Cassini Saturn mission ending with spectacular death plunge

Nasa scientists are preparing to kill off the Cassini space probe with a spectacular suicidal dive into Saturn's atmosphere today.

The 22ft robot craft will break into fragments and burn up as it ploughs into the ringed planet's cloud tops, ending a 20-year mission that cost £2.9 billion.

Cassini was launched in 1997 and took seven years to travel two billion miles to Saturn, before embarking on a 13-year journey of discovery that delivered a wealth of scientific data on the planet and its moons.

Now with the spacecraft running out of fuel, and soon to become impossible to steer, controllers have chosen to bring the mission to a fiery end.

Scientists expect to lose contact with the probe at around 12.55pm UK time as Cassini begins to feel the effects of drag from Saturn's atmosphere and starts to tumble, causing its dish antenna to lose sight of Earth.

At this point the craft will be roughly 930 miles above the planet's cloud tops. From then on, Cassini will start to burn like a meteor and tear apart. Within two minutes of signal loss the probe will be completely consumed.

During the dive Cassini will be travelling at around 70,000mph. Its plunge to destruction will mark the end of a series of 22 daring orbits that allowed the probe to slip between Saturn and its rings.

Because Saturn is so far away, the spacecraft's last gasp transmissions will take 83 minutes to reach Earth.

Earl Maize, Cassini project manager at Nasa's Jet Propulsion Laboratory in Pasadena, California, said: "The spacecraft's final signal will be like an echo. It will radiate across the solar system for nearly an hour and a half after Cassini itself has gone.

"Even though we'll know that, at Saturn, Cassini has already met its fate, its mission isn't truly over for us on Earth as long as we're still receiving its signal."

Right up until it beams its final signals to Earth eight of the spacecraft's 12 scientific instruments will be gathering data from the top of Saturn's atmosphere and transmitting information about its structure and composition.

Cassini's cameras will capture their final images of looming Saturn and its moons several hours earlier. They will be radioed to Nasa's Deep Space Network antenna complex in Canberra, Australia, before being posted on the mission website.

A high point of the mission came in January 2005 when a small European Space Agency lander called Huygens detached from Cassini and descended to the surface of Saturn's largest moon Titan.

The probe touched down on a pebble-strewn surface with the consistency of wet sand. It was the first successful landing on a world in the outer solar system.

As it parachuted down through Titan's thick nitrogen and methane atmosphere, Huygens captured images of features that looked like shore lines and river systems on Earth.

Scientists now know Titan has lakes and seas filled with liquid methane and ethane.

Another key discovery made by Cassini was a global ocean under the icy surface of another moon, Enceladus, that may sustain life.

The decision to send Cassini to its fiery grave was taken in order to avoid any chance of the spacecraft crashing onto Titan or Enceladus and possibly contaminating the potentially life-hosting worlds with Earth bugs.

British scientists played a major role in the mission, wholly or partly contributing several of the spacecraft's instruments.

Open University planetary scientist Professor Simon Green, who helped develop Huygens' surface science package, said: "The Cassini-Huygens mission has transformed our understanding of the second largest planet in our solar system, Saturn, with its vast ring system and its unique moons.

"Landing on Titan revealed a cold, but surprisingly Earth-like landscape, shaped by the flow of methane rather than water, and the icy volcanoes of Enceladus hint at a habitat for life below its frozen surface.

"The scientific legacy of the mission will extend long beyond its fiery end in the clouds of Saturn."

PA