The Market Place

           Cometsaresmall,fragile,irregularlyshapedbodiescomposedofamixtureofnon-volatilegrainsandfrozengases.TheyusuallyfollowhighlyelongatedpathsaroundtheSun.Mostbecomevisible,evenintelescopes,onlywhentheygetnearenoughtotheSunfortheSun’sradiationtostartsublimingthevolatilegases,whichinturnblowawaysmallbitsofthesolidmaterial.Thesematerialsexpandintoanenormousescapingatmospherecalledthecoma,whichbecomesfarbiggerthanaplanet,andtheyareforcedbackintolongtailsofdustandgasbyradiationandchargedparticlesflowingfromtheSun.Cometsarecoldbodies,andweseethemonlybecausethegasesintheircomaeandtailsfluoresceinsunlight(somewhatakintoafluorescentlight)andbecauseofsunlightreflectedfromthesolids.Cometsareregularmembersofthesolarsystemfamily,gravitationallyboundtotheSun.Theyaregenerallybelievedtobemadeofmaterial,originallyintheouterpartofthesolarsystem,thatdidn’tgetincorporatedintotheplanets—leftoverdebris,ifyouwill.Itistheveryfactthattheyarethoughttobecomposedofsuchunchangedprimitivematerialthatmakesthemextremelyinterestingtoscientistswhowishtolearnaboutconditionsduringtheearliestperiodofthesolarsystem.

           Everycometthenreallyhastwotails,adusttailandaniontail.Ifthecometisfaint,onlyoneorneithertailmaybedetectable,andthecometmayappearjustasafuzzybloboflight,eveninabigtelescope.Thedensityofmaterialinthecomaandtailsisverylow,lowerthanthebestvacuumthatcanbeproducedinmostlaboratories.In1986,theGiottospacecraftflewrightthroughCometHalleyonlyafewhundredkilometersfromthenucleus.ThoughthecomaandtailsofacometmayextendfortensofmillionsofkilometersandbecomeeasilyvisibletothenakedeyeinEarth’snightsky,asCometWest’swerein1976,theentirephenomenonistheproductofatinynucleusonlyafewkilometersacross.