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NASA's Cassini Spacecraft Begins Its Final Mission Before Plunging Into Saturn (popsci.com)
NASA has announced that their Cassini spacecraft will begin its final mission before slamming into Saturn on April 23rd. The "final mission" consists of a series of dives through a 1,500-mile-wide gap between Saturn and its rings. "No spacecraft has ever gone through the unique region that we'll attempt to boldly cross 22 times," said Thomas Zurbuchen, associate administrator for the Science Mission Directorate at NASA Headquarters in Washington. "What we learn from Cassini's daring final orbits will further our understanding of how giant planets, and planetary systems everywhere, form and evolve. This is truly discovery in action to the very end." The spacecraft will then dive into the gas giant's atmosphere, where it will "break apart, melt, vaporize, and become a part of the very planet it left Earth 20 years ago to explore," Cassini project manager Earl Maize said. Popular Science explains in its report why Cassini has to die: Some space probes are allowed to keep orbiting their targets in perpetuity after their mission ends -- like the Dawn spacecraft at the dwarf planet Ceres. But things are a lot more complicated around Saturn. Whereas Ceres is essentially just a really big rock with no moons, Saturn has 62 satellites, at last count. The gravitational push and pull from those moons -- especially the largest, Titan -- wreak havoc on Cassini's trajectory, which it normally corrects by burning fuel. But the spacecraft's fuel is running out, and ultimately its fate is sealed by its own discoveries; scientists don't want to risk the spacecraft crashing into Titan and Enceladus, which may be capable of supporting life. Although Cassini launched 20 years ago, experiments on the Space Station have suggested microbes can survive for years in the extreme temperatures, radiation, and airless vacuum of space. If NASA were to accidentally put water bears on Enceladus, the tiny Earthlings could potentially wipe out any native lifeforms that the moon may harbor, and/or complicate the search for those alien organisms later. This is why Cassini must die now, while NASA can still control its last swan dive.
Prime Directive and all....
There isn't enough fuel left to do that. They are using just about every last drop to do science.
"While we feared a resumption of direct attacks since the missile strike least year (their code name for the mission was "Schiaparelli"), the 3rd planet merely continues their siege of Mother Mars: their land-attack rovers plowing implacably above our heads on the Martian surface, continue to drill/probe to find an entrance to our homes. Their reconnaissance orbiters taking constant pictures forcing our defense forces to remain camouflaged. All Martian citizens are ordered to remain on alert! Do not go outside! Do not approach these murderous killing machines!
However, Martian Military Intelligence has also reported a pending assault on our peace-loving allies of the Ringed Planet. The Water Planet has sent a massive Death-Star which has heretofore patrolled above the Skies of Saturn, seeking any opportunity to slaughter any innocent transport vehicle it would come across, compelling a complete cessation of Qrgrzantik shipments and triggering the Qrgrzantik shortage which we continue to suffer. New information gained by our brave operatives in place on the 3rd planet suggests that in frustration at their inability to engage the Saturnian Space Navy, this death-ship (code name: Cassini) is now intending a suicidal death strike into the Saturnian home-cloud. This is the exact sort of attack such frustrated barbarians would finally attempt.
We offer our brethren-in-arms in Saturn our hopes and prayers that all will be safe.
We now return you to regularly scheduled programming.
MSN out."
-Styopa
You are absolutely right that it definitely could have gotten enough delta-v to get to any of the outer planets by doing several maneuvers while doing a slingshot orbit around Titan and Saturn. They explored this option pretty extensively with all of the outer planets, including Pluto.
The problem came down two-fold. One, they would have had to propose a somewhat costly extension to a spacecraft already on borrowed time. And Two, they would have had to leave the Saturn system a while ago, cutting out the last year to year and a half worth of observations to send it off onto another mission that it may not make it to.
Ultimately they decided, the maximum benefit from the spacecraft would be gained from keeping it in the Saturn system and destroying the spacecraft at the end to protect Titan and Enceledus.