody.)

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Fig.2Quasi–satelliteappearstohaveanoblongorbitasseenfromtheplanet.Earth’squasi–satelliteshavebeendiscoveredinthelastdecade;it'slikelythatquiteafewmorewillbefound.Therearethreeconfigurations:‘horseshoe’and‘tadpole’and‘quasi-satellite’orbits(Figure3).Theirorbitsarestableforcenturiesormuchlonger.TheyarenotEarth-crossingasteroids(Apolloasteroids),whichfollowfarlargerorbitsandspendmostoftheirtimefarfromEarth.

Figure3Threetypesofco-orbitalorbits.Examplesofco-orbitals:

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• ‘SecondmoonofEarth’Cruithne(3753),estimatedsize2km,closestapproachtoEarth0.080AU=12Mkm.[9].Originunknown,asitexperiencedcloseencounterwithMars2500yearsago.Itispronouncedkru-EEN-ya.

• ‘EarthTrojan’(2010TK7)size0.3-0.5km,oscillatingaboutSun-EarthLagrangianpointL4,so0.81to1.19AUfromSun,21degreesfromellipticin‘tadpole’orbit,closestapproach0.13AU=20Mkm,50timesMoondistance[10].

• ‘Earth'sConstantCompanion‘2016HO3(469219,nowcalledKamoʻoalewa)is40-100mindiameter.Itiscurrentlythesmallest,closest,andmoststable(known)quasi-satelliteof Earth, discovered in 2016. Minimum distance is0.0348AU=5.2Mkm[11].

• Otherquasi-satellites∗are:(164207),2015SO2,(227810)2006FV35,2013LX28,2014OL339,2010SO16and(469219).

Figures4-6showaspectsoftheirorbits.Figure7showstheorbitsofseveraloftheseobjectsrelativetoEarth.Table1givessomeofthekeyparametersofseveralofthem.Otherquasi-satellites∗are:(164207),(227810)2006FV35,2013LX28,2014OL339,2010SO16and(469219).

Fig.4Cruithneorbitistiltedtotheecliptic.

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Fig.5EarthTrojanorbit[2010TK7].

Fig.6Orbitof2016HO3aroundEarth.

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Figure7showstheorbitsofseveraloftheseobjectsrelativetoEarth.Table1givessomeofthekeyparametersofseveralofthem.

Fig.7OrbitsofseveraloftheseobjectsrelativetoEarthingeometricequatorialcoordinates.TheY-labeloftherightascension(alpha),theunitsarehours(h)notdegrees(o)[11].

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Name OrbitType Size ClosestApproach

Albedo Magnitude

3753Cruithne

Horseshoe 2km 12Mkm 0.365 15.6

‘EarthTrojan’2010TK9

Tadpole 0.4km 1.5Mkm 0.06 20.8

‘Earth’sConstantCompanion’2016HO3

Tadpole 0.04km 5.2Mkm 0.25 24.3

2006FV35 Horseshoe 0.14-0.32km 15Mkm 0.22 21.12014OL339 Tadpole 0.17km 2.7Mkm (0.19) 22.92004GU9 Tadpole 0.22km 0.0004Mkm 0.22 21.12010SO16 Tadpole 0.36km 0.029Mkm 0.084 20.52015SO2 Horseshoe 0.05-0.11km 0.019Mkm 0.108 23.9

• Table1Parametersofseveralco-orbitalobjects.Albedosinparenthesisare

estimatesusedwiththeobservedmagnitudetoderivethediameter.

3.StabilityofCo-orbitals

Thelongtermstabilityoftheseobjectsisanareaofsubstantialstudy[12-15].Alargenumberofhorseshoeco-orbitalsofEarth,quasi-satellites,Trojansandhorseshoeorbits,appeartobelong-termstable.MoraisandMorbidelli,usingmodelsofmainasteroidbeltsourcesprovidingtheco-orbitalsandtheirsubsequentmotions,concludethatthemeanlifetimeforthemtomaintainresonancewithEarthid0.33millionyears[16].Thereareinterestingexamples:OtherexamplesnearEarthare:

• Cruithnemayleaveitsorbit5000yearsfromnowbyinteractionwiththegiantplanets.

