Page last updated at 15:41 GMT, Wednesday, 16 April 2008 16:41 UK

Nasa extends Saturn probe mission

Artist's impression of Cassini spacecraft during Enceladus flyby (Nasa/JPL/SSI)
The extended mission will include further flybys of Saturn's moons

The US space agency (Nasa) has extended the international Cassini-Huygens mission by two years.

The unmanned Cassini-Huygens spacecraft entered orbit around Saturn in 2004 on a mission that was supposed to come to an end in July this year.

The two-year mission extension will encompass some 60 extra orbits of Saturn and more flybys of its moons.

These will include 26 flybys of Titan - its biggest moon - seven of Enceladus, and one each of Dione, Rhea and Helene.

Bob Mitchell, programme manager for Cassini-Huygens at Nasa's Jet Propulsion Laboratory (JPL), in California, commented: "The spacecraft is performing exceptionally well and the team is highly motivated, so we're excited at the prospect of another two years."

Dr Rosaly Lopes, also from JPL, told BBC News: "We're very pleased. We were expecting Nasa to extend Cassini for another two years, we had been told to plan for it, so we had already done a lot of the planning and decided what the tour was going to look like.

"But it's nice to actually have the news out, because you never know up until the point when they sign on the dotted line."

'Earth-like' world

The mission has made stunning discoveries about the Saturn system since it arrived at the ringed planet four years ago.

Its studies of the largest moon, Titan, have provided a glimpse of what Earth might have been like before life evolved. Conditions on the moon are believed to resemble those on our own planet 4.6 billion years ago.

The European Huygens probe was built to explore Titan's atmosphere, weather and its surface. Huygens piggybacked on Cassini, separating from the orbiter in December 2004 to begin its journey to the orange-tinged satellite.

In January 2005, Huygens parachuted through Titan's thick haze and touched down on the surface, and returned data for several hours before succumbing to the cold.

Saturn and its rings (Nasa/JPL/SSI)
The mission has revealed new targets for future exploration

Cassini's observations of the moon from space have revealed Earth-like features such as lakes, rivers, channels, dunes, rain, snow, clouds, mountains and possibly volcanoes.

Unlike Earth, Titan's lakes, rivers and rain are composed of methane and ethane, and temperatures reach a chilling -180C (-290F).

Although Titan's dense atmosphere limits viewing the surface, Cassini's high-resolution radar coverage and imaging by the infrared spectrometer have given scientists a better look.

"We're going to have a lot more Titan flybys," Dr Lopes said of the extended mission.

"These flybys are highly contested because everyone wants to look at Titan with the different instruments, and so the more flybys the better. With radar, it's going to allow us to map much more of the surface."

New tricks

The Enceladus moon, regarded as "just another ball of ice" until Cassini arrived, has now become a high priority for further exploration.

The spacecraft found evidence for geysers of water-ice jetting from the surface.

These geysers, which shoot out at a distance three times the diameter of the moon itself, feed particles into Saturn's outermost ring.

Artist's impression of Huygens probe (Esa)
Huygens is shown on Titan's surface in this artist's impression

In the extended mission, Cassini could come as close as 24km (15 miles) from the moon's surface.

Other activities for Cassini scientists during the extended mission will include monitoring seasons on Titan and Saturn, observing unique ring events - such as the 2009 equinox when the Sun will be in the plane of the rings - and exploring new places within Saturn's magnetic "envelope" - or magnetosphere.

Jim Green, director of Nasa's planetary science division in Washington DC, said the extended mission would allow the science community and the public to continue to share in "unlocking Saturn's secrets".

Nasa said three of the science instruments on Cassini were suffering from minor ailments, but the impact on data gathering was minimal.

The spacecraft will have enough propellant left after the extended mission to potentially allow a third phase of operations.

Science from the extended mission could lay the groundwork for possible new robotic missions to Titan or Enceladus, which are under study by Nasa and the European Space Agency (Esa).

Cassini-Huygens was launched on 15 October 1997, from Cape Canaveral, Florida, taking seven years to make the 3.5 billion km (2.2 billion miles) journey to Saturn.

The Cassini-Huygens mission is a cooperative project between Nasa, Esa and the Italian Space Agency (Asi).

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