Jupiter delights Cassini's European scientists
14 January 2000Saturn and its mysterious moon Titan are the primary target of the NASA/ESA CassiniHuygens mission, but the final destination is still a long way away. The spacecraft, which has just passed the closest approach in a swing-by of Jupiter, will take four more years to reach the 'king of the rings' and start studying its atmosphere, rings, interior and magnetic field environment, as well as Titan and the planet's other moons. Huygens, ESA's first planetary probe, will have to wait longer, until after arrival at Saturn, to enter Titan's atmosphere and explore this mysterious cold world.
The Cassini Orbiter is carrying onboard, in addition to Huygens, a suite of 12 scientific instruments. Some of them had already gathered scientific data during the swing-bys of Venus and Earth. More unique data are being gathered by 10 out of the 12 instruments during the 6-month long Jupiter observation campaign which started on 1 October 2000.
The planetary swing-bys were required to boost the spacecraft further towards its final destination, Saturn. But they also provide unique opportunities for calibrating the complex instruments in preparation for arrival at Saturn. Exciting new observations were made during the Venus and Earth fly-bys. For example, the Visual Infrared Mapping Spectrometer (VIMS) has detected for the first time thermal emissions from the surface of Venus at 0.85 and 0.90 um, an observation recently published in Icarus, (Icarus 148, 307-311, 2000). Dr. Angioletta Coradini, from the University of Rome, who is associated with that work said, "Of course this was not the first time we have observed Venus, but the unique and new capability of the Cassini/VIMS instrument told us something about Venus we did not know before". The main scientific results obtained by Cassini during the Venus and Earth flybys will be published in a special section of the Journal of Geophysical Research before mid-2001.
Now it is Jupiter's turn, and for the next couple of months - until March 2001 - Cassini Orbiter instruments are conducting a detailed investigation of the Jupiter system. For the first time, Cassini and its brother spacecraft, NASA's Galileo, are performing dual spacecraft observations of Jupiter. On its approach to Jupiter, Cassini looked at the dynamics of the planet's atmosphere and recorded spectacular movies. Cassini also studied the Io-torus and analysed the chemical composition of the Jovian dust streams. A joint Galileo/Cassini observation of the Io dust stream was performed on 30 December. During the Jupiter observation campaign the Cassini spacecraft had to keep a safe distance from the planet. The closest approach, which took place on 30 December 2000, was at 10 million kilometers, roughly 130 times Jupiter's radius. This is however what was required to get the right boost to send Cassini-Huygens to Saturn. At this distance, Cassini flew on the flank of Jovian magnetosphere. The dual observations by Cassini and Galileo will truly provide a unique view of how the Solar Wind influences Jupiter's magnetic field environment and how Jupiter's magnetic field reacts to Solar Wind variations.
Roughly, half of the scientists involved in Cassini Orbiter investigations are European. Together with their American colleagues, they share the same feelings of excitement now that, for the first time ever, two spacecraft are simultaneously observing Jupiter, giving a unique stereoscopic study of the Jovian system. In 1992, the ESA/NASA Ulysses flyby provided the first opportunity for European scientists to be directly involved in the exploration of Jupiter. This time, once again, expectations are very high. The ESA/NASA Hubble Space Telescope also joins Cassini and Galileo, in particular for observations of the Jovian aurora.
Scientists at the Max Planck Institut in Heidelberg, who are responsible for the Cassini Cosmic Dust Analyzer (CDA), were particularly impatient for the Joint Galileo/Cassini Io dust stream observations to take place. The CDA was developed under the leadership of the German group has been designed to study the Saturnian dust environment. However, during the cruise phase, the instrument, which was switched on in March 1999, has already provided exciting data on interplanetary dust. Dust grains were a key ingredient during the formation of the Solar System. That is why studying the interplanetary dust environment may shed light on the processes that led to the formation of moons and planets in our Solar System. Cassini's Jupiter flyby offers the unique opportunity to observe and characterize the chemical composition of the Jovian dust streams first discovered by Ulysses.
These dust streams are very interesting objects. Until Ulysses approached Jupiter in 1992 there was no information about their existence. "Previous NASA spacecraft that flew by Jupiter, Pioneer 10 and 11, carried a dust detector that was about 100 000 less sensitive (in mass) than the Ulysses instrument. They could only detect very big (mass > 1/1000 microgramme) particles which did not show any of the phenomena we now see with Galileo and Cassini", says Dr. Eberhard Gruen, Head of the Heidelberg group and responsible for the Ulysses, Galileo and Cassini dust instruments.
Ulysses detected collimated dust streams originating from the Jovian system. Recently an international team of scientists, using data from Ulysses and Galileo, suggested Io as the source of the streams. The volcanoes on Io eject small particles, which charge up in the magnetosphere and are subsequently accelerated to very high speeds in the inner Jovian system before they escape on straight trajectories. According to this mechanism, Io's volcanoes and dust streams are closely connected, and by studying the dust streams it might be possible to monitor Io volcanic activity.
Therefore, the Jupiter flyby provides the Cassini CDA, together with Galileo's dust detector, with a unique opportunity to further characterize the Io-generated dust streams and to measure their elemental composition. Just before the Cassini Jupiter flyby on 30 December, both spacecraft were linked by a single dust trajectory and the particles ejected from Io intersected both spacecraft.
Beside the CDA, there is another instrument onboard Cassini that is under European leadership. That is the magnetometer, or in short MAG, the result of an international cooperation headed by Imperial College, London. The instrument's main objectives are to measure Saturn's magnetic field and to characterize the planet's magnetic environment. Cassini's magnetometer is mounted on an 11 metre long boom to keep it away from the magnetic disturbance generated by the spacecraft. The MAG boom was deployed a few days before the Earth flyby, which allowed for calibration of the sensors in the well-known Earth magnetic field. As the spacecraft neared Jupiter, the MAG made its contribution to the understanding of the Solar Wind/Jupiter magnetic field interaction. This provided an opportunity for European scientists to make another significant contribution to the international investigation of the Jovian system.
CDA and MAG symbolize only the tip of the iceberg of the European involvement in Cassini-Huygens. Most of the instruments onboard the Orbiter and the Probe are the fruits of international collaboration between European and American institutes.
Dr. Angioletta Coradini
Istituto di Astrofisica Spaziale Reparto Planetologia CNR
Area di Ricerca di Tor Vergata - Via del Fosso del Cavaliere
Phone: +390 6 4993 4184
Fax: #00390 6 2066 0188
Dr. Eberhard Gr|n
Max-Planck-Institut f|r Kernphysik
Phone: +49 6221 516 478
Fax: #0049 6221 516 324
Prof. D. Southwood, IC, London
Space and Atmospheric Physics
Prince Consort Road
London SW7 2BZ
Phone: +44 20 7594 7757
Mobile: Fax: #0044 20 7594 7772
Dr. Jean-Pierre Lebreton
Huygens Project Scientist
+(33) 71 565-3600