Comet ISON Sized Up By NASA's Swift Telescope

NASA’s Swift Telescope has now set its sights on comet C/2012 S1 (ISON), also known as Comet ISON, and is providing preliminary data on the comet, helping to improve estimates on its future brightness, and other things. Comet ISON has been predicted to possible become “the comet of the century” when it nears the Sun later this year, possible becoming brighter than the Full Moon at its peak.

By utilizing images taken by Swift’s Ultraviolet/Optical Telescope (UVOT) over the past two months, researchers from the University of Maryland at College Park (UMCP) and Lowell Observatory have come up with some initial estimates of “the comet’s water and dust production and used them to infer the size of its icy nucleus.”

“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,” said Lead Investigator Dennis Bodewits, an astronomer at UMCP.

“Additional factors, including an encounter with Mars followed by a scorching close approach to the sun, make comet ISON an object of special interest.” Towards the end of February, NASA requested that a “team of comet experts initiate the Comet ISON Observing Campaign (CIOC) to assist ground- and space-based facilities in obtaining the most scientifically useful data.”

Comet ISON, like all other comets, is essentially a frozen block of gases, ices, and dust. Which is why comets are often referred to as “dirty snowballs”. As a result of their makeup, when comets near the Sun they begin emitting significant quantities of gas and dust. The frozen gases sublimate (think dry ice), and when in combination with the dust present produce the “tail” that you often see on comets, and also increase the brightness of the comet.

“Typically, a comet’s water content remains frozen until it comes within about three times Earth’s distance to the sun. While Swift’s UVOT cannot detect water directly, the molecule quickly breaks into hydrogen atoms and hydroxyl (OH) molecules when exposed to ultraviolet sunlight. The UVOT detects light emitted by hydroxyl and other important molecular fragments as well as sunligreflected from dust.”

“The Jan. 30 UVOT observations reveal that ISON was shedding about 112,000 pounds (51,000 kg) of dust, or about two-thirds the mass of an unfueled space shuttle, every minute. By contrast, 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.”

“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,” Bodewits said. “Other more volatile materials, such as carbon dioxide or carbon monoxide ice, evaporate at greater distances and are now fueling ISON’s activity.”

“At the time, the comet was 375 million miles (604 million km) from Earth and 460 million miles (740 million km) from the sun. ISON was at magnitude 15.7 on the astronomical brightness scale, or about 5,000 times fainter that the threshold of human vision.”

“While the water and dust production rates are relatively uncertain because of the comet’s faintness, they can be 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.”

So, knowing this, how bright will comet ISON actually get?

“It looks promising, but that’s all we can say for sure now,” said Matthew Knight, an astronomer at Lowell Observatory in Flagstaff, Ariz., and a member of the Swift and CIOC teams. “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.”

ISON is expected to make its closest approach to the Sun on November 28th, passing within 730,000 miles of it. “In late November, its icy material will furiously sublimate and release torrents of dust as the surface erodes under the sun’s fierce heat, all as sun-monitoring satellites look on. Around this time, the comet may become bright enough to glimpse just by holding up a hand to block the sun’s glare.”

Even though comets that pass that close to Sun often lose large amounts of their mass, it’s still something of a mystery what will actually happen with ISON. Especially considering that this is thought to be ISON’s first journey to the inner solar system.

“We estimate that as much as 10 percent of the comet’s diameter may erode away, but this probably won’t devastate it,” explained Knight. Nearly all of the energy reaching the comet acts to sublimate its ice, an evaporative process that cools the comet’s surface and keeps it from reaching extreme temperatures despite its proximity to the sun.

“Following ISON’s solar encounter, the comet will depart the sun and move toward Earth, appearing in evening twilight through December. It will swing past Earth on Dec. 26, approaching within 39.9 million miles (64.2 million km) or about 167 times farther than the moon.”

Image Credit: NASA’s Goddard Space Flight Center/Axel Mellinger


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