Following the success of the Pioneer and Voyageur projects which flew past Jupiter in the 1970s, NASA started work on the Galileo project. Their plan was to develop a probe which would traverse close to the giant planet and conduct a thorough study of the planet and its many moons. The mission lasted for 14 years and made many major discoveries.
The Galileo mission was conceived as a fly-by project. The plan was to launch the space probe from the back of the Space Shuttle and allow it to make a minimum energy path to Jupiter using a slingshot manoeuvre based upon the interactions of the orbits of Earth and Venus. Upon arrival, a detachable probe would be fired into the dense Jovian atmosphere. The project was planned to be completed within two years. The achievements of Galileo after that deadline were far in excess of expectations.
The Galileo spacecraft was launched from the Kennedy Space Centre on October 18, 1989 using the Shuttle ''Atlantis''. This followed two significant delays. The first intention was to launch Galileo using the Shuttle Columbia in January 1982. It was abandoned due to development work on the shuttle and gave Galileo engineers time to build a more sophisticated probe. The second attempt to launch the Galileo mission using the Shuttle Atlantis in 1986 was abandoned due to the catastrophic failure of the Shuttle Challenger. The failure of the Challenger led to a new safety culture at NASA. The powerful, but potentially dangerous, Centaur-G liquid hydrogen fuelled booster which was planned to propel Galileo to Jupiter was swapped for a lower powered solid fuel design. The new design meant that the several gravitational slingshots using the orbit of Venus were vital for the flight to Jupiter.
Galileo was an international project. The German company Daimler Benz Aero Space AG (DASA) developed the propulsion system. The Jet Propulsion Laboratory built the “Galileo” spacecraft and managed the spaceflight. The Hughes Aircraft Company designed the probe used in descent into the Jovian atmosphere. The descent itself was managed by the Ames Research Centre. Galileo was powered through the radioactive decay of 7.8 kilograms of Plutonium 238 and was jam packed with advanced instrumentation.
The Galileo spacecraft swept passed Venus of February 10,1990 and completed two passes of the Earth on the route to Jupiter. The closest approaches to Earth took place on 8 December 1990 and on 8 December 1992. During the first fly past Earth Galileo conducted an experiment devised by Carl Sagan which looked for signs of life on Earth. The experiment which proved that evidence for life can be seen from space is a benchmark in the quest for life elsewhere.
During that first flyby the high gain antenna failed to fully deploy. Throughout the remaining mission flight engineers had to rely upon the auxiliary low gain antenna which was not able to transmit information at nearly the same rate. The Deep Space Network antennas, data compression techniques and the on board tape recorder were all used to maximise the limited data transmission facilities that were available for the remaining mission
Between the two Earth flybys Galileo made history by passed close to the asteroid 951 Gaspra on the inner rim of the asteroid belt. This was the first ever close approach to an asteroid by a terrestrial spacecraft. Gaspra was a tiny cratered and irregular body about 19 by 12 by 11 kilometres in extent. After the second pass of Earth Galileo revealed, on August 28, 1993 that the asteroid Dactyl was in orbit around 243 Ida, a larger asteroid. This was the first discovery of a natural satellite orbiting an asteroid. Dactyl appears to have a different geologic origin to that of Gaspra.
In the latter part of the voyage In July 1994 the Galileo spacecraft observed the descent of remnants of Comet Shoemaker-Levy 9 into the Jovian atmosphere.
On 13 July 1995 the Galileo spacecraft released a probe into the Jovian atmosphere. This part of the mission was extremely technically demanding and is regarded as the most complex atmospheric re-entry mission ever attempted. The probe, measuring 1./3 metres in diameter, weighed 339 kilograms and carried a 152kg heat shield. The probe slowed from a speed of 47 kilometres per second to subsonic speed in less than two minutes. Some 80 kg were burnt from the heat shield in the process. Despite conditions the probe was able to transmit data for 57.6 minutes before disintegrating under conditions of intense heat and pressure. The probe descended 150 kilometres into a Jovian atmosphere that was hotter and more turbulent than the engineers has expected.
In October 1995 Galileo experienced a somewhat alarming malfunction. The tape recorder, which had become essential for efficient data transfer because of the failure of the high gain antenna to deploy failed. For fifteen hours, until superseded by a signal from Earth the recorder was jammed in rewind mode
On December 8, 1995 the Galileo spacecraft finally entered into orbit around Jupiter. Thereafter it completed 38 circuits of the Red Planet. Each orbit lasted approximately 2 months and was structured to be highly elliptical allowing each pass to sample a different part of the magnetosphere. In the main mission Galileo examined the composition of giant clouds of ammonia that encircle the Red Planet and made extensive studies of the magnetosphere. During the orbits the mission established that the rings of Jupiter are made from dust created when meteoroids crash into the four small inner moons of Jupiter. The mission was also able ton make close observations of the principal moons of Jupiter: Io, Ganymede, Callisto and Europa.
After two years in orbit the spacecraft had completed its primary mission and directed to more speculative studies. These included spectacularly close flybys of the Jovian moons Io and Europa. These were dangerous encounters were left to the end of the mission because the spacecraft would be exposed to high levels of radiation which might damage scientific instruments and cause the spacecraft to malfunction.
The end of mission flybys produced dramatic results, both in terms of visual images and in terms of science. They showed that Io was exposed to tremendous tidal forces from the enormous mass of Jupiter and was a place of intense volcanic activity.
With respect to Europa, the science is intriguing. Galileo produced dramatic images which suggest the presence of surface ice. The surface of Europa is ruptured with similar patterns to those found in terrestrial regions in the Artic and Antarctic. If ice is present, the enormous mass of Jupiter could produce sufficient tidal heating to support subterranean water resources. Liquid water is a prerequisite for life. As a result of the Galileo mission Europa is regarded as the leading contender on which to look for extra-terrestrial life in the Solar System. Similar saltwater layers might be present under the surfaces of Ganymede and Callisto, The observations of Io and Europa continued until Galileo’s cameras were turned off due to radiation damage on January 17, 2002
When Galileo was running out of fuel in 2003 mission control decided to destroy it in the Jovian atmosphere. The decision was taken to avoid any possibility that Galileo could crash into Europa and contaminate the ice with terrestrial bacteria. The decent took place on September 21, 2003, 9 days before the completion of the 35th orbit of Jupiter.
The Galileo mission has generated an enormous wealth of information about Jupiter and its satellites. The dramatic images of Shoemaker-Levy 9, Jupiter, Io and Europa will inspire many generations of scientists. The science and interpretation behind this mission will occupy the minds of many scientists for years to come.