Media playback is unsupported on your device Media caption Jim Green: The Cassini discoveries 'blew our minds'

Engineers now have a precise expectation of when they will lose contact with the Cassini probe.

The spacecraft is being ditched in the atmosphere of Saturn on Friday, bringing to an end 13 amazing years of discovery at the ringed planet.

The team hopes to receive a signal for as long as possible while the satellite plummets into the giant world.

But the radio will likely go dead at about 6 seconds after 04:55 local time here at mission control in California.

That is 11:55:06 GMT (12:55:06 BST). This is the time that antennas on Earth lose contact.

Because of the finite speed of light and the 1.4 billion km distance to Saturn, the event in space will actually have occurred 83 minutes earlier.

"The spacecraft's final signal will be like an echo. It will radiate across the Solar System for nearly an hour and a half after Cassini itself has gone," said Earl Maize, the US space agency's (Nasa) project manager.

"Even though we'll know that, at Saturn, Cassini has already met its fate, its mission isn't truly over for us on Earth as long as we're still receiving its signal."

Cassini: 'Our Saturn years'

Image copyright NASA

An epic journey to the ringed planet, by the people who helped make it happen

Media playback is unsupported on your device Media caption Earl Maize: "The wonder and mystery" call us to go back

A giant dish in Canberra, Australia, will be in prime position to track the probe. Others across the globe will be working in support.

Cassini is in the process of taking its final pictures of the Saturnian system - some last postcards for the scrapbook, if you will.

These will include views of the moons Enceladus and Titan, which harbour huge volumes of liquid water beneath their icy surfaces and where scientists believe simple lifeforms might be able to eke out an existence.

"And then… we're going to look on the dark side of Saturn at that point where Cassini will be plunging into the atmosphere, looking in the near-infrared and the ultraviolet, trying to get some pictures of Cassini's final home inside the planet Saturn itself," explained Nasa project scientist Linda Spilker.

Image copyright NASA/JPL-CALTECH/SSI Image caption Just returned from Cassini: An image of Titan and its northern seas of liquid methane

All pictures must be downlinked and the cameras switched off before the death plunge can begin.

The data rates flowing back from Saturn will not support imaging on the way down. Instead, Cassini will be configured to run only those instruments that can sense the planet's near-space environment, such as its magnetic field, or that can sample the chemical composition of its gases.

Media playback is unsupported on your device Media caption Why scientists are so excited about Saturn's icy moon Enceladus.

Hunter Waite leads the probe's Ion and Neutral Mass Spectrometer (INMS).

"We'll be able to look at some important constituents that we know are there because we've been measuring them, but we'll get a better idea, for example, of the hydrogen to helium ratio, and that's important in terms of understanding the formation and evolution of Saturn," he said.

In the final three hours or so before impact on Friday all data acquired by the spacecraft will be relayed straight to Earth, bypassing the onboard solid state memory.

Image copyright NASA/JPL-CALTECH/SSI

Media playback is unsupported on your device Media caption Hunter Waite: It would be exciting to search for life at Enceladus

The end when it comes is going to be a bittersweet moment. The probe has dazzled all those who have followed its progress, including Jim Green, the head of planetary science at Nasa.

He said Cassini had re-written the textbooks on where one might try to find life beyond Earth.

"No-one ever thought that we could go to the outer part of the Solar System," he told BBC News. "That's where water, because the Sun hardly shines there, should be frozen solid - and you have to have liquid water to be able to have life.

"And now we're finding whole moons with oceans of liquid water that have been that way for 4.5 billion years. That blew our minds."

Looking to the future, many of Cassini's scientists are eager to return with new, more capable spacecraft.

Mission proposals have been submitted to Nasa that include instrumented boats that could float on Titan's seas, and life-detection probes that could be flown through the jets of water that spout from the southern polar region of Enceladus.

In the near term, though, many of the same researchers will be working on America's Clipper mission to Europa, a moon of Jupiter that in many ways is just a big version of Enceladus. And in tandem, European Cassini scientists will be targeting Ganymede, another jovian moon that is bigger even than Titan and which is suspected also of hiding a huge ocean of liquid water under its icy shell.

The Cassini-Huygens mission is a joint endeavour of Nasa, and the European and Italian space agencies.

BBC News will have live coverage of the ending of the mission on both TV and radio. Inside Science will preview the climax this Thursday at 16:30 BST on Radio 4. A Horizon documentary will also review the mission and the final hours in a special programme to be broadcast on Monday 18 September at 21:00 BST on BBC Two. And you can still watch the Sky At Night programme Cassini: The Gamechanger on the iPlayer. This is being repeated on Thursday on BBC Four at 19:30 BST.

Media playback is unsupported on your device Media caption It took Cassini seven years just to reach Saturn

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos</s>The Cassini spacecraft, after spending 13 nearly flawless years revealing a complex, ringed gas giant along with its extensive array of enigmatic moons—and finding two worlds capable of supporting life—died on Friday morning. It expired after setting its engines on full thrust and flying directly into the maw of Saturn. It was 19 years old.

