The comet of the century?

Comet ISON is on its way, and a variety of space and ground-based telescopes, along with thousands of amateur astronomers, are preparing for its arrival. Discovered on 21st September 2012, by two Russian astronomers using the International Scientific Optical Network telescope near Kislovodsk, Comet ISON (C/2012 S1) is expected to be the brightest ‘comet of the century’.

Comets are composed of dust, rock and ice, and are often on highly eccentric orbits around the Sun. The shape of their orbit means that they spend most of their time in the cold, outer regions of the Solar System.   But as their orbits carry them into the inner Solar System, the  Sun heats and vapourises the icy outer layers of the nucleus and a giant atmosphere or coma forms around the comet. Sunlight, as well as the solar wind, push on the material in the coma and create the cometary tail, which can be tens of millions of kilometres long.

Comet tails are produced when sunlight and the solar wind collide with material in the comet’s coma. Because of this, comet tails always point away from the Sun. (Credit: Swinburne)

Comet ISON is a long period comet believed to have originated from the Oort Cloud. The Oort Cloud is an enormous spherical cloud of icy bodies which surrounds our Solar System, first postulated in 1950 by Jan Hendrik Oort but not directly observed since comets are not visible when far from the Sun due to their small size (cometary nucleus are generally 20 km is size) and enormous distance fro the Sun (the Oort cloud is thought to extend some 20,000 to 100,000 AU from the Sun).  The long period comets, with orbital periods greater than 200 years, are thought to be disturbed from their orbit within the Oort Cloud by a passing star or the gravitational effect of stars within the Milky Way and moved onto orbits that carry them into the inner Solar System.  Oort cloud comets that enter the inner Solar System for the first time are called ‘dynamically new’ comets.

Comet ISON is believed to be a dynamically new comet, which also suggests that it is composed of pristine material that has been largely unaltered since the formation of the Solar System.  It is hoped that with many ground and space-based observatories  focused upon the journey of the comet, information about its composition as more and more material is vapourised as it approaches perihelion (its closest approach to the Sun) will provide insight about the conditions of the early Solar System.

Comet ISON photographed on 10 November 2013 from Austria. (Credit: Michael Jäger)

NASA’s Near Earth Object Program has been plotting the trajectory of the comet and NASA’s Deep Impact Spacecraft has been monitoring the progress of the comet since January 2013. During January, NASA’s Swift mission has observed that the comet is shedding about 50,800 kg of dust and 58 kg of water every minute. In February, the comet passed inside the orbit of Jupiter and, while still in the outer solar system, the comet was discovered to be active with a tail extending more than 64,400 kilometres from its nucleus. By April 2013, the images from Hubble determined that the nucleus was four to six km across, and was traveling at approximately 75,640 km/hr.

Its orbit will carry it very close to the Sun, making ISON a sungrazing comet.  Its perihelion passage will occur on 28 November 2013, bring ISON within 1.8 million km from the surface of the Sun – effectiviely within the Sun’s extended atmosphere. This is when the comet ill be most active, shedding vast quantities of water, carbon monoxide and carbon dioxide. By closely monitoring this activity from outside the Earth’s atmosphere at near-infrared, ultraviolet and visible wavelengths, astronomers hope to establish  the temperature at which the comet and more accurately determine where in the Solar System the comet originated.

As it approaches the Sun, there are three scenarios for the fate of comet ISON;

  1. Disintegration well before perihelion – given that little is still know about the formation and structure of comets, this is a possibility. A small fraction of comets (less that 1%) spontaneously disintegrate near periastron.
  2. Destruction near perihelion – due to the extreme temperatures of the Sun’s surface as comet ISON approaches perihelion, the dust, ice and rock within the coma may totally vaporize. Another possibility is that the tidal influences of the Sun’s gravitational force will rip the comet apart.
  3. Survival – comet ISON may also survive its journey around the Sun and re-emerge from its ‘close encounter’ with enough nucleus material to remain active. If the comet does survive, following its interaction with the Sun,] the amount of dust that it will have lost in the encounter is likely to produce an enormous tail which may span half of the night sky and have a coma that could then be visible for months.

For those living in the southern hemisphere, the comet will be visible during November, an hour and a half before local sunrise, situated about five hand spans diagonally east of Mars and moving towards the star Spica, which is the brightest star in the constellation of Virgo. If the comet survives, on 26 December it will make its closest approach to Earth and will be visible to the naked eye from the northern hemisphere throughout the night during late December 2013 and January 2014, with predictions of the comet being as bright as the full moon!

Comet ISON as seen from the southern hemisphere. (Credit: ABC Science)

Comet ISON could be the ‘Comet of the Century’ or a fizzle. Either way the interest that this comet has generated has assisted in bringing a greater focus on the work of the Near Earth Objects Program, and the vast amount of data and images from all the observations will enhance our understanding of both the formation of comets and the Solar System. Thousands of people over the next couple of months will be ‘stepping out and looking up’ at the night sky – enjoy!

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[Sheridan Lacey & Sarah Maddison]

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