The European Space Agency bid a poignant farewell Friday to intrepid space explorers Philae and Rosetta, brought to "life" as loveable cartoon characters that it has had to abandon on a cold, dark comet surface.

As the real steel and bolts Rosetta spacecraft made a planned crash-landing on Comet 67P/Churyumov-Gerasimenko, its fairytale counterpart is depicted going through a range of emotions as she approaches her end.

Cartoon Rosetta grins widely as she beams photos and data home to Earth, then breaks out in a gasp of pleasant surprise as she spots robot lander Philae, her "little brother", asleep between large boulders on the comet surface.

A resigned smile follows as Rosetta sends a final text message to Earth: "Mission complete!" before waving in our direction with a spindly arm.

A green-and-blue cartoon Earth replies by holding aloft a large banner proclaiming: "Goodbye Rosetta" as the craft, its solar panels stretched out wing-like from either side, drifts slowly out of view.

Our protagonist is next depicted lying eyes closed on the comet surface, her panels bent, battered and slightly dirty.

She clutches a story book entitled "Once upon a Time", which was also the title of the cartoon series about her exploits.

And in the background, Philae is fast asleep on a mat, under a green blanket, at the foot of the ledge where he came to rest after repeatedly bouncing upon landing.

Both are smiling in repose, reunited after nearly two years apart.

"And so ends the amazing adventure of our two extraordinary explorers Rosetta and Philae at Comet 67P," a narrator says.

"Farewell, dear friends."

The European Space Agency's Rosetta mission gained a loyal fan following around the world through its social media outreach campaign.

Trouble is, many children, even some adults, came to think of the machines as cute cartoon characters - siblings bravely exploring a comet hundreds of millions of kilometres from Earth.

To be continued...?
And when Rosetta started running out of power, communications managers were presented with a conundrum: how to depict the orbiter's demise in a controlled comet crashlanding in cartoon form?

With Philae it had been easier.

The robot did not need to be "killed" after its initial mission, as it was expected to wake up from hibernation, and did. So cartoon Philae simply fell asleep.

But with Rosetta's demise, the pair "had to be finished. We didn't want to give false hope they were on the surface sleeping like we did with Philae," ESA scientist Mark McCaughrean told AFP at mission control in Darmstadt.

"We really wanted to give closure."

The fairy tale began in January 2014, when Rosetta "woke up" after hibernating for nearly three years to conserve energy on her 10-year journey from Earth to 67P - a veritable deep-space Sleeping Beauty.

ESA released the first cartoon video entitled: "Once upon a time."

Subsequent videos followed Rosetta's six-billion-kilometre journey to the comet, Philae perched on her head and jumping up and down in childlike fashion as he insists: "Are we there yet? Are we there yet?"

When Rosetta released Philae onto the comet surface, the eager explorer was depicted strapping on a pair of studded boots and a hard hat, stuffing a cheese sandwich and compass into a backpack, and bidding his sister a fond farewell before leaping boldly into space.

The pair's exploits have made them rare science celebrities, each with its "own" Twitter account - Philae has nearly 450,000 followers.

They sell T-shirts, hoodies and plush toys.

In the final video, the parting words: "To be continued", are followed by ellipsis and a question mark.

The duo are seen being covered in dust as time passes.

And then... a pair of futuristic humans appear, peering out of what appears to be a spacecraft window.

"We found them," a woman pronounces.

"It was precisely to take the sting out of it," explained McCaughrean - though only in the very distant future, " a million years from now, maybe."

"We don't want to give any sense that it will happen tomorrow, that we are planning a mission to go there. Why would we bother? We know what they've done. They're museum pieces."