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Last Updated: Friday, 19 September, 2003, 16:10 GMT 17:10 UK
Jupiter probe set for impact
By Dr David Whitehouse
BBC News Online science editor

Copyright astroart.org
Impression of Galileo's end
The US space agency (Nasa) is about to destroy its long-serving Galileo spacecraft in Jupiter's atmosphere.

On Sunday, the probe will plunge into the giant planet's stormy clouds to be consumed by heat and pressure.

Despite some technical glitches, Galileo has studied Jupiter and its moons for far longer than was planned.

It has to be destroyed so that any stowaway microbes on the probe do not contaminate Europa, one of Jupiter's moons, on which there may be life.

Planetary protection

Protecting Europa is important as it is widely believed to be a promising habitat for life.

Were bacteria from Earth to infect Europa, perhaps in pools of water warmed by the radioactive plutonium the spacecraft uses to generate electricity, it would compromise future attempts to investigate the moon for indigenous life.

"It seems like a good place where, potentially, you can have life and it also seems like a place where Earth life would find it a nice place to live," says John Rummel, Nasa's planetary protection officer.

Volcano on Io
A sulphur plume rises above Io
Nasa usually scrubs its spacecraft clean of microbes to prevent contamination of other places in the Solar System.

But that was not done with Galileo, which Nasa originally intended to leave in orbit around Jupiter.

However, the promise of Europa, and the possibility that Galileo could have accidentally collided with it, convinced Nasa to destroy the spacecraft.

"We in our infinite wisdom thought nothing could survive in those harsh environments, but we are learning every day about things that can," says Claudia Alexander, Galileo's latest project manager.

Successful mission

The 14-year mission has been among Nasa's most successful, despite a few technical problems.

Shortly before Galileo arrived at Jupiter in 1995, it released a small probe that plunged into the planet's atmosphere to return a "weather report" for the gas giant.

Astronomers believe that the probe plunged into an unusually hot and dry region of the planet's clouds.

During its very first orbit of Jupiter, Galileo passed Io and revealed how the tiny moon had changed since the Voyager flyby of 1979.

In particular, close-up images of the Ra Petara volcano showed sulphur lava flows emerging from a central caldera.

The ice plains of Europa
Life could exist on ice-crusted Europa
Above the volcano, Galileo, complemented by observations made by the Hubble Space Telescope, scrutinised a blue plume 100 kilometres (60 miles) high, composed of sulphur dioxide gas and snow.

Even on its final orbit, Galileo observed Io's most powerful volcano, Loki, in infrared light, seeing freshly exposed material on the shores of a lava lake.

Another highlight has been the moon Europa. This ice-crusted world, covered in ridges and plates, may have a warm water ocean, and perhaps life, beneath its icy surface.

'The flavour of a wake'

Galileo also looked at small, elongated moons that, judging from the large craters that pepper them, have barely survived to the present time.

The swirling atmosphere of Jupiter was also a key object of study. Galileo made some extensive observations of its equatorial cloud belts and circulating ovals of clouds, some of which had long lifetimes.

Nasa hopes to obtain some more scientific measurements from Galileo before its demise.

When the end does come, scientists and engineers associated with the mission are expected to gather to mark the occasion.

"It will have some of the flavour of a wake," says Claudia Alexander.

Sue Nelson reports

Galileo's Jupiter mission winds down
27 Feb 03  |  Science/Nature
Europa's ice crust probed
18 May 03  |  Science/Nature
More moons for Jupiter
07 Apr 03  |  Science/Nature

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