verysoofteninourSolarSystem,a
cometswingsinfromtheouterreaches
andmakesa briefvisittotheinner
portionofthesystembeforeheading
outagain– eitherona longsweepingorbit,ornever
tobeseenagain.Suchcomets,knownaslong-
periodcomets,areofgreatinteresttoscientists,but
we’veonlyeverseenthemfromafar.Whatifwe
couldseeoneupclose?
That’stheideabehinda newmissioncalled
CometInterceptor,whichinJune 2019 wasselected
bytheEuropeanSpaceAgency(ESA)forfurther
development,witha potentiallaunchplanned
in2028.Themissionwillbeunprecedented,
launchingwithnoparticulartargetinmind,but
insteadwaitinginspacefora suitablecometto
approachfromtheOortCloud,thevastarrayof
trillionsofcometsthoughttosurroundourSolar
System.Ifonegetsnudgedfromitsorbitanddoes
approach– andit’shopeditwill– thespacecraftwill
raceoffandf lypast,returningimagesanddatato
Earthintheprocess.
“It’sa uniquetarget,”saysDrColinSnodgrass
fromtheUniversityofEdinburghintheUK,the
deputyleadontheCometInterceptormission
proposal.“Goingtoa dynamicallynewcometfrom
theOortCloudissomethingwe’veneverdone
before.Andit’ssomethingwecouldn’tnormally
conceiveofdoing,becausethesecometsare
discovereda coupleofmonthsoryearsbeforethey
comepast,andthat’stooshorttolauncha mission.
Soit’suniquebecausewe’reabletowaitinspace
andinterceptthisnewtarget.”E
Comet Interceptor is known as an F-class
mission, or ‘fast’ mission. This isn’t in reference to
its plans in space, but rather its development time.
Having been selected, the team will now plan to
have the mission ready to launch in eight years.
The mission will have a mass of less than 1,
kilograms and will head to space in 2028 as a
secondary mission on launch of the Atmospheric
Remote-sensing Infrared Exoplanet Large-survey
(ARIEL) telescope, designed to hunt for planets
beyond the Solar System.
The two missions will be sent 1.5 million
kilometres (1 million miles) from Earth beyond the
Moon, to a location known as Lagrange point 2 (L2).
Here the gravity of the Moon and Earth essentially
cancel out, allowing spacecraft to remain in one
position in space without expending too much fuel.
While ARIEL will be deployed here permanently to
look for exoplanets, Comet Interceptor will be ready
and waiting for its mission to begin.
In the build-up to the mission, and after its
launch if necessary, the team will be keeping a very
close eye on the skies to look for potential targets
to visit. They will make use of vast surveys of the
sky like the Large Synoptic Survey Telescope (LSST)
in Chile, which will switch on in 2020, to look for
any long-period comets entering the Solar System.
The team will be on the hunt for a comet that
approaches roughly to Earth’s orbital plane, which
would make an interception possible.
“It has to get close to Earth’s orbit,” says
Snodgrass. “It doesn’t necessarily have to get
close to Earth, but it has to come to about oneWhystuudy
a comett?
What we can learn about
these icy bodies as they
make their way around
theSun
Water, water everywhere
Comets contain vast amounts of
water-ice, both on their surface
and underground. We think they
may have been responsible for
delivering water to Earth in its past,
so studying them can tell us more
about whether this is true or not.
Origins of life
It’s also thought that comets may
contain some of the key ingredients
for life, such as amino acids. Again,
like water, it’s possible that these
ingredients were brought to the
young Earth by comets hitting our
planet – perhaps giving rise to all life
as we know it!
The giant planets
As comets are icy, they likely formed
in the Solar System’s outer reaches
early in its history. This is the same
location that some of our larger
planets like Saturn and Neptune
are thought to have formed, so by
studyingcometswecanlearnmore
aboutthoseworlds.
Whyaretheydifferent?
Ifallcometsformedintheouter
SolarSystem,youwouldexpect
themtobemostlythesame.
Butwearerapidlyfindingthatis
notthecase,withmanycomets
havinguniquecompositionsand
abundancesofwater.Findingout
whyiskeytounderstandingthe
SolarSystem’sevolution.
CouldonehitEarth?
Acometisthoughttohavewiped
outthedinosaurs 66 millionyears
ago,butcouldsuchaneventoccur
again?Findingcometscanhelp
uslookforanythatmightbeon
a collisioncoursewithEarth–
althoughsofar,thecoastisclear.
Scienceinaction!
Cometsarefascinatinglaboratories
ofsciencewherewecanseesome
amazingprocessestakingplace.As
theyapproachtheSuntheyform
largetailsthatextendlongbehind
themastheybecomemoreactive.
Studyingthemisquitesimply
scientificallyinteresting!
Right
(clockw ise):
In 2014
Europe’s last
comet mission,
Rosetta, entered
orbit around
Comet 67P/
Churyumov-
Gerasimenko,
ending in 2016Comet 73P/
Schwassmann-
Wachmann 3,
which has
broken apart,
is a backup
target for Comet
InterceptorComets may
have brought
water, and
even life, to
Earth early in
its history© ESAComet Interceptor