[meteorite-list] Comet ISON: Incoming Sungrazer

[Ron Baalke]

http://www.space.com/19973-comet-ison.html 

Comet ISON: Incoming Sungrazer
Elizabeth Howell
space.com
February 26, 2013

Comet ISON will make an appearance in Earth's sky in 2013. While it's 
hard to predict just how bright the comet will be when it arrives, some 
astronomers are saying that it could be as bright as the full moon or 
perhaps, even visible in daylight.

The comet is 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.

ISON is considered a "sungrazer," meaning that it will pass very close to the 
sun when it gets into the inner solar system in November 2013.

Discovery and naming

Amateur astronomers Vitali Nevski and Artyom Novichonok spotted the comet in 
photographs taken by an ISON telescope in September 2012.

Even from a great distance, the comet does appear bright, making it possible 
that its nucleus is somewhere between 0.6 miles and 6 miles (1 to 10 kilometers) 
wide, according to astronomer Matthew Knight of the Lowell Observatory and NASA.

ISON is expected to get as close as 800,000 miles (1.2 million km) from the sun's 
surface, providing it survives the gravitational forces or the sun's radiation. 
That closest approach will take place on Nov. 28, 2013.

Traditionally, comets are named after the people who find them, such as 
Shoemaker-Levy 9 that crashed into Jupiter in 1994, or Hale-Bopp that brightened 
Northern Hemisphere skies in 1997.

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 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 the time of its discovery in late September 2012, Comet ISON was 625 million 
miles (1 billion km) from Earth in the constellation of Cancer.

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.

A deep-frozen comet could blaze spectacularly as it whips past the sun at 
Thanksgiving, 2013.

"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 has 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.

Preparing for a sky show

With more than a year to get ready, NASA and other organizations are busy making 
plans for how to observe the comet.

The agency's twin Solar TErrestrial RElations Observatory (STEREO) spacecraft 
currently does continuous observations of the sun from orbit. This should afford 
the observatories a good view when the comet swings by the sun in November 
2013, NASA stated.

"If Comet ISON works out as expected, the STEREO spacecraft should have a 
spectacular view," NASA stated.

"During the period when Comet ISON is closest to the sun, it will actually pass 
in front of the sun as seen from behind [one of the satellites]," the agency added. 
"This opens up the exciting possibility that we might see extreme-ultraviolet 
emission from the comet, as was seen recently with the bright sungrazing 
Comet Lovejoy."

Comets are a notoriously fickle sky object, however, and can flare up or die at 
times that are difficult to predict. Many observers of a certain age recall the 
tale of Comet Kohoutek in 1973, which was billed as the "Comet of the Century."

However, the comet did not live up to expectations. It was a visible comet 
observed by, among others, one of the crews aboard the Skylab space station. But 
it was nowhere near as stunning as some astronomers predicted.

What's known for sure, however, is ISON poses no threat to Earth. At its closest 
approach it will still be 40 million miles (64 million km) from Earth, just under 
half the distance between the planet and the sun.

Multiple comet discoverer David Levy, who was on the team to spot Shoemaker-Levy 9, 
offered up this bit of advice to SPACE.com concerning comets: "Comets are like cats; 
they have tails, and they do precisely what they want."