Sky and Telescope - January 2017 - 12

NEWS NOTES
MISSIONS

Rosetta's Grand Finale
THE HISTORIC ROSETTA mission to

explore Comet 67P/Churyumov-Gerasimenko ended in dramatic fashion on
September 30th, when the spacecraft
touched down on the comet's nucleus.
Rosetta accompanied Comet 67P for
two years, orbiting the bilobed nucleus
as it approached and survived its perihelion pass in August 2015 (S&T: Aug.
2014, p. 20). The spacecraft kept a good
distance from the nucleus in order to
protect itself from outbursts, but in its
last days Rosetta spiraled in as close as
1 km from the surface.

Mission operators programmed the
craft to come to rest in the Ma'at region,
on the smaller of the nucleus's two lobes.
Several sinkhole-style pits, measuring
about 100 m wide by 50 m deep (300
by 160 ft), dot the region. Rosetta was
aimed to touch down on a smooth plain
between two depressions, at a velocity of
about 1 mps (2.2 mph), equivalent to a
slow walking pace. Instruments continued to measure gas, dust, and plasma all
the way down during Rosetta's descent,
and to take images like the one at right.
But even if the spacecraft survived its
landing, we won't hear from it again: it's
been commanded not to try to recontact
Earth, says project scientist Matt Taylor
(European Space Agency). "As Jim Morrison once said: 'This is the end, beautiful friend.'"
Rosetta won't be alone. On September 2nd, the team ﬁnally found
the long-lost Philae lander, wedged in
a dark crack about a third of the way
around the smaller lobe from Rosetta.
Philae bounced and disappeared after
its attempted landing (S&T: Feb. 2015,
p. 12), and its resting place shielded it
from the sunlight needed to recharge its

batteries and enable it to phone home.
Rosetta's data show that the comet's
water ice contains three times more
deuterium than water on Earth, ruling
out primordial comets as the predominant source of Earth's water. But the
glycine and other complex compounds
discovered on Comet 67P suggest that
these bodies might have delivered
organic compounds to our planet.

Pisces A and B are puny, each only a
couple thousand light-years across and
holding on the order of 10 million suns.
The Milky Way contains approximately
100 billion stars, and its disk spans more
than 100,000 light-years.
The team's observations reveal that
the dwarfs lie on the edge of a nearby
cosmic desert called the Local Void, one
of the gaps in the cosmic web of galaxies
that make the large-scale universe look
like a sponge. Each dwarf also boasts a
couple dozen young, blue stars. In fact,
it looks like the galaxies both experienced upticks in their star production
within the last 300 million years.
Dwarfs are notoriously on-again,
off-again in their star formation, so the
waves of starbirth might be internally
triggered. But the team suspects that
they're instead catalyzed by the galaxies' migration, moving from the Local

Void into the strand of the cosmic web
they're currently in. More gas ﬂoats
around in the web's ﬁlaments than in
voids, and as a galaxy entered a strand,
the pressure of ramming into this ambient stuff could compress the gas inside
the galaxies, triggering star formation.
Indeed, several astronomers have suggested that this process could kick-start
starbirth or even kill it by tearing the gas
out of a galaxy (S&T: Feb. 2013, p. 14).
But as the team explains in the
August 20th Astrophysical Journal, it
hasn't measured the dwarfs' motions -
the researchers only know the locations
of Pisces A and B, not where they've
been or where they're going. The idea
that their star formation is being
spurred by entering the web is only
an extrapolation based on where the
dwarfs currently lie.

■ DAVID DICKINSON

Rosetta's OSIRIS telephoto camera captured
this image of Comet 67P/Churyumov-Gerasimenko on September 30, 2016, from a point
16 km (10 mi) above the surface during the
spacecraft's controlled descent. The resolution
is about 30 cm (12 in) per pixel.

Pisces B

Coming in from the Void?
LIVING IN A COSMIC VOID might be
responsible for two small galaxies'
recent ﬂurries of starbirth. Erik Tollerud
(Space Telescope Science Institute) and
colleagues used Hubble to spot the two
dwarfs, Pisces A and B. They lie about
20 and 30 million light-years away,
respectively, beyond the Local Group.

J A N U A R Y 2 0 1 7 * SK Y & TELESCOPE

■ CAMILLE M. CARLISLE

MISSIONS A RT: ESA / N ASA; MISSIONS IM AG E: ESA / ROSE T TA / MPS FOR OSIRIS TE A M;
G A L A XIES: N ASA / ESA / E. TOLLERUD (STSCI)

GALAXIES

Sky and Telescope - January 2017

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