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 between its rings.
"This is the final chapter of an amazing mission, but it's also a new beginning,” said Thomas Zurbuchen, associate administrator for NASA's Science Mission Directorate. "Cassini's discovery of ocean worlds at Titan and Enceladus changed everything, shaking our views to the core about surprising places to search for potential life beyond Earth."
Cassini launched from Cape Canaveral, Florida in 1997, then spent seven years in transit followed by 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.
The 22-by-13-foot (6.7-by-4 meter) spacecraft is also credited with discovering icy geysers erupting from Enceladus, and eerie 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.
"The mission has changed the way we think of where life may have developed beyond our Earth," said Andrew Coates, head of the Planetary Science Group at Mullard Space Science Laboratory at University College London.
"As well as Mars, outer planet moons like Enceladus, Europa, and even Titan are now top contenders for life elsewhere," he added. "We've completely rewritten the textbooks about Saturn."
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Linda Spilker, Cassini project scientist, likened Cassini's mission to a marathon.
"For 13 years we have been running a marathon of scientific discovery, and we are on the last lap," she said early Friday.
Eight of the spacecraft 12 scientific instruments were on, capturing data, in Cassini's last moments, before it disintegrates like a meteor, she said.
"We are flying more deeply into Saturn than we have ever flown before," she said. "Who knows how many Ph.D. theses might be in just those final seconds of data?"
Already, some 4,000 scientific papers have been based on data from the mission, said Mathew Owens, professor of space physics at the University of Reading.
"No doubt scientists will be analyzing the information from its final, one-way trip into Saturn's atmosphere for years to come," Owens said.
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