The spacecraft made its final significant maneuver on Monday, flying near enough to Saturn's largest moon Titan to nudge Cassini into a collision course with the planet. Cassini took its final image on Thursday and on Friday morning began accelerating to more than 140,000km/hour. There was no return.

Cassini had to die this way. Its fuel reserves were nearly gone. Scientists didn't want to take a chance that, however improbably, some life from Earth had survived on board the spacecraft for 20 years and might one day contaminate one of the planet's moons, such as Titan or Enceladus.

Early Friday, as Cassini began flying deeper into Saturn's atmosphere, molecules from the planet struck the spacecraft. Slowly, it began to heat up. The spacecraft's attitude control thrusters, fighting to keep Cassini's antenna pointed back toward large arrays on Earth amid the turbulence, lost their struggle. At about 1,500km above the planet's cloud tops, Cassini began to tumble, falling, falling... falling. NASA scientists at the Jet Propulsion Laboratory lost contact with the spacecraft at 7:55am ET. It was burning and soon would break apart, becoming part of the gas giant it has so closely studied for so long.

NASA's Cassini orbiter, along with the Huygens lander built to descend to the surface of Titan, launched on October 15, 1997, aboard a Titan IVB/Centaur rocket and reached an orbit around Saturn on July 1, 2004. That marked the beginning of the end of a more than two-decade journey, beginning in the 1980s, to get the probe into space and onto Saturn.

Voyager 1 had flown by Saturn in 1980, and Voyager 2 followed in 1981. These two iconic probes raised more questions about Saturn than they answered, especially about its moons. Titan, in particular, intrigued scientists with its thick nitrogen atmosphere—the only other world in the Solar System to have a nitrogen-rich atmosphere like Earth's. What might lie there?

"Voyager was a tease," planetary scientist Carolyn Porco, who worked on the imaging team for the Voyager mission, and led it for Cassini, told Ars. "It was like a fleeting moment in time when you could just see, briefly, what was there. We were left with all these questions. And they were important questions."

Questions like: Why is one half of that moon dark, but the other light? Why are they so weirdly shaped? What are all these things in the rings of Saturn? What could it tell us about the formation of the Solar System? And might the atmosphere on Titan allow for some kind of exotic life forms to exist?

During its 13 years at Saturn, Cassini sent back more than just dazzling photos. It found that Titan's nitrogen-rich atmosphere had a surface pressure 1.6 times that of Earth and had large, methane lakes. The presence of nitrogen along with methane, and ultraviolet light reaching the surface, created conditions where rich organic chemistry might flourish.

Similarly, Cassini answered key questions about the upper atmosphere of Saturn—mostly hydrogen but about 7 percent helium—and elucidated the nature of its fine rings. It explored the more than five dozen moons orbiting the planet. And upon closer inspection, Cassini found small moons dancing in gaps between the rings, gaps those tiny moons had created with their own small gravity fields.

And Cassini discovered a huge surprise at one of Saturn's smaller moons, Enceladus. Large, icy geysers were spewing into space. Later, scientists confirmed that a relatively large ocean must exist on the moon, beneath the ice. Cassini's mission was modified to fly through the geysers, and that small world, only about 500km across, is now considered one of the best places in the Solar System to look for extant life.

For the scientists who study Saturn and the outer Solar System, Cassini represents a glorious achievement. While they are sad about Cassini's demise, it has been a tremendous run. "It's the finality of this enormous effort that we all personally have put into Cassini," Porco said. It was three decades of her life. "It’s kind of like a death in the family. It will feel that way."

Cassini leaves few survivors out in the Solar System. There is the Juno spacecraft, exploring the Jovian atmosphere for clues about the inner workings of the largest planet in the Solar System. It will fly into Jupiter within a couple of years. On January 1, 2019, the New Horizons spacecraft will fly by the Kuiper Belt object 2014 MU69 before heading ever deeper into space and following the Voyagers out of the Solar System.

And beyond that? NASA is developing an orbiter mission to Jupiter's moon Europa and, potentially, a lander. And while the agency is making progress, neither of these $3 billion to $4 billion missions is guaranteed to fly. Therefore, after Cassini's final transmission this morning, scientists will have lost more than a long-time, faithful friend in the outer Solar System. The worlds out there will have become quite a bit darker.</s>In October of 1997, nearly 20 years ago, NASA launched the last of its great probes to the outer planets. A joint mission with the European Space Agency, a single rocket sent Cassini and Huygens on a meandering path through the Solar System. Huygens plunged into the atmosphere of the moon Titan well over a decade ago, but the Cassini orbiter has been looping around Saturn for over 13 years. But in less than 24 hours, its time at the ringed planet will come to a close as Cassini plunges into Saturn's atmosphere.

This was the end that NASA had always planned for its hardware. Some of Saturn's moons are thought to be capable of harboring life. So, rather than risk contaminating those moons with life from Earth, Cassini and any microbes it harbors will burn up on entry into Saturn's atmosphere.

