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Last Updated: Tuesday, 11 March 2008, 16:34 GMT
Cassini makes audacious flyby
By Jonathan Amos
Science reporter, BBC News

Artist's impression of Enceladus (SPL)
Activity on Enceladus has been the big discovery on the Cassini mission

Such is the interest in Enceladus that Nasa directed its Cassini spacecraft to pass just 50km from the Saturnian moon.

The flyby took the probe through the plumes of icy particles emanating from the enigmatic cracks at the south pole dubbed the "tiger stripes".

The cause of this activity has developed into the big scientific question of the flagship mission.

The pass on Wednesday was designed to allow Cassini's instruments to sample the plume particles directly.

This should help scientists address the tantalising issue of whether there is an ocean under Enceladus' icy crust.

Trade off

However, commanding a probe more than a billion km from Earth to fly so close to this 500km-wide moon was no small feat.

The Viking mission may have gone to within 20km of the Martian moon Phobos, but there is nothing in deep Solar System exploration history to compare with Cassini's pass.

If you are going to take a risk, you've got to see some benefit
Bob Mitchell, Cassini programme manager
And at one stage in the planning, Cassini was going to go even closer to the surface.

"The scientists went to the navigators and said, 'hey, how close can you go?" recalls Cassini programme manager, Bob Mitchell. "Well, they thought that was a great sport and said, 'we can go to 25km'. And the scientists said, 'well, let's do it then'."

But Mitchell said the flyby distance was eventually raised to provide some extra margin.

He told BBC News: "We concluded that 25km was safe, but the increase in the science you get between 25km and 50km isn't all that great; and so my view was, if you are going to take a risk, you've got to see some benefit. I didn't see the gain being big enough to take any risk."

'No worry'

It should be said that at the moment of closest approach, Cassini was not actually in the plumes.

Closest approach occured just below the equator, as the spacecraft swept down from the north on a trajectory that eventually ducked under the moon at the south pole.

The tiger stripes, or sulci, are now the focus of intense study

At the plumes, Cassini should have had an altitude no lower than about 190km.

The particles it will have flown through are very small - about the width of a human hair - and moving in a stream of gas that leaves the surface at some 1,300km/h.

The encounter should not have presented any danger to Cassini.

"There's nothing to worry about," said Professor Carl Murray, a UK scientist working on the mission.

"The particles are essentially E-Ring particles and we go through the E-Ring all the time; these are micron-sized. The risk to the spacecraft comes from centimetre-sized particles and there's no indication that Enceladus is producing anything like that."

Closing theories

The spacecraft was turned to give the prime view to its Ion and Neutral Mass Spectrometer (INMS) and its Cosmic Dust Analyzer (CDA).

These instruments will have measured the density, size, composition and speed of the gas and the particles.

Enceladus (Nasa/JPL/SSI)
The plumes are thought to be the source of Saturn's E-Ring
"There are two types of particles coming from Enceladus: one pure water-ice, the other water-ice mixed with other stuff," explained Sascha Kempf, the CDA's deputy principal investigator from the Max Planck Institute for Nuclear Physics in Heidelberg, Germany.

"We think the clean water-ice particles are being bounced off the surface and the dirty water-ice particles are coming from inside the moon. This flyby will show us whether this concept is right or wrong."

Several gases, including water vapour, carbon dioxide, methane, perhaps a little ammonia and either carbon monoxide or nitrogen gas, make up the gaseous envelope of the plume.

"We want to know if there is a difference in composition of gases coming from the plume versus the material surrounding the moon. This may help answer the question of how the plume formed," explained Hunter Waite, from the Southwest Research Institute, San Antonio, US, who leads the INMS team.

Picture limitation

There is an intense debate in the scientific community about the origin of the jets and their associated temperature "hot spots" on the surface. Several competing theories are in play.

Some have suggested the idea of occasional venting from a pressurised sub-surface reservoir of water that is maintained by the heat of tidal forces and residual radioactivity in the rocky core.

Others prefer the idea of ice and gases that are compressed and trapped in clathrates, frozen bundles that suddenly release their gases when they are exposed as the tiger stripes open and flex.

If there is a substantial body of liquid water below the surface being sustained by an energy source, it means Enceladus would have at least some of the key ingredients needed for microbial life. Hence, the need to learn more.

The flyby occured just after 1900 GMT on Wednesday. There will be no really close-up images from the pass.

In order to give the INMS and the CDA the ideal observing position, the spacecraft had to be orientated in a way that resulted in its optical imaging equipment being turned towards space.

The cameras will have imaged Enceladus on the way in and out, however.

These pictures will reveal northern regions of the moon previously not captured by Cassini.

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