Comet ISON was a sungrazing comet that was expected to put on a spectacular show in Earth's sky in late 2013. However, shortly after rounding the sun's far side on Nov. 28 (U.S. Thanksgiving), the comet faded, torn apart by the star's immense gravity.
ISON's behavior shortly after the close pass confused astronomers because it appeared brighter than what one would expect of a comet that had just broken up. It was later determined, however, that the increased activity was a trick of orbital dynamics.
Discovery and naming
The comet was named after a telescope for the International Scientific Optical Network. Two Russians spotted ISON through a 15.7-inch (0.4-meter) reflecting telescope from that organization.
Amateur astronomers Vitali Nevski and Artyom Novichonok spotted the comet in photographs taken by an ISON telescope in September 2012.
Comet ISON, however, is part of a newer trend that sees the name of the comet after the project rather than the individuals who discovered it. This means that several comets could have the same name, leading to confusion.
For that reason and also because the newer method is less personal, Peter Jedicke, past president of the Royal Astronomical Society of Canada, is calling for a return to the old naming convention.
Every comet also has a name assigned to it by the International Astronomical Union (IAU) that includes features such as the year of discovery. ISON's official name is Comet C/2012 S1.
Similar orbit to 1680 'Great Comet'
At 584 million miles (939 million km) from the sun, the comet was shining at magnitude 18.8 on a scale used by astronomers to gauge how bright sky objects are. (Brighter objects have a lower number.) This is about 100,000 times fainter than what the naked eye can see.
"The most exciting aspect of this new comet concerns its preliminary orbit, which bears a striking resemblance to that of the 'Great Comet of 1680,'" wrote Space.com skywatching columnist Joe Rao in a September 2012 article. [Gallery: Incredible Photos of Comet ISON]
"That comet put on a dazzling show; it was glimpsed in daylight and later, as it moved away from the sun, it threw off a brilliantly long tail that stretched up from the western twilight sky after sunset like a narrow searchlight beam for some 70 degrees of arc." (A person's clenched fist, held at arm’s length, covers roughly 10 degrees of sky.)
When the comet was still a long ways from Earth, in February 2013, NASA's Deep Impact spacecraft took a series of images of the comet. Deep Impact snapped close-up pictures of two comets before — Tempel 1 and Hartley 2 — but the astronomers were fascinated by how much activity was taking place on ISON despite its great distance from the sun.
"Preliminary results indicate that although the comet is still in the outer solar system, more than 474 million miles (763 million km) from the sun, it is already active. As of Jan. 18, the tail extending from ISON's nucleus was already more than 40,000 miles (64,400 km) long," NASA stated in a February 2013 press release.
Shortly after ISON's close shave with the sun on Nov. 28, 2013, its appearance brightened considerably. This confounded astronomers who had previously declared the comet dead.
More tracking of the comet in the days afterward revealed a rapid fading, however, and by Dec. 11 astronomers confidently called the comet dead. The strange brightening likely was due to an orbital dynamics phenomenon, said Geraint Jones of University College London at the time.
As the comet approached the sun, its fragment cloud was pulled out considerably, with the pieces closest to the sun moving faster than those far behind. After the comet dimmed, it then brightened up briefly when the pieces clumped together again after passing the sun.
Astronomers noted that it likely fell apart due to its small size; its nucleus was between 330 feet and 3,300 feet (100 to 10,000 meters), according to observations from NASA's Mars Reconnaissance Orbiter.
"It was probably smaller than maybe 600 meters [in] diameter," said Alfred McEwen of the University of Arizona, principal investigator of MRO's HiRISE camera, at the time. "And from past sungrazing comets, those smaller than about half a kilometer, they don't survive."
In October 2014, two Lowell Observatory scientists — Matthew Knight and David Schleicher — published their findings from ISON observations. They concluded that the nucleus had a "significant mass loss" before Nov. 1, 2013, that “catastrophically weakened the nucleus prior to perihelion.” The paper is available on prepublishing site Arxiv and is accepted by the Astrophysical Journal.