Spectroscopic exploration of the three H-comets: Halley, Hyakutake and Hale-Bopp

On March 9, 1986, the Vega 2 spacecraft approached Halley's comet at a distance of 8030 km. During the very brief encounter time, the inner coma was scanned by the optical axis of the TKS spectrometer. This permitted a multi-spectral description of the coma to be obtained at small nucleocentric distances. Two jets and a region called the "valley" were identified. Radial profiles originating at the nucleus were drawn for the cometary emissions: OH, NH, CN, C3, CH, and C . Using ratios at2 different wavelengths, evidence for a diffuse source was shown. The excitation processes differ if the emission comes from a parent molecule issued by the nucleus or by a diffuse source. For instance, the C2 radical was found to be "hot" if emitted from a jet. The analysis of the color of dust showed that dust particles suffered fragmentation at distances less than 6000 km. In the inner coma, several bands in the 342-384 nm spectral range progressively emerged when the nucleocentric distance became less than 1000 km. It was suggested that these UV fluorescence bands might be due to polycyclic aromatic hydrocarbons. The same type of analysis was employed during an observation program of comets Hyakutake and Hale-Bopp. Multispectral imagery was obtained using four narrow-band filters to measure the excitation temperature of C and the color of dust. A diffuse source could be2 identified in Hyakutake. The S molecule could be identified during one of the last2 observation sessions of IUE. In the case of Hale-Bopp, its activity was monitored from October 1996 until March 1997. An important increase in the comet activity was recorded when its heliocentric d istance became less than 2 AU.