THE EXPLORATION OF JUPITER

Jupiter has been observed through telescopes since their first use in Europe. The four large moons of Jupiter are called the Galilean moons in honour of Galileo who first observed them on 7 January, 1610. Jupiter�s most famous feature, the Great Red Spot, was first observed by Newton�s great rival Robert Hooke, in 1664.

The first spacecraft to fly close to Jupiter was Pioneer 10. It passed within 132,000 kilometres of Jupiter and returned over 500 images of Jupiter and its moons. Pioneer 10 also collected data about Jupiter�s magnetic field: 4,000 times stronger than that of the Earth.

Pioneer 11 flew within 43,000 kilometres of Jupiter on 1 December 1974, en route to Saturn. Results from the Pioneer 11 flyby provided the first suggestion of Jupiter�s ring system. The existence of a ring system was later confirmed by direct Voyager images.

The most spectacular discoveries about Jupiter and its moons came with the Voyager missions. Voyager 1 flew within 350,000 kilometres of Jupiter on 5 March 1979. Voyager 2�s closest encounter was on 9 July 1979 at a distance of 71,400 kilometres.

The Voyager spacecraft were programmed to photograph the area where a ring system had been detected by Pioneer 11. They also acquired the better images of the Great Red Spot, than had been obtained by the Pioneer craft. Voyager 1 imaged two small moons close to the rings: Metis and Adrastea.

Io

Voyager 1 entered the Jovian system close to Io and Callisto. 35% of Io�s surface was photographed at high resolution. Scientists were amazed by images showing violent volcanic activity on Io. Nine active volcanoes were seen during the Voyager 1 flyby (more than 300 active and inactive volcanoes have now been identified). Sulphurous gases and rocks were detected spraying 300 kilometres above the surface.

Callisto

Images transmitted by the Voyagers showed Callisto to have a heavily cratered surface, but with relatively little variation in height apart from the effect of impact craters. Callisto is more heavily cratered than even the oldest areas on Ganymede implying an older geological history.

Ganymede

Voyager 2 passed closer to Ganymede and Europa complementing the data collected by Voyager 1. About half the surface of Ganymede was photographed during the Voyager flybys. Voyager images enables scientists to identify light circular patches several hundred kilometres across. These are called palimpsests and may be scars from impacts to Ganymede�s icy surface early in its history.

Europa

The Voyager spacecraft failed to acquire any high resolution pictures of Europa, however the images that were taken show a relatively young surface with few impact craters. The Voyager pictures show a reflective surfaces, criss-crossed with dark lines. It is now believed that a layer of ice or liquid water may lay just beneath the surface.

Shoemaker- Levy 9 Impact

Over the last few years our understanding of the Jovian system has advanced dramatically, thanks to Galileo, a joint US and European mission comprising an orbiter and atmospheric probe. Launched on October 18th 1989, the spacecraft imaged asteroids (Ida and Dactyl), our Moon (confirming the existence of a suspected impact basin on the Moon's far side, and revealing the Moon to have been more volcanic active than previously thought) and took a look back at Earth on its five year Voyage to Jupiter. Galileo arrived in time to observe the impact of the Shoemaker- Levy 9 comet fragments, studied the radiation environment at Jupiter, and the probe released into the atmosphere of Jupiter returned valuable new data on the composition and conditions within the upper layers of the planet.

Images of the Jupiter and the moons returned by the Galileo orbiter from nearly 30 flybys through the system are a great improvement on the Voyager pictures, and reveal surface features crucial to understanding the geological evolution, and surface processes at work on the Galilean satellites. In images of Europa, terrain resembling pack-ice, and features now regarded as evidence of materials upwelling from a comparatively warm liquid interior below the frozen crust. Images of Io reveal even more volcanic activity and clues to how the moon is being resurfaced; Io was found to possess a magnetic field, generated by its mobile interior.

Callisto, the outermost of the Galileans, and at first glance rather dull, has a very important part to play. Galileo images of the surface will provide useful crater counts, enabling researchers to determine impact history and asteroid populations in Jupiter area through time. Because the type and profile of craters are related to the strength and physical characteristics of the surface- as well as excavating material from within - the new pictures of the cratered satellites will reveal much about their geology.

Despite a few technical hitches Galileo performed remarkably, and coped very well with the high radiation environment encountered near Io. The Galileo mission was given several extensions to carry out further mapping and experiments, and only wound down in 2003, the September of which, Galileo was sent on a final journey into the atmosphere of Jupiter itself.

The discoveries of the Galileo mission have returned images and data that will keep investigators busy for years to come, and will be vital to planning future missions to the Jupiter system. The most promising target is Europa, and it is proposed to send an orbiter to carry out a detailed survey and act as a communications relay for a probe, which will burrow through the crust to explore the moon's subsurface ocean.

Spacecraft Launch date Arrival date Mission Pioneer 10 3 Mar 1972 3 Dec 1973 Jupiter Flyby Pioneer 11 5 Apr 1973 2 Dec 1974 Jupiter/Saturn Flyby Voyager 2 20 Aug 1977 5 Mar 1979 Jupiter/Saturn/Uranus/Neptune Flyby Voyager 1 5 Sep 1977 9 July 1979 Jupiter/Saturn Flyby Galileo 18 Oct 1989 7 Dec 1995 Jupiter Orbiter/Probe Ulysses 06 Oct 1990   Jupiter Flyby and Solar Polar Orbiter