BBC Focus 02.2020

“THE SURFACE OF THE

COMET ACTUALLY CHANGED

AS WE WERE WATCHING

WITH ROSETTA”

2 Agency(ESA).It gaveusthefirstimageofa
comet’sicynucleusandshowedthat gaswas
shootingintospacefromjetsonthesurface.
ESA’s more recent cometmission, Rosetta,
rendezvousedwiththecomet67P/Churyumov-
Gerasimen ko, a nd followed it for two yea rs
between 2014 and2016.It revealedunprecedented
informationabouthowa cometbehavesasit draws
closetotheSun,andthenheadsintodeepspace
again.Onethingbecameveryclear.“Therearea
lotofevolutionaryprocessestakingplaceonthe
surfacelayersofa comet.Thesurfaceactually
changedaswewerewatchingwithRosetta,”saysDr
ColinSnodgrass,anastronomerfromtheUniversity
ofEdinburghwhowaspartoftheRosettateam.
Theywatchedasareaswereerodedaway,while
otherswereburiedinmaterialfallingbacktothe
surface.“Wealwaysknewthatcometserodedbut
theextenttowhichtheychangedeachtimethey
passedtheSunwassomethingthatwe really learned
from Rosetta,” says Snodgrass.

THEREALROCKSTARS
That’sbothgoodandbadnewsforastronomers.
Whileit meansthatweknowmoreabouthowa
cometworksthaneverbefore,it alsotellsusthat
theirsurfacesarenota pristinerecordof theoriginal
planetarybuildingblocks.Allof thecometswehave
visitedsofararecalledshort-periodcomets.They
areancientobjectsthatformedintheouterreaches
oftheSolarSystem,beyondthegiantplanets,and
havesincefallenintocloserorbitsaroundtheSun.
Heretheyhavebeenperiodicallysunblastedand
altered,perhapsbeyondrecognitionfromtheir
originalstates.
Thankfully,forhelpingusunderstandhowthe
planetsformed,thereisanotherclasscalledthe
long-periodcomets.Theseareongiganticorbits.In
1997,cometHale-Boppappearedinourskiesand
wasvisibletothenakedeyefor 18 months.Based
onitscurrentorbit,it mayhavelastbeencloseto
Eartharound4,200yearsago.A yearbefore,in1996,
cometHyakutakepassedbyonanorbitthatwill
takearound70,000yearstocomplete.Allofthese
cometshavesurfacesthatmustbemuchcloserto
theirpristinestates.
Thereisalsotheoccasionalcometthatfalls
backtowardstheSunforthefirsttime.Theseare
theoneswereallyneedtolookatif wearegoing
tounlockthesecretsoftheearlySolarSystem.
Aswellasbeingpristine,astronomersthinkthat
theyoriginallyformedbetweenthegiantplanets,
making them much more representative of the

BELOWThedumbbell

shapeofcomet67P/

Churyumov-Gerasimenko

is theresultoffusion

betweentwocomets,

capturedherebythe

Rosea mission in 2015

actualbuildingblocksthatmadeuptheplanetsin

thefirstplace.Gravitationalinteractionswiththe

newlyformedplanetsthenthrewthemouttovast

distances,wheretheintensecoldhaspreservedthem.

If wecouldseeone upclose,it wouldbe an

opportunitytoexaminea completelydifferent

regionoftheearlySolarSystem.But wenever

knowwhentheyarecoming,andspacecrafttake

thebestpartofa decadetodesignandbuild,by

whichtimethecomethasbeenandgone.So,how

willweevergettoseeoneupclose?

That’s where Comet Interceptor comes in. ESA

FEATURE COMET MISSIONS