Flash of Fizzle
Ken Tapping, December 4th 2013
Comet ISON has just passed behind the Sun and should now be on its way back to the outer reaches of the Solar System. As we go through December it will gradually move out of the Sun's glare and become visible in the eastern sky shortly before dawn. Unfortunately, despite the hype that comets always attract, we do not know whether this will be one of those “comets of the century” events that spectacularly dominate the sky, or a faint thing that we will be able to see only if we have the right equipment, a dark sky and know exactly where to look, or something between. Although we can calculate exactly where a comet will be, we are not able to look at it while it is still far from the Sun and estimate how spectacular it will be when it moves in close, and warms up.
Thanks to several close approaches by space probes, we now know what the bodies that produce comets look like. The body or nucleus of a comet is a lump of greyish, blackish stuff a few kilometres in size that might be smooth or covered with craters. It might be a rounded lump, resembling a potato, or dumbbell shaped, where two rounded lumps have become stuck together. That greyish-blackish material is a mixture of ice, dust, trapped gases and organic chemicals. It has much in common with what we find by Canadian city streets in late winter. Whether a comet will be a spectacle in the sky, like Comet Hale-Bopp (1995), or a fizzle, like Comet Kohoutek (1973), is dictated by the relative proportions of those ingredients and how they are distributed within the lump. Both these comets got enormous hype while still far off, and moving towards the Sun. The former lived up to it, the latter definitely did not.
These lumps are leftover materials from the formation of the Solar System. Far from the Sun, out in the cold and darkness, there are millions orbiting quietly. On occasion, a collision or some sort of gravitational interaction with another body sends one of these lumps on a path taking it into the inner Solar System, where our Earth is located. As the lump gets closer to the Sun, it starts to warm up, and eventually, the ice and other volatile materials in it start to melt and evaporate.
Unlike large, massive objects like the Earth or Moon, the tiny lumps of material that produce comets have gravitational attractions that are too weak to pull them strongly together, so they remain friable and porous. So when the material inside starts to evaporate, there are lots of paths to the surface, where it can escape into space as jets of dust and vapour. Material also evaporates from the surface. Since the gravity of the lump is so weak, the main thing holding the body of the comet together is ice. As this melts and jets of water vapour blast into space, the glue disappears and the lump starts to come apart.
At first the lost material forms a cloud around the disintegrating lump. This glows in the sunlight, making the comet visible to astronomers for the first time. As the comet gets closer to the Sun and it continues to get warmer, the rates of evaporation and disintegration increase. Now the expanding cloud surrounding the lump gets caught by the Solar Wind, which pushes it out to form a great tail, which might become millions of kilometres long. This is the point where the comet might or might not become a spectacle. If lots of dust gets released, the tail with be bright, if not, it will be much less conspicuous. Unfortunately we cannot use the observations we make when a comet is far from the Sun to meaningfully forecast what will happen when it gets closer. So, amidst the excitement when a new comet is discovered, the hype happens before we know if it's justified.
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