Comets

 The Path of Comet ISON

Comet McNaught as seen from a Dark site with huge tail and Moon shining bright above horizon

Comet ISON is expected to be visible just like this comet.

Comet ISON was discovered on Sept 21, 2012, by Russian astronomers Vitali Nevski and Artyom Novichonok using a telescope of the International Scientific Optical Network (ISON) located near Kislovodsk.

Shortly after discovery, it was noted that some of the orbital elements of C/2012 S1 were similar to that of the Great Comet of 1680, which suggested the two comets may have fragmented from the same parent body. However, further observations of ISON have shown that the two comets are not related.

Like all comets, ISON is a clump of frozen gases mixed with dust. Often described as "dirty snowballs," comets emit gas and dust whenever they venture near enough to the sun that the icy material transforms from a solid to gas, a process called sublimation. Jets powered by sublimating ice also release dust, which reflects sunlight and brightens the comet.

On Nov. 28, ISON will make a sweltering passage around the sun. The comet will approach within about 1.2 million km of its visible surface, which classifies ISON as a sungrazing comet. 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.

Sungrazing comets often shed large fragments or even completely disrupt following close encounters with the sun, but for ISON neither fate is a foregone conclusion.

C/2012 S1 will come to perihelion (closest approach to the Sun) on 28 November 2013 at a distance of 0.012 AU (1,800,000 km) from the centre point of the Sun. Accounting for the solar radius of 695,500 km, C/2012 S1 will pass approximately 1,100,000 km above the Sun's surface. Its trajectory is nearly parabolic, which suggests that it may be a dynamically new comet coming freshly from the Oort cloud. On its closest approach, C/2012 S1 will pass about 0.0724 AU (10,830,000 km) from Mars on 1 October 2013, and it will pass about 0.429 AU (64,200,000 km) from Earth on 26 December 2013.

Following ISON's solar swing by, the comet will depart the sun and move toward Earth, appearing in morning twilight through December. The comet will swing past Earth on Dec. 26, approaching within 64.2 million km or about 167 times farther than the moon.

Comet C/2012 S1, better known as comet ISON, may become a dazzling sight as it traverses the inner solar system in late 2013. During the weeks before its Nov. 28 close approach to the sun, the comet will be observable with small telescopes, and binoculars. Observatories around the world and in space will track the comet during its fiery trek around the sun. If ISON survives its searing solar passage, which seems likely but is not certain, the comet may be visible to the unaided eye in the pre-dawn sky during December.

Earth will pass near the orbit of C/2012 S1 on 14–15 January 2014, well after the comet has passed, when micron-sized dust particles blown by the Sun's radiation may cause a meteor shower, or noctilucent clouds. However, both events are highly improbable. Because Earth just passes near C/2012 S1's orbit, not actually through the tail, the chances that a meteor shower will occur are very slim. In addition, meteor showers from long period comets that make just one pass into the inner solar system are very rare, if ever recorded.

The possibility that small particles left behind on the orbital path—almost one hundred days after the nucleus has passed—could form noctilucent clouds is also slim. A significant amount of dust should enter Earth's atmosphere, and besides that, no such events are known to have taken place in the past, under similar circumstances.

5 Things to know about ISON

Comet ISON may put on a show when it skims through the sun's atmosphere later this year. Right now, it's still far away, but we're keeping track and will give you regular updates. Here are five key facts about ISON as we await its arrival:

1.      What's with the funky name?

Comet ISON was named after their night-sky survey program, the International Scientific Optical Network, a group of observatories in 10 countries organized to track objects in space.

2.     How big is it?

Measurements taken by the Hubble Space Telescope in April indicate ISON has a nucleus that is 5 to 6.5 kms across. The comet's head, or coma, is estimated to be 5000 kms across, or 1.2 times the width of Australia. The Hubble team says its dust tail extends more than 93,000 kms -- more than twice the circumference of Earth, and far beyond the telescope's field of view.

3.     OK, it's a comet. Aren't there lots of comets? Why is this one special?

Some early comet prognosticators have tagged ISON "the comet of the century."

"Comet ISON has the potential to be among the brightest comets of the last 50 years," Dennis Bodewits, an astronomer at University of Maryland at College Park, told NASA.

Bodewits and other astronomers used NASA's Swift satellite to estimate ISON's water and dust production.

4.     How to tell space rocks apart

"Comet ISON belongs to a class of comets called Sungrazing comets," Karl Battams of the Naval Research Lab told CNN. This means it will fly relatively close to our sun. These comets "teach us not only about comets, but also yield valuable and unique results about the sun," he said.

“But before you get too excited, other experts caution it's too early to know what ISON will do”

"Predicting the behavior of comets is like predicting the behavior of cats -- can't really be done," Don Yeomans of NASA's Near-Earth Object Program told CNN.com in March.

5.     When can I see it?

In November, ISON is expected to fly through the sun's atmosphere at about 700,000 miles above the surface. If it survives the sun's heat, experts say it might glow as brightly as the moon and be briefly visible in daylight. Its tail might stretch far across the night sky. Or the sun could cause it to break apart.

6.     What if ISON breaks apart? Is Earth in danger?

No. Experts say the comet won't threaten Earth. In fact, even if it breaks up, Battams says it could put on a big show.

"If Comet ISON splits, it might appear as a 'string of pearls' when viewed through a telescope," Battams told NASA. "It might even resemble the famous Comet Shoemaker-Levy 9 that hit Jupiter in 1994."

Whatever happens to ISON, sky watchers in the Northern Hemisphere should have a good view for several months. NASA says it will pass almost directly over the North Pole and will be visible all night long.

Track comet ISON all October using the following illustrations:

Comet ISON as seen on October 1, 2013 in due East just before Sunrise

Comet ISON close to Mars and Moon as seen on Oct. 11, 2013 in due East just before sunrise

Comet ISON on October 21, 2013 near to Mars as seen in the East just before sunrise

Orbital Parameters for Comet ISON as seen from various points in Space

Orbital parameters for Comet ISON*

Position

Right ascension                                                 9h 42.6m

Declination                                                        16° 50'
Constellation                                                    Leo
Distance from Earth                                        2.043 AU
Last observed magnitude                              15.0
Date of last reported observation                03/10/2013
Angular separation from Sun                      49.1°
Ecliptic latitude                                              3.8°

Orbit

Distance from Sun                                          1.581 AU
Perihelion                                                         0.012 AU   (28/11/2013)
Eccentricity                                                       1.000002
Inclination to ecliptic                                   62.4°
Speed relative to Sun                                     33.500 km/s


* Source: www.heavens-above.com