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Back to Hyakutake and Hale Bopp
First size. Hale Bopp's neucleus was probably around 30km or more in diameter. Hyakutake's was a mere 1 to 2km. however there wasn't a lot of difference in the brightness. Hale Bopp reached a magnitude of around -1 and Hyakutake zero. The reason for that was that Hyakutake came much closer to the Earth - about 10 million miles or 0.1 of an Astronomical Unit. Hale Bopp passed by at between 1 and 3 AUs. If it had come as close as Hyakutake it would have been around magnitude -6. Interestingly, Hyakutake faded rather sooner than predicted, and it's thought that this may have been because some fragments were seen to break off the nucleus, exposing more volatiles to solar heating. If so the maximum brightness was a flare rather than the more rapid than expected fading a fizzle out.
Hale Bopp had a much better tail than Hyakutake, and not just because it became the 'classic' comet tail. Hyakutake's tail was almost exclusively an ion tail with a very weak dust tail, although the SOHO observatory observed a third tail at perihelion. Hale Bopp had a sodium tail as well as an ion tail and a massive dust tail, and no-one's sure yet how the sodium tail formed. Both comets emitted X rays which were detected by the ROSAT satellite, and again no-one's sure yet how they came to be there. One theory is that the radiation is retransmitted from the Sun, another that they are formed during collisions between cometary material and the supersonic wind of plasma and particles from the Sun. Again this shows that there is a lot we don't know about comets and all our modern technology seems to do it throw up lots of questions and very few answers.
Another feature Hale Bopp showed quite dramatically through a telescope was the bow wave effect on the Sun side of the nucleus. It's thought that this is material that is ejected from the hotter side of the nucleus in fountains or jets which pulse as the nucleus rotates and then get caught by the solar wind and turned back into the tail.
Hyakutake may return after 12,000 years. Hale Bopp's period is somewhat shorter. We know its last appearance was 4200 years ago but it has been influenced by Jupiter and its period has been shortened still more to 2400 years. Who knows - maybe one day it will be the new Shoemaker Levy 9.
And finally, a thought on Jupiter and its influence.
The Earth is, as we know, occasionally hit by fairly large bits of debris from space. If there was no giant planet like Jupiter in our solar system it seems likely that bits would hit Earth more often, and might prevent any long term species development. That is, we humans might not be here. This certainly has implications for the likelihood of intelligent life in the solar systems of other stars, in that you don't just need an Earth type planet the right distance from the star - you also need a huge planet whose gravity will 'mop up' most of the debris left lying around and allow life to evolve relatively undisturbed for long enough to develop what we would recognize as intelligence.
When I gave this article as a talk, I ended with a paradox. Something brought the organic compounds necessary for life to begin to the Earth, and comets are a strong contender for being the guilty party. However as I spoke, there was an asteroid on its way which, it was thought, might just hit the Earth in thirty years' time and deal with homo sapiens in the same way as the dinasaurs. Current theory is that that one, at least, will miss.