Close to Halley’s comet
To prepare for the five missions to Halley’s comet, professional and amateur astronomers grouped to watch it from the ground as well as they could. Their international collaboration, named the International Halley Watch, was exemplary. First, one had to find the comet. This was the subject of a great competition, won in October 1982 by the 5-meter-diameter Palomar telescope. The comet was only 10 seconds of a degree from the position predicted by celestial mechanics, a remarkable feat. The comet was approaching, but was much less brilliant than during its previous passage in 1910. It remained far from the Earth and was at its perihelion on the other side of the Sun. Expectations had been high, so that the public was disappointed that the comet was so hard to see.
Cliché de la redécouverte de la comète de Halley au Mont Palomar en octobre 1982. La comète est le petit point au centre du cercle ; les étoiles du champ sont surexposées. La caméra CCD ayant pris ce cliché, une grande nouveauté à l’époque, est semblable à celle qui équipera plus tard le Télescope spatial Hubble (HST). L'Astronomie, 1982.
This was fortunately not the case for the probes that approached the comet. The Giotto and Vega 2 probes provided the first images of a comet nucleus. The Vega’s images were of poor quality, but they showed that this nucleus was far from spherical. The first image of Giotto was saturated, because the very bright dust emitted by the comet nucleus was centered by the camera.
Fortunately, after the initial fear, one managed to get a good image. Surprise: the nucleus was very dark, almost black, and that was why it had been so hard to see. It was probably covered by a dust layer, under which the ice was buried. It sublimated in some areas under the effect of heating by the Sun, forming jets whose extensions were the famous plumes described by Arago.
Le noyau de la comète de Halley vu par la sonde Giotto en mars 1986. Le noyau lui-même est très sombre dans les parties qui ne sont pas éclairées par le Soleil, car il est recouvert de matériaux solides surmontant la glace. Des points brillants à gauche, chauffés par le Soleil, sont les endroits d’où s’échappent les produits de sublimation de la glace interne, accompagnés de poussières, formant les jets brillants.
Crédit : ESA/ Max-Planck-Institut (MPS)
The scientific observation devices installed in the different probes generally worked well. A particularly interesting result was obtained by the infrared spectrometer IKS built by France, which detected for the first time several parent molecules directly emitted by the nucleus, before they were destroyed or ionized by the solar ultraviolet radiation. One could see the signature of water vapor (already observed with great difficulty from a stratospheric plane), carbon monoxide, carbon dioxide and formaldehyde H2CO. The encounter with Halley’s comet decidedly marked a new era in our understanding of comets.
Le spectre infrarouge de la comète de Halley, obtenu avec le spectrographe IKS placé sur la sonde VEGA. On y voit les émissions de la vapeur d’eau, du monoxyde et du dioxyde de carbone et du formaldéhyde H2CO, ainsi qu’une émission vers 3,3 micromètres (repérée par la mention CH-X) caractéristique de molécules carbonées complexes.
d’après M. Combes et al., Icarus 76, 40, 1988.
La comète de Halley vue de l’Observatoire de Haute-Provence, le 15 janvier 1986. Cette photographie est peut-être la meilleure de la comète à son passage de 1986. Elle présente un aspect classique avec sa queue de poussières. La queue de gaz est trop peu lumineuse pour être visible.
Crédit : Observatoire de Haute-Provence/CNRS