Illustration of Cassini during its flyby of Jupiter. Photo: NASA/JPL

As the Cassini spacecraft starts its approach of Jupiter, the Huygens Probe and all its onboard instruments remain dormant. However, Huygens is not going to be totally passive. The role of Huygens in acting as a sunshield will be crucial in protecting Cassini's instruments from the heat of the Sun.

Helped on its way by an Earth swing-by in August 1999, the Cassini Orbiter is now heading towards the outer Solar System for a final gravity-assisted manoeuver at Jupiter. This final planetary swing-by is vital in acquiring the velocity needed to reach Saturn, the final destination of the seven-year interplanetary cruise. The ESA/NASA Cassini-Huygens spacecraft is now approaching the giant planet. The closest approach to Jupiter will happen at the end of December this year at a distance of ten million kilometres. Even at such a vast distance the gravity of Jupiter will be sufficient to bend Cassini-Huygens' trajectory and re-direct it to Saturn. All the scientists involved in the Cassini-Huygens mission will remember 2000 as the year of the approach, observation and flyby of Jupiter. Moreover, for the first time ever, two spacecraft, namely Cassini and NASA's Galileo, will simultaneously observe Jupiter.

Around the time of Cassini's closest approach, Galileo will be in the magnetosphere, while Cassini will be in the Solar Wind. Huygens will also play an important role in this encounter. The Huygens heat shield will be used to shade the Orbiter and its instruments from the Sun's heat. Huygens has happily been filling this role since early October 2000, as shown by the changes in temperature of key Huygens parts, monitored by the Orbiter, which are well within what the Probe is able to withstand.

Except for short periods during manoeuvres, the Probe has been shadowed by Cassini's High Gain Antenna, which until February 2000 had always been pointed towards the Sun. The Probe is equipped with a robust thermal subsystem, designed to maintain the temperature of the instruments within the allowed range throughout the mission. On the other hand, the Probe dissipates about 200 W of power during a Probe checkout, during which it gets pretty warm inside (about 35°C). Because of the super-insulation, it takes a few days for the Probe interior to cool down after a checkout. The overall temperature variations from early January until end of September 2000 of the monitored key points in the Probe.

An artist's concept of Huygens dropping toward the moon Titan. Photo: ESA

Although the actual Jupiter flyby is scheduled for December 30 this year, the instruments onboard Cassini have been collecting data on the giant planet since early October. Prior to the closest approach, Cassini is outside Jupiter's magnetosphere and is providing reference measurements on the Solar Wind for Galileo, which in the meantime is flying inside Jupiter's magnetic field. Through simultaneous collection of data from both spacecraft, scientists will be able to observe, for the first time, both the environment outside and that within the planetary magnetic field of a giant gas planet. Jupiter's magnetosphere dynamics are believed to respond to changes in Solar Wind conditions. The combined data from the two spacecraft will bring a better understanding of how the Solar Wind interacts with Jupiter's magnetic field. However, Cassini and Galileo will not be close enough to see each other, even at the time of Cassini's closest approach to Jupiter they will be separated by more than seven million kilometers.

Ground based telescopes will join Cassini and Galileo in studying Jupiter, in particular in observing the planet in the radio window, and in mapping the synchrotron emissions which are due to the interaction of energetic electrons with Jupiter's intense magnetic field. Furthermore, the ESA/NASA Hubble Space Telescope will be studying Jupiter's aurora in coordination with Cassini, starting 20 days before the closest approach until 20 days after the flyby.

The results of all these studies will greatly improve our understanding of the largest planet in the Solar System, and for the second time after the Ulysses Jupiter flyby in 1992, a significant European participation is distinguishing itself in the observation of Jupiter and its environment. For now Huygens is just a helper in this important moment in the history of the mission, but the Probe is waiting patiently for it's big moment, which will be in four years time when it is released into the atmosphere of Titan, Saturn's enigmatic moon.