Just when you had high hopes of getting your hands on the latest gizmos and trendy gadgets in time for Christmas… and Boom! You’re being given a comet! Not just any comet. Comet ISON (C/2012 S1). It’s 4.6 billion years old! And it will pass within 40,000,000 miles of Earth.
Astronomers are getting excited. In a few days, we could witness one of the most spectacular astronomical sights of the night sky for a whole generation or more. Or it could just turn out to be a disappointment…
Meet the Sungrazing Comet of the Century ISON C/2012 S1
Astronomers and the media have high hopes that on 3 December 2013, a comet and its tail will appear on the Eastern horizon – Comet ISON. Throughout December, millions of people in the Northern hemisphere may be able to see the comet’s tail, which is several millions of kilometres long, and stretching across the dawn sky looking East.
Comet ISON (C/2012 S1) has come from the Oort cloud, a belt of comets at the very edge of the Solar System, where it has been for the last 4.6 billion years. Many comets pass through the Solar System every decade, but very few go through the corona of the Sun. Ison will do just that. What makes Comet Ison so special is that it is a “sungrazer”.
The Kreutz Sungrazers are a category of sun-grazing comets, characterised by orbits that take them extremely close to the Sun at perihelion. In plain English, the perihelion is the point in the orbit of a planet, asteroid or comet where it is at its nearest point to the Sun. The opposite is the aphelion, when the orbit of the object is at its farthest point from the Sun. The sungrazer comets are believed to be fragments of one large comet that broke up several centuries ago and are named after German astronomer Heinrich Kreutz (1854-1907), who first demonstrated that they were related.
Observations by Swift in January 2013 have revealed that C/2012 S1’s nucleus was about 5 kilometres (3 miles) in diameter. Expected to approach our star at a distance of just 1.2 million kilometres from the surface, the encounter could cause Ison to break up completely, but if it survives, the comet could put on a bright display in the sky during December.
Literally, Comet Ison brushes past the Sun on 28 November, in a region of solar space where the heat at “perihelion” is expected to exceed 2,000 C. It is not known exactly what effect this great heat and the gravitational force of our Sun will have on the comet. The passage through the Sun’s corona, which happens tonight, will be watched with great interest by many astronomers across the World.
What Fate for Comet Ison?
Dr Matthew Knight, from the Lowell Observatory (Arizona, U.S.A.), has been watching the comet over the past year and has worked out three scenarios to describe what fate awaits Comet Ison in the coming week.
Scenario 1 for Comet ISON: It could suffer the same fate as Comet Lovejoy (C/2011 W3)
Another Kreutz sungrazer, comet Lovejoy went around the Sun in autumn 2011. The gravity of the Sun pulled one side of the comet’s nucleus more strongly than the other, stretching it apart. As comet Lovejoy emerged from the corona, it exploded. Lovejoy was disappointing. 🙁
Whether this will happen to ISON depends on its size. A nucleus of two kilometres or under puts it at great risk. Astronomers estimate that ISON is almost exactly two kilometres, so it’s right on the borderline. It could go either way.
Scenario 2 for Comet ISON: It might behave like Comet Encke (2P/Encke)
Encke has orbited the Sun about 70 times since it was first observed a few centuries ago. Comet Encke holds a special role in scientific history in the generally discredited concept of luminiferous aether. As its orbit was observed to be perturbed and shortened, the shortening of Encke’s orbit could only be attributed to the drag of an “ether” through which it orbited in outer space. One reference from American chemist Benjamin Silliman (yes!) in First principles of chemistry, for the use of colleges and schools (1860), reads:
“Encke’s comet is found to lose about two days in each successive period of 1200 days. Biela’s comet, with twice that length of period, loses about one day. That is, the successive returns of these bodies is found to be accelerated by this amount. No other cause for this irregularity has been found but the agency of the supposed ether.”
On 20 April 2007, observations of the tail of Comet Encke showed it was temporarily torn off by magnetic field disturbances caused by a coronal mass ejection (a blast of solar particles ejected from the Sun). However, the tail grew back due to the continuous shedding of dust and gas by the comet.
Comet Encke is believed to be responsible for several related meteor showers, known as the Taurids, and near-Earth object 2004 TG10 may be a fragment of Encke.
Like Encke, Comet Ison could fizzle out in the solar flares and regain brightness later.
Scenario 3 for Comet ISON: Its display could be a party in the sky like for Comet Ikeya-Seki (C/1965 S1)
Comet Ikeya–Seki was a long-period comet discovered independently by Kaoru Ikeya and Tsutomu Seki, first observed as a faint telescopic object on 18 September 1965. Initial calculations of its orbit forecasted that on 21 October 1965, it would pass just 450,000 kilometres above the Sun’s surface, and that it probably become extremely bright as a result.
In Japan, it reached its perihelion at noon local time. At the time, it was seen shining at magnitude −10. Ikeya–Seki proved to be one of the brightest comets seen in the last thousand years, and is sometimes known as the Great Comet of 1965.
The comet was seen to break into three pieces just before its perihelion passage. The three pieces of Comet Ikeya–Seki continued in almost identical orbits, and the comet re-appeared in the morning sky in late October, showing a very bright tail. By early 1966, it had faded from view as it receded into the outer solar system.
Ikeya-Seki was an impressive sight. And there is a relative chance that Comet Ison could also emerge from the perihelion into a blaze of glory…
Observing Comet ISON (C/2012 S1)
You could check this ultimate Guide on Comet Hunting on NightSkyHunter, but…
All you really need is the will to get up and go outside in the cold weather before dawn, a good clear view of the Eastern sky horizon, a pair of binoculars, maybe a telescope, or just the naked eye, and some warm clothes and hot beverages to keep you from the cold.
Safety First: Dry ice can be dangerous. It can and will burn your skin. It can and will explode a sealed container. Respect the stuff.
At the time of writing, there were already news that Comet ISON’s nucleus may be buckling under the heat and gravitational pull of the Sun at around 5,000,000 kilometres from the Sun. It’s one of those things…
“Comets are like cats; they have a tail and they do exactly what they want.”