BBC Focus 02.2020

FEATURE

INTERSTELLAR

VISITORS

In2017,astronomersidentifieda
cometfroma completely
dierentsolarsystempassing
throughourown.Discoveredby
thePan-STARRStelescopeat
HaleakalaObservatory,Hawaii,it
wascalled‘Oumuamua,the
Hawaiianwordfor‘scout’.Atfirst,
it lookedlikeanasteroidbecause
it didnotdisplaytailslikea
comet,nora surroundinggas
cloudknownasa coma.
Butlater,it begantoaccelerate
ina waythatcouldnotbe
describedbythegravitational
pulloftheSunortheplanets.
Althoughthisledtosomelurid
headlinesaboutit beinganalien
spacecra, suchbehaviouris
preciselywhatcometsdo.It
occursbecausetheheatfromthe
Sunreleasesgasfromtheices
belowthecomet’ssurface,and
theescapinggasactslikea small
rocketmotor.
In2018,anotherinterstellar
cometwasidentifiedcruising
throughtheSolarSystem.Called
Borisov,it madeitsclosestpassto
theSuninDecember 2019 and
wasestimatedtobelosingtwo
kilogramsofdustand 60
kilogramsofwatereverysecond.
TheideaofusingComet
Interceptertoseesomething
fromanothersolarsystemis
clearlytantalisingforthescience
team,buttheyarealsorealistic
abouttheirchancesofactually
targetingone.
“It’sapreyremotechanceto
bequitehonest,”saysDrColin
Snodgrass,anastronomerfrom
theUniversityofEdinburgh.This
is becausewehavenoideahow
frequentlytheypassby.It may
simplybeblindluckthatwehave
identifiedtwoinasmanyyears.
Astronomerswillknowmore
whentheLargeSynopticSurvey
Telescope(LSST) begins work
in 2022.

ABOVEThiscigarshaped

cometis‘Oumuamua,thefirst

everobjectconfirmedas

originatingfromaroundanother

star.Itshighlyelongatedshapeis

unlikeanythingwe have in our

Solar System