An artist's concept of Cassini during flyby of Venus. Photo: NASA/JPL

A very successful session entitled "The Jovian and Saturnian systems: surfaces and atmospheres - The Cassini/Huygens mission to Saturn and Titan" took place from April 28 to 29, as part of 2000 Assembly of the European Geophysics Society in Nice. The audience included mission scientists and their collaborators, who shared the same feelings that the swingbys around Venus and the Earth had not only allowed an unique opportunity for calibration operations, but had also been bringing a considerable amount of science data.

"The Huygens Probe has just landed at Nice airport!" With this announcement J.-P. Lebreton, Huygens Project Scientist, opened a very successful session on "The Jovian and Saturn systems" which took place as part of XXV General Assembly of the European Geophysics Society, held in Nice last week. In fact, the actual Probe is still on its cruise toward Saturn and Titan aboard the Cassini Orbiter, but for this special occasion a joint effort by ESA and ALCATEL from Cannes, France, had enabled the display of a full-scale model of the ESA's Huygens probe at the international airport of the Cote Azur.

The session was convened by J-P Lebreton (ESA) and co-convened by D. Matson (JPL) and covered many topics regarding surfaces and atmospheres of the Jovian and Saturnian systems. A general review of the most recent data from Cassini/Huygens mission was also presented, as well as future perspectives for Titan exploration. Two-day oral presentations were followed by a very lively poster session, which concluded a well attended gathering.

Dennis Matson, the Cassini Project Scientist, gave the opening talk, in which he pointed out the great event, scheduled for the end of this year (30 December), when for the first time two spacecraft, namely Cassini and Galileo, will simultaneously observe Jupiter. An unique stereoscopic study of the Jovian atmosphere, ionosphere and magnetosphere will be performed, also covering the morphology, geology and magnetic field nature of various Jupiter's Moon. HST and some terrestrial telescopes will also perform simultaneous observations. The great planet will be the special guest at the New Year's day party which many scientists will be celebrating in laboratories and mission control centres.

Many papers dealt with the scientific data that have already been obtained during the swingbys of Venus and Earth. Mission scientists and their collaborators shared the same feelings that the two swingbys allowed an unique opportunity for calibration operations. Without the swingby opportunity, a considerable amount of time orbiting Saturn and Titan would have had to be spent, in calibrating all the instruments.

An artist's concept of Cassini during flyby of Earth. Photo: NASA/JPL

During the poster session included presentations on calibration results for the Huygens Atmospheric Structure Instrument (HASI), the main instrument aboard the Huygens Probe, obtained during the periodical checkouts since its launch in October 1997. The zero-gravity data from the cruise phase is of particular interest for an assessment of the errors that occurred in accelerometer output. The cruise checkouts have therefore made it possible to estimate uncertainties in HASI accelerometers and this will be very useful during the mission phase.

One of the main issues to be examined was the investigation of lightning on Titan, namely the sudden high-current discharge caused by atmosphere's electrical breakdown, which occurs in regions of strong vertical atmospheric convection, very common on the Earth. During the Voyager flyby of Titan, no terrestrial-like lightning discharges were observed, whether because lightning flashes occur only locally or because the discharges may release energies below the Voyager detection threshold. However this lack of evidence does not rule out its existence. The Cassini radio and plasma wave science instrument (RPWS) onboard the Orbiter and HASI (Huygens Atmospheric Structure Instrument) onboard the Probe have to investigate more precisely the existence of lightning and, eventually, to determine the geographical areas where the probability of lightning generation is higher. Meanwhile models and predictions have been developed, some of them were also discussed in Nice. Lightning on Titan should be a local phenomenon which occurs only near the sub polar point, where the energy input is large. This means that Cassini would have no chance to detect Earth-like lightning signals via nighttime flybys. An important contribution came from the observations performed by RPWS during the two Venus flybys, which showed no evidence of lightning signals. Due to the very good sensitivity of the observations, there are strong constraints on possible Venusian lightning, at least like those on Earth, due to the absence of strong vertical convection in the dense atmosphere.

Many among scientists convened in Nice are gathering again in Padua where the HSWT is meeting for the annual session, in order to review the 5th checkout results and to prepare next checkout test, scheduled for September this year. There will also be discussion about latest science features related to the mission and assessment of the payload's performance.