• 2010TK9orbitalparametersindicatethatmostprobablytheasteroid

becameaTrojan1.800yearsagoandwillremaintherefor~15,000years,whenitwilljumptoahorseshoeorbit.

• 2016HO3hasbeenastablequasi-satelliteofEarthforalmostacentury,and

itwillcontinuetofollowthispatternasEarth'sclosestcompanionforcenturiestocome.Itneverwandersfartherawaythanabout100timesthedistanceofthemoonorcomescloserthan38timesthat.2016HO3swamintoitsorbitclosetoEarthonlyacenturyago,aremarkablecoincidenceincosmictime.

• 2006FV35hasoccupieditsorbitstateforabout100,000yearsandwillstayinthisorbitforabout800moreyears.

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• 2014OL339becameaquasi-satelliteatleast775yearsagoandwillstop

beingthat165yearsfromnowaftera"close"encounterwithEarthat~0.13AU.Thisquasi-satelliteepisodewillhavehadadurationof<2,500years.ItisthemostunstableoftheknownEarthquasi-satellites.

• Twoclosequasi-satellites,2015SO2and2016HO3,havetheunusual

coincidentalfeatureoforbitsthathavebeenalmostidenticalformanyyearsandapproachEarthatthesametimeeveryyear[11].

4.SETISearchesofCo-orbitals

Severalapproachestostudytheseobjects,startingwithpassiveobservations:

1)Planamultiyearprogramofobservationsbyradioandopticaltelescopesandplanetaryradarsaroundtheworld. Centraltotheprojectwouldbeopticaltelescopes,suchastheLickObservatoryandotherplatformsparticipatingintheBreakthroughListenproject,todiscerntheirsize,shape,rotationperiods,andopticalproperties,suchasspectra.WewouldneedtodiscerntheiropticalspectraouttoatleastJ-band(to1.2µ).ABracewellprobecouldalsogivespecularreflectionofsunlightfromitsartificialsurfaces[17].2) ConductpassiveSETIobservationsofthesenearer-Earthobjectsinthemicrowave,infraredandoptical.3)Useactiveplanetaryradartoinvestigatethepropertiesoftheseobjects.Theseobjectshavenotbeenpingedorimagedbyanyplanetaryradarasyet.Recentdevelopmentsinplanetaryradarshaveshowntheycandetectthepresenceandtrajectoriesofspacecraftinlunarorbit,eventhoughtheirsizeisafewmeters[18].The‘glinty’reflectionsfromspacecraft,largerapidchangesinsignal-to-noiseratio,wouldbeanindicatorofanartificialobject[18].Whethertheseradarsaresensitiveorpowerfulenoughtogetareturnsignalforimagingfromanyofthepresentlyknownco-orbitalobjectsrequiresanalysis.Iestimatethat2016HO3isdetectablewithasignal-to-noiseratio>100.Inanycase,theycan"ping"theobjects,meaningthatasignalreachesthem,butthereturnsignalmaybetooweaktodetectatEarth.IfthereisanETprobethere,itmightsensethatithadbeennoticedbyus.Wecouldimprintamessageonthesesignals.

4)Conductactivesimultaneousplanetaryradar‘painting’andSETIlisteningoftheseobjects.Thiswouldbe‘ActiveSETI’,whichcouldsolicitaresponsefromahypotheticalprobe.Thisdoesnotincurtheobjectionstosendinginterstellarmessages,messagingtoETI(METI),becauseanysuchalienlurkerswouldalreadyknowwearehere.Ofcourse,thisisatveryshortrangecomparedtotheinterstellar

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ambitionsofMETIenthusiasts.WepresumethatLurkersalreadyknowthatwehaveradar,butmightnotrespondtoasinglesimpleradarpaintingsuchaswehavedonetomanyasteroids.Ifwewanttosendamessage,asPaulDaviessuggestedfortheLagrangepointsin2010,howwouldasignalbedesignedtoelicitsucharesponse[4,19]?ThisisthebasicquestionofMETI.Figuringoutthisnear-termpossibleuseofMETIcandrivediscussionandresearchideasinthisfieldbecauseit’saconcreteproblem:whatmessagewoulddrawthemoutoftheirpassivestatetointeractwithus?Onestraightforwardmessagetosendwouldbeaphotographoftheobjectwearesendingthemessageto.Takingthehighestresolutionpicturesofitasitrotateswouldsimplysay“Weseeyou.”

