What’s a comet?
Comets are changeable by nature. When far from the sun, they’re frozen, inert bodies that look like points of light in large telescopes. No warm, fuzzy outside, no tail. But once a comet’s orbit takes it “downtown” to the inner solar system, heat from the sun vaporizes dust-laced ices to form a hazy atmosphere around the comet called a coma. Comas can grow up to 60,000 miles or more across or nearly as big as Jupiter. While impressive in size and appearance, they’re extremely rarified and possess little mass.
How a tail develops and grows as a comet approaches and then recedes from the sun along its orbit. Ion tails always point directly away from the sun. Being made of nearly nothing, makes it easy for the sun to fashion them into tails by the sun. Radiation pressure – literally the pressure of sunlight – pushes back dust inside the coma to form a yellow-hued dust tail.
Ultraviolet light from the sun ionizes or electrifies atoms and molecules inside the comet’s temporary atmosphere. One of the most common gases found in comets is carbon monoxide. Yep, the same stuff that comes out of your car’s tailpipe. As the sun’s magnetic field washes across the solar system like so many waves rippling a pond, it sweeps ionized carbon monoxide molecules out of the coma to form a second, blue-colored ion tail.
Ion tails always point directly away from the sun much like a wind vane, while dust tails tend to follow the curve of the comet’s orbit. Depending on where the comet is in relation to Earth, tails can appear long and narrow, short and spiky, fan-like or even hide for a time behind the coma.
Comet C/2012 S1 (ISON) is a sungrazing comet that was discovered by Russian astronomers Vitali Nevski and Artyom Novichonok in September 2012. It had been named ISON after the Russian astronomers’ night-sky survey program, the International Scientific Optical Network. Defined as “an upcoming rare space event”, astronomers are now following its path. At present it’s still now known exactly how big and bright the comet will be but it is estimated that it will be 15 times brighter than the moon and will be clearly visible even during daytime.
By the second half of October, Comet ISON will approach Mars and then Earth and by November it will be visible to the unaided eye. The real show will start on November 28 when the comet will reach perihelion, i.e. the shortest distance from the centre point of the sun. In fact it is estimated that ISON will be just 1,800,000 kilometres (1,100,000 miles or 0.012 AU) from the sun’s center point. With the sun having a diameter of 1,391,000 kilometres, the sungrazing comet will pass through the sun’s corona at just 400, 000 kilometres above the surface. If ISON survives the close shave with our sun then it will have an amazing tail and could possibly already become visible at perihelion.
ISON will be first seen coming out from the constellation of Leo when it will appear close to Regulus. It will then flyby Spica in the constellation of Virgo on November 18 and by November 25 it will be over the constellation of Libra (Figure 1)
Assuming that ISON survives the sun’s flyby, it could be displaying an spectacular tail of bright light which would be crossing the constellation of the Serpent and subsequently it would reach for the constellation of the Crown before it dies out as it moves away from the sun (See Figures 3 and 4 – click images to enlarge).
Beginning of December 2013 Comet ISON will be visible during the evening and morning skies.
On 26th December 2013 the comet will come closest to our planet as it will pass about 63,000,000 kilometres (39,000,000 miles or 0.42 AU) from Earth (see Figure 5).
Earth is expected to pass through the orbit of the comet on 14–15 January 2014, which may result in the creation of a spectacular meteor shower.
Below find two animations of the path of Comet ISON: