(CNN) The Rosetta space probe has crash-landed on the surface of a distant comet, ending a remarkable 12-year mission that made a series of breakthrough discoveries.

It was the final act of an epic project to chase a comet in its orbit around the sun, put a sister spacecraft on the surface and gather information about its behavior and chemical composition.

After confirmation that Rosetta had landed, the mission controllers sent out a simple tweet in many languages, just saying: "Mission complete."

'Sleep well Rosetta'

Rosetta lander system engineer Laurence O'Rourke paid his own emotional tribute to the project, telling CNN it had been and "exceptional" mission.

"Sleep well Rosetta. You leave a legacy of science for generations to come," he said.

"To have seen the signal lost was a sad moment for all of us on the project and yet to have ended it in such an extraordinary way has made that moment also very special for us too.

"I was watching the images come down during the morning and each one was better than the last. The final ones taken were the cherry on the cake of this exceptional mission; they really showed the surface at amazing resolution."

Engineers and scientists celebrate in the Rosetta control room in Darmstadt

The European Space Agency (ESA) which led the mission consortium, including NASA, decided that setting down the orbiter on Comet 67P/Churyumov--Gerasimenko was the best way to close the project.

Airbus Defence and Space, which built the Rosetta probe, said the impact was at a sedate walking pace of about one mile per hour.

O'Rourke earlier told CNN that Rosetta was too far away for its solar panels to be effective in running the heaters or the computers.

The orbiting spacecraft was not designed to land on the comet but by making a controlled descent and impact, it was thought possible to gather more pictures and data on the way down.