The decision to do this now is based on the dwindling supply of fuel for the probe's maneuvering engines, which will eliminate NASA's ability to make further adjustments in its orbit. With the chaotic gravitational interactions of a giant planet and multiple moons, there'd be no way to determine where Cassini would end up. So NASA is acting while it still manages the hardware's destiny.

The process started in April, with small adjustments in Cassini's orbit that gradually sent it closer to the planet, eventually plunging through the gap inside the rings. With each successive orbit, the hardware got closer to Saturn. The process will end over the course of the evening tonight. Accounting for the hour-plus delay that comes from the time it takes light to travel between Saturn and Earth, the final signals from Cassini will arrive early Friday morning, US time. In this case, success will be indicated by the probe going permanently silent.

NASA TV is devoting its entire schedule to Cassini today, but it's hard to summarize all the discoveries that have come out of having a constant presence at Saturn for over 13 years. Geysers on Enceladus, oceans on Titan, the bizarre polar hexagone of Saturn itself; all of these were dramatic subjects for detailed study. But Cassini was also able to capture things like the shifting of seasons, details of ring dynamics and the moons that shape them, and countless other details that will keep planetary scientists busy for decades. The mission's end is a loss for an entire scientific community.

It's also a loss for many of us humans who don't depend on it for scientific data. The astonishing photos it has returned provided a steady drip of amazement that will be missed by a lot of people.</s>NASA probe Cassini relays final data as it plunges into Saturn’s atmosphere

Madrid, Sep 13 (EFE). The National Aeronautics and Space Administration’s Cassini space probe completed its extraordinary journey of exploration on Friday and ended its mission almost 20 years after takeoff by taking a final, fiery plunge at high speed into Saturn’s atmosphere, NASA’s Jet Propulsion Laboratory in Pasadena, California said.

Cassini had spent the last 13 years circling around Saturn’ s ring and satellite system, busily transmitting data and images right to the last second prior to destruction in the gas giant’s outer reaches.

“After two decades in space, NASA’s Cassini spacecraft is nearing the end of its remarkable journey,” NASA said.

“Having expended almost every bit of the rocket propellant it carried to Saturn, operators are deliberately plunging Cassini into the planet to ensure Saturn’s moons will remain pristine for future exploration,” the statement added.

Scientists had been eager to keep Saturn’s moons, in particular Enceladus, as well as other space objects, uncontaminated.

Cassini’s last orbit came after a final 22 weeks of orbital dives, hurtling through a gap between Saturn and its rings, that began in April.

No spacecraft had ever ventured so close to the gas giant.

NASA’s JPL Mission Control’s final calculations predicted the loss of contact with Cassini would take place Sept. 15 at 4:55 am local time.

Cassini would have entered Saturn’s atmosphere one minute earlier, some 1,190 miles (1,915 kilometers) above the planet’s cloud tops at around 70,000 mph (113,000 km/h).

When Cassini encountered Saturn’s upper atmosphere, its control thrusters fired in short bursts attempting to keep its saucer-shaped high-gain antenna pointed towards Earth to relay the mission’s final data.

As the atmosphere thickened, its thrusters went from 10 percent capacity to full power in around a minute.

At that point, the spacecraft was no longer capable of fighting Saturn’s dense atmosphere and tumbled, spinning out of control.

As soon as the antenna swayed some fractions of a degree away from Earth, all communications were cut for good.

 NASA predicted this would happen at some 930 miles (1,500 km) above Saturn’s cloud tops and the spacecraft became a meteor.

It was calculated that Cassini began disintegrating 30 seconds later.

NASA scientists would only get confirmation of events 83 minutes after they occurred, given that is how long radio signals take to reach mission control.

Earl Maize, Cassini project manager previously explained the spacecraft’s final signal would be its swan song: “It isn’t truly over for us on Earth as long as we’re still receiving its signal.”

Cassini’s last data relays were picked up by NASA’s Deep Space Network complex in Canberra, Australia.

Cassini was expected to provide groundbreaking scientific observations of Saturn, with eight of its 12 science instruments collecting pioneering data in the final plunge, sampling the planet’s atmosphere’s composition and structure.

Before Cassini, the only brief glimpses of Saturn were the Pioneer 11 and Voyagers 1 and 2 flybys decades ago. They briefly took pictures and data as they zoomed past.

These missions gave us insights on Saturn’s complex ring system, discovering new moons and took sketchy measurements of Saturn’s magnetosphere, but that was all.

Cassini changed all that when in 2004 it relayed new and surprising data, especially relating to Saturn’s moons.

 Cassini’s visit to Saturn’s largest moon, Titan, gave scientists a glimpse of what Earth may have looked like before life evolved.

Also noteworthy was the data provided by the European Space Agency’s Huygens probe, which rode piggyback on Cassini all the way to Saturn before plunging in 2005 through Titan’s dense atmosphere and landing on its surface.

Enceladus was another gem: A spray of icy particles from surface jets forming massive plumes extending way above Enceladus.

The spacecraft came as close as 15 miles (25 km) from its icy surface revealing the presence of a global subsurface ocean and conditions that may prove suitable for life.

