Comets are small, icy bodies that travel around the Sun often on highly eccentric orbits, spending much of their time in the cold, outer Solar System. They are sometime called ‘dirty snowballs’ (or ‘icy dirtball’!) because they are composed of dust and ice (where the ice is often a mixture of water, carbon monoxide, ammonia and methane). When their orbits carry them into the inner Solar System, the warm Sun sublimates their outer layers of icy material and the comet becomes visible to the naked eye. The name comet comes from the Greek word κομήτης (komētēs), meaning “long-haired” or the “hairy one”, due to the long tail which comets display when close to the Sun.
As the cometary nucleus approaches the Sun, its first sign of activity is the appearance of a large temporary coma of gas and dust, which can be 104–105 km in size. The coma interacts with solar photons and the solar wind, producing a long cometary tail. Solar radiation pressure drives dust particles in the coma away from the Sun, resulting in an extended dust tail, which can be several 107 km long. Comets also have a gas tail (or ion tail) which results when solar UV radiation ionised the molecular gas from the coma.
These small celestial objects are often very faint and most comets are not visible without the aid of binoculars or a telescope. However, we are occasionally treated with a comet bright enough to be seen with the naked eyes. Astronomers have dubbed 2013 the “year of comets”, because two comets are going to be visible with the naked eye this year.
On 6 June 2011, a new comet was discovered using the 1.8 m Pan-STARRS (Panoramic Survey Telescope & Rapid Response System) telescope in Hawaii. As it was the first comet discovered with this telescope, it was given the name Comet Pan-STARRS (though its official designated name is C/2011 L4). In late August 2011, its orbit was calculated and it was determined to be a long period comet, with an extremely high eccentricity and large orbit, presumably originating from the Oort cloud. Its perihelion (point of closest approach to the Sun) was calculated to occur on 10 March 2013, when its orbit carries it closer to the Sun than Mercury. Comet Pan-STARRS is clearly visible in the northern hemisphere, but for the people in the southern hemisphere it is only visible for short time after sunset, just above the western horizon. It was bright enough and easily identifiable on 12 March just above the new moon.
The comet will be visible in the western sky after sunset until the end of this month, but won’t be easy to identify for the naked eye. In order to watch Comet Pan-STARRS, you’ll want to find a hilly area from where the western horizon is clearly visible.
If you don’t get a chance to see Comet Pan-STARRS this month, you should get another chance to catch a comet in November this year, when Comet ISON (C/2012 S1) will be visible. Comet ISON is another highly eccentric long-period comet, discovered in September 2012 with the 0.4-m International Scientific Optical Network (ISON) telescope near Kislovodsk in Russia. Comet ISON’s perihelion passage on 28 November 2013 will be at a distance of just 0.012 AU from the Sun and it is predicted to be as bright as the full moon. This is going to be a visual treat for all the sky watchers!
For more information, see
- Comet Pan-STARRS: Still on Track (Kelly Beaty, Sky & Telescope)
- Everything you need to know: How to see Comet PANSTARRS (Deborah Byrd, Earth Sky)
- A Naked-Eye Comet in March 2013 (ScienceCasts @ NASA)
[Sreeja Kartha & Sarah Maddison]