5)Launchroboticprobesandmannedmissionstoconductinspections,takesamples.Forexample,2016HO3atcloseapproachhasarelativevelocityof3-5km/sec,soiswithinpresentcapability.2006FV35requires~12km/sectorendezvous[20].

Perhapsprobesarewaitingontheseobjects,listeningtousandwaitingforustofindthem.Theymayremainsilentandsimplyreportbacktowherevertheycommunicateto.

5.NotFavoredForLurkingNearEarth

ThereareseveralotherregionsthataLurkermightlocate.HereIdescribesomethatarelesspromisingthantheco-orbitals.

• Earthorbit:OrbitsverynearEartharenotstableoverthelongtermduetodrag.Geosynchronousorbitsarestable.Theyarequiteobservablebyopticalandradarmeans,butallthathavebeenseenareours.

• Lagrangianpoints:TheEarth-MoonL4andL5LagrangianpointscontainonlyinterplanetarydustinwhatarecalledKordylewskiclouds.Atleastoneasteroid,2010TK7,detectedinOctober2010,oscillatesabouttheSun–EarthL4Lagrangianpoint(~60degreesaheadofEarth.Seesection2.)ThereareprobablymanyothersattheSun–EarthLagrangianpoints[21].

• Moon:Themoonofcourseisthenearestobject. Fromorbiterswehavephotographsofalmostallthemoonatresolutionsof~1meter,andteamsofpeoplelookingatthemcarefully.Nothinghasbeenseenexceptourownartifactswesentthere.Sothenonewouldhavetopresumethatobserverswouldhavebuiltinaverydisguisedway.(Asin2001,wheretheMonolithwas“deliberatelyburied",toensurethatonlyacivilizationcapableofspaceflightwouldbeabletodiscoverit.)AnypointontheMoonisinshadowhalfthetime,soiscoldandhastohavestoragetooperatethroughthenight

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ifitissolarpowered.Moreover,ifaprobeisgoingtorespondtous,itwouldsurelyhavedonesobynow,respondingthemanytransmissionsthatbeenmadetothemoonforcommunicationswithourorbitersandlanders.

• Earth-crossingasteroids(Apolloasteroids),whichfollowfarlargerorbitsandspendmostoftheirtimefarfromEarth,soarenotasusefulforEarth-observingasaretheco-orbitals.

• Asteroids:SomeoftheswarmofasteroidsintheBeltmightbeusedbyLurkers,buthavemanydrawbacksasalocation:Ofcourse,asteroidsaremuchfurtheraway.AndatseveralAU,thesolarfluxisreducedbyafactorof4-10.Thereforesolarpanelswouldhavetobeaverylarge.Nuclearpoweredsystemswouldbepreferred.(ThisisanotherreasontolurknearEarth:highersolarflux.)Theasteroidsareverycoldforthesamereason.Thatmeansthatmechanicalsystemsandelectricalsystems,aswellaslubricantsandeverythingassociatedwithchemistrywouldbefarmoredifficulttokeepworkingoverlongtimes.However,therearepotentialreasonstolurkthere:(i)availabilityofmetalandvolatile-richmaterials,(ii)safetyfromorbitalperturbationsovermuchlongertimescales...tensorhundredsofmillionsratherthanmeretensofthousandsofyears,(iii)opportunitytoevaluateanewspacefaringcultureinsafetyfromdiscoveryforlongerperiods.

6.DiscussionGeneralObservations:

• AnoverallreasontolookclosertoEarthisthatwehaven'tseenanythingintherestofthesolarsystem.Andwehaven'tseenanythingcommunicatingfromthenearbystarsouttoabout100lightyears.

• Thebasicfactiswedonotknowalienlogic,alieninstinct,alienintentionoranythingelseaboutthem.

• Thecivilizationthatsenttheprobemaybeverylong-lived,meaningthatindividualsmaylivemuchlongerthanwedoorthattheyareinfactAI's.Ifproperlypowered,andcapableofself-repair(vonNeumannprobes),theycouldreportscienceandintelligencebacktotheiroriginoververylongtimescales.Therefore,lookinglocallynearEarthnotonlyexploresanewspace,butalsoDeepTime[22,23].

• Tostudythesenearbyobjectschangesthemeansofinspectionfromlisteningtothestarstoastronomicaldiagnosticssuchasimagingandspectroscopy.Thisapproachinvolvestechniqueswelldevelopedforthestudyofasteroidsandplanets.Thatmeansusingothertechnologiesandotherinstitutionstopursuethissearch.

• Thispossibilityallowsalocaltestofmessaging(METI)prospects.Wecanconstructmessagesandtrythemagainstnearbyobjects,thuscircumventingargumentsthatwemightbenoticedbyorencouragehostileforcesinthe

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stars.TheSETIandMETIlandscapeisthustransformedintoalocalexperiment.ManyoftheargumentsagainstinterstellarMETI(e.g.drawingattentiontoourselves)seemlesscompellingwhenitcomestoattemptedmessagingofpotentialLurkersitesthataresocloseby,sincesuchnearbyLurkerwillalreadyknowofus.

• Civilizationswouldnotneedtobeveryclosetothesolarsystemtosendaprobehereinthelastfewcenturies.Herearesomenumbers:Supposeaprobecanbelaunchedat1/10thspeedlight,whichiscertainlysomethingwehavefiguredouthowtodoconceptuallybyeitherbeam-drivensails(photonbeams,neutralparticlebeams)ornuclearfusionrockets.Travelingover300years,suchprobescouldreachusfrom30lightyearsout.About3000mainsequencestarsliewithin100lightyears,soplentyofstarsthatmighthavecivilization.

• Forsocietieswithtimehorizonsbeyondacentury-longpayoff,lowerspeedslike0.01cdemandamillenniumtotakeupresidenceandstudyEarth.Worldsthattakeevenlongerperspectivescanuselowerspeedsandsolowertheirinitialcosts.Notethatthisimpliesthatbysearchinglocally,weextendourquestoverbothlongdistancesinspaceandgreatspansintime,generalizingtheentireSETIapproach.Evenaliensocietiesthathaveceasedtheirinterstellarinterestsorareevenextinctcanstilltellussomething,throughtheirancientprobes.

Whathavewetolosebycheckingouttheseobjects?Certainlyresourcessuchastimeontelescopes,radioandoptical.Butwewouldbestudyingnewlyfoundobjects,whichcouldwellbeinterestingastronomy.Nobodyhasreallylookedattheseco-orbitals,otherthanorbitalcalculationsandfaintimages.Weknowalmostnothingaboutthem.

7.Conclusion

Co-orbitalsareattractivetargetsforSETIsearchesbecauseoftheirproximity.Weshouldmoveforthrightlytowardobservingthem,bothbyobservingthemintheelectromagneticspectrumandplanetaryradar,aswellasvisitingthemwithprobes.Themastattractivetargetis‘Earth'sConstantCompanion‘2016HO3,thesmallest,closest,andmoststable(known)quasi-satelliteofEarth.GettingtherefromEarthorbitrequiresadelta-vofabout4.5km/sec.ItapproachesEarthannually,inOctober.Chinahasannouncedtheyaregoingtosendaprobeto2016HO3[24].

Acknowledgements IgratefullyacknowledgefundingbytheBreakthroughFoundation.

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ThankstoDominicBenford,GregoryBenford,DavidBrin,KeithCooper,PaulDavies,CarlosdelaFuenteMarcos,JohnGertz,PaulGilster,AlJackson,TomKuiper,JoeLazio,GeoffMarcy,AndrewSiemionandMartySladeforcomments. References

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