The probe flew within 120,000km of the giant moon Titan on Monday - an encounter that bent its trajectory just enough to put it on a collision path with the ringed planet.
Nothing can now stop the death plunge in Saturn's atmosphere on Friday.
Cassini will be torn to pieces as it heads down towards the clouds.
Ever since it arrived at Saturn 13 years ago, the probe has used the gravity of Titan - the second biggest moon in the Solar System - to slingshot itself into different positions from which to study the planet and its stunning rings.
It has been a smart strategy because Cassini would otherwise have had to fire up its propulsion system and drain its fuel reserves every time it wanted to make a big change in direction.
As it is, those propellants are almost exhausted and Nasa is determined the spacecraft will not be permitted to just drift around Saturn uncontrolled; it must be disposed of properly and fully.