28 AUSTRALIAN SKY & TELESCOPE July 2017
COMET: ESA / ROSETTA / MPS FOR OSIRIS TEAM; ROSETTA CONCEPT: ESAThis mission didn’t end up the way the
European Space Agency conceived it in the
early 1990s. The original plan called for a
spacecraft able to return a sample of comet
nucleus. Then it became a mission with two
landers (NASA’s Champollion and ESA’s
RoLand) before morphing to the orbiter-lander
configuration finally built by ESA.
Even the original target, a comet named
46P/Wirtanen, had to be changed. The failure
of an Ariane V rocket just one month prior
to Rosetta’s planned January 2003 launch
caused a year-long postponement. That delay
put a rendezvous with Comet Wirtanen out ofreach. So the team scrambled to find another
target that was scientifically interesting,
similarly sized (Philae had been designed
specifically for the mass of Comet Wirtanen),
and in an orbit that could be reached.
The decision to go to 67P/Churyumov-
Gerasimenko was a compromise. This body,
like Wirtanen, is also a Jupiter-family comet
— but one that’s at least three times larger
and up to 30 times more massive. So the
lander did have to be modified a bit. And
while 67P could be reached with decent
fuel reserves, it also took the longest time to
reach — which made it a more risky choice.
Comet 67P was amazing, so imagine what
that ‘other’ mission would have been like.Rosetta, circa 1992
dry, some types ofcarbonaceous chondritemeteorites can have
a water fraction as high as 20%. But whether these could have
deliveredenoughwatertoEarthisstilldebated.
Meanwhile,themostdistant,longest-periodcomets—
thosecomingfromtheOortCloud—allhaveD:Hratiosthat
aretoohigh,comparedwithourplanet.However,thevalue
measuredinashort-periodcomet,103P/Hartley2,hasavery
Earth-like value. This object is aJupiter-family comet(JFC), an
objectwitharelativelyshort-periodorbitthatisdynamically
controlledbyJupiter.Sowewereeagertoknowif67P,which
alsoisaJFC,likewisehasanEarth-likeD:Hratio.
Thefirstandstrongestresultregardingthisquestion
came from Rosetta’s ROSINA mass spectrometer, which
foundaD:Hratio3½timeshigherthanEarth’s—avalue
evenhigherthanthoseofOortCloudcomets!Thisconfirms
whatmanyplanetaryscientistshadalreadycometosuspect:
Perhapscometsofanytypearenottheprimarysourceof
Earth’swater.Instead,maybemostofitreallydidcomefrom
asteroids—orwasherefromtheoutsetinthematerialsfrom
which Earth formed.
Still,cometshaveandwillcontinuetohitEarthanddeliver some water. So every glass of water you drink has
comet water in it, just not as much as once thought.
An exciting implication of the different D:H values in
comets Churyumov-Gerasimenko and Hartley 2 is that
JFCs might have originated from widely different regions
of the Solar System. Moreover, apparently the water ice in
67P is original to the comet — it was not ‘reprocessed’ and
mixed with water from other sources in the primordial
solar nebula.
So do we have any clue where Comet 67P/Churyumov-
Gerasimenko formed? One hint comes from the ROSINA and
Alice instruments, which detected molecular oxygen (O 2 ).
This is surprising because O 2 is very reactive — it’s hard to
understand how it could have been retained in a 4.5-billion-
year-old comet.
One idea that’s become a lively topic of discussion among
Rosetta’s scientists is that the O 2 became trapped in ‘cages’
of ice (called clathrates). This would keep the oxygen from
escaping or reacting until the ice sublimates whenever the
comet comes close to the Sun.
ROSINA also discovered molecular nitrogen (N 2 ). This
finding, the detection of O 2 , and the high D:H ratio all indicate
that 67P formed at very cold temperatures (25 to 30 Kelvin).
This would place its origin in the outer fringe of our planetary
system, likely in the Kuiper Belt. That’s also dynamically
consistent with current models of Solar System formation.
So let’s recap: Measurements from Rosetta and dynamical
models imply that 67P formed in the Kuiper Belt, and that its
two lobes were separate bodies that gently collided and stuck
together. But how did those 2-km-wide bodies come to exist
so far from the Sun in the first place?
One clue comes from the ‘goose bumps’ observed on the
nucleus — perhaps an appropriate term considering how cold
it is out there. On the walls of pits and cliffs we identified
textures that look like they are built of many small boulders
just a few metres wide. Could these be our first peek at the basicPLAN A ESA’s original concept for Rosetta.
Note the sample-return capsule (brown).SPOUTING OFF A key
objective for Rosetta
was capturing the
comet when it was
most active. The
misshapen nucleus,
here silhouetted by
dust in its own coma,
released one very
strong jet and several
weaker ones on
August 12, 2015 — one
day before the comet
reached perihelion.ROSETTA RESULTS