Why is Comet Hale-Bopp now so bright?
One possible cause for the unusual brightness of Comet Hale-Bopp at its present location, more than 200 million kilometres outside the orbit of Jupiter, is that it possesses a very large nucleus, that is the `dirty snowball’ of dust and ice at the centre of a comet. The larger the diameter of the nucleus, the more sunlight will be reflected from its surface and the brighter will it appear. A corresponding estimate indicates that the diameter of its nucleus would be nearly 100 kilometres, as compared to about 10 kilometres for Comet Halley. However, it is also important to consider that – due to the heating action of the sunlight on its surface – the nucleus of a comet that is not too far from the Sun will emit dust particles of which many assemble as a cloud around it (the `dust coma’). These particles are moved outwards by the pressure of gas molecules emanating from the melting ice(s) in the nucleus. That this is indeed the case for Comet Hale-Bopp can be clearly seen on the first hi
