Comet ISON Details Beginning to Emerge


Comet ISON

Scientists are unraveling more information on Comet C/2012 S1 (ISON) as it continues on its journey toward the Sun. 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 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."

An image of Comet ISON is available at

Comet 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 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 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 ISON provides astronomers a rare opportunity to study a fresh comet preserved since the formation of the solar system," said Li. "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."

PIO Contact:
Alan Fischer
PSI Public Information Officer
+1 520-382-0411, +1 520-622-6300

Science Contact:
Jian-Yang Li
PSI Research Scientist
+1 301-367-6315

NASA and the Space Telescope Science Institute funded the project.

The Planetary Science Institute is a private, nonprofit 501(c)(3) corporation dedicated to solar system exploration. It is headquartered in Tucson, Arizona, where it was founded in 1972. PSI scientists are involved in numerous NASA and international missions, the study of Mars and other planets, the Moon, asteroids, comets, interplanetary dust, impact physics, the origin of the solar system, extra-solar planet formation, dynamics, the rise of life, and other areas of research. They conduct fieldwork in North America, Australia and Africa. They also are actively involved in science education and public outreach through school programs, children's books, popular science books and art. PSI scientists are based in 20 states and the District of Columbia, Australia, Canada, France, Germany, Ireland, Latvia, Russia, South Africa, Switzerland and the United Kingdom.

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