|
The Vega 2 spacecraft of the Halley's exploration mission was equipped with a rotatable platform designed to accurately follow the cometary inner region during the observation sessions. Among the three optical instruments fixed on this platform was a multi-element spectrometer designed to obtain a detailed spectroscopic description of the inner part of the coma which could not be observed from a ground or satellite-based telescope due to the high optical thickness of the comet head. The instrument was composed of two parts. The first one was a Cassegrain telescope of focal length 350 mm which formed an image of the comet on the entrance slit of the spectrometer. Its hyperbolic secondary mirror could be sequentially rotated around two perpendicular axes to explore a 2°x1.5° field of view with a moderate spatial resolution. The second part was a static spectrometer with a concave holographic grating and an intensified linear photodiode array. The spectral response function presented a width at half maximum equal to 4.5 pixel widths when the entrance slit aperture was equal to 0.050 mm (2 pixel widths). The elementary exposure time was set equal to 400 ms. The video signal was digitized on 12 bits. Eight consecutive spectra were accumulated inside the RAM in order to transmit a complete spectrum every five seconds. During the cometary encounter sessions on March 8, 9, 10, 1986, about 3000 spectra were transmitted to the Earth. The following emissions could be identified: CO2+, OH, NH, CN, C3, OH, C2, NH2, H2O.+ Very close to the nucleus the ejection rate of water vapor was directly deduced from its absorption at 121.6 nm. After an interplanetary journey of 15 months the instrument worked satisfactorily and provided a large quantity of new data about Halley's comet
|
