[meteorite-list] C/2012 S1 (ISON) Imaged on Way to Inner Solar System Arrival

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 23 Apr 2013 12:33:11 -0700 (PDT)
Message-ID: <201304231933.r3NJXBsn014313_at_zagami.jpl.nasa.gov>

FROM:
Alan Fischer
Public Information Officer
Planetary Science Institute
520-382-0411
520-622-6300
fischer at psi.edu

Newly Discovered Comet Imaged on Way to Inner Solar System Arrival

April 23, 2013, Tucson, Ariz. -- Scientists eagerly await the arrival of
a recently discovered, highly active comet that will skim 730,000 miles
above the Sun's surface on Nov. 28 and has the potential to be readily
visible from Earth.

The comet, C/2012 S1 (ISON), is highly unusual in that it comes to the
inner solar system for the first time and will skirt around the Sun within
less than two solar radii from the Sun's surface on Nov. 28.

Comet C/ISON was discovered in September 2012 when it was farther away from
the Sun than Jupiter, and was already active at such a great distance. This
is distinct from most other sungrazers - comets that pass extremely close
to the sun - that are only discovered and remain visible for several hours
nearest the Sun. At such a close perihelion distance from the Sun,
sungrazers are expected to be intensely heated by the Sun, and sublimate
not only ice but also silicates and even metals, releasing a tremendous
amount of dust. The expectation is high that Comet C/ISON will be much
brighter and more spectacular than most other sungrazers when it puts on a
show late this year.

"As a first-time visitor to the inner solar system, Comet C/ISON provides
astronomers a rare opportunity to study a fresh comet preserved since the
formation of the solar system," said Planetary Science Institute Research
Scientist Jian-Yang Li, who led a team that imaged the comet. "The expected
high brightness of the comet as it nears the Sun allows for many important
measurements that are impossible for most other fresh comets."

Comet C/ISON was imaged with the Hubble Space Telescope on April 10 using
the Wide Field Camera 3, when the comet was slightly closer than Jupiter:
386 million miles (621 million kilometers) from the Sun and 394 million
miles (634 million kilometers) from the Earth.

An image of Comet C/ISON is available at
http://www.psi.edu/news/cometc/ison.html

The team is using these images to measure the activity level of this comet
and determine the size of the nucleus, in order to predict the comet'2s
activity when it passes perihelion, or closest to the Sun, later this
year. Preliminary measurements from the Hubble images suggest that the
nucleus, the solid, icy body at the center of the comet, is no larger than
three or four miles across. This is remarkably small considering the high
level of activity observed in the comet so far. This small size also means
that the outcome from its close perihelion passage near the Sun is
extremely hard to foresee.

The comet is active as sunlight warms the surface and causes frozen
volatiles to sublimate. The comet's dusty coma, or head of the comet, is
currently approximately 3,100 miles (5,000 kilometers) across, or 1.2 times
the width of Australia. A dust tail extends more than 57,000 miles (92,000
kilometers), far beyond Hubble's field of view.

A detailed analysis of the dust coma surrounding the nucleus reveals a
strong jet blasting dust particles off the sunward-facing side of the
comet's nucleus. This jet, as projected on the sky, extends at least 2,300
miles (3,700 kilometers).

More careful analysis is currently under way to improve these measurements
and to predict the possible outcome of the sungrazing perihelion passage of
this comet.

Whether Comet C/ISON will become a "Comet of the Century" and outshine all
other bright comets in the past still remains to be seen. But the new
Hubble images of Comet C/ISON have revealed much valuable information about
this highly unusual comet.

NASA and the Space Telescope Science Institute funded the project.

CONTACT:

Jian-Yang Li
Research Scientist
571-488-9999
jyli at psi.edu
Received on Tue 23 Apr 2013 03:33:11 PM PDT