[meteorite-list] Comet C/ISON Details Emerge as it Races Toward the Sun

[Ron Baalke]

http://www.psi.edu/news/ISON3.html

Comet C/ISON Details Emerge as it Races Toward the Sun
Planetary Science Institute
Oct. 9, 2013

Tucson, Ariz. -- Scientists are unraveling more information on Comet C/2012 
S1 (ISON) as it continues on its journey toward the Sun. Comet C/ISON 
will skim 730,000 miles above the Sun's surface on Nov. 28 and has the 
potential to be readily visible from Earth starting in early December.
 
"We measured the rotational pole of the nucleus.  The pole indicates that 
only one side of the comet is being heated by the Sun on its way in until 
approximately one week before it reaches it closest point to the Sun," 
said Planetary Science Institute Research Scientist Jian-Yang Li, who 
led a team that imaged the comet.
 
"Since the surface on the dark side of the comet should still retain a 
large fraction of very volatile materials, the sudden exposure to the 
strong sunlight when it gets closer to the Sun than Mercury could trigger 
huge outbursts of material," Li said.
 
Li presented the findings today at the American Astronomical Society's 
Division for Planetary Sciences 45th Annual Meeting in Denver.
 
Comet C/ISON was imaged with the Hubble Space Telescope using the Wide 
Field Camera 3 on April 10.
 
"We measured the color of the coma, and found that the outer part of the 
coma is slightly redder than the inner part," Li said. "This color change 
is unusual in comets, and seems to imply that the inner part contains 
some water ice grains, which sublimate as they move away from the nucleus."
 
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 at 
most several days when 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," Li said. "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."
 
NASA and the Space Telescope Science Institute funded the project.