Cassini cannot “rust” in peace, after its incendiary demise but has earned a place of honor in NASA’s hall of (robotic) fame, and changed the course of our future planetary exploration, the space agency said.</s>NASA’s Cassini spacecraft disintegrated in the skies above Saturn early on September 15 in a final, fateful blaze of cosmic glory, following a remarkable journey of 20 years.

Confirmation of Cassini’s expected demise came about 7.55 a.m. EDT (5.25 p.m. IST). That’s when radio signals from the spacecraft its last scientific gifts to Earth came to an abrupt halt. The radio waves went flat, and the spacecraft fell silent.

Cassini actually burned up like a meteor 83 minutes earlier as it dove through Saturn’s atmosphere, becoming one with the giant gas planet it set out in 1997 to explore. But it took that long for the news to arrive at Earth a billion miles away.

The only spacecraft to ever orbit Saturn, Cassini showed us the planet, its rings and moons up close in all their glory. Perhaps most tantalising, ocean worlds were unveiled by Cassini and its hitchhiking companion, the Huygens lander, on the moons Enceladus and Titan, which could possibly harbour life.

Cassini snapped its “last memento photos” of the Saturn system on September 14. Dutiful to the end, the spacecraft sampled Saturn’s atmosphere on September 15 morning as it made its final plunge.

Programme manager Earl Maize made the final announcement: “This has been an incredible mission, an incredible spacecraft and you’re all an incredible team,” Mr. Maize said. “I’ll call this the end of mission.”

More than 1,500 people, many of them past and present team members, had gathered at California’s Jet Propulsion Laboratory for what was described as both a vigil and celebration. Even more congregated at nearby California Institute of Technology, which runs the lab for NASA.

Project scientist Linda Spilker noted Cassini has been running “a marathon of scientific discovery” for 13 years at Saturn. “So we’re here today to cheer as Cassini finishes that race,” she said.

The spacecraft tumbled out of control while plummeting at more than 122,000 kmph. Project officials invited ground telescopes to look for Cassini’s last-gasp flash, but weren’t hopeful it would be spotted from a billion miles away.

This Grand Finale, as NASA calls it, came about as Cassini’s fuel tank started getting low after 13 years exploring the planet. Scientists wanted to prevent Cassini from crashing into Enceladus or Titan and contaminating those pristine worlds. And so in April 2017, Cassini was directed into the previously unexplored gap between Saturn’s cloud tops and the rings. Twenty two times, Cassini entered the gap and came out again. The last time was last week.

The leader of Cassini’s imaging team, Carolyn Porco, a visiting scholar at the University of California, Berkeley, was so involved with the mission for so long that now, “I consider it the start of life, part two.”

Cassini departed Earth in 1997 and arrived at the solar system’s second largest planet in 2004. The European Huygens landed on big moon Titan in 2005. Nothing from Earth has landed farther.

In all, Cassini collected more than 453,000 images and traveled 4.9 billion miles. It was an international endeavour, with 27 nations taking part. The final price tag was $3.9 billion.

5.32 p.m.: NASA says Cassini spacecraft has burned up in the skies over Saturn as planned, ending a 20-year mission. Earth received Cassini’s final signal at 7:55 a.m. ET (5.25 p.m. IST). Cassini is now part of the planet it studied. Thanks for the science.

5.30 p.m.: Our Cassini spacecraft is now one with the planet it studied for so long. The rest is science. Goodbye Cassini, tweets @NASAJPL.

5.27 p.m. IST: NASA announces end of mission. Our spacecraft has entered Saturn's atmosphere, and we have received its final transmission, tweets @CassiniSaturn

4.53 p.m. IST: @CassiniSaturn tweets: Cassini’s final transmission is currently traveling at the speed of light past Jupiter.

4.01 p.m. IST: Cassini's final dive is happening at Saturn, with the last signal expected on Earth at 5.25 p.m. IST. This means the spacecraft is entering the planet’s atmosphere. (NASA’s Cassini Grand Finale update)

2.25 p.m. IST: Cassini engineers have received the signal that Cassini has started a five-minute roll to point the instrument that will sample Saturn's atmosphere into the optimal direction, facing the direction of the oncoming gases. Along with this roll, the spacecraft is reconfiguring its systems for real-time data transmission at a rate of 27 kilobits per second (3.4 kilobytes per second). Final, real-time relay of data starts immediately after. That relay marks the beginning of Cassini's final plunge. (NASA’s Cassini Grand Finale update)

Why did we end @CassiniSaturn mission with this #GrandFinale dive into #Saturn? To protect moons from contamination: https://t.co/mzKW5uDsTipic.twitter.com/vJJvhkSnPR — NASA (@NASA) September 15, 2017

Cassini flew by Titan one last time on September 12 before transmitting images and scientific data from the flight.

Mission engineers will use the information gathered from the encounter they dubbed "the goodbye kiss" to make sure the vessel is following the right path to plunge into the gas giant's atmosphere.

