Recently, we have seen some changes in the brightness of Comet ISON.
Notice how the magnitude (a measure of the brightness) has gone from around 10 at the end of October to near 4 now. For the everyday person, that means that ISON has become around 200 times brighter in a matter of weeks and has been visible to the naked eye in many locations. Magnitude 6 is the general threshold for visibility to the naked eye. The smaller the magnitude number, the brighter an object is. For example, the Full Moon has a magnitude of about -12, and Venus, a bright "star," is around -4.
The latest light-curve for Comet ISON (compiled Nov. 19, 2013 by Matthew Knight).
So, what happened to make it so much brighter? Some scientists hypothesized that ISON has started to break up, or small fragments have broken away and that this may have caused an increase in its brightness. However, there is no hard evidence to prove or disprove that. The more likely reason is that ISON has had a series of "outbursts," in which gases and water are expelled from the comet, which causes the brightness to go up (the magnitude down). This reason is supported by the European Southern Observatory's TRAPPIST telescope.
To understand further what is going on now and what will happen with ISON, let's take a look at its orbit around the sun.
Image of the orbit of Comet ISON thanks to NASA
Ok, so where is ISON? It is currently in the lower part of the orbit shown in this image, approaching the Sun. Because it is getting closer to the Sun, ISON is getting lower and lower in the sky in the pre-sunrise hours. ISON will soon disappear from view to the naked eye in the pre-sunrise hours as it will be too close to the Sun and drowned out by its brightness.
On Thanksgiving Day, Nov. 28, ISON will be reaching perihelion, its closest approach to the Sun at 2 p.m. EST.
Photo courtesy of www.damianpeach.com
From here, there are many questions about ISON and what its "health" is going to be. What I mean by that is that some comets do not survive their trip around the Sun, some break up a bit and some stay intact. Two factors are causing scientists headaches about how ISON will look as it nears the perihelion with the Earth in December.
First, because it is a "new" comet and has never interacted with the Sun before, there is a lot of uncertainty about what will happen to it as it nears the Sun. An "old" comet, like Halley's, is fairly predictable in terms of how it will interact with the sun, as most of its volatile parts have already been stripped away. Scientists are not entirely sure about ISON's makeup and there are already questions about its behavior, as mentioned above. These questions will only increase as it continues to encounter a harsher thermal and gravitational environment as it nears the Sun.
Secondly, it is going to have a relatively close encounter with the Sun, passing as close to the Sun as the Sun’s own diameter. That is much closer than Mercury, the closest planet. How it will react to such a harsh environment is unknown.
Not all of the possible changes are bad for viewing here on Earth. The worst case scenario for viewing is that it completely disintegrates and is not able to be seen in December at all. Possibly the best case scenario is that it breaks into several pieces but doesn’t disintegrate and is much brighter than it is now. Scientists are just unsure of the exact result right now and will likely not know until changes actually occur.
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