How will Comet ISON perform?

Experts all over the planet are eagerly tracking Comet ISON (C/2012 S1), the so called "Comet of the century". However since June 22, 2013 the comet is so near to the Sun, less than 18 degrees, making it not visible against the dark sky. The comet will stay "invisible" up until August 8, 2013, when it moves more than 18 degrees from the Sun.

ISON is currently traveling close to the stars of the constellation Gemini, moving slowly towards east and will reach the constellation Cancer on August 1, 2013.

Based on comet's orbit, astronomers believe the comet is coming from Oort cloud, making its first visit to inner solar system. ISON was discovered on September 21, 2012, by Russian astronomers Vitali Nevski and Artyom Novichonok using a telescope of the International Scientific Optical Network (ISON) located near Kislovodsk, Russia.

From now through October, comet ISON tracks through the constellations Gemini, Cancer and Leo as it falls toward the sun. (Credit: NASA's Goddard Space Flight Center/Axel Mellinger)

Recently John Bortle, an experienced comet researcher, posted his opinion on the future behavior of Comet ISON on Comets-ml (a mailing list for comet observers).

Throughout the period from January to May, 2013 Comet ISON stayed stuck at magnitude +16, so Bortle predicts that it will brighten more slowly than initially forecasted. The comet will not be visible to the naked eye up until mid-November, which is just a week or two before it makes its closest approach to the Sun. At perihelion which falls on November 28, 2013, Bortle predicts that ISON will for a short period reach magnitude -6, making it visible to the naked eye in even at daylight.

Interestingly Bortle expects that after perihelion, the comet will start to fade rapidly, and will stay brighter than magnitude +2 or +3 only for a short period, probably for just a few days. Bortle also forecasts that Comet C/2012 S1 ISON will produce an impressive tail, 30° or even 45° in length. The comet`s tail will stay spectacular only for a week to ten days, and will diminish rapidly after that period.

John Bortle also said that any further, more detailed, predictions of ISON`s future behavior will have to be postponed until at least early September, 2013. 

David Seargent, also a well known comet expert and author, commented on Bortle`s forecasts for ISON, and for the most part agreed with Bortle. However he pointed out that observing the comet with the naked eye before perihelion doesn`t seem likely, especially with an inconvenient mid-month full moon and the comet's position low in the morning twilight. He concluded that it is the post-perihelion performance that will probably make or break Comet C/2012 S1 ISON`s reputation.

Comet ISON will pass very near Regulus in constellation Leo on October 14/15, 2013 and near Spica in mid-November. On November 23, 2013 ISON will pass very near a planetary pair of Mercury and Saturn and on November 28, 2013 Comet ISON will be at closest point to the Sun. It will be visible on both evening and morning skies for mid-Northern observers and circumpolar for the far North through December. ISON makes its closest approach to Earth at 39.6 million miles on December 26, 2013.

Sources: Comets-ml, NASA Science

Featured image:  Comet ISON seen by Hubble Telescope  on April 10, 2013, when the comet was 386 million miles from the sun. (Credit: NASA, ESA, J.-Y. Li (Planetary Science Institute/NASA’s Hubble Space Telescope)