[meteorite-list] Comet ISON's Dramatic Final Hours

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 17 Jul 2014 12:20:36 -0700 (PDT)
Message-ID: <201407171920.s6HJKaae025184_at_zagami.jpl.nasa.gov>


Comet ISON's dramatic final hours
European Space Agency
16 July 2014

A new analysis of data from the ESA/NASA Solar and Heliospheric
Observatory (SOHO) spacecraft has revealed that comet 2012/S1
(ISON) stopped producing dust and gas shortly before it raced past
the Sun and disintegrated.

SOHO/LASCO view of Comet ISON, 27-30 November 2013. /(Further
details and other formats available here
</science-e/www/object/index.cfm?fobjectid=54346>.) Credit: SOHO

When comet ISON was discovered in the autumn of 2012, astronomers
hoped that it would eventually light up the night sky to become a
"comet of the century". Orbital analysis showed that the
sungrazing intruder from the outer reaches of the Solar System
would pass only 1.2 million kilometres above the Sun's visible
surface on 28 November 2013.

Based on its early brightness, the comet promised to be a unique
research object and, should it survive its flyby of the Sun, a
stunning celestial phenomenon in the weeks preceding Christmas.
However, it soon became clear that these hopes and expectations
would not be met.

During the final phase of the approach to perihelion (its nearest
approach to the Sun), the comet's tail became increasingly faint.
It was clear that ISON's activity had ceased or that the nucleus
had completely disintegrated.

Hours before ISON reached perihelion, stunning images taken by
SOHO's Large Angle and Spectrometric Coronagraph (LASCO) showed
the bright, elongated tail of the onrushing comet. Unfortunately,
ISON's trajectory took it so close to the Sun - about 1.2 million
kilometres above its visible surface - that the final phase of the
encounter was obscured by LASCO's occulting disc, which blocks
light from the Sun to create an artificial solar eclipse.

Researchers at the Max Planck Institute for Solar System Research
(MPS) turned to the Solar Ultraviolet Measurements of Emitted
Radiation (SUMER) instrument in order to reconstruct the comet's
behaviour during its final hours. The SUMER spectrograph on board
SOHO was the only instrument capable of obtaining comet data
during the final minutes of its approach to the Sun.

SOHO/SUMER view of Comet ISON, 28 November 2013. /Credit: MPS/

"The only instrument that could obtain serviceable data at this
time was SUMER," says Werner Curdt from the MPS, first author of
a paper in the latest issue of the journal Astronomy &
Astrophysics. Curdt has been head of the SUMER team since 2002.

"For everyone involved, this was a huge challenge," he adds.
"The instrument was designed to investigate plasma flows,
temperatures, and density in the Sun's hot outer atmosphere, not
to detect a comparatively faint comet."

By operating the instrument in camera mode, the researchers were
able to record images of the comet's tail in far ultraviolet light
with a wavelength of 121.6 nanometres. This light was emitted from
the solar disc and reflected by the dust particles into space.

The SUMER data showed the dust tail between 17:56 and 18:01 (GMT)
on 28 November, shortly before closest approach. The images showed
a slightly curved, pointed tail with a length of at least 240 000
kilometres. No signs of a particularly bright area were found at
the position where the comet's active nucleus was predicted to be.

To understand what processes generated this tail shape, the
researchers compared the images with computer simulations. They
calculated what the tail would look like, after making certain
assumptions about the size of the dust particles, their speed and
the time of their emission.

Spectral information was also obtained by SUMER, starting at 18:02
(GMT). The instrument was repointed every ten minutes to track the
comet, but most of these spectra had an extremely low signal,
indicating an absence of cometary gas or plasma.

"Our measurements and calculations indicate that ISON ran out
of steam before perihelion," says Curdt.

"During our modelling of the event, we were not able to
reconstruct anything resembling our images, assuming that ISON
was still active during the SUMER observations," says comet
researcher and co-author Hermann Bohnhardt from the MPS.

Instead, the MPS model most consistent with the observations
indicates that the comet stopped producing dust and gas hours
earlier. Whether the nucleus completely disintegrated cannot be
settled for certain, according to Bohnhardt. However, several
pieces of evidence indicate that this was the case.

The arrow-shaped coma of the comet indicates that there was a
short, violent outburst which released a great amount of dust, 8.5
hours before it was due to pass by the Sun. Calculations show that
the comet must have emitted around 11 500 tonnes of dust at this time.

"It is most likely that the final break-up of the nucleus
triggered this eruption, abruptly releasing gas and dust trapped
inside the nucleus," says Werner Curdt. "Within a few hours the
dust production stopped completely."

"SOHO has now been sending back a stream of data about our
nearest star and thousands of sungrazing comets for more than 18
years," says Bernhard Fleck, ESA's SOHO project scientist.

"Observations such as those made of comet ISON show that the
observatory still has an important role to play in improving our
understanding of the Sun and its influence on the planets and
other objects which orbit around it."

More information

"Scattered Lyman-?? radiation of comet 2012/S1 (ISON) observed by
SUMER/SOHO", by W. Curdt, H. Boehnhardt, J.-B. Vincent, S. K.
Solanki, U. Sch??hle and L. Teriaca, is published in Astronomy &
Astrophysics, Volume 567, L1:

SOHO (SOlar Heliospheric Observatory) is a project of
international cooperation between ESA and NASA to study the Sun,
from its deep core to the outer corona, and the solar wind.

SOHO monitors the Sun constantly from a privileged point in the
Sun-Earth system, the first Lagrangian Point (L1). Located between
the two celestial bodies, at a distance of about 1.5 million
kilometres from the Earth, SOHO enjoys an uninterrupted view of
the Sun - something impossible to achieve from a ground-based
observatory due to our planet's rotation.


Werner Curdt
SUMER Principal Investigator
Max Planck Institute for Solar System Research
Gottingen, Germany
Email: Curdt at mps.mpg.de
Phone: + 49 551 384 979-420

Hermann B??hnhardt
Max Planck Institute for Solar System Research
Gottingen, Germany
Email: Boehnhardt at mps.mpg.de
Phone: +49 551 384 979-545

Bernhard Fleck
ESA SOHO Project Scientist
Directorate of Science and Robotic Exploration
European Space Agency
Email: bfleck at esa.nascom.nasa.gov
Phone: +1 301 286 4098
Received on Thu 17 Jul 2014 03:20:36 PM PDT

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