Astrophysicist Brian Koberlein talks about the possible outcomes of Comet ISON.
Comet ISON is showing up in the news recently because it has a chance of being quite bright. This irregular mass of rock, dust, ice and frozen gases is currently at about magnitude 13 in the constellation Gemini, but there are some predictions that it will be a "comet of the century," with an apparent magnitude of -10 or more (about as bright as the moon). A more realistic prediction is that it will peak at -3 to -5, or about as bright as Jupiter or Venus.
Predicting the maximum brightness of a comet is a challenge, because it depends not only on its path relative to the Earth and Sun, but also its structure and composition. Comets can flare up in brightness quickly as volatiles are outgassed suddenly, or they can remain unexpectedly dormant during their close approach to the Sun. ISON is no different in this respect.
The best we can do to estimate its maximum brightness is to compare its apparent magnitude over time with similar comets of the past. Typically this is done by a measurement known as Afrho. If you consider a line of sight from Earth to the comet, then rho is the radius of the dust surrounding the comet, and Af is a measure of the albedo. Using Afrho you can get an idea of just how much material is being ejected from the comet, as well as the brightness of the material.
What we've found is that ISON has a higher Afrho value than is typical for comets at its distance. This means it is more active than typical comets, with more gas and dust being ejected from its surface. It's this ejected gas and dust that forms the coma and tail that we see as a comet. Its level has also stayed fairly constant for several months, so this activity is not simply a short-lived outburst. It is likely that ISON will continue to be more active (and therefore brighter) as it nears the Sun.
That might seem like a good sign that it will become visible to the naked eye later this year, but ISON's orbit is almost parabolic, which means this is likely its first visit to the inner solar system. Comets that have visited the inner solar system before typically have elliptical orbits (just as planets and asteroids have elliptical orbits). Comets with parabolic orbits tend to come from the farthest reaches of our solar system. And since this is ISON's first trip into the warmth of the inner solar system we have to consider what that heat will do to ISON. The answer depends significantly on its makeup.
If ISON is fairly fragile, the higher than average activity could mean that it will disintegrate as it gets nearer to the Sun. This has happened to similar comets in the past, and would mean that ISON would move to the pile of forgettable dim comets that have passed through our solar system. If it remains intact, then its activity level would give it a naked eye brightness in November and December, providing a great view in the early mornings, just before sunrise.
Of course the outcome we all hope for is that its activity steadily increases as it journeys through the inner solar system, becoming visible to the naked eye in October, culminating in a spectacular, almost moon-bright vision in the December morning skies. This last case isn't particularly likely, but it is within the realm of possibility.
We'll just have to keep our eyes on it to find out.
What do you think will happen to Comet ISON when it enters our solar system? Will it put on a show or be a bust? Tell us in the comments!