"The Cassini mission has been packed full of scientific firsts, and our unique planetary revelations will continue to the very end of the mission as Cassini becomes Saturn's first planetary probe, sampling Saturn's atmosphere up until the last second," said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, California. "We'll be sending data in near real time as we rush headlong into the atmosphere — it's truly a first-of-its-kind event at Saturn."

Cassini is expected to lose communications with Earth one or two minutes into its final dive, but 10 of its 12 scientific instruments will be working right up until the last moment to analyze the atmosphere's composition. That data could help understand how the planet formed and evolved.

On the eve of its final descent, other instruments will make detailed observations of Saturn's aurora borealis, temperatures and polar storms.

Cassini's final maneuvers began at 7.14 a.m. GMT (12.44 pm IST) on September 15, although the signal will only reach NASA 86 minutes later.

At 10.31 a.m. GMT (4.01 pm IST), the spacecraft is due to enter Saturn's atmosphere with its antennas pointed toward Earth and its motors running full blast in order to hold its trajectory. Just a minute later, at some 1,500 km above Saturn's clouds, the probe's communications will stop before Cassini begins to disintegrate moments later, NASA predicts.

"The Grand Finale represents the culmination of a seven-year plan to use the spacecraft's remaining resources in the most scientifically productive way possible," said Earl Maize, Cassini project manager at the Jet Propulsion Laboratory. "By safely disposing of the spacecraft in Saturn's atmosphere, we avoid any possibility Cassini could impact one of Saturn's moons somewhere down the road, keeping them pristine for future exploration."

The mission is a cooperative project of NASA, the European Space Agency (ESA) and Italy's space agency. NASA's European and Italian partners built the Huygens probe Cassini carried until dropping it on Titan.

The Cassini-Huygens mission's total cost is about $3.26 billion, including $1.4 billion for pre-launch development, $704 million for mission operations, $54 million for tracking and $422 million for the launch vehicle.

The United States contributed $2.6 billion to the project, the European Space Agency $500 million and the Italian Space Agency $160 million.

Italian astronomer Giovanni Cassini discovered four of Saturn moons in the 17th century, although scientists have since identified more than 60. During the same era, Dutch mathematician Christiaan Huygens found that Saturn had rings. He also was the first person to observe Titan.

A brief look back at Cassini

Cassini rocketed from Cape Canaveral, Florida, on October 15, 1997, carrying with it the European Huygens lander. The spacecraft arrived at Saturn in 2004. Six months later, Huygens detached from Cassini and successfully parachuted onto the giant moon Titan. Cassini remained in orbit around Saturn, the only spacecraft to ever circle the planet. In April 2016, NASA put Cassini on an ever-descending series of final orbits, leading to the September 15 swan dive. Better that, they figured, than Cassini accidentally colliding with a moon that might harbour life and contaminating it.

Travelling too far from the sun to reap its energy, Cassini used plutonium for electrical power to feed its science instruments. Its separate, main fuel tank, however, was getting low when NASA put the spacecraft on the no-turning-back Grand Finale. The mission already had achieved great success, and despite the chance of pounding Cassini with ring debris, flight controllers directed the spacecraft into the narrow gap between the rings and Saturn’s cloud tops. Cassini successfully sailed through the gap 22 times, providing ever better closeups of Saturn.

Cassini discovered swarms of moonlets in Saturn’s rings, including one called Peggy that made the short list for final picture-taking. Scientists wanted one last look to see if Peggy had broken free of its ring. Data from the spacecraft indicate Saturn’s rings which consist of icy bits ranging in size from dust to mountains may be on the less massive side. That would make them relatively young compared with Saturn; perhaps a moon or comet came too close to Saturn and broke apart, forming the rings 100 million years ago. Or perhaps multiple such collisions occurred. On the flip side, more massive rings would suggest they originated around the same time as Saturn, more than 4 billion years ago.

Saturn has 62 known moons, including six discovered by Cassini. The biggest, by far, is the first one discovered way back in the 1655 — Titan, which slightly outdoes Mercury. Its lakes hold liquid methane, which could hold some new, exotic form of life. Little moon Enceladus is believed to have a global underground ocean that could be sloshing with life more as we know it. Incredibly, geysers of water vapor and ice shoot out of cracks in Enceladus’ south pole. Project scientist Linda Spilker said if she could change one thing about Cassini, it would have been to add life-detecting sensors to sample these plumes. But no one knew about the geysers until Cassini arrived on the scene.

Scientists would love to return to Enceladus or Titan to search for any potential life. Nothing is firmly on the books right now. But there are proposals to go back, submitted under NASA’s New Frontiers programme. So stay tuned.</s>Nasa’s Saturn probe Cassini is ending its twenty year mission by plummeting into the atmosphere of the second-biggest planet in the solar system and burning up in its atmosphere.

This ends Cassini’s 13-year vigil of the ringed plant. With its fuel supplies exhausted, the probe is expected to enter Saturn’s atmosphere at the speed of 120,000km/h (76,000mph), where it’ll be torn to pieces before it hits the ground.

Before its demise, however, Cassini is expected to beam back images and data gathered during its descent, which will be useful to scientists back at Nasa. Even in its death throes, Cassini is hard at work, it seems. Now that’s dedication.

