In the six years it has orbited Jupiter, Galileo has studied the planet’s atmosphere, satellites and surrounding magnetosphere. Launched in 1997, Cassini-Huygens is heading for Saturn where it will make similar observations.

To boost its energy for the journey to Saturn, Cassini-Huygens performed a fly-by of Jupiter in January 2001, when the planet’s magnetosphere was unusually large. This enabled the craft to make direct observations of Jupiter’s large magnetic field. One team found that variations in the polar aurorae on Jupiter arose from shock waves propagating out from the Sun (D Gurnett et al p985). This process is similar to that on Earth, where the solar wind can cause magnetic storms.

Another group has shown that it is valid to use the Earth’s magnetosphere as a model for Jupiter’s magnetosphere (W Kurth et al p991). The changing magnetic field of Earth has previously been measured by several space-based experiments, and the conjunction of the Cassini-Huygens and Galileo spacecraft has allowed the astronomers to obtain a dynamic picture of Jupiter’s changing magnetosphere for the first time.

Synchrotron radiation emitted by Jupiter arises from electrons spiralling in its magnetic field, and has previously been observed with Earth-based telescopes and on previous missions. But Cassini-Huygens detected synchrotron emission with a frequency of 13.8 GHz on its fly-by, which suggests that ultra-relativistic electrons in Jupiter’s magnetosphere have energies of 50 MeV – some 30 MeV more energetic than previous studies had indicated (S Bolton et al p987).

This discovery challenges existing theories of how charged particles are accelerated in the magnetospheres of planets, and the researchers speculate that electrons in Jupiter’s radiation belts may be accelerated by processes similar to those on Earth. New theories were needed to explain the existence of relativistic electrons in the Van Allen radiation belts – two doughnut-shaped regions of charged particles above the equator.

Other achievements made by the Cassini-Huygens and Galileo missions include the detection of a hot wind of neutral particles originating from the Jovian moon Io, and the discovery of ultraviolet emission from Jupiter’s atmosphere caused by the magnetic effect of the moons Ganymede and Europa.