ITS power systems are drained. Its attitude jets almost out of fuel
After 13 long years ducking and weaving between the celestial giant's moons and rings, Cassini was originally going to be left to slowly waste away.
But the significance of its discoveries changed things.
Enceladus. Titan. The space probes' close-up observations revealed the very real potential that life - as alien as it may be - could be clinging to their surfaces.
And Cassini could be contaminated.
If it were to eventually topple into one of those moons in an uncontrolled fashion, those Earth microbes that have somehow survived the incredible challenges of space could spark interplanetary genocide.
So today, NASA is taking steps to prevent that.
Cassini is about to kamikaze into Saturn's upper atmosphere at high speed. The intense heat generated by the friction of re-entry will slag the space probe - and vaporise any unwanted stowaways it may have on board.
After more than a decade of science, Cassini's death will provide yet another opportunity to learn.
Cassini has already had two stays of execution. It was originally designed to circle Saturn for four years, after spending seven years getting there. But careful husbanding of its fuel has seen its time above Saturn stretched out to 13 years.
In April, that all began to end. The probe was set on a death-spiral, with an ever-tightening orbit plunging between the planet and its rings. It has so far completed 22 of these dives. This, the 23rd, will take it into Saturn's atmosphere.
"The Cassini team has fine-tuned the ring-plane crossings to be at slightly different distances from rings and Saturn each time in order to allow the particle-tasting instruments to check out how the environment varies with distance from Saturn," the Planetary Society's Emily Lakdawalla says. The final five orbits, which began on August 14, actually took Cassini through the uppermost atmosphere.
On Friday, the probe will empty its data banks and configure itself to relay its observations live. There will be no 'later' for data to be transmitted back to Earth.
Prompted by commands from CSIRO's Canberra radio telescopes, the probe will use the last of its fuel to maintain a stable orientation. It will strive to point its antenna back to Earth as long as possible, relaying fresh measurements and images as it passes between the planet and its rings for the last time.
But once Cassini touches Saturn's atmosphere, it will all be over within minutes.
First the friction will cause the probe to tumble - its attitude jets no longer able to correct against the drag.
The probe's signal is expected to stutter. Then it will be lost.
Cassini's death throes will be silent.
Within a minute the probe will start to fragment. Seconds later, it will begin to melt.
And this 1300C meteor-like flash will be closely watched as it streaks over Saturn's clouds.
Its brightness. Its colours. This will help scientists understand what the chemical make-up of Saturn's upper atmosphere is.
The flare's trail will also be revealing. Shifts in its shape and course will expose what winds are at play at different heights.
Only the most powerful Earth-based telescopes will be able to see this, however.
And Saturn itself won't even notice. Two minutes after it first kisses the atmosphere, all that will be left of Cassini will be a fine cloud of metallic dust.
"Don't be sad that Cassini is ending," Ms Lakdawalla says. "Be mad that there is no new outer planet flagship mission on its way to exploring a giant planet, like Uranus or Neptune."
Cassini's contribution to astronomy has been incredible. Scientists now have several lifetimes of data to dissect and understand.
But there are several highlights amid the overwhelming wealth of new knowledge.
The incredible complexity of Saturn's enormous rings was never suspected. It's a living structure, shepherded by moonlets and rippling under the gentle caress of gravity and magnetic waves. Far thinner than anticipated, the rings appear to be made up of pure ice. There's little rubble to be found.
Then there's Saturn's mysterious weather. What forces could possibly combine to create the colossal - and unexpected - hexagonal storm at its pole? Planetary scientists are only now beginning to unravel its mysteries. Without Cassini, they would never have guessed such as broiling structure of gas could even exist.
But Saturn's moons may have turned out to have been the most interesting of all.
There's Titan, its surface concealed by a thick haze. Cassini's Huygens probe pierced these for the first time, landing on a primeval landscape of ice-rock remarkably similar to what Earth would have been like 3.5 billion years ago. It has rivers, lakes and seas of liquid ethane and methane. Cassini's radar watched these fill and drain, as ethane rains swept across Titan's surface.
There's Enceladus, its icy surface streaked by deep cracks. Surprisingly, these were found to be warm. Some were pumping huge geysers of liquid water into space - creating Enceladus' own icy ring. Cassini flew through these clouds, 'tasting' their composition. It found the water to be rich in hydrocarbons - indicating they came from hydrothermal vents of the type so abundant with life deep beneath Earth's oceans. And Enceladus may have its own oceans, under the ice, warmed by a molten core.
It was the presence of all this rich organic chemistry which sealed Cassini's fate.
Contamination is a very remote risk. But every opportunity to avoid interrupting the natural processes of evolution around Saturn has to be taken.
Including the destruction of the space craft itself.
It was built in a clean room. It was 'baked' in a sterilising oven. Even greater care was taken with its lander, Huygens.
NASA is 99.9 per cent certain no potential contaminant remained anywhere within Cassini's complex structure.
But life can be surprising. And persistent.
Tiny creatures called tardigrades - or water bears - have proven to be almost miraculously resilient. These microscopic eight-legged creatures have been shown to survive huge variations in temperature, the vacuum of space - even extreme radiation - and still live.
This is why NASA cannot guarantee Cassini is not contaminated.
If the probe was allowed to simply run out of fuel and be left to tumble out-of-control amid the push and pull of gravity, and the gentle breeze of solar winds, it could eventually tumble into a moon.
If that moon was Titan, any Earth-originating life would likely not survive. It is a freeze-dried landscape where even the oceans of liquid methane are incredibly cold.
If that moon was Enceladus, it's another matter. It has liquid water. And that means temperatures - and ingredients - in which Earth's bacteria and microbes (such as water bears) could potentially thrive.
Such contamination could end evolution on Enceladus before it even begins. And future scientists may not be able to tell what microbial life originally emerged independently on Enceladus, or arrived from Earth.
This cannot be allowed to happen.
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