rior. “Rosetta will be far more impressive in
terms of advancing our science than Deep
Impact,” he says.
The Rosetta orbiter is not just about seeing; it also sniffs. The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis
(ROSINA) has beenaiming its spectrometers
at the gas molecules in 67P’s thin
halo of an atmosphere,
called the coma.
In addition to detecting expected
gases such as water, methane, and
hydrogen, it has started to find rarer species, including formaldehyde and hydrogen
cyanide. Getting the list of comets’ primordial ingredients is important because many
scientists think comets helped jump-start
life on early Earth with an infusion of water
and organic molecules.
ROSINA has the sensitivity to detect isotopes, too. In one of the most important
mission results so far, published online on
10 December in Science, the ROSINA team
found an exceptionally high ratio of heavy
hydrogen (deuterium) to regular hydrogen.
Because this D-to-H ratio is so much higher
than that found in Earth water, it suggests
that comets like 67P—part of a group that
hails from the Kuiper belt, a region beyond
Neptune—could not have played a major role in delivering water to Earth. The
high ratio also buttresses the classical view
of where the two main groups of comets
formed. In this view, Kuiper belt comets
like 67P formed beyond Neptune before being flung farther out into the Kuiper belt
by the gravitational influence of Jupiter.
Other comets, with lower D-to-H
ratios, formed closer to the sun.
There, solar system dynamics gave
them an even bigger push, strong
enough to scatter them out to an
even more distant solar suburb
known as the Oort cloud.
Rosetta has its limitations.
It struggles to detect complex
chains of organic molecules, such as amino
acids, especially on the comet’s solid surface. As a result, it is unlikely to tell scientists conclusively whether the organics at
the comet’s surface are the pristine ingredients that 67P started with, or products of
chemical reactions sparked by earlier close
encounters with the sun. That’s why mission scientists are disappointed that Philae
was unable to perform its most ambitious
experiment: drilling a sample from below
the comet’s surface and baking it in an
oven for analysis.
Mission managers haven’t completely
ruled out trying again. As 67P approaches
the sun, more light will fall on Philae’s
Science is celebrating as 2014’s Break-
through of the Year.
Merely getting the €1.4 billion mission to
the comet was a massive undertaking for
the European Space Agency. After launch in
2004, flight engineers put Rosetta through
a decade of orbital tricks: Mars and Earth
were used as gravitational slingshots to
bring the spacecraft in line with the comet’s elliptical 6.5-year orbit. When Rosetta
caught up with 67P, the comet was still far
from the sun and cold. As it plunges sunward, its subsurface ice deposits have begun to sublimate, powering jets of gas and
dust. Peak activity should come in August
2015 at perihelion, when the comet is halfway between the orbits of Earth and Mars.
By watching the jets develop and change,
scientists can learn how comets are altered
each time they approach the sun. Then, by
subtracting those processes, they can turn
back the clock and understand how comets
formed some 4.5 billion years ago.
Much of Rosetta’s power comes from its
ability to inspect the comet at close range
for months on end. The half-dozen or so previous missions to
comets were all flybys that were
over in hours. Another ballyhooed
flyby of an icy body—NASA’s New
Horizons mission to Pluto—will
whiz past the dwarf planet on
14 July 2015. At its closest, New
Horizons will pass Pluto at a distance of 10,000 kilometers, close enough to
make out mountain-sized features (with a
camera resolution of 12 kilometers per
pixel). By contrast, Rosetta’s camera can
discriminate between objects just centimeters apart. Already, Rosetta has tracked
arcs of dust emerging from active jets.
“The breakthrough is yet to come, and it
will come from having the orbiter stay with
the comet,” says Michael A’Hearn, a planetary scientist at the University of Maryland,
College Park. A scientist on two of Rosetta’s
instruments, A’Hearn was also the principal investigator for Deep Impact, a NASA
comet flyby that in 2005 lobbed a projectile
into the comet Tempel 1 to probe its inte-
Visitors to Science’s website
picked their top breakthroughs
of 2014. The results:
Rosetta will continue to orbit
and study 67P throughout 2015.
For more on the
the Year, including
a video and a
podcast, go to
ON OUR WEBSITE
Giving life a bigger genetic
Young blood fixes old 32%
Comet rendezvous 17%
Cells that might cure diabetes 11%
An easy cure for hepatitis C 6%
Since 1996, Science’s writers
and editors have assembled the
Breakthrough of the Year section
by holding meetings, revising lists,
and even installing a suggestion jar
in the office kitchen. This year, we
decided to give the public a say. In
November, we posted our “long list” of
19 breakthrough candidates online at
www.sciencemag.org and let visitors
vote on them for 2 weeks. In December,
we took the top five semifinalists and
posted them for a second, weeklong
round of voting.
A science-themed horserace ensued. The Rosetta comet mission—
which was making headlines worldwide
as the voting began—finished the first
round with a healthy lead, garnering
more than 16% of the 24,947 votes
cast. Next came “Young blood fixes old”
with 11% of the votes, followed by “Cells
that might cure diabetes” with 10%.
In the second round, things got
interesting. “Young blood” took an
early lead and looked like a shoo-in.
But in a late surge, “Giving life a bigger
genetic alphabet” pulled even with
the rejuvenated mice. The two results
ran neck and neck in the final stages
of voting, but the expanded genetic
alphabet pulled ahead and won by a
nose. The Rosetta mission—Science’s
own choice—finished third. Kudos
to all contestants, and thanks to
everyone who cast a vote.
solar panels. In its last tweet, the lander
raised the possibility of a resurrection.
“My #lifeonacomet has just begun @ESA_
Rosetta. I’ll tell you more about my new
home, comet #67P soon… zzzzz.” Regardless
of whether Philae wakes up, Rosetta’s life
at comet 67P has indeed just begun—and it
heralds a new age of comet science. ■