[meteorite-list] A Tale of Two Comets: MESSENGER Captures Images of Encke and ISON

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 25 Nov 2013 10:00:14 -0800 (PST)
Message-ID: <201311251800.rAPI0E1G001699_at_zagami.jpl.nasa.gov>

http://messenger.jhuapl.edu/news_room/details.php?id=246

MESSENGER Mission News
November 25, 2013

A Tale of Two Comets: MESSENGER Captures Images of Encke and ISON

On November 18, NASA's Mercury-orbiting MESSENGER spacecraft pointed its
Mercury Dual Imaging System (MDIS) at 2P/Encke and captured this image
<http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?image_id=1303>
of the comet as it sped within 2.3 million miles (3.7 million
kilometers) of Mercury's surface. The next day, the probe captured this
companion image
<http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?image_id=1304>
of C/2012 S1 (ISON), as it cruised by Mercury at a distance of 22.5
million miles (36.2 million kilometers) on its way to its late-November
closest approach to the Sun.

MESSENGER's cameras have been acquiring targeted observations
<http://messenger.jhuapl.edu/the_mission/movies/MESSENGER_observes_Encke_and_ISON.mov>
of Encke since October 28 and ISON since October 26, although the first
faint detections
<http://messenger.jhuapl.edu/news_room/details.php?id=245> didn't come
until early November. During the closest approach of each comet to
Mercury, the Mercury Atmospheric and Surface Composition Spectrometer
(MASCS) and X-Ray Spectrometer (XRS) instruments also targeted the
comets. Observations of ISON conclude on November 26, when the comet
passes too close to the Sun, but MESSENGER will continue to monitor
Encke with both the imagers and spectrometers through early December.

The spacecraft has a view of the comets very different from that of
Earth-based observers. "MESSENGER imaged Encke only a few days before
its perihelion when it was at its brightest," explains Ron Vervack, of
the Johns Hopkins University Applied Physics Laboratory, who is leading
MESSENGER's comet-observation campaign. "That we are so close to the
comet at this time offers a chance to make important observations that
could shed light on its asymmetric behavior about perihelion."

In contrast, ISON did not pass as close to Mercury, but the comet was
between the Earth and Mercury when it passed closest to MESSENGER. "We
saw the side opposite to that visible from Earth," says Vervack, "so our
images and spectra are complementary to observations from Earth made at
the same time and could aid in understanding the variable activity of
the comet as it approached the Sun."

On the day that Encke was closest to Mercury, the MDIS wide-angle camera
scanned the comet with all of its 12 filters while the instrument's
narrow-angle camera (NAC) snapped images of the rotating comet every 10
minutes to capture a full 360-degree view. The imaging campaign for ISON
was similar, with the NAC capturing a series of stills every 30 minutes.

Several ground- and space-based NASA observatories, as well as many
other observatories around the world, are collecting data on the comets.
However, none will be able to collect simultaneous images and spectra
from X-ray through near-infrared wavelengths when the comets are so
close to the Sun, as will MESSENGER. Vervack expects MESSENGER to gather
15 hours worth of data on Encke and another 25 hours on ISON. "These
observations of Encke and ISON fill a gap in heliocentric coverage to
which most other observatories don't have access," Vervack says.

Scientists are still combing through the data collected by MASCS, but
there are already confirmed detections of several molecules and atoms,
including OH, NH, CS, oxygen, carbon, sulfur, and hydrogen.
"Far-ultraviolet observations can't be made from ground-based
observatories, and only a few instruments in space have been able to
look at the comets in the ultraviolet," says Vervack. "The MASCS
observations are therefore of great interest."

Scientists were also hoping to obtain the first definitive detections of
cometary X-ray emission from silicon, magnesium, and aluminum. "NASA's
Chandra X-ray space telescope has observed ISON and Encke and seen X-ray
emission from them both," Vervack says. "We are able to make these
observations when both comets are closer to the Sun, so the X-ray
emissions have the potential to be much more intense." However, a series
of large solar flares during the observations increased the
contaminating background in the X-ray spectra and have complicated the
analysis. "We can't help what the Sun does," says Vervack, "but we're
going to analyze the data carefully to see if there are any detections
to be had."

Taken together, the MESSENGER observations offer a varied science
investigation of the comets. "Whereas the MDIS images will provide a
global picture of the comet coma morphology, MASCS observations will
inform us about the composition of the cometary ices and XRS may be able
to tell us what the dust is made of," Vervack says.

"Comet encounters were not considered when the MESSENGER mission was
designed," adds MESSENGER Principal Investigator Sean Solomon of
Columbia University. "If Encke and ISON share a few of their secrets on
the formation and evolution of the Solar System, the MESSENGER team will
be delighted with the scientific bonus."

------------------------------------------------------------------------

MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and
Ranging) is a NASA-sponsored scientific investigation of the planet
Mercury and the first space mission designed to orbit the planet
and entered orbit about Mercury on March 17, 2011 (March 18, 2011 UTC),
to begin a yearlong study of its target planet. MESSENGER's extended
mission began on March 18, 2012, and ended one year later. A possible
second extended mission is currently under evaluation by NASA. Dr. Sean
C. Solomon, the Director of Columbia University's Lamont-Doherty Earth
Observatory, leads the mission as Principal Investigator. The Johns
Hopkins University Applied Physics Laboratory built and operates the
MESSENGER spacecraft and manages this Discovery-class mission for NASA.
Received on Mon 25 Nov 2013 01:00:14 PM PST


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