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Thursday, 17 January, 2002, 16:24 GMT
Galileo bids farewell to Io
Jupiter's moon Io (Nasa)
The probe will get its best view of Jupiter's moon Io
By BBC News Online science editor Dr David Whitehouse

Nasa's Galileo orbiter has made its last and closest flyby of one of Jupiter's four major moons, Io.

Io's volcanoes have revealed many surprises since they were discovered in 1979 by the Voyager spacecraft.

We're hoping to see areas we haven't seen well since Voyager imaged them back in 1979

Dr Torrence Johnson, Galileo project scientist
Scientists hope this final encounter will reveal how several of them have changed over the years.

"Galileo's days are numbered now, so it's especially exciting to visit Io one last time," said Dr Eilene Theilig, Galileo project manager at Nasa's Jet Propulsion Laboratory (JPL) in California.

"An orbital mission like Galileo gives you the advantage of getting to examine interesting places repeatedly over a period of time. That's been great for studying Io, since it keeps changing so much."

Space survivor

For its encore, the Galileo flight team at JPL aimed the orbiter just 100 kilometres (62 miles) above Io's surface at 1509 GMT on 17 January.

"The reason we're going so close is to put Galileo on a ballistic trajectory for impact into Jupiter in September 2003," said Dr Theilig.

Although there is sadness at the drawing to a close of Galileo's highly productive six years at Jupiter, scientists are delighted that it operated in orbit more than three times longer than its originally planned mission - having survived exposure to Jupiter's intense radiation far longer than expected.

Galileo space craft (Nasa)
Galileo will end its life by crashing into Jupiter
During its 33 loops around Jupiter, it has flown near Io six times and near the other three of Jupiter's planet-sized moons - Europa, Ganymede and Callisto - a total of 27 times.

The tour relied on expert navigators to use each moon's gravity to help adjust the spacecraft's trajectory toward its various encounters.

However, the fuel needed for steering the spacecraft and keeping its antenna pointed towards Earth is now almost exhausted. To avoid even a slim chance that Galileo could crash into Europa after its mission ends, Nasa will send it into the crushing pressure of Jupiter's atmosphere.

Galileo has found evidence that Europa has a deep ocean of melted saltwater under its frozen surface, heightening the need to keep the moon sterile for later studies of its potential for harbouring extra-terrestrial life.

Best-ever view

Before its final plunge, Galileo will make the first close flyby of Amalthea, a small, inner moon of Jupiter, in November 2002.

Io, with its cold, sulphur volcanoes is the most volcanically active world ever found in our Solar System. Like our Moon, it always keeps the same side facing toward its planet.

Eruption on the surface of Io (Nasa)
Eruption on the surface of Io
During its close encounter with Io, Galileo was in position for its best-ever look at the Jupiter-facing side of the moon.

"We're hoping to see areas we haven't seen well since Voyager imaged them back in 1979," said JPL's Dr Torrence Johnson, Galileo project scientist. "We'd like to know more about rates of change for volcanic features on Io."

New observations were planned for a previously inactive volcano that unexpectedly lofted a tall plume last summer.

On this swing through the inner portion of the Jovian system, Galileo was also due to examine storms on Jupiter itself and the Io torus, a doughnut-shaped band of charged particles encircling Jupiter at Io's distance from the planet.

See also:

23 Aug 01 | Sci/Tech
Galileo sends new Jupiter moon views
03 Mar 00 | Sci/Tech
Crash plan for Galileo spaceprobe
23 Feb 00 | Sci/Tech
Galileo's brush with volcanic moon
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