What do you do with a 20-year-old spacecraft that has spent 13 years orbiting Saturn, logged 6.6 billion kilometres in space and is about to run out of fuel?

You could crash it into one of the dozens of moons that orbit the ringed planet. Or you could let it hang out in a wide orbit around the gas giant where it would stay out of the way.

But the planners of Cassini's mission had a different idea.

On Wednesday, NASA's running-on-empty space probe will dive into the mysterious space between the cloud tops of Saturn and the inner edge of its majestic rings.

Over the next several months, Cassini will fly through this uncharted territory 22 times. Along the way, it will collect data that could answer important questions about Saturn's interior, its mysterious storms, the age of its rings and the length of its day.

It's a noble end to a long trip. Then, after the spacecraft's final pass through this narrow region, it will hurl itself into Saturn's atmosphere, where it will vaporise in a matter of minutes.

"I like to say it's going out in a blaze of glory," Cassini project scientist Linda Spilker, of the Jet Propulsion Laboratory in Southern California, said. "It will be trailblazing until the very last second."

NASA has dubbed this final series of manoeuvres the grand finale. But in Spilker's view, it's more like the beginning of a new mission.

"Getting this close to the rings and the planet, that's a once-in-a-lifetime experience for a scientist like me," she said. "We've wanted to do this for a long time."

In the past, mission planners have been hesitant to explore this part of the Saturn system because it was deemed too risky.

"We're going to be going 70,000mp/h [112,000km/h] into a 1200-mile [1900-kilometre] gap - and oh, by the way, we'll be driving this thing from a billion miles away," Earl Maize, Cassini's project manager at Jet Propulsion Laboratory in La Canada Flintridge, said.

But it isn't the navigation that has NASA officials worried.

"Our concern is not accuracy, it's whether we modelled the system correctly," Maize said.

At Cassini's high velocity, even a small piece of dust could take out one of its instruments. If a particle the size of a grain of sand struck it in the wrong spot, it could trip up the entire spacecraft.

Models of the Saturn system suggest that the narrow region between the rings and the planet should be free of dust, but it's always possible that the models are wrong. (In fact, that's one thing Cassini's instruments will test.)

Therefore, on the spacecraft's first pass through the gap, its large, dish-shaped antenna will go in first to act as a shield for the rest of its instruments.

The odds are in Cassini's favour.

"Our most conservative, dire models where the engineers awful-ise everything puts us at 97 per cent chance of success," Maize said.

That may sound pretty safe to you, but NASA really hates risk.

"We would never take a flagship mission on that kind of course on any other time in the mission except when it's about to end," Maize added.

In the unlikely event that the models are wrong and Cassini's path is not sufficiently dust-free, contingency plans are in place. And in the even more unlikely event that the models are really wrong and the spacecraft encounters BB-sized material that causes serious damage, Cassini will still end up vapourised in Saturn's atmosphere.

Spilker, who leads Cassini's 300 scientists, said the decision to send the spacecraft on this final mission was a no-brainer.

"We took one look at the possibility of going between the planet and the rings and said this is what we're going to do," she said. "No discussion needed."

The spacecraft arrived at the Saturn system in July 2004, and it has been collecting data on the planet, its moons and its rings ever since. However, there are fundamental questions about Saturn that have yet to be answered.

For example, scientists believe that, deep beneath the planet's atmosphere, lies a rocky core - but whether it is the size of one Earth or two remains unclear.

"Seven hundred and sixty-four Earths would fit inside Saturn," Spilker said. "So imagine trying to find just one Earth with all this mass on top of it."

Knowing the mass of the core would help scientists determine the mass of Saturn's iconic rings. This, in turn, could indicate how old they are.

"If the rings are more massive than we expect, then perhaps they are as old as Saturn itself, because they had enough mass to survive micro-meteor bombardment and erosion," Spilker said.

"On the other hand, if they are less massive, perhaps they are very young, forming as little as 100 million years ago."

Spilker said data collected during the grand finale could help answer other lingering mysteries as well - including the make-up of Saturn's atmosphere, how deep beneath the surface its winds blow and how fast the planet's interior is spinning.

"All we see on Saturn is its atmosphere, with different bands rotating in different directions and different speeds," Spilker said. "We don't have any way of knowing at what speed the interior is rotating."

New data could give researchers a better understanding of the strange hexagon-shaped jet stream at the planet's north pole.

And, of course, Cassini will beam back the highest-resolution pictures yet of the planet and its rings.

While the Cassini team is eagerly anticipating a new batch of science, some members are also starting to feel the first pangs of sorrow that their time at Saturn will soon come to an end.

"We, humankind, through this mission, have been at Saturn for 13 years," Maize said. "Unfortunately, there's not going to be a substitute for that for a long time."

He paused, and added: "It's been the ride of a lifetime."

Los Angeles Times