Cassini left earth 20 years ago in in 1997 and travelled eight billion kilometres past Jupiter and Venus before taking up its position in Saturn’s orbit. In its time in space, Cassini has orbited Saturn nearly 300 times, beaming back information that helped astronomers and scientists gain a better understanding of the ringed planet.

“We’ve done amazing science; it’s an amazing team,” Former US space agency chief scientist Ellen Stofan told the BBC. “And I think we can celebrate that we’ve really eked every little bit of science that we could out of the Cassini spacecraft. But then it’s what’s next?

“We want to go back to Titan, we want to go back to Enceladus; there’s so much we don’t know about the interior of Saturn, so people have talked about Saturn probe missions. There’s a lot more to be done,” she added.

So farewell, then, Cassini. It’s been quite a trip.</s>Launched 20 years ago to circle Saturn, spacecraft runs out of fuel as expected after journey of 7.9 billion kilometres.

Global astronomers are bidding farewell to NASA's famed Cassini spacecraft, which launched 20 years ago to circle Saturn and transformed the way we think about life elsewhere in the solar system.

Cassini, an international project that cost $3.9bn and included scientists from 27 countries, runs out of rocket fuel on Friday as expected after a journey of 7.9 billion kilometres.

Its plunge into the ringed gas giant - the furthest planet visible from Earth with the naked eye - is scheduled for shortly after the spacecraft's final contact with Earth at 11:55 GMT.

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

READ MORE: Why discovering new Earth-like planets matters

"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 recognise 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 was launched from Cape Canaveral, Florida in 1997, following which it spent seven years in transit and then another 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 6.7x4 sq metre spacecraft is also credited with discovering icy geysers erupting from Saturn's moon Enceladus, and 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.

About 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.</s>One of the most ambitious and successful space missions ever undertaken is to come to an end on Friday morning, with the Cassini space probe set to crash into Saturn. The 20-year mission to get up close and personal with Saturn and its moons will end when the 22-foot robot burns up in the ringed planet's cloud tops. Launched in 1997, Cassini made the two billion mile journey to the second largest planet in the solar system, delivering a wealth of scientific data. With it running out of fuel, scientist have decided to crash it into Saturn, and expect to lose contact around lunchtime tomorrow. Here's what you need to know about the Cassini mission.

With Cassini running out of fuel, therefore being out of our control, they want to avoid the remote possibility of a collision with the moons Titan or Enceladus, which could both conceivably host life. Nasa did not want to risk contaminating the environments with Earth bugs.

It took seven years from its 1997 launch to reach Saturn, travelling two billion miles. On the way it made fly-bys of Venus, the Earth, and Jupiter to receive gravitational "kicks" that boosted its speed to more than 42,500mph.

It found a global watery ocean beneath the icy surface of Saturn's moon Enceladus that scientists believe could harbour simple life. It also found seven new moons, six of which have been named, carried out detailed studies of Saturn's rings, and spotted raging hurricanes at both of Saturn's poles. A smaller probe, Huygens, also landed on the surface of Titan.

The European lander provided the high point of the mission when it successfully touched down on the surface in 2005. It was the first time any spacecraft had landed on an outer solar system world. As it parachuted down through the moon's atmosphere, Huygens captured images of surprisingly Earth-like features such as shorelines and river systems. Scientists now know Titan has rivers, lakes and seas filled with liquid methane and ethane.

It was powered by a small plutonium-fuelled nuclear reactor. There were protests leading up to the launch in 1997 over fears about what would happen if the rocket crashed on takeoff, spreading radioactive plutonium over Florida and beyond.

Right up until it beams its final signals to Earth eight of the spacecraft's 12 scientific instruments will be gathering data from the top of Saturn's atmosphere and transmitting information about its structure and composition. Cassini's cameras will capture their final images of Saturn and its moons several hours earlier. They will be radioed to Nasa's Deep Space Network antenna complex in Canberra, Australia, before being posted on the mission website.</s>PASADENA, Calif. — Contact has been lost with the Cassini spacecraft after it completed a “death dive” into the upper atmosphere of Saturn and transmitted its final signal, according to NASA.

The spacecraft deliberately sank into Saturn’s upper atmosphere at a high speed and plunged itself into the planet just after 6:30 a.m. ET Friday. Given the amount of time it takes signals to reach Earth, the final signal and last bits of data reached the Deep Space Network’s Canberra Station in Australia about an hour and a half later.

NASA confirmed the spacecraft’s demise at 7:55 a.m. ET, as predicted.

For about a minute, Cassini was able to transmit new data about the planet’s composition as long as its antenna remained pointed toward Earth, with the assist from small thrusters. Then, the spacecraft burned and disintegrated due to the heat and high pressure of the hostile atmosphere. It became part of the planet it set out to explore.

“It was a perfect spacecraft,” said Julie Webster, spacecraft operations chief. “Right to the end, it did everything we asked it to. It’s perfect, it’s perfect.”

No spacecraft has ever been so close to Saturn. The last few seconds of the Cassini mission provided the “first taste” of the atmosphere of Saturn, NASA said.

