Comet ISON still intrigues and inspires, even after its demise

Comet ISON moves toward the sun in this composite image from ESA and NASA's Solar and Heliospheric Observatory, or SOHO, and NASA's Solar Dynamics Observatory, or SDO,captured at 9:30 a.m. EST on Nov. 28, 2013.



Comet ISON is dead, but its memory will live on." That eulogy for the much-discussed “comet of the century" was delivered today by Karl Battams of the Naval Research Laboratory in Washington, D.C., during a series of reports presented at the annual meeting of the American Geophysical Union (AGU) in San Francisco, Calif. In response to queries from reporters seeking a definitive statement on the fate of the comet that rounded the sun in late November, Battams noted that both amateur and professional astronomers had combed the area of the sky where ISON would now be had it survived, and there is simply no trace of it. 

Since its discovery in September 2012, Comet ISON captured the imagination of millions around the world. It was unusually bright for an object so far away, and calculations of its trajectory soon revealed that it was a first-time visitor to the inner solar system, that it would be a “sungrazer,” closely approaching the sun, and that it originated in the Oort Cloud. Located between 5,000 and 100,000 AU (one AU equals 150 million kilometers, Earth’s distance from the sun), the Oort Cloud is a little known or understood region of outer space, but it is the source of comets that require more than 200 years to orbit the Sun.

Because it was the first known sungrazing comet originating in the Oort cloud, scientists studied ISON intensely as it approached the sun and circled around it, Battams said. Many spacecraft-based instruments were temporarily repurposed to observe it. NASA’s Soho and Stereo satellites focused on ISON and provided ongoing images of perihelion, its closest approach to the sun. Even before perihelion, however, scientists noticed that ISON was dimming, suggesting a loss of mass. Although some observers cheered when a bright object reappeared from behind the sun, the experts realized that it was merely a dust cloud, formed of ISON remnants, and it quickly dissipated and faded.

On its way toward the sun, ISON passed much closer to Mars than to Earth, which permitted a telescope there to attempt to measure its size. The High Resolution Imaging Science Experiment on the Mars Reconnaissance Orbiter captured 10 images of ISON over several days. Although its lens could not resolve the comet’s nucleus, the instrument’s principal investigator, Alfred McEwen of the University of Arizona in Tucson, said that analysis of the best images suggest a nucleus width of between 100 and 1,000 meters.

Another satellite pressed into ISON observations was MESSENGER, currently orbiting Mercury. “It was in the right place at the right time,” said Ralph McNutt Jr. of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. He showed two MESSENGER images of ISON, taken 21 hours apart, during its closest approach to Mercury. They revealed a noticeable brightening of the comet during that period. He noted that much of the ISON-related data is still aboard the spacecraft, as its download speed is slow, so additional revelations may come later.

Battams suggested several possibilities for why ISON did not survive its encounter with the sun, while other sungrazers have. Other comets that have made multiple circuits of the solar system may have been toughened, or “heat treated,” during their previous passages, he said, and were better able to withstand their close encounter. ISON, as an Oort cloud comet, may also have been more vulnerable to solar radiation. Additionally, if ISON's size was actually toward the lower end of the calculated range, that too would have increased its vulnerability. Most known sungrazers have, in fact, perished at perihelion, he added.

In the long-term, scientists want to “develop a picture of how comet ISON was put together,” Battams said. Comets are quite varied, said Geraint Jones of University College London, and studying them helps round out the picture of the formation of the solar system.

“Before you can build a planet, you have to build a comet, and we don’t know how comets are built,” Battams said. “We know how to take very fine dust and make little chunks of rock, and we know how to take lots of comets and stick them together and make a planet, but we don’t know how to take a couple of little rocks and have them ultimately form a comet.”

The ongoing study of data from ISON — its elemental composition, why it fell apart, when it fell apart — is all part of building that picture, he concluded.

Harvey Leifert
Wednesday, December 11, 2013 - 16:30

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