There has been a lot of interest in the Comet 2012/S1 (ISON), more commonly known as Comet ISON. This comet has been predicted to become the comet of the century; this of course, may not eventuate. For those of us fortunate to have witnessed C/2006 P1 (McNaught) in January 2007 it will sure take a lot of beating.
The comet was discovered on 24 September, 2012, by two amateur astronomers in Belarus and Russia, using a 40-cm (16-inch) telescope. As it took them a day to confirm that the object was a comet, the organisation International Scientific Optical Network with which they are associated was credited with the discovery.
Over the past decade, hundreds of sungrazing comets have been discovered, but they are usually small, short-lived, and only seen by spacecraft designed to observe the Sun. Most sungrazing comets do not survive their trip around the Sun as they are disrupted by its intense radiation and gravity. Most of these, however, are quite small being only tens of metres across. The nucleus of Comet Ison is believed to be around 4km in diameter and hopefully its larger size will insulate the interior from the Sun’s energy.
One reason this is considered to be such an interesting comet is that it is believed to be making its first approach to the inner solar system. Along with this belief, comes the reason for all the uncertainty in the predictions as scientists can only guess at how bright it will become at the time it is closest to the Sun. There will be professional and amateur astronomers eagerly observing the comet from the ground and via the internet as a whole fleet of spacecraft have turned their cameras in its direction.
The comet has been gradually brightening as it speeds up on its journey towards the Sun. Unfortunately for us in the Southern Hemisphere just before and just after it passes behind the Sun on November 29, it will be below the horizon in both the morning and evening sky. This is the time that the comet is most likely to be at it brightest. For northern hemisphere observers, at these times, it may even be naked eye or perhaps visible in binoculars. A few weeks later the comet’s outward trajectory, providing it survives its passage around the Sun, will bring it to just 0.4 times the distance from the Earth to the Sun.
But perhaps, the greatest unknown is, in fact whether or not the comet will actually survive its passage through the corona which is the Sun’s atmosphere. At the time it passes the Sun it will only 1.1 million kms away and the nucleus of the comet will be subject to a combination of both extreme heating approximately 5000 degrees Celsius, which is hot enough to melt iron as well as constant radiation bombardment. Add to that the fact that the Sun’s strong gravitational pull will be trying to tear it apart. Still the best guess from scientists is that the nucleus or at least a large chunk of it will manage to sweep around the Sun and start making its way out of the solar system.
Comets are believed to be the frozen left-overs from the formation of our Solar System, originating in the Oort cloud. While comets have been in a deep freeze for the past 4 billion years, planets and asteroids have changed a lot from their original compositions. Better understanding of their ices, dust, and organic matter, and how they have changed over the past billions of years, tell us about the origins of our Sun, the planets, and, possibly, life on Earth. To astronomers, every bright comet is an opportunity to learn more about our Solar System.
NOTE that it is always dangerous to look directly at the Sun. Do not use telescopes or binoculars to search for the comet, just your unaided eyes and block the Sun with a post or other convenient object. Take extreme care!