NASA has begun to receive the last photos ever taken by its doomed Cassini probe at Saturn, and will soon upload them to its public servers.

Cassini - a bus-size, nuclear-powered robot - launched toward the planet in 1997. It took seven years to reach Saturn and, since the probe's arrival, it has recorded more than 450,000 pictures.

However, Cassini has run low on propellant, and will become an artificial meteor at Saturn on Friday morning as it plunges to its death.

If NASA had risked letting the propellant tanks run dry, it would have lost control of the US$3.26-billion mission, leaving open a slim but significant 1-in-a-million chance (over the next 50 years) that Cassini would crash into and contaminate Enceladus: an icy moon of Saturn that hides a salty ocean and possibly alien life.

Titan, the planet's largest moon, may also have a habitable ocean.

"Because of planetary protection, and our desire to go back to Enceladus, and go back to Titan … we must protect those bodies for future exploration," Jim Green, the leader of NASA's planetary science program, told reporters on Friday.

This led NASA to plan Cassini's "Grand Finale": a series of 22 dives between the planet and its rings, with one final orbit to destroy the probe on Friday.

Cassini spacecraft saturn grand finale final orbit dive nasa jplNASA/JPL-Caltech

Cassini snapped its final of many images at Saturn on Thursday around 3:58pm EDT. Even though its data signal travels at light-speed, it took more than an hour for NASA to start receiving those photos.

That data began trickling in on Thursday around 5:45pm EDT, according to the Deep Space Network

Fully transmitting all of the photos should take about 11 hours, Earl Maize, an engineer at NASA's Jet Propulsion Laboratory (JPL) who manages the Cassini mission, told reporters on Wednesday.

"We'll be able to share those with you sometime Friday morning," Maize said.

You can find all photos being uploaded from Cassini in NASA's raw image gallery here.

What Cassini's final images will show

Cassini was speeding toward Saturn's atmosphere, about to take its 78,000-mph (125,000 km/h) plunge, when it took its final pictures.

Linda Spilker, a Cassini project scientist and a planetary scientist at NASA JPL, walked reporters through all of those last planned images on Wednesday.

"In that last period of time, looking around Saturn, what we're doing is taking our final picture postcards of the Saturn system, looking at our favourite targets to put these images into our Cassini scrapbook," Spilker said.

From a distance, the probe will take multiple photos of Saturn and its rings in colour - "basically our last look at the entire system," she said.

There will be photos of Enceladus setting across the northern limb of Saturn. Titan - which has liquid-hydrocarbon lakes, an atmosphere twice as thick as Earth's, and seasonal rain - also got some attention.

Cassini saturn rings titan enceladus moons nasa jpl caltech ssi PIA14604Enceladus (foreground) drifts in front of Saturn's rings while Titan looms behind. Image: NASA/JPL-Caltech/Space Science Institute

"We're going to take some goodbye pictures of Titan, a last look to see if there's any weather or clouds going on," she Spilker said.

NASA will also show images of Cassini's last close-up look at Saturn's rings.

One focus was "Peggy", a group of particles that "might break free to become a moon," she said, as well as propeller objects that are "trying to open up gaps in Saturn's rings, but are not quite big enough to do that."

The final images to trickle onto NASA's servers will show the spot on Saturn's dark side toward which Cassini will dive - "Cassini's final home inside the planet Saturn itself," Spilker said.

Watch the Cassini mission end live

Cassini is scheduled to disintegrate starting around 6:22am EDT on Friday, though it will take until 7:55am EDT for NASA to receive the probe's last packet of data.

While it's unlikely that any telescope will see the probe die, you can tune in to NASA TV's live broadcast of mission control here.

The space agency's coverage will start around 7am EDT.

This article was originally published by Business Insider.

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