About the Mission
After two decades in space, NASA's Cassini spacecraft is nearing the end of its remarkable journey of exploration. Having expended almost every bit of the rocket propellant it carried to Saturn, operators are deliberately plunging Cassini into the planet to ensure Saturn's moons will remain pristine for future exploration—in particular, the ice-covered, ocean-bearing moon Enceladus, but also Titan, with its intriguing pre-biotic chemistry.
Beginning in 2010, Cassini began a seven-year mission extension in which it completed many moon flybys while observing seasonal changes on Saturn and Titan. The plan for this phase of the mission was to expend all of the spacecraft's propellant while exploring Saturn, ending with a plunge into the planet's atmosphere. In April 2017, Cassini was placed on an impact course that unfolded over five months of daring dives—a series of 22 orbits that each pass between the planet and its rings. Called the Grand Finale, this final phase of the mission has brought unparalleled observations of the planet and its rings from closer than ever before.
On Sept. 15, 2017, the spacecraft will make its final approach to the giant planet Saturn. But this encounter will be like no other. This time, Cassini will dive into the planet's atmosphere, sending science data for as long as its small thrusters can keep the spacecraft's antenna pointed at Earth. Soon after, Cassini will burn up and disintegrate like a meteor.
To its very end, Cassini is a mission of thrilling exploration. Launched on Oct. 15, 1997, the mission entered orbit around Saturn on June 30, 2004 (PDT), carrying the European Huygens probe. After its four-year prime mission, Cassini's tour was extended twice. Its key discoveries have included the global ocean with indications of hydrothermal activity within Enceladus, and liquid methane seas on Titan.
And although the spacecraft may be gone after the finale, its enormous collection of data about Saturn—the giant planet itself, its magnetosphere, rings and moons—will continue to yield new discoveries for decades.
The Grand Finale
Since April 2017, NASA's Cassini spacecraft has been writing the final, thrilling chapter of its remarkable 20-year-long story of exploration: its Grand Finale.
Every week, Cassini has been diving through the approximately 1,200-mile-wide (2,000-kilometer-wide) gap between Saturn and its rings. No other spacecraft has ever explored this unique region.
A final close flyby of the moon Titan on April 22 used the moon's gravity to reshape Cassini's trajectory so that the spacecraft leapt over the planet's icy rings to pass between the rings and Saturn. During 22 such passes over about five months, the spacecraft's altitude above Saturn's clouds varied from about 1,000 to 2,500 miles (1,600 to 4,000 kilometers), thanks to occasional distant passes by Titan that shifted the closest approach distance. At times, Cassini skirts the very inner edge of the rings; at other times, it skimmed the outer edges of the atmosphere. During its final five orbits, its orbit passes through Saturn's uppermost atmosphere, before finally plunging directly into the planet on Sept. 15.
Cassini's Grand Finale is about so much more than the spacecraft's final dive into Saturn. That dramatic event is the capstone of six months of daring exploration and scientific discovery. (And those six months are the thrilling final chapter in a historic 20-year journey.)
At times, the spacecraft will skirt the very inner edge of the rings; at other times, it will skim the outer edges of the atmosphere. While the mission team is confident the risks are well understood, there could still be surprises. It's the kind of bold adventure that could only be undertaken at the end of the mission.
As Cassini plunges past Saturn, the spacecraft will collect some incredibly rich and valuable information that was too risky to obtain earlier in the mission:
- The spacecraft will make detailed maps of Saturn's gravity and magnetic fields, revealing how the planet is arranged internally, and possibly helping to solve the irksome mystery of just how fast Saturn is rotating.
- The final dives will vastly improve our knowledge of how much material is in the rings, bringing us closer to understanding their origins.
- Cassini's particle detectors will sample icy ring particles being funneled into the atmosphere by Saturn's magnetic field.
- Its cameras will take amazing, ultra-close images of Saturn's rings and clouds.
Discoveries to the End
Cassini’s final images will have been sent to Earth several hours before its final plunge, but even as the spacecraft makes its fateful dive into the planet's atmosphere, it will be sending home new data in real time. Key measurements will come from its mass spectrometer, which will sample Saturn's atmosphere, telling us about its composition until contact is lost.
While it's always sad when a mission comes to an end, Cassini's finale plunge is a truly spectacular end for one of the most scientifically rich voyages yet undertaken in our solar system. From its launch in 1997 to the unique Grand Finale science of 2017, the Cassini-Huygens mission has racked up a remarkable list of achievements.
Why End the Mission?
By 2017, Cassini will have spent 13 years in orbit around Saturn, following a seven-year journey from Earth. The spacecraft is running low on the rocket fuel used for adjusting its course. If left unchecked, this situation would eventually prevent mission operators from controlling the course of the spacecraft.
Two moons of Saturn, Enceladus and Titan, have captured news headlines over the past decade as Cassini data revealed their potential to contain habitable – or at least "prebiotic” – environments.
In order to avoid the unlikely possibility of Cassini someday colliding with one of these moons, NASA has chosen to safely dispose of the spacecraft in the atmosphere of Saturn. This will ensure that Cassini cannot contaminate any future studies of habitability and potential life on those moons.
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