“You can think of Cassini as the first Saturn probe,” said Linda Spilker, Cassini project scientist.

On Thursday, Cassini took its last images and transmitted all the data on its recorder to prepare for the final plunge. The final data from Friday is already being processed and analyzed.

For the team behind the mission, losing their spacecraft is bittersweet: Many are sad that the mission is over but excited to see the groundbreaking science provided by unprecedented proximity to Saturn during the final dive.

“The Cassini operations team did an absolutely stellar job guiding the spacecraft to its noble end,” said Earl Maize, Cassini project manager. “From designing the trajectory seven years ago, to navigating through the 22 nail-biting plunges between Saturn and its rings, this is a crack shot group of scientists and engineers that scripted a fitting end to a great mission. What a way to go. Truly a blaze of glory.”

The team members that have worked together from concept to the final minutes exchanged hugs and tears in Mission Control at NASA’s Jet Propulsion Laboratory in California after the loss of signal was confirmed.

“Things never will be quite the same for those of us on the Cassini team now that the spacecraft is no longer flying,” Spilker said. “But, we take comfort knowing that every time we look up at Saturn in the night sky, part of Cassini will be there, too.”

After launching in 1997, traveling nearly a million miles and reaching the Saturn system in 2004, Cassini spent 13 years exploring the planet and its moons. The data and images led to numerous discoveries that changed how scientists think about our solar system.

“A superb machine in an amazing place doing everything possible to reveal the mysteries and secrets of our solar system,” Maize said. “This morning, a lone explorer, a machine made by humankind, finished its mission 900 million miles away. To the very end, the spacecraft did everything we asked. We believe we got every last second of data. We have indeed accomplished everything we set out to do.”

Cassini had its closest approach with Saturn’s moon Titan on Monday, dubbed a “goodbye kiss” by the mission’s engineers because it provided the gravity assist that sent the spacecraft on its final encounter with Saturn.

Mission scientists and operators gave Cassini this fiery send-off on purpose. Although many other options were considered — such as “parking” the spacecraft in orbit — they didn’t want to risk Cassini colliding with any of Saturn’s moons.

Cassini data and observations revealed that while seemingly inhospitable to us, the moons Enceladus and Titan could be habitable for some form of life. And NASA didn’t want to risk contaminating the moons or any future studies of them with Earth particles. Although Cassini has been in space for 20 years, microbes from Earth could still exist on the spacecraft without air, water or protection from radiation.

Although the mission itself has ended, the data and observations provided by Cassini will provide new details about the planet, its unique rings and its moons for decades to come.

“Cassini may be gone, but its scientific bounty will keep us occupied for many years,” Spilker said. “We’ve only scratched the surface of what we can learn from the mountain of data it has sent back over its lifetime.”

During the final plunge, the Ion and Neutral Mass Spectrometer acted as the “nose” of the spacecraft, directly sampling the composition and structure of the atmosphere — something that can’t be done from orbit, said Hunter Waite, team lead for the spectrometer.

This was in the hopes of investigating the “ring rain” phenomenon discovered by NASA’s Voyager mission in the early 1980s, in which it appeared that the rings were raining down material on the planet and causing changes in the atmosphere. The spectrometer could determine what material is from the rings and what material is part of the atmosphere.

Cassini’s grand finale actually began in April, with a series of dives between Saturn’s rings, close to the planet and its moons, providing unprecedented insight. This is another reason the mission scientists decided on Cassini’s particular end-game: The final dive was a dramatic conclusion to this long and scientifically valuable goodbye.

What Cassini taught us and what’s next

Inspired to learn more after flybys of Saturn by NASA’s Voyager missions, the Cassini mission was designed to be an international effort that united NASA, the European Space Agency and the Italian Space Agency.

It is known as the Cassini-Huygens mission because it delivered the European agency’s Huygens probe to Titan, the “first descent and landing on a world in the outer solar system,” according to NASA.

The Cassini mission has been extended twice and finally used up the last of its rocket propellant this week.

In the end, Cassini witnessed about half of a Saturn year. When the craft arrived, Saturn’s northern hemisphere was emerging from winter. As seasons on Saturn last about seven Earth years each, Cassini was just able to witness summer in the northern hemisphere before the mission ended.

Over the years, Cassini has revealed insights about Saturn, its rings and how they operate, the complexities of its moons, the history of the solar system and planet formation and even the other places in our solar system where life might exist: ocean worlds. Cassini has collected more than 450,000 images using a visible light camera.

It has traveled nearly five billion miles, executed 2.5 million commands, conducted 162 targeted flybys of Saturn’s moons, completed 294 orbits and its collected data has led to the publication of nearly 4,000 research papers.

When Cassini arrived, it witnessed a giant storm circling the planet for nine months. We learned that there are 3-D structures in the rings. Serendipitous observations showed that icy jets erupt from Enceladus. And Titan not only has seas and lakes of liquid ethane and methane, it has an atmosphere of chemicals that rain down, forming a unique chemistry that could lead to life.

