Monday, October 11, 1999 Published at 10:15 GMT 11:15 UK
Galileo sweeps over Io's volcanoes
Scientists hope Galileo will have flown through a volcanic plume
By BBC News Online Science Editor Dr David Whitehouse
At 0600 GMT on Monday, the Galileo spacecraft swept within a few hundred kilometres of Io, the innermost moon of Jupiter. It was its closest ever approach to this strange volcanic moon.
Galileo remained in contact with ground controllers at Nasa's Jet Propulsion Laboratory in California and appears to be making its science observations. If all goes as planned, it will return data to Earth over the coming weeks.
The sulphur is billowed hundreds of kilometres up into the sky by at least a dozen active volcanoes, making Io the most volcanically-active world in the Solar System.
As well as the active vents scattered over its surface, there are extensive frozen plains, mountain ranges and volcanic rings the size of England.
Io's activity comes from a gravitational tug-of-war with mighty Jupiter.
The result is a massive input of energy into the world that becomes heat inside it providing the volcanic energy.
In 1979, the Voyager spaceprobe that flew through the system detected these alien volcanoes. More recently, the Hubble Space Telescope has been able to observe plumes above them.
Astronomers are hoping that Galileo will actually fly through a plume.
Io is also the hottest place in the Solar System outside the Sun. Parts of the volcanoes can reach temperatures of 1,500 degC (2,800 degF).
It is the first time that Galileo has flown so close to Io. Until now the closest images taken of the volcanic moon were from a distance of 129,000 km (80,000 miles).
When Galileo passes within 600 km (380 miles), objects as small as 12 metres (40 feet) will be visible.
A second Io flyby is planned for 25 November as the finale to Galileo's two-year, extended mission orbiting Jupiter.
Galileo reached Jupiter in December 1995, entering orbit and dropping a probe into the Jovian atmosphere. It completed its main mission in December 1997 and then began its current extended mission.
Engineers hope that it will go on, if it survives the intense radiation field around Io. Its fuel will run out towards the end of next year but before that it will be moved into a wide circular orbit around Jupiter where its sensors could continue working for many years.