Photos: Rosetta: The comet chaser The Rosetta probe sent an unexpected final image back to Earth shortly before it made a controlled impact onto the surface of Comet 67P last September. Hide Caption 1 of 40 Photos: Rosetta: The comet chaser Philae has been found! – The Rosetta spacecraft's high-resolution camera took this image of the Philae lander on September 2, 2016. The lander is wedged into a dark crack on a comet, named 67P/Churyumov--Gerasimenko, hurtling through space. The discovery comes less than a month before the Rosetta mission's end. Hide Caption 2 of 40 Photos: Rosetta: The comet chaser Philae is wedged in a dark corner of Rosetta – "We are so happy to have finally imaged Philae, and to see it in such amazing detail," says Cecilia Tubiana of the OSIRIS camera team. She was the first person to see the images when they were downlinked from the Rosetta probe, according to the European Space Agency. Hide Caption 3 of 40 Photos: Rosetta: The comet chaser Philae found using images from Rosetta – The image is detailed enough that viewers can pick out features of Philae's 3-foot-wide (1 meter) body. Even two of its three legs can be seen. Hide Caption 4 of 40 Photos: Rosetta: The comet chaser One of the primary objectives of the Rosetta mission was to drop the Philae lander onto the comet. The probe was successfully deployed in November 2014, becoming the first probe to land on a comet. But Philae failed to grab onto the comet and bounced around. It fell silent a few days later. Then on June 13, 2015, Philae came out of hibernation and "spoke" to mission managers at the European Space Agency for 85 seconds. This photo above was taken by the lander's mothership, the Rosetta orbiter, after the lander started its descent to the comet. Hide Caption 5 of 40 Photos: Rosetta: The comet chaser The Rosetta spacecraft captured this image of a jet of white debris spraying from Comet 67P/Churyumov--Gerasimenko on July 29, 2015. Mission scientists said this was the brightest jet seen to date in the mission. The debris is mostly of ice coated with dark organic material. Hide Caption 6 of 40 Photos: Rosetta: The comet chaser This image of Comet 67P/Churyumov-Gerasimenko was taken by Rosetta on July 8, 2015 as the spacecraft and comet headed toward their closest approach to the sun. Rosetta was about 125 miles (201 kilometers) from the comet when it took this image. Hide Caption 7 of 40 Photos: Rosetta: The comet chaser Philae wakes up! Mission managers posted this cartoon of the lander yawning after it came out of hibernation on June 13, 2015. They also sent a series of tweets between the lander and its mothership, Rosetta. Hide Caption 8 of 40 Photos: Rosetta: The comet chaser Comet 67P/Churyumov-Gerasimenko is a Jupiter-family comet. Its 6.5 year journey around the Sun takes it from just beyond the orbit of Jupiter at its most distant, to between the orbits of Earth and Mars at its closest. The comet hails from the Kuiper Belt, but gravitational perturbations knocked it towards the Sun where interactions with Jupiter's gravity set it on its present-day orbit. Hide Caption 9 of 40 Photos: Rosetta: The comet chaser This image of Comet 67P/Churyumov-Gerasimenko was taken by Rosetta on June 5, 2015, while the spacecraft was about 129 miles (208 kilometers) from the comet's center. Hide Caption 10 of 40 Photos: Rosetta: The comet chaser Rosetta's navigation camera took this image of the comet on June 1, 2015. Hide Caption 11 of 40 Photos: Rosetta: The comet chaser The Rosetta Mission is tracking Comet 67P/Churyumov-Gerasimenko on its orbit around the sun. This image was taken on May 3, 2015 at a distance of about 84 miles (135 km) from the comet's center. Hide Caption 12 of 40 Photos: Rosetta: The comet chaser This image of Comet 67P/Churyumov-Gerasimenko was taken on April 15, 2015. Hide Caption 13 of 40 Photos: Rosetta: The comet chaser Rosetta snapped this wide-angle view of Comet 67P/Churyumov-Gerasimenko in September 2014. Rosetta was about 107 million miles (172 million kilometers) from Earth and about 92 million miles (148 million kilometers) from the sun when the photo was released. Hide Caption 14 of 40 Photos: Rosetta: The comet chaser A camera on Rosetta took this picture of Comet 67P/Churyumov-Gerasimenko on November 22, 2014, from a distance of about 19 miles (31 kilometers). The nucleus is deliberately overexposed to reveal jets of material spewing from the comet. The 2.5-mile-wide (4-kilometer) comet has shown a big increase in the amount of water its releasing, according to NASA. The space agency says about 40 ounces (1.2 liters) of water was being sprayed into space every second at the end of August 2014. Hide Caption 15 of 40 Photos: Rosetta: The comet chaser Rosetta took this picture of a section of the comet's two lobes from a distance of about 5 miles (8 kilometers) on October 14, 2014. Hide Caption 16 of 40 Photos: Rosetta: The comet chaser The Rosetta spacecraft's Philae lander is shown sitting on Comet 67P/Churyumov-Gerasimenko after becoming the first space probe to land on a comet on November 12, 2014. The probe's harpoons failed to fire, and Philae bounced a few times. The lander was able to send back images and data for 57 hours before losing power. Hide Caption 17 of 40 Photos: Rosetta: The comet chaser Rosetta's lander, Philae, wasn't able to get a good grip on the comet after it touched down. This mosaic shows Philae's movements as it bounced across the comet. Hide Caption 18 of 40 Photos: Rosetta: The comet chaser Philae snapped these images after landing, and mission scientists used them to create a panoramic view of the landing site. A graphic shows where the probe would be sitting in the photograph. Hide Caption 19 of 40 Photos: Rosetta: The comet chaser The image of Comet 67P/Churyumov-Gerasimenko was taken by a camera on the Philae lander during its descent to the comet on November 12, 2014. The lander was about 1.9 miles (3 kilometers) from the surface at the time. Philae touched down on the comet about seven hours later. Hide Caption 20 of 40 Photos: Rosetta: The comet chaser Rosetta's OSIRIS camera captured this parting shot of the Philae lander after separation. Hide Caption 21 of 40 Photos: Rosetta: The comet chaser Rosetta's lander Philae took this parting shot of its mother ship shortly after separation on November 12, 2014, as Philae headed for a landing on Comet 67P. While Philae is the first probe to land on a comet , Rosetta is the first to rendezvous with a comet and follow it around the sun. Hide Caption 22 of 40 Photos: Rosetta: The comet chaser This mosaic is made of four individual images taken about 20 miles (31.8 kilometers ) from the center of the comet on November 4, 2014. Hide Caption 23 of 40 Photos: Rosetta: The comet chaser Rosetta took this image of comet 67P/Churyumov-Gerasimenko on September 15, 2014. The box on the right shows where the lander was expected to touch down. Hide Caption 24 of 40 Photos: Rosetta: The comet chaser The spacecraft sent this image as it approached the comet on August 6, 2014. From a distance of nearly 81 miles (130 kilometers), it reveals detail of the smooth region on the comet's "body" section. Hide Caption 25 of 40 Photos: Rosetta: The comet chaser This image, captured August 7, 2014, shows the diversity of surface structures on the comet's nucleus. Hide Caption 26 of 40 Photos: Rosetta: The comet chaser The comet's "head" can be seen in the left of the frame as it casts a shadow over the "body" in this image released August 6, 2014. Hide Caption 27 of 40 Photos: Rosetta: The comet chaser This image of the comet was taken on August 1, 2014, as Rosetta closed in its target. Hide Caption 28 of 40 Photos: Rosetta: The comet chaser Rosetta's mission started on March 2, 2004, when it was launched on a European Ariane 5 rocket from Kourou, French Guiana. Hide Caption 29 of 40 Photos: Rosetta: The comet chaser Rosetta is named after the Rosetta Stone, the black basalt that provided the key to deciphering Egyptian hieroglyphs. Scientists think the mission will give them new clues about the origins of the solar system and life on Earth. The mission is spearheaded by the European Space Agency with key support from NASA. Hide Caption 30 of 40 Photos: Rosetta: The comet chaser This photo shows Rosetta being tested before it was wrapped in insulating blankets and loaded on a rocket for launch. Hide Caption 31 of 40 Photos: Rosetta: The comet chaser Rosetta has massive solar wings to power the spacecraft. They were unfurled and checked out at the European Space Agency's test facilities before being packed up for liftoff. Hide Caption 32 of 40 Photos: Rosetta: The comet chaser After its closest approach to Earth in November 2007, Rosetta captured this image of the planet. Hide Caption 33 of 40 Photos: Rosetta: The comet chaser Rosetta snapped this image of Earth in November 2009. The spacecraft was 393,328 miles from Earth. Hide Caption 34 of 40 Photos: Rosetta: The comet chaser Rosetta passed asteroid Steins in September 2008, giving scientists amazing close-ups of the asteroid's huge crater. The asteroid is about 3 miles in diameter. Hide Caption 35 of 40 Photos: Rosetta: The comet chaser Rosetta took this image of Mars as it looped through the solar system. Hide Caption 36 of 40 Photos: Rosetta: The comet chaser This image was taken by an instrument on Rosetta's Philae lander just minutes before the spacecraft made its closest approach to Mars. Part of Rosetta and its solar arrays are visible. Hide Caption 37 of 40 Photos: Rosetta: The comet chaser On July 10, 2010, Rosetta flew about 1,864 miles from asteroid Lutetia, which is 10 times larger than asteroid Steins. Hide Caption 38 of 40 Photos: Rosetta: The comet chaser Look closely at the top of this picture. See that dot? That's Saturn. Rosetta snapped the picture of asteroid Lutetia and captured Saturn in the background. Hide Caption 39 of 40 Photos: Rosetta: The comet chaser After taking pictures of Earth, Mars and asteroids, Rosetta was put into hibernation in May 2011 after it reached the outer part of the solar system. Mission managers woke it January 20, 2014. Hide Caption 40 of 40

