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Wednesday, 23 February, 2000, 12:44 GMT
Galileo's brush with volcanic moon
Passing close to volcanic Io (Nasa impression)
Passing close to volcanic Io (Nasa impression)
By BBC News Online science editor Dr David Whitehouse

The Galileo spacecraft, in orbit around Jupiter, has made its third and closest flyby of the planet's moon Io. This is the most volcanic body in the Solar System.

Observations from Earth-based telescopes by the International Jupiter Watch suggest that a volcanic outburst was underway on Io as Galileo passed by.

A volcanic eruption on Io
A volcanic eruption on Io
Galileo flew only 200 km (124 miles) above Io's surface at 15:32 GMT on Tuesday. During its previous close flybys of Io, Galileo has gathered a wealth of data as well as stunning pictures.

Io has volcanic vents that spew sulphur plumes over 100 km (60 miles) above the surface. The sulphur falls back and covers Io, creating a dramatic red, yellow and black landscape.

"Io's volcanoes are so active that the moon's surface is always changing, and with each flyby we get new and different observations," said Dr Torrence Johnson, Galileo project scientist at Nasa's Jet Propulsion Laboratory.

"This time we expect to be able to observe the effects of the eruptions we saw in the October and November Io flybys."

Risky mission

Io orbits so close to Jupiter that it is bombarded by radiation from the giant planet's radiation belts. The radiation can disrupt Galileo's systems or even disable its instruments, but mission planners believe potential gains in scientific knowledge outweigh the risks.

Nonetheless, the encounters were planned near the end of Galileo's mission when the spacecraft had already returned volumes of pictures and information from Jupiter and its moons.

Third approach to the volcanic moon
This was the third approach to the volcanic moon
"Although we gathered some great images and data during the previous Io flybys, the radiation did cause some problems, and we won't be surprised if that happens again this time," said Galileo Project Manager Jim Erickson of JPL.

"Galileo has already survived more than twice the radiation it was designed to withstand, so we're keeping our fingers crossed that it will complete this encounter with flying colours."

Galileo has been in orbit around Jupiter since December 1995. It was originally intended to spend two years studying Jupiter, its moons and its magnetic environment. When that original mission ended in December 1997, it was followed by a two-year extended mission, which ended in January 2000. This Io flyby is part of an additional extension, called the Galileo Millennium Mission.

End of mission

Galileo has two close flybys of the frozen Jupiter moon Ganymede planned for later this year. After that, it may be commanded to crash into one of Jupiter's moons or even into the atmosphere of Jupiter itself.

According to scientists, such a dramatic end may yield more scientific returns than just letting it run out of fuel in orbit.

But there is one moon that Galileo will be kept well away from, the frozen, ice-crusted world called Europa. Astronomers believe that there is the possibility that in Europa's under-ice oceans there may be primitive forms of life.

Even though it has been in space for over a decade Galileo may still have viable Earth microbes on it, so it is important not to contaminate Europa.

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09 Feb 00 |  Sci/Tech
Jupiter's thunderous rising damp
11 Jan 00 |  Sci/Tech
Spacecraft finds alien ocean
20 Dec 99 |  Sci/Tech
Io's fountains of fire
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