by Pat Browne
Understanding Comet ISON – Where , When and Why the Observing Interest (Nov 2013)
If we are lucky some of us will get to see Comet ISON (C2012) before it takes a nose-dive into the Sun. So much controversy surround this ‘cosmic hairball’ because of the varying predictions of its potential brightening as it approaches the Sun. Some of the uncertainty about the comet has been analyzed by comet specialist John E Bortle – ( see http://www.icq.eps.harvard.edu/bortle.html) According to John Bortle, the accuracy of the brightening predictions for Comet ISON depend on:
- Whether comet ISON is
- a returning traveller to the inner solar system (like Comet Halley – a periodic comet of 76 years),
- or a brand new one fresh from the Oort Cloud (see background information below).
- How close comet ISON will approach the Sun (the perihelion distance)
Points to Consider
- If the Comet has brushed past the Sun in a previous orbit, much of the volatile icy stuff has burned off. In this case, the comet tends to shine more consistently in our night sky, and predictions are closer to actual visual magnitude observations.
- Conversely, if the comet is making its close encounter with the Sun for the first time, it is coated with a volatile icy material of a thin outer crust which can vaporizes off early on. Hence early predictions of brightness were possibly biased by this transitory phenomenon. Extrapolation of this kind of brightness to perihelion could give rise to brighter than Moon (ridiculous) predictions
- The perihelion distance of Comet ISON is .012 A.U . We have few comparisons with comets who have come this close to the Sun. It is possible that the Comet will not survive this solar encounter. According to John Bortle (Dec 2013, Sky and Telescope article Comet ISON – Part 1) , he has developed an empirical relationship between the intrinsic brightness necessary to withstand and remain intact during the ablationary trajectory near the Sun. He calculates that the comet must have an intrinsic brightness ( indicating a certain material composition) of magnitude 7.0 to remain intact. Current estimates of ISON’s brightness place it above this threshold.
So we do know that the Comet is brightening rapidly, and we know why its brightness maximum is not predictable. Now to catch it in the night sky before dawn..
Where to Look this Week For Comet ISON (before dawn – 5 AM)
Since Comet ISON is a pre-dawn object now, it graces the Spring Constellation Virgo, and happens to be placed near the brightest star in Virgo, Spica (or Alpha Virginis). So this may be your opportunity to crawl out of bed, don the binoculars and stand outside looking at an unobstructed SouthEast horizon. It is Low! Here are some images from the program Stellarium for Nov 16 through to Nov 20 2013. Notice the adjacent Spring quadrilateral constellation Corvus. Each night ISON decreases in altitude for the same observing time, indicating it rises later in the morning and closer to the sunrise. At 6am for example, on Nov 19, the altitude is 11 degrees and the azimuth is 120 degrees (more South than East). Click on each image to get a full view.
Background Information: What is a comet?
A comet or “cometes” in Greek means ‘hairy‘ because comets look like ‘hariy’ stars. Indeed, famous comet hunter David Levy is said to ‘patiently sweep the heavens for those cosmic hairballs called ‘comets'”, – courtesy, Deep Sky Objects, David Levy (Forward by Stephen James O’Meara).
A comet has the following properties:
- The fuzzy envelope known as the coma, which varies in size with each comet as well as with its distance from the sun. The ability of the comet to form a coma distinguishes it from an asteroid. The coma is usually tens of thousands of kms in diameter.
- Most comets have a ‘tail’ – often there are 2 parts to that – the gas tail and the the dust tail . The tails of comets are the largest things in the solar system reaching millions of kms in length. Comet tails are formed when the pressure of light from the sun and solar wind drive gases from the core nucleus into the coma and then into the tail. The main tail is thus pointing away from the Sun. “As the comet approaches the sun, it drags its tail after it, but as the comet leaves the sun’s region, its tail precedes. In other words, the tail then wags the dog.” (courtesy – Helen Sawyer Hogg, The Stars Belong to Everyone.
- A comet’s trajectory is a very wide and often one way trip around the solar system. Comets orbits are good examples of ‘conic sections’. They can trace elliptical, parabolic or hyperbolic paths as they round the Sun. Earthlings have recorded periodic ones like Comet Halley, but there are others that we see only once. Their origin is believed to be out past the orbit of Neptune in the Kuiper belt or Oort cloud as shown in this diagram:
Here is a small video of a favourite comet that appeared in the Spring of 2012: Comet Garadd over Miss. Mills Skies