Physics and chemistry of comets: recent results from comets Hyakutake and Hale–Bopp Answers to old questions and new enigmas

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Abstract
1996 and 1997 years were very prolific years for cometary studies. In March 1996, C/1996 B2 (Hyakutake), a medium-bright comet, passed at only 0.10 astronomical units (AU) from the Earth. On 1st April, 1997, C/1995 O1 (Hale–Bopp) passed at perihelion. This comet was 10 to 100 times brighter than comet Halley when compared at the same distance. It could be studied and monitored from its discovery in July 1995 as far as 7 AU from the Sun until its perihelion at 0.9 AU, where it was the object of extensive observations. It was thus possible to study these two exceptional comets with modern instrumentation covering the electromagnetic spectrum from radio to X-ray domains. Our knowledge of the composition and nature of comets has been transformed. New cometary molecules were observed, as well as isotopic species. New physical processes could be studied, owing to the large amounts of matter escaping from these objects, to the proximity of Earth allowing a detailed investigation of the region close to the nucleus (comet Hyakutake), or to the unusual physical conditions far from the Sun (comet Hale–Bopp). Selected results of these observations, relevant to the chemistry and physics of molecules and grains, are discussed. It is shown that whereas some old problems are now solved, new ones have emerged. The needs for laboratory data and theoretical work are pointed out.

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