226 SOLAR SYSTEM/Asteroids, Comets and Space Dust sodium It was last at perihelion about 4200 years ago, but planetary perturbations mean that it should return in about 2350 years, though of course all periods of this kind of length cannot be given accurately Its orbital inclination is 89 , so that its path lies at almost a right angle to that of the Earth During its period of visibility there were marked changes in the tails, and a spiral structure developed in the coma Comet Ikeya-Zhang of 2002 was much less striking – it became no brighter than the fourth magnitude – but is notable because it was found to be a return of the comet of 1661, and is therefore the longest-period comet to have been seen at more than one apparition It will be back once more in 2343 Figure Head of Halley’s Comet, imaged from the Giotto space craft The dark coating and the active dust jets are well seen (Photograph from the HMC [Halley Multi colour Camera]), Giotto passed 605 km from the nucleus on the night of, 13 14 March 1986 Figure Comet Hyakitake, C/1996 B2 This beautiful comet was conspicuous object briefly in April May 1996; it was obvi ously greenish, and had a long tail It was in fact a small comet, but made a fairly close approach to the Earth It will next come to perihelion in 14 000 years! time, look out for it then Life in Comets? The ‘panspermia’ theory was due to the Swedish scientist Svants Arrhenius, whose work won him the Nobel Prize for Chemistry in 1903 Arrhenius believed that life on Earth was brought here in a meteorite, but the theory never became popular, because it seemed to raise more problems that it solved The same sort of theme has been followed up much more recently by Sir Fred Hoyle and C Wickramasinghe, Table Some Great Comets Year Name 1744 1811 1843 1858 1882 1910 1927 1947 1965 1976 1996 1997 de Che´seaux Flaugergues Great Comet Donati Great Comet Daylight Comet Skjellerup Maristany Southern Comet Ikeya Seki West Hyakutake Hale Bopp Multi tailed comet; max magnitude Mag 0; 24 degree ion tail Period 3096 years Mag Sun grazer Period 517 years Mag, Most beautiful of all comets, with ion and dust tails Period 1951 years Reached mag Period 760 years Magnitude Immensely long period Magnitude 6; 35 degree tail Magnitude 5, 25 degree tail Magnitude 10; seen very near the Sun Magnitude Multiple dust tail Briefly reaches magnitude Q Very long tail The green comet Magnitude 0.5; naked eye object for over a year