Cassini, the golden nuclear-powered spacecraft launched in October 1997, fell into orbit around the gas giant Saturn in July 2004, and has been documenting the planet and its dizzying variety of moons ever since. For nearly three decades, researchers have worked to design, build, launch, and operate this unique mission to explore Saturn.

Cassini is now about to begin the final chapter of its remarkable story. On Wednesday, April 26, the spacecraft will make the first in series of dives through the 2,400km-wide gap between Saturn and its rings as part of the mission's grand finale.

"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."

During its time at Saturn, Cassini has made many dramatic discoveries, including a global ocean that showed indications of hydrothermal activity within the icy moon Enceladus, and liquid methane seas on its moon Titan.

But all good things must come to an end, and it is now 20 years since launching from Earth, and after 13 years orbiting the ringed planet, Cassini is running low on fuel. To avoid crashing into and contaminating a nearby moon that is suspected to harbour alien life, NASA is going to destroy the $US3.26 billion robot, ending its mission on Sep 15 this year. Cassini engineers designed a flight plan that will maximise the scientific value of sending the spacecraft toward its fateful plunge into the planet, and as it ticks off its final orbits during the next five months, the mission will rack up an impressive list of scientific achievements.

Earl Maize, an engineer at NASA's Jet Propulsion Laboratory, talked about the warm, saltwater ocean that Cassini found hiding beneath the icy crust of Enceladus, a large moon of Saturn that spews water into space. NASA's probe flew through these curtain-like jets of vapour and ice in October 2015, "tasted" the material, and indirectly discovered the subsurface ocean's composition and it's thought entirely possible that it may support alien life.

However, Maize and a collaboration of researchers from 19 nations aren't going to let their plucky probe go down without a fight. They plan to squeeze every last byte of data they can from the robot, right up until Cassini turns into a brilliant radioactive comet above the swirling storms of Saturn.

According to Maize, there is no way that the agency could risk inadvertent contact with such a pristine celestial body. "Cassini has got to be put safely away. And since we wanted to stay at Saturn, the only choice was to destroy it in some controlled fashion," he said.

Resources on Cassini's grand finale, including images and video, are available at: https://saturn.jpl.nasa.gov/mission/grand-finale/grand-finale-resources