In order of arrivals, the two Soviet VEGA 1 & 2 probes flew by the comet on March 6th and 9th at respectively 9.000 km and 8.000 km.
In the meantime, the Japanese probe Suisei passed by the comet some 151.000 km away in a bid to study the solar wind-comet interactions.
Since the Americans wanted to be part of the running, they diverted a terrestrial plasma study probe and made it pass 28 million km away from the comet on March 25th.
Moreover, an unprecedented international cooperation of many countries - the International Halley Watch - was implemented by NASA in order to optimize the observations from ground-based observatories especially to study the temporal evolution of its activity since the comet came out in the sky in 1982 to its return at the boundary of the solar system approximately ten years later. Surprising activity bursts were detected until spring 1991 when the comet was beyond the Saturn's orbit.
Pinpoint the comet's chemical composition
The main objective of this armada was to pinpoint the chemical composition of the comets' nucleus material.
Indeed, from the ground, the observations of the region close to the nucleus were limited by the Earth's atmosphere transparency and dusts as well as the photochemistry process of the molecules ejected from the nucleus by the Sun's UV rays.
For that reason, even if the visible, infrared and radio ground observations did detect some molecules resulting from the photochemistry (such as radical CN detected in spring and OH detected by radio in Nançay in autumn 1985 resulting from H2O), only the mother molecules CO and S2 were identified.
For the VEGA probes, the challenge was huge. Since they were the first, their observations also had to enable the European Giotto probe's path to be refined in order to get the closest to the nucleus.
They mainly featured remote analysis instruments for cometary materials and Giotto was focusing on in situ analysis of gas and comet's seeds.
One of the many French contributions to the Soviet missions was the IKS infrared spectrometer (IKS standing for "IR spectrometer" in Russian) developed by LESIA (previously DESPA) and the IAS (previously LPSP) with the contribution of other laboratories. This instrument was composed of two spectrometric channels at 2.5-5 µm and 6-12 µm and an image channel (without providing any picture) the objective of which was to pinpoint the nucleus' dimensions, temperature and emissivity. The infrared detectors were cooled down by a gas pressure relief device.
IKS was mounted on a pointing platform jointly with the French TKS instrument (UV, visible and near-infrared spectrometer developed by the observatory of Besançon, France) and the TVS camera to which LAM and LATMOS contributed.
The VEGA 1's flyby was perfect for the IKS instrument. In the 2.5-5µm range, the spectra showed H20, CO2, CO and even H2CO and OCS as well as an intense emission band at 3µm related to the vibration of the C-H link in non-formally identified complex hydrocarbon molecules. These hydrocarbons could be PAHs (polycyclic aromatic hydrocarbons) or small carbonaceous seeds. Between 6 and 12 µm, a large band was attributed to silicates the signature of which was specific to olivine.
TKS completed this list by detecting PAHs such as naphthalene (C10H8) and anthracene (C14H10). Moreover, considerations about the isotopic ratio of hydrogen and carbon, the values of which did not correspond to the on-Earth measured ratio, suggested that the material of the Halley's comet did not result from solar power but would have formed in the interstellar medium like the carbon chondrite-type meteorites.
The flyby of the Halley's comet confirmed the Fred Whipple's model which described the comets as "dirty snowballs" and the VEGA and Giotto probes confirmed that they are really dirty. But still, the mystery of the cometary material composition remained.
The recent discovery of olivine, a material which forms at high temperature, in the Wild-2 comet's seeds brought back on Earth by the Stardust mission in 2006, as already suggested by IKS, showed that the turbulence mechanism in the primitive solar nebula was underestimated until now. This fact forced the solar system formation's model to be reconsidered.
Now, the quest to this primitive material is still on with the Rosetta mission which is planned to reach the vicinity of the Churyumov-Gerashimenko comet in 3 years from now. This time the spacecraft will orbit the comet in order to study its activity emergence and evolution.
Rosetta is going to drop a lander module - Philae - which is planned to land on the nucleus in order to sample the ground and the near-underground to in situ analyse them. The Philae analysis means should finally give us the chemical composition of this such primitive material. The comet formation mechanisms should be better understood and the pre-solar material existence should be clarified.
So there was a before and an after this appointment with Halley in 1986 and there will probably be a before and an after Rosetta in 2014.
- CNES Solar System programme scientist: Francis Rocard