10 March 2014 sky & telescopeNews Notes
COMETS I ISON’s Untimely Demise
As many experts feared, Comet ISON
(C/2012 S1) choked. The comet fi zzled out
as it skimmed the Sun at perihelion on
November 28th, despite the spike in its
brightness in the preceding weeks that
raised observers’ hopes. No naked-eye
spectacle emerged to light December’s
dawn, nor did its remains appear in back-
yard telescopes.
As the comet raced toward its fateful
pass, several Sun-watching spacecraft
relayed pictures of its progress almost in
real time. Excitement peaked in the fi nal
24 hours, when the comet’s starlike headbrightened to dazzle the SOHO cameras
at an estimated magnitude −2. Then, just
as rapidly, the head dwindled away.
At the comet’s closest approach (less
than one solar diameter from the Sun’s
photosphere), the extreme-ultraviolet cam-
eras on the Solar Dynamics Observatory
saw nothing, suggesting the nucleus had
ceased to emit gas.
Then out the other side came a head-
less dust-and-rubble stream. It regrouped
somewhat, as expected for a stream decel-
erating away from the Sun. It gradually
expanded and faded to nothing as it left
the Sun behind.
Still, astronomers have pulled some sci-
entifi c results from the comet. Research-
ers originally estimated the nucleus’s
diameter was a couple of kilometers
wide. But Alfred McEwen (University ofArizona) reported December 10th at the
annual American Geophysical Union
(AGU) meeting in San Francisco that
the nucleus was probably much smaller.
Observations by the HiRISE camera on
the Mars Reconnaissance Orbiter, taken
during the comet’s Red Planet fl yby at the
end of September, suggest that the diam-
eter of the dirty iceball was only between
100 and 1,000 meters (300 to 3,000 feet).
Albedo measurements favor the middle of
that range, or around 600 meters.
Due to a happy accident, NASA’s
Messenger spacecraft, currently in orbit
around Mercury, was in the right place
at the right time to study both ISON and
the periodic Comet 2P/Encke as they both
passed through the inner solar system.
The orbiter’s ultraviolet spectrograph
performed about 9,000 scans of ISON. As
of the AGU meeting, most of those data
were still onboard the spacecraft, await-
ing download and analysis. But from what
scientists have, it looks like the fi rst-timer
ISON contained lots of carbon compared
with the repeat-visitor Encke. That sug-
gests organic grains might exist on pri-
mordial comets but are burned off during
solar passages. It’s also possible that this
“heat treatment” somehow protects peri-
odic comets during subsequent passes.
Scientists think Comet ISON stopped
expelling dust at perihelion, even though
it briefl y brightened a bit right afterward.
One theory for this unexpected brighten-
ing is that when the comet rounded the
Sun it was pulled apart like a Slinky, with
its pieces separated but still traveling
together as a cloud of debris. That separa-
tion would increase the surface area that
was refl ecting sunlight and would create
an illusion of nucleus brightening.
Although it’s unclear why some sun-
grazers survive perihelion and others do
not, the consensus is that Comet ISON
was too small, too volatile, and too new to
survive its close encounter with our star.
Oh well. That’s comets for you.
■ ALAN MACROBERT & EMILY POOREComet ISON plunges sunward toward perihelion, and then its wreckage emerges, in
this composite of frames taken by the LASCO C3 instrument on the Solar and Helio-
spheric Observatory (SOHO). The dark blue disk in the center is a metal mask blocking
the Sun’s glare; the thick black line (upper right) is the stalk holding the mask.Watch video of ISON’s demise at
skypub.com/ISONdies. We will
announce the winners of our Comet ISON
photo contest in next month’s issue.NASA / ESA / SOHONov. 29Nov. 28Nov. 30SunNN layout.indd 10 12/23/13 11:38 AM