In deep space exploration, as in life, planning gets you only so far. Preparation is essential, but you have to be ready to improvise when you encounter surprises – such as a world constantly erupting with ice volcanoes, or a system of rivers and lakes made from liquid methane, or giant dunes made from plastic.

The dunes and methane lakes are on Titan and the ice volcanoes are on Enceladus – both moons of the giant ringed planet Saturn, and part of the frontier that NASA is exploring with its epic Cassini mission. Cassini's scientists have had to think on their feet many times over the nearly 20-year endeavour, which will end this spring when the spacecraft runs out of fuel.

"We had to change plans to make observations we didn't know we wanted to make until we saw things we didn't expect to see," says Jeffrey Moore, planetary scientist at NASA's Ames Research Centre in California.

In the early 1980s, NASA's twin Voyager spacecraft had flown by and photographed Saturn and its largest moons, but these were brief encounters and with mid-20th-century technology. They left only hints at what a sustained mission like Cassini might observe, Moore says.

So Cassini was built with some flexibility. It bristles with instruments, he says, "like a flying Swiss army knife". You may start out thinking the blade or screws or scissors are meant for one thing, but once you're out hiking, you realise they might be used for something else.

Instruments designed to study particles trapped in the magnetic field of Saturn were repurposed to measure the composition of the material erupting from fields of active volcanoes on Enceladus.

That turned out to reveal something important. Enceladus receives meagre sunlight, but it gets internal energy from friction generated from the motion of massive tides. So while its crust is icy, this internal heat warms a sea of liquid water underneath. Cassini's measurements showed that the water erupting from the ice volcanoes is mixed with materials from the moon's rocky interior – organic matter and minerals. Enceladus now joins Jupiter's moon Europa as a possible abode for extraterrestrial life.

Flybys of Titan, Saturn's biggest moon, revealed something even more surprising. It's the only other body in the solar system with Earth-like surface features – lakes, seas, rivers, rain, wind and sand dunes. "Titan is an explorer's utopia," says Alex Hayes, a planetary scientist at Cornell University. People can see Titan from ground-based telescopes, he says, and in the early 20th century astronomers saw features they attributed to a thick atmosphere. Pioneer 2 and the Voyagers flew by Titan and took pictures, which revealed nothing more than a sphere of orange haze.

But what was underneath? This was tantalising, Hayes says, because remote sensing techniques showed the atmosphere contained a lot of methane, and at the temperature and pressure expected on Titan, methane should act a lot like water does on Earth – existing in liquid, solid and gas forms.

Cassini is in the news this year because it's almost out of fuel and will die by September. But how NASA would use the craft's dying weeks wasn't decided until last year.

And so Cassini is now death spiralling into Saturn. Over winter it should get some unprecedented close-ups of those rings – beautiful in a way no computer-generated drawing can equal. On September 15, it will plunge into Saturn's stormy atmosphere.