NASA's Cassini spacecraft has completed its death plunge into Saturn, ending a fruitful 20-year mission to explore one of the solar system's most intriguing outer planets.

Confirmation of Cassini's demise came at about 11:55 GMT. That's when radio signals from the spacecraft - a final burst of scientific data - came to an abrupt halt.

The radio waves went flat, and the spacecraft fell silent.

"Cassini showed us the beauty of Saturn. It revealed the best in us. Now it's up to us to keep exploring," NASA tweeted.

The density of the atmosphere caused the spacecraft to tumble and sever its radio link. Soon after, friction with the atmosphere should have caused Cassini to burn up and disintegrate, much like a meteor.

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The Cassini-Huygens mission, a cooperative project of NASA, the European Space Agency and the Italian Space Agency, was launched in October 1997 and reached Saturn's orbit in 2004.

Cassini's planned demise is a way of preventing any damage to Saturn's moons Titan and Enceladus, which scientists want to keep pristine for future exploration because they may contain some form of life.

"It will be sad to see Cassini go on Friday, especially as the instrument we built is still working perfectly," said Stanley Cowley, professor of solar planetary physics at the University of Leicester.

"But we recognise that it is important to bring the mission to an end in a tidy and controlled manner."

Three other spacecraft

Three other spacecraft have flown by Saturn - Pioneer 11 in 1979, followed by Voyager 1 and 2 in the 1980s.

But none have studied Saturn in such detail as Cassini, named after the French-Italian astronomer Giovanni Domenico Cassini, who discovered in the 17th century that Saturn had several moons and a gap in between its rings.

Cassini was launched from Cape Canaveral, Florida in 1997, following which it spent seven years in transit and then another 13 years orbiting Saturn.

In that time, it discovered six more moons around Saturn, three-dimensional structures towering above Saturn's rings, and a giant storm that raged across the planet for nearly a year.

INFOGRAPHIC: Goodbye, Cassini

The 6.7x4 sq metre spacecraft is also credited with discovering icy geysers erupting from Saturn's moon Enceladus, and hydrocarbon lakes made of ethane and methane on Saturn's largest moon, Titan.

In 2005, the Cassini orbiter released a lander called Huygens on Titan, marking the first and only such landing in the outer solar system, on a celestial body beyond the asteroid belt.

Huygens was a joint project of the European Space Agency, Italian Space Agency and NASA.

About 4,000 scientific papers have been based on data from the mission, said Mathew Owens, professor of space physics at the University of Reading.

And its final plunge will reveal even more about the makeup of Saturn's atmosphere.