ESA Science & Technology - The flybys around Venus and the Earth provided a calibration opportunity for the instruments aboard Huygens and Cassini

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 new ground based observations of Titan and other mission related work. 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. Cassini will be used as a solar wind monitor, while Galileo will be within the magnetosphere. Cassini and Galileo roles will be inverted after closest approach. Moreover, combined observations of the Io dust streams will be carried from two different locations. HST and some terrestrial telescopes will also perform simultaneous observations. The giant 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.

Special presentations 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 a unique opportunity for calibrating the instruments and getting ready for Jupiter and Saturn observations. Without the swingby opportunity, a considerable amount of time orbiting Saturn and Titan would have had to be spent, in calibrating all the instruments and learn how to use them in a new environment. The swingbys around Venus and the Earth have also been bringing in a considerable amount of new and unique science data. For instance, the fastest ever Earth magnetosphere crossing made possible a snapshot of the whole magnetosphere in about 3 hours and this had never been done before.

The poster session included a presentation on calibration results for the Huygens Atmospheric Structure Instrument (HASI), one of the six instruments 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 accuracy of the accelerometer sensors. The Huygens cruise checkouts have therefore made it possible to estimate uncertainties in HASI accelerometers measurements and this will be very useful during the mission phase.

One of the fascinating 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 have released signals 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 will investigate more precisely the existence of lightning and, eventually, determine the local time of lightning activity. 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-solar point, where the solar energy input is largest. 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, this no-detection on Venus constraints on possible Venusian lightning, at least like those on Earth, due to the absence of strong vertical convection in the dense Venusian atmosphere.

The Cassini/Huygens session was very exciting. Nevertheless, many among participant scientists arranged to meet at the end of March next year, still in Nice, for the 2001 EGS annual meeting. Hopefully, an even more exciting session on Jupiter should take place at that time, in order to review the observations that Cassini will have performed during the flyby at the end of this year. Many of the scientists from theNice Assembly gathered again in Padua the first week of May for the Huygens Science Working Team (HSWT) annual meeting, in order to review the fifth checkout results, to prepare for the next checkout in July, and to discuss latest science issues related to the mission and assessment of the payload's performance.