Jonathan Amos
Science correspondent, BBC News, in Pasadena

It's a tradition here at mission control that peanuts are eaten at important moments - such as during a landing on Mars, or, as in this case, the ending of a spectacular orbiter. The voice loop has just suggested the nuts be passed around with just 15 minutes left to impact.

Cassini has just passed 30 degrees North latitude. Even closer now.

Jason Major, who is well known for processing the raw images from space missions, has tweeted this image - Cassini's final one from Saturn.

As the final signal races towards Earth, Cassini-watchers are grappling with the fact that the much-loved spacecraft, a part of our lives for 20 years, is now gone forever.

Jonathan Amos
Science correspondent, BBC News, in Pasadena

Controllers report that the spacecraft has just passed 40 degrees North latitude. The impact zone is at 10 degrees. Not long now.

The probe's demise is leading to some moving commentary from observers - including this free-form poetry from Emily Lakdawalla of the Planetary Society.

Image source, @eladakwalla

David Shukman
Science editor, BBC News

As we wait for the loss of signal from Cassini, I’m reminded of a similar tension around a very different highlight of the mission: the stunning achievement of Europe’s Huygens lander touching down on Saturn’s moon Titan on January 14 2005.

For all of us at mission control at ESA that day, no one was taking bets on whether the attempt would succeed. This was a venture far beyond anything previously tried and it remains the most distant landing ever made by a spacecraft. I happened to be filming in the control room just minutes after the first pictures arrived from Titan’s surface.

There was a scene of what looked like boulders, a landscape no human eye had ever seen before, and it was exhilarating to share this moment of discovery. It took a far-sighted vision, along with extraordinary patience and some incredible engineering, to make this revelation possible, as with so many others from the remarkable Cassini project.

Jonathan McDowell, an astronomer at the Harvard-Smithsonian Center for Astrophysics, says Cassini has now sent its last radio signal and been destroyed in Saturn's atmosphere.

The final signal is travelling at the speed of light and at 12:07 BST was 5.8 astronomical units (AU) from Earth. An astronomical unit refers to the average distance between the centre of Earth and the centre of the sun.

British planetary scientist Leigh Fletcher has posted a photo of some of those working on the mission over the years - people who have gone through different stages of life together as Cassini has continued sending back information.

With the focus on Nasa's Cassini spacecraft, it's worth remembering the other big part of the mission. The Huygens probe was the big European contribution to this very international venture.

The lander travelled to Saturn attached to the Cassini "mothership" and was released to begin its journey to the moon Titan in December 2004

On the 14 January, the lander began its descent into Titan's thick atmosphere, profiling its density, temperature and composition - as well as measuring the strength and direction of winds.

Huygens survived for at least 90 minutes on the surface of this frozen moon, and is still lying there - perhaps for some future mission to examine.

You can read about the Huygens landing here. And the European Space Agency have compiled science highlights from Cassini and Huygens here:

As Cassini makes its final descent, Australian astrophysicist Katie Mack has been reflecting on its mission and what it says about the human species.

Cassini is due to be entering Saturn's atmosphere right about now. It will all end in the spacecraft's destruction, with the loss of radio contact with Earth set to be received at around 12:55 BST (04:55 PDT).

It takes a whopping 67 to 85 minutes for transmissions from Cassini to reach Earth. By comparison, the Mars-Earth signal delay is between four and 24 minutes.

The variation is dependent on how far away the planet is in relation to Earth (this changes based on the orbits of the two planets) and is ultimately determined by the speed of light, which travels at 1,079,000,000 km/hour.

Jonathan Amos
Science correspondent, BBC News, in Pasadena

Cassini will enter the atmosphere at about 10 degrees north of Saturn's equator. At the time it will be travelling at over 120,000km/h (75,000mph).

As the density of gas molecules increases, the probe will experience more and more drag. Its long magnetometer boom will act like a rudder and will try to turn the spacecraft. Cassini’s thrusters will fight this in an effort to keep the radio antenna pointing accurately at Earth. But there will come a point when the forces will be too great, the thrusters will be overwhelmed and Cassini will lose lock on home and begin to tumble.

Engineers here talk about the equivalent altitude of the International Space Station on Earth. That's to say the density of gas molecules will be roughly comparable. Yes, the ISS encounters residual air at 400km in height, and this drags on the platform and pulls it down. The station has to boost back up every so often to stay in orbit. The ISS travels at 27,000km/h. Cassini will be going four to five times faster, and it will rapidly get out of shape.

Once contact is lost with Earth, it should then be a matter of seconds before Cassini breaks apart. The iridium encapsulating the plutonium pellets in the probe’s battery will likely be the last material to completely melt, dissociate and spread out in Saturn’s atmosphere.

Image source, NASA-JPL

The Cassini spacecraft sent back a deluge of images from the Saturn system over 13 years. We've compiled a few of the most amazing pictures for you in our gallery page, which can be found at this link.