Scientists were keen to find out more about the curious lumpy structures they are calling "goosebumps" in the wall of pits on the surface of 67P.

Breakthrough discovery

They believe they may be the pieces that clumped together to form the comet when the solar system was young.

The Rosetta orbiter returned spectacular images when the comet made its closest approach to our star and tails of material were driven off.

And the landing craft Philae discovered 16 organic compounds including four that had never been detected on comets before the lander ran out of power. It was an important step because some of those chemicals form the building blocks for the ingredients of life.

One theory is that comets may have seeded the Earth with the necessary components for life to start.

Ahead of the last stage of the mission, astronomer Dan Brown, who lectures at Nottingham Trent University in the UK, told CNN that Rosetta had been an "astonishing" and inspirational venture.

"The presence of complex molecules, some of which were previously unknown to exist on comets, still allows comets to be a possible source of introducing complex molecules and enable the formation of life on Earth," he said.

Millions have followed the twists and turns of the mission through some of the social media interactions between the orbiter and lander.

Philae made a dramatic descent on to the comet's surface after its thruster and grappling harpoons designed to anchor it in the weak gravity failed and it bounced.

Mission controllers greeted it as a blessing as the lander was able to gather data from two sites.

Philae ended up trapped between a rock and a cliff and it was only very recently that the orbiter discovered its final resting place.

ESA said the mission has cost about 1.4 billion euros ($1.57 billion) from the start of the project in 1996.

The spacecraft covered billions of miles to make its rendezvous with the comet and follow its trajectory around the sun.

Rosetta needed four gravity assists from Mars and the Earth -- the so-called slingshot effect -- to accelerate and meet its target. This circuitous route took 10 years and was so far away from a source of solar power that the craft had to be put in hibernation for part of the journey.

ESA's mission overview

When Rosetta first arrived at 67P in August 2014 project scientist Matt Taylor told CNN that by following the comet on its journey they could see how it changed from its inert phase to when it was highly active as it approached the sun. As it got close, ice melted and turned it into an ionized gas tail and the dust produced a separate curving tail.

O'Rourke said the discoveries Rosetta has left a legacy that "will be there for many generations."