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Comet of the century? ISON has 'potential' to be visible all day.

As sun-grazing comet ISON approaches the sun, it's getting progressively brighter – and might even flare into a dazzling object bright enough to be visible in broad daylight.

By Joe Rao / April 25, 2013

Comet ISON was photographed by the Hubble telescope on April 10, 2013 when the comet was 386 million miles from the Sun (slightly closer than Jupiter's orbit). Around the time the 'comet of the century' makes its closest approach to the Sun, on November 28, it may briefly become brighter than the full Moon, say NASA researchers.

NASA Hubble telescope / Courtesy of NASA / Reuters

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An exceptional comet flying ever closer to the sun may offer an amazing naked eye sight to Earth dwellers this fall as it gradually brightens.

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As comet C/2012 S1 (ISON) continues to approach the sun, it is slowly responding to the increasing warmth of the sun and getting progressively brighter. The comet is getting considerable scrutiny from both amateur and professional scientists because it's a rare sungrazing comet, destined to approach to within 730,000 miles (1.17 million kilometers) of the surface of the sun on Nov. 28. Because of this extremely close approach, comet ISON holds the "potential" to flare into a dazzling object — possibly becoming bright enough to be briefly glimpsed in broad daylight. 

Still, at this early stage in the comet's development, we can't be sure if this will actually happen.

A Swift look

Although still quite far from the sun and very faint, the comet has been imaged by two orbiting observatories. Astronomers from the University of Maryland at College Park and Lowell Observatory used NASA's Swift satellite to check out the comet during January and February. Using images acquired from Swift's Ultraviolet/Optical Telescope, the team was able to make initial estimates of the comet's water and dust production and then used these values to determine the size of ISON's icy nucleus.
 These observations revealed that each minute ISON was shedding about 56 tons (51,000 kg) of dust, or about two-thirds the mass of an unfueled space shuttle. Jets powered by sublimating ice also release dust, which reflects sunlight and brightens the comet.

By contrast, however, the comet was producing only about 130 pounds (60 kg) of water every minute, or about four times the amount flowing out of a residential sprinkler system. At the time, however, the comet was nearly half a billion miles from the sun. Typically, a comet's water content remains frozen until it comes within about three times Earth's distance to the sun — about 280 million miles (450 million km) away. ISON won't be this close to the sun until early July at which time the water production rate should markedly increase.

The water and dust production rates from Swift were used to estimate the size of ISON's icy body. Comparing the amount of gas needed for a normal comet to blow off dust at the rate observed for ISON, the scientists estimate that the nucleus is roughly 3 miles (5 km) across, a typical size for a comet. This assumes that only the fraction of the surface most directly exposed to the sun, about 10 percent of the total, is actively producing jets.

Hubble's turn

More recently, Planetary Science Institute research scientist Jian-Yang Li led a team that imaged comet ISON with the Hubble Space Telescope on April 10 using the Wide Field Camera 3. At this point, the comet was slightly closer than Jupiter at 386 million miles (621 million kilometers) from the sun and 394 million miles (634 million kilometers) from Earth.

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