Here's a taster of some of the spacecraft's great shots:

Image source, NASA

Image source, NASA

Image source, NASA

Jonathan Amos
Science correspondent, BBC News

Let's talk timings. The moment controllers expect the radio signal to fall off, as received here on Earth, has changed a bit in recent weeks. That's because of the uncertainty - about the precise course of the probe and the nature of the upper layers of the atmosphere.

But after Cassini's recent pass of the moon Titan, which had the effect of slowing the spacecraft ever so slightly, we can now be pretty sure when the veteran probe will flatline.

Engineers here at mission control at the Jet Propulsion Laboratory are predicting loss of signal to occur at 6 seconds after 04:55 local California time.

That is 11:55:06 GMT, or 12:55:06 BST. As stated previously, this is the time that antennas on Earth lose contact. In reality, Cassini will have been destroyed much earlier. There is an 83-minute light travel-time from Saturn currently.

Cassini's end-call travels like an echo across the vastness of space.

The BBC asked Prof Andrew Coates from the Mullard Space Science Laboratory in Surrey to sum up the appeal of Saturn and its moons for scientists.

The scientist, who is a member of the Cassini Plasma Spectrometer instrument team, says: "Saturn is the most spectacular planet in our Solar System. The incredible rings, visible even in binoculars or a small telescope, make it stand out compared to all the rest. But it has other tantalising features too - the moons make it a mini planetary system in itself."

Of these moons, Titan has long been one of the most appealing for exploration - and Cassini delivered fascinating insights. Prof Coates explains: "At Titan, we've found rivers, lakes, dunes and dried-up lake beds on the alien surface all carved by methane and other hydrocarbons, as well as complex chemistry in the upper atmosphere where we found unexpectedly large organics - pre-biotic building blocks - and a sub-surface water ocean."

But Enceladus was the big surprise of the mission: "At Enceladus, the amazing plumes have let us literally fly through material from the sub-surface ocean below the icy surface.

"As well as water, the plumes include sodium indicating a salty ocean, silicates from hydrothermal vents, hydrocarbons and hydrogen. This indicates a habitable environment now, bringing Enceladus up there with Mars, Europa and maybe Titan as suitable past, present and future locations for life beyond Earth."

Image source, NASA

Jonathan Amos
Science correspondent, BBC News

Is it really necessary to destroy Cassini? The US-European mission did have options. It could have sent the probe far from Saturn. It could have put it in a stable orbit a long way beyond the rings and the icy moons. But none of these ideas was as attractive scientifically as going really close to Saturn.

These past five months Cassini has been flying through the gap between the rings and the top of the atmosphere. This orbit has provided a unique vantage point from which to nail certain outstanding questions. These include getting the age of the rings. This is done by making a detailed map of the gravity field in which the contributions from the huge world and the rings can be teased apart.

Cassini has essentially been trying to weigh the rings.

The more massive they are, the older they are likely to be. Some scientists think they could even have formed with Saturn itself 4.6 billion years ago. They would certainly need a large mass to withstand the forces that might erode them over time, such as collisions from tiny meteoroids. But it is looking like the opposite may actually be true - that their mass is less than previously estimated.

If confirmed it points to the rings being the remnants of some object that has broken apart around Saturn in the recent past.

And then there is the issue of "planetary protection". With so little fuel left in Cassini (about 30kg), it will soon become uncontrollable, and space agency officials do not want anything bumping into Enceladus or Titan that might contaminate them with Earthly microbes. It's a very slim possibility but has to be considered. It's just safer therefore to dispose of the probe fully in Saturn's atmosphere.

Cassini passed the orbital distance of Enceladus at 05:08 GMT (06:08 BST) as it raced in towards the planet. Just over half an hour ago, it would have rolled to put its Ion and Neutral Mass Spectrometer directly in the line of travel. This is the priority instrument for the plunge. It will sample the composition of the atmosphere.

One of the things the INMS wants to determine is the exact ratio of hydrogen to helium in the upper layers of gases. This number says something about the formation and evolution of Saturn and its atmosphere. The ratio can be compared and contrasted to Jupiter.

As of this posting, Cassini is now essentially an atmospheric probe. It has been reconfigured so that any data acquired by the instruments is immediately relayed to Earth. The information bypasses the onboard solid state memory and is transmitted with just a 2-3-second delay.

The radio signal does of course take 83 minutes to get to Earth - because of the finite speed of light and the 1.4 billion km separation between Saturn and us.

Dr Carolyn Porco, the lead scientist on the Cassini imaging team - which is responsible for many of the amazing images from Saturn we've marveled at over the years - has written her final blog entry of the mission.

The researcher, who has been documenting the tour of Saturn online, says: "It is with both wistful, sentimental reflection and a boundless sense of pride in a commitment met and a job well done that I now turn to face this looming, abrupt finality."

You can read the full blog entry here:

Another keen image processor is Emily Lakdawalla, the senior editor at The Planetary Society, external. Here, she's run a series of the images together from Cassini showing Enceladus disappear from sight.