“This is the final chapter of an amazing mission, but it’s also a new beginning,” said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate. “Cassini’s discovery of ocean worlds at Titan and Enceladus changed everything, shaking our views to the core about surprising places to search for potential life beyond Earth.”

And the spacecraft itself, as well as its instruments, are informing future missions like NASA’s Europa Clipper mission to explore Jupiter’s icy moon, launching in the 2020s.

“Cassini has enabled those future missions to be possible,” said Jim Green, NASA’s director of planetary science.

NASA’s New Frontiers competition has called for proposals that include future missions to the Saturn system. The proposals include a Saturn probe that explore the planet deeper than Cassini. Intrigued by Cassini’s discoveries, scientists have submitted concepts for future “spacecraft to drift on the methane seas of Titan and fly through the Enceladus plume to collect and analyze samples for signs of biology,” according to NASA.

Finalists will be announced at the end of the year, and NASA will explore those mission concepts further.

“For future missions to Saturn, there are a lot of possibilities,” Spilker said. “One certainly involves the tiny moon Enceladus and the potential for life in that global ocean. The potential for astrobiology in the ocean on Titan. There’s questions about the rings — can we get even closer and understand those individual ring particles? Lots of questions remain, so that’s why we have to go back.”</s>After 20 years spent traveling the solar system and 13 years spent circling Saturn, the remarkable Cassini mission is coming to a bittersweet end.

On Friday, Sept. 15, mission scientists and enthusiasts will witness the self-destruction of the Cassini probe, as it burns up and disintegrates in the atmosphere of the ringed planet that it observed over the course of three total missions. [Cassini's Saturn Crash: How to Watch Its 'Grand Finale']

Cassini has gathered new insights and imagery from Saturn, a planet that has long fascinated people's imaginations. To gain a solid understanding of the ways in which this distant spacecraft has contributed to the scientific understanding of Saturn and its more than 60 moons, it's worthwhile look at the numbers:

453,048: The number of images Cassini has taken. One of the many instruments on board the Cassini spacecraft is the Imaging Science Subsystem camera, which uses red, green and blue spectral filters to capture the marvels of the Saturn system. The colors are later combined to make a composite image. Cassini also carries various spectrometers and radar instruments that allowed the probe to observe features that were obstructed, such as the surface of Saturn's moon Titan, which was otherwise hidden underneath the clouds.

635: The amount of data, in gigabytes, that Cassini has collected. The complex scientific data Cassini has sent back to mission scientists on Earth includes electromagnetic spectrum observations, information on dust particles and a collection of Saturn's plasma characteristics.

890 million: The average distance in miles between the Earth and Saturn. It translates to roughly 1.43 billion kilometers.

83: Total minutes it will take the light from Saturn to reach Earth during the end of Cassini's mission. Since the speed of light is quite fast, it is used to measure the distances of faraway objects, such as stars and galaxies. The distance between Earth and Saturn can change, depending on where the planets are in their respective orbits, and therefore the time for light to travel from one world to the other will also change.

2: Number of oceans that the spacecraft discovered. Cassini found that both Titan and Enceladus are home to underground oceans. Many scientists now believe that Earth resembled Titan before life formed here and that Titan and Enceladus could be home to alien microbes. Preventing Cassini from contaminating these worlds is one key reason the spacecraft is being disintegrated on purpose, before its fuel runs out.

3: The total number of missions that the Cassini spacecraft completed. Cassini's journey began in 1997, and it reached Saturn in 2004. It completed its first mission in four years, and in 2008, the spacecraft extended its coverage of the planet with the two-year Equinox mission. In 2010, Cassini began the mission that it ends tomorrow, the Solstice mission.

6,504: The amount of propellant Cassini already used up in lbs., which translates to 2,950 kilograms. The probe set out with 6,565 lbs., or 2,978 kg, of fuel. This narrow gap between the numbers indicates Cassini is running low on fuel. If the probe were allowed to orbit Saturn without any way to for NASA to navigate it, Cassini would be at risk of crashing onto a Saturnian moon and disturbing that body's natural processes.

12,593: Weight in lbs. when Cassini set off with the Huygens probe, fuel, adaptor and other instruments. That translates to 5,712 kg.

4,685: Cassini's current weight in pounds (2,125 kg) after using up just about all of its fuel and having released the Huygens probe onto the surface of Saturn's moon Titan.

12: The total number of scientific instruments used by Cassini to collect and send data about Saturn, its rings and its moons. Those instruments include the composite infrared spectrometer (CIRS), imaging system (ISS), ultraviolet imaging spectrograph (UVIS), visual and infrared mapping spectrometer (VIMS), imaging radar (RADAR), radio science (RSS), and plasma spectrometer (CAPS).

Visit Space.com for complete coverage of Cassini's crash into Saturn on Friday, Sept. 15.

Follow Doris Elin Salazar on Twitter @salazar_elin . Follow us @Spacedotcom , Facebook and Google+ . Original article on Space.com.
After 20 years "floating" the Solar System, the Cassini space probe to the Saturn system ends the mission with a controlled fall into the atmosphere of the planet.