Cassini, an international project that cost $3.9 billion and included scientists from 27 nations, has run out of rocket fuel as expected after a journey of some 7.9 billion kilometers.

Cassini’s well-planned demise is a way of preventing any damage to Saturn’s ocean-bearing moons Titan and Enceladus, which scientists want to keep pristine for future exploration because they may contain some form of life.

Just before 0500 GMT NASA wrote on Twitter that Cassini was reconfiguring “to transmit its final observations to Earth in real time.”

According to NASA estimates, Cassini has already crossed the orbital distance of the F-ring – outermost of Saturn’s main rings – for the last time.

“It will be sad to see Cassini go on Friday, especially as the instrument we built is still working perfectly,” said Stanley Cowley, professor of solar planetary physics at the University of Leicester.

“But we recognize that it is important to bring the mission to an end in a tidy and controlled manner.”

Three other spacecraft have flown by Saturn – Pioneer 11 in 1979, followed by Voyager 1 and 2 in the 1980s.

But none have studied Saturn in such detail as Cassini, named after the French-Italian astronomer Giovanni Domenico Cassini, who discovered in the 17th century that Saturn had several moons and a gap in between its rings.

Cassini launched from Cape Canaveral, Florida in 1997, then spent seven years in transit followed by 13 years orbiting Saturn.

In that time, it discovered six more moons around Saturn, three-dimensional structures towering above Saturn’s rings, and a giant storm that raged across the planet for nearly a year.

The 6.7 by 4 meter spacecraft is also credited with discovering icy geysers erupting from Saturn’s moon Enceladus, and eerie hydrocarbon lakes made of ethane and methane on Saturn’s largest moon, Titan.

In 2005, the Cassini orbiter released a lander called Huygens on Titan, marking the first and only such landing in the outer solar system, on a celestial body beyond the asteroid belt.

Huygens was a joint project of the European Space Agency, Italian Space Agency and NASA.

“The mission has changed the way we think of where life may have developed beyond our Earth,” said Andrew Coates, head of the Planetary Science Group at Mullard Space Science Laboratory at University College London.

“As well as Mars, outer planet moons like Enceladus, Europa and even Titan are now top contenders for life elsewhere,” he added, AFP reported.

“We’ve completely rewritten the textbooks about Saturn.”

Some 4,000 scientific papers have been based on data from the mission, said Mathew Owens, professor of space physics at the University of Reading.

And its final plunge will reveal even more about the make-up of Saturn’s atmosphere before Cassini disintegrates like a meteor.

“No doubt scientists will be analyzing the information from its final, one-way trip into Saturn’s atmosphere for years to come,” Owens said.

The nuclear-powered robot was launched in 1997 to deeply study Saturn and its mysterious collection of moons. Cassini arrived in 2004, dropped off a lander on one moon, began orbiting Saturn, and has beamed back data and images to Earth ever since.

Scientists would love to keep the $3.26-billion mission going, but knew their robot would eventually run too low on propellant to safely control.

Extending the mission would risk crashing Cassini — which is contaminated by trace amounts of earthly bacteria — into Enceladus or Titan. These two moons of Saturn hide oceans of water and may be habitable to or even host alien life.

Instead of chucking the probe into deep space, like the twin Voyager spacecraft, NASA decided to destroy Cassini by sending it on a months-long death spiral into Saturn.

This daring maneuver — what NASA calls “the Grand Finale” — gave Cassini 22 unprecedented dives between Saturn and its gossamer-thin rings.

As NASA prepares to lose one of its most storied space probes, many space enthusiasts are wondering whether Cassini’s final moments will be visible from Earth, 930 million miles away.

Business Insider posed that question to Linda Spilker, a Cassini project scientist and a planetary scientist at NASA JPL. Her answer: “It’s gonna be tough, but I’m hopeful.”

When Cassini plunges into Saturn’s outer atmosphere at about 76,000 mph, it should produce bursts of light. But that’ll be tough to see from Earth for a few reasons.

First, the brightest parts of those bursts will be in ultraviolet — the same wavelength of light that can cause sunburn. Because Earth’s ozone layer soaks up gobs of ultraviolet light, however, any UV flashes will appear dramatically dimmed to anyone watching from the ground.

Another challenge is that Cassini’s two nexuses of control — NASA and the European Space Agency — won’t “see” the event in the dark of night, making the signal all the much dimmer as western telescopes fight twilight.

“There are really no big assets that we could be to bear either in the United States or in Europe to look,” Spilker said. “It’s going to be five in the morning here, and of course even later in the day in Europe.”

Because of those factors, Spilker said this moment in space history won’t be like seeing the Shoemaker-Levy 9 comet wallop Jupiter in 1994.

“Those objects were so much bigger and so much more massive than Cassini, and a lot of those strikes were on the night side. We’ve got sort of the double-whammy of a little tiny spacecraft that’s really not that massive hitting on basically the day side of Saturn,” Spilker said. “So it’s unlikely, but it’s definitely worth looking.”

Space is the ideal place to record Cassini’s fiery death, since the sun and Earth’s atmosphere wouldn’t be in the way. So, the Cassini mission asked NASA to use the Hubble Space Telescope to stare down Saturn for signs of a flash.

This could have worked, but Spilker said a recent discovery at Saturn led to a poor “luck of the draw” with Hubble.

During Cassini’s last handful of dives between Saturn and its rings, the probe revealed that the planet’s outer atmosphere extends farther than previously thought. Plowing through those sparse gases gradually slowed down Cassini — and this, in turn, bumped up the probe’s time of death by about 15 to 20 minutes, Spilker said.

“It’s really just in the last few weeks that we’ve started to get a better handle on what that time of impact would be,” Spilker said.

When NASA studied this new timing, it realized Cassini would burn up while Hubble is flying over Earth’s South Atlantic anomaly — a chink in the armor of our planet’s protective shield where radiation levels and electric currents spike.

“We were trying to get some time on Hubble, but it looks like … the instrument we want to use will have to be turned off when we’re flying across that region,” Spilker said. “You don’t want to have currents flowing at high voltage. You worry about damage to your instruments, so the best thing you can do is turn off high voltage.”

She added: “I was sad to lose Hubble, because above [Earth’s] atmosphere looking in the ultraviolet, that was a good chance. And it just didn’t work.”

But Spilker thinks all hope is not lost.

She said there’s a chance that professional, ground-based telescopes in the southern hemisphere — perhaps those in Australia— might be sensitive enough and in the right place to record Cassini’s death.

She also said there’s a large community of space fans with powerful telescopes and refined techniques sprinkled across the globe that could help.

“There’s still some really great, really capable amateur ground astronomers, in particular, that have returned some beautiful images of Saturn,” Spilker said, reiterating that it will be a “tough one” to capture.

“Maybe the last of the fuel tank will suddenly brighten and we’ll be able to see that. I think it’s definitely worth looking,” she said. “We might be surprised.”