One of the most remarkable chapters in the history of planetary exploration will come to end on Sunday 21 September, when NASA’s Galileo spacecraft plunges into the dense atmosphere of Jupiter.

Galileo’s dramatic demise will bring down the curtain on 14 years of exploration that began with a launch from the Space Shuttle in October 1989. Despite the handicaps of a main communications antenna that failed to deploy and a temperamental tape recorder, Galileo went on to complete the first detailed reconnaissance of a giant gas planet.

In December 1995, it became the first spacecraft to enter orbit around the largest planet in our Solar System and the first to release a probe into its hydrogen-rich atmosphere.

Since then, the orbiter has been returning hundreds of images and huge amounts of other scientific data on the Jovian system. Researchers from a number of UK universities have played leading roles in this groundbreaking mission.

Jupiter is famous for its colourful, swirling clouds. The most notable feature in this ever-changing scene is the Great Red Spot, a giant storm system which could swallow up three Earths and which is known to have existed through for more than three centuries.

Professor Fred Taylor and Dr. Simon Calcutt of Oxford University are both co-investigators on Galileo’s Near-Infrared Mapping Spectrometer (NIMS), an instrument designed to study Jupiter’s cloud formations and atmospheric processes.

With the support of the Particle Physics and Astronomy Research Council (PPARC) and the former Science and Engineering Research Council (SERC), Professor Taylor and his colleagues helped to design and develop NIMS, the first of a new class of imaging spectrometer capable of acquiring three dimensional pictures of planetary atmospheres and measuring their temperature, composition and cloud properties.

“NIMS has enabled us to probe Jupiter’s cloud structure and map its atmospheric composition for the first time,” said Professor Taylor.

“Galileo has confirmed that no less than four different cloud layers make up the visible face of the planet, from water clouds at depth, through frozen ammonia and hydrogen sulphide to hydrocarbon haze on top,” he said. “Isolated thunderstorms associated with water clouds rise through the ammonia and appear at the cloud tops.”

NIMS images of the Great Red Spot also showed that it consists of spiralling ribbons of reddish cloud with gaps of relatively clear ‘air’ between. The main vortex is much smaller than the spot itself. Surrounding the Great Red Spot is a ring of white clouds which lie 3 to 7 km lower, and outside that a turbulent region with giant plumes of cloud and high winds.

Galileo also found that Jupiter’s atmosphere contain more heavy elements than expected.

“This suggests that Jupiter – unlike the Sun – can’t have formed all in one go,” said Professor Taylor. “It seems to be the result of lots of separate pieces coming together at different times. Some of these components, particularly the noble gases (argon etc.), originated in very cold regions, so either they migrated inwards, towards the Sun, or Jupiter formed further out than it is today.” 

Among the other aspects of the Jovian environment being studied by Galileo are the streams of dust particles surrounding the planet and influenced by its powerful magnetic field. Co- investigator on the Dust Detection System experiment is Professor Tony McDonnell of the Open University.

The experiment detects impacts from dust particles which hit a target and vaporise. Not only does this provide information on the mass of the incoming particle but an approximate orbit can be derived from the particle’s estimated direction of travel and velocity.

More than 100 dust impacts per day were sometimes recorded prior to Galileo’s arrival at Jupiter in December 1995, evidence of discrete dust streams that originate in the planet’s vicinity. The impacts were caused by tiny particles, no larger than specks of cigarette smoke, that probably came from the Jovian moons, particularly from volcanic eruptions on Io. The dust instrument also recorded hits from larger, micrometer-sized grains, some of which seem to be interstellar dust which has arrived from outside the Solar System.

The instrument is expected to operate through the final hours of Galileo’s approach to Jupiter, sending back unique information on the nature of the particles that make up the thin gossamer ring as the spacecraft dives through the innermost region of the planet’s dark ring system.

One of the most intriguing scientific puzzles investigated by Galileo is the link between Jupiter’s enormous magnetic field and its four large moons. The magnetic field, which is the  largest and most powerful of any planet, was carefully studied by the spacecraft’s magnetometer during each flyby of one of these satellites.

To everyone’s surprise, analysis of the flyby results shows that, whereas Ganymede, Europa and Callisto possess internally generated magnetic fields, this not true of the smaller satellite Io. Spacecraft data also suggest that saltwater oceans exist beneath the icy exteriors of the three outer moons. Some scientists speculate that alien life forms may survive in the dark depths of a Europan ocean.

One of the designers of the magnetometer on Galileo was Professor David Southwood of Imperial College, London, now the Director of Science for the European Space Agency. Dr. Michele Dougherty of Imperial College is also a member of the magnetometer team.