Năm 2008 bắt đầu vào một cách khá bất ngờ. Một sao chổi dài nổi tiếng, Holmes, ngạc nhiên tất cả chúng ta bằng cách đột nhiên bùng lên đến độ sáng triệu lần bình thường của nó, và vì vậy tôi nghĩ rằng điều này sẽ là một thời điểm tốt để nói về các mảnh vụn vũ trụ. | Chapter 30 Cosmic Debris Encke s Comet Credit Spitzer The year 2008 began in a rather unexpected way. A long-known comet Holmes surprised us all by suddenly flaring up to a million times its normal brightness and so I thought that this would be a good moment to talk about cosmic debris -the bits and pieces of the Solar System. I was joined by two leading experts Dr Simon Green and Dr. Richard Greenfield. P Moore The Sky at Night DOI 978-1-4419-6409-0_30 Springer Science Business Media LLC 2010 117 118 30 Cosmic Debris The Sun s family is dominated by the eight planets from Mercury out to Neptune together with their satellites. But we must not forget that there are other members too comets and asteroids for example. There is also a vast amount of cosmic dust. We have just seen a comet take us by surprise so let us begin with these strange wraithlike objects which can sometimes look so much more important than they really are. A comet is not a solid substantial body similar to a planet. The only fairly substantial part is in fact the nucleus seldom more than a few miles across and made up of rocky particles and ice the mass is very small compared with that of a planet or even a satellite such as the Moon. When the comet moves in towards the Sun and is heated it may develop a tail of either gas or dust some comets may produce both. Tails always point away from the Sun. Gas-tails do this because the particles are driven out by what is called the solar wind and dust-tails because of the pressure of sunlight and light does produce a pressure albeit a very weak one . Most comets move in very eccentric orbits and there are many with short periods so that we always know when and where to expect them. Encke s Comet has a period of only just over 3 years so that it is an old friend. Like most short-period comets it seldom becomes bright enough to be seen with the naked eye. The only bright comet which returns reasonably often is Halley s whose period is 76 years it .
