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Comet ISON, center, imaged on January 30, 2013. Credit: NASA/Swift/D. Bodewits, UMCP Comet ISON, center, imaged on January 30, 2013. Credit: NASA/Swift/D. Bodewits, UMCP

NASA's Swift satellite examines Comet ISON

Sen— NASA's Swift telescope has been capturing data on Comet ISON, which could be one of the brightest comets to pass through the Solar System for decades when it grazes the sun later this year.

Swift's Ultraviolet Optical Telescope has been observing the comet for the last two months. Astronomers from the University of Maryland College Park (UMCP) and Lowell Observatory have been studying the rate at which the comet is emitting dust and water.

Lead Investigator Dennis Bodewits from UMCP said: "Comet ISON has the potential to be among the brightest comets of the last 50 years, which gives us a rare opportunity to observe its changes in great detail and over an extended period."

Comets are often thought of as dirty snowballs. They emit a tail of gas and dust when they fly near enough to the Sun to melt the frozen elements into gases, a process called sublimation. The jets powered by sublimating ice also release dust, which reflects sunlight and makes the comet brighter.

Swift's observations in January were made at a distance of 604 million kilometres (375 million miles) from Earth and 740 million kilometres (460 million miles) from the sun. At this distance, Comet ISON was observed shedding about 51,000 kg - over 50 tonnes - of dust every minute. The comet was also shedding water, though only about 60 kg a minute. Similar observations were made in February. The small amount of water being shed at this stage was expected; the vapour trail will increase dramatically when the comet's frozen core gets closer to the sun.

Bodewits observed: "The mismatch we detect between the amount of dust and water produced tells us that ISON's water sublimation is not yet powering its jets because the comet is still too far from the sun, Other more volatile materials, such as carbon dioxide or carbon monoxide ice, evaporate at greater distances and are now fueling ISON's activity."

The rate at which the comet is emitting water and dust have enabled observers to estimate the size of the comet's icy nucleus as being about 5 kilometres (3 miles) across, a typical size for a comet. This calculation is based on the assumption that only the fraction of the surface most directly exposed to the sun, about 10 percent of the total, is actively producing jets.

The comet originated in the Oort cloud, a belt of perhaps a trillion icy bodies beyond the orbit of Neptune in the outer Solar System. Based on its orbit, astronomers believe that this is ISON's first trip to the inner Solar System.

On October 1 the comet will pass within 10.8 million km (6.7 million miles) of Mars. Data may be captured by spacecraft operated by NASA and the European Space Agency as it makes its way past the Red Planet. 

After Mars it will head towards its close encounter with the sun, passing within 1.2 million km (730,000 miles) of the the sun on November 28. Comet ISON will pass close enough to be classified as a sungrazing comet. The comet is expected to become very bright as it furiously sublimates under the sun's heat.

It will make its closest approach to Earth on December 26, coming within 64 million km (40 million miles).

The comet, officially designated C/2012 S1 (ISON), was discovered on Setpember 21, 2012 by Russian astronomers Vitali Nevski and Artyom Novichonok using the International Scientific Optical Network (ISON) telescope. 

It is too early to say if the comet will be as bright as expectations.

"It looks promising, but that's all we can say for sure now," said Matthew Knight, an astronomer at Lowell Observatory. "Past comets have failed to live up to expectations once they reached the inner solar system, and only observations over the next few months will improve our knowledge of how ISON will perform."

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