On Tuesday (19th of November) I was on the BBC just before 7am talking about Comet ISON. I am going to be on TV this next Tuesday evening (26th of November) talking about the same topic, so I thought it would be a good idea to do a blog about it.
What is a comet?
Back in this blog, I talked about Comet PANSTARRS. I briefly mentioned that comets come from either the Kuiper belt or from the Oort Cloud. But I did not really say what a comet is. A comet is a ball of ice, rock and dust, sometimes referred to as “a dirty ball of ice”. The ice includes water-ice (astronomers can also refer to carbon dioxide and ammonia and methane as “ice”, as they can freeze at the low temperatures found in parts of the Solar System). It is the boiling away of the water-ice as a comet approaches the Sun which gives it its distinctive appearance.
In fact, a comet has two tails. This picture of the comet Hale-Bopp, taken in 1997, shows the two tails quite distinctly.
The more distinctive white tail is also called the “dust tail”, and is due to dust and ice boiling away from the solid body of the comet as it gets heated up in the inner parts of the Solar System. This material forms a coma around the solid body of the comet, and as the comet hurtles through space it leaves some of this material behind in its wake. This tail points in the opposite direction to the comet’s direction of travel, directly behind where it is heading.
The less distinctive blue tail is called the “ion tail”. It is also due to ice and dust, but rather than being left behind by the comet’s motion through space, the material in the ion tail has been driven away from the comet by the Solar wind, a stream of charged particles coming from the Sun. So, the ion tail always points away from the Sun, which may not be in the same direction as the direction in which the comet is travelling.
What is Comet ISON?
Comet ISON was discovered by Valili Nevski and Artyom Novichonok on the 21st of September last year (2012) using the 0.4m International Scientific Optical Network (ISON), from which it gets its name. From studies of its path it is felt that it has an open, hyperbolic orbit, which suggests it does not come from the Kuiper belt, but rather from the Oort Cloud. It was presumably nudged out of the Oort Cloud by a passing star in our Galaxy. Because it has come from the Oort Cloud, it is a new comet in the sense that it has not been to the inner parts of our Solar System before, not will it return if it really is in an open (hyperbolic) orbit.
Comet ISON is rapidly approaching its closest approach to the Sun, something called perihelion. This will be on 28th of November, when it will be only 0.0124 AUs (Astronomical Units) from the Sun, which translates to 1.86 million kms. By Solar System standards, this is pretty close, and Comet ISON has been referred to by some as a “Sun-grazing comet”, so close to the Sun is its closest approach. About a month later, on the 26th of December (Boxing Day to those of us in the Disunited Kingdom) it will pass at its closest to the Earth, 0.43 AUs (64 million kms) away.
Where should I look to see Comet ISON?
As comet ISON approaches the Sun it will be moving pretty quickly through the sky. That is to say, its position from night to night will be quite different. Here is a chart showing its position in the early morning sky at the beginning of December. Currently, as of today the 21st of November, it has just passed the bright star Spica in the constellation Virgo and has entered the constellation Libra. You should look for it in the sky just before Sunrise. It is not visible in the evening or middle of the night at present, only in the pre-dawn sky.
It has been said that Comet ISON could be one of the best comets for many many years. We do not really know yet, but given how close it is approaching to the Sun, and the observations of it so far, it could indeed put on a spectacular display for us. We will find out over the next few weeks, so keep your fingers crossed!