First, a brief bio: Cassini launched on its billion-mile journey from Earth to Saturn on October 15, 1997. It was named for the astronomer Giovanni Cassini, who discovered four of the planet's moons and a gap in its rings. Cassini also carried a single passenger: The Huygens lander, built by the European Space Agency and named for the Dutch scientist who first spotted the moon Titan.
Cassini and Huygens arrived in Saturn's orbit seven years after launch, in July 2004. Several months later, Huygens split off and touched down on the shore one of Titan's lakes of liquid methane. It was humankind's first-ever landing on a moon other than our own, and the first landing of any kind in the outer solar system. Cassini, meanwhile, was the first probe to orbit Saturn. (Pioneer and Voyager had simply flown by, in 1979 and 1980, respectively.)
Cassini has been one of the most successful planetary missions ever. Its flight was smooth, its instruments worked, its software rarely acted up. In addition to Huygens' perfectly stuck landing, Cassini probed the formation and behaviour of Saturn's ring system, discovered an 8000km-wide hurricane at Saturn's south pole and got the first close-up view of the planet's hexagonal North Pole storm.
Cassini revealed that Saturn's rings have a lot of three dimensional texture and contain bumps as big as the Rocky Mountains, solved the mystery of the moon Iapetus's two-tone black-and-white cookie coloration and photographed its odd equatorial bulge. Roughly 4000 papers have been written using the 635 gigabytes of data collected by Cassini in nearly 300 orbits of Saturn.
Best of all were the revelations about Saturn's ocean moons. In the haze around Titan, Cassini discovered molecules that could be precursors to - or even indicators of - biological activity on that methane-rich planet. Zooming past the icy moon Enceladus, it found evidence of an underground ocean of water, and spotted geysers spewing out ingredients for life.
The spacecraft is not equipped with life-detecting instruments but these moons are now considered two of the best places in the solar system to look for alien organisms, and they are the focus of several proposals for new Nasa missions.
"There's this tremendous legacy," said project scientist Linda Spilker, who has worked on the mission since 1988. "Cassini has certainly rewritten the textbooks."
Cassini is a victim of its own success. It's precisely because of Cassini's revelations about Titan and Enceladus that the spacecraft has been sentenced to die. Back in 2009, when it became apparent the spacecraft was running out of fuel, scientists at Nasa's Jet Propulsion Laboratory got together to assess their options. The craft couldn't be left to float around in space, on the off chance that it might be knocked out of orbit and crash into one of the potentially habitable moons. If that happened, Cassini could potentially contaminate those worlds with Earthling microbes.
The mission team considered moving Cassini to a more distant orbit, or sending it off to another planet. But then it came up with a proposal that would launch Cassini into one last flyby past Titan and use the moon's pull to sling the craft into 22 close-in orbits of Saturn that would explore the gaps between the planet's rings, then end by crashing into Saturn itself.
It was the obvious choice, Spilker said. She compared these ring-grazing orbits to a "brand new mission".
During the orbits, Cassini has mapped Saturn's gravity and magnetic fields to reveal the internal structure of the planet. It got close-up views of the rings and even sampled some of the icy particles that constitute them. It's expected to finally figure out the length of a Saturn day - a measurement that has eluded scientists for decades.
The "Grand Finale" began in April, and ends with a fiery plunge into Saturn's atmosphere on September 15. On that day, scientists who have worked on the mission during the past 30 years will converge at the Jet Propulsion Laboratory in Pasadena, California. The spacecraft will point its instruments in the direction of Saturn's atmosphere and start rapidly transmitting real-time data about what it sees. It will hit the atmosphere three hours later. A minute after that, it will start tumbling through the increasingly dense clouds of gas and will lose the ability to send a signal back to Earth. Because of the time delay in communication between Saturn and Earth, that final message won't arrive at JPL until 83 minutes later.
At that point, Cassini will already have burned up in Saturn's atmosphere, a tiny, bright meteorite streaking across an alien sky.
The end of Cassini is also the end of humankind's presence at Saturn - for now. Nasa has no additional missions planned for the ringed planet, though several proposals are in the works.
