It's a curtain call of the spectacular kind: On September 30, the European Space Agency's (ESA) historic space probe Rosetta is to land gently on a comet 720 million kilometres away, taking some final pictures and measurements before going silent.
The landing of the probe comes more than a dozen years after it was launched into space and two years after the probe dropped Philae, a lander filled with laboratory instruments, onto the comet - 67P/Churyumov–Gerasimenko - the first-ever landing by a man-made probe on a comet.
"We have never had Rosetta so close to Churi as this," says ESA flight director Paolo Ferri at the agency's satellite control centre in Darmstadt, using the shortened nickname for the comet discovered by Russian astronomers Klim Churyumov and Svetlana Gerasimenko in 1969. "We will be trying to take measurements and pictures right up to the final seconds."
If all goes to plan, the Rosetta will be landing at 1140 GMT on September 30, bringing an end to the 12-year mission. "When the space probe touches the surface of Churi, it will shut itself down," Ferri said. "We won't hear from it ever again."
The landing is to be extremely slow, slower than at walking speed.
"We are going to try to make the touchdown as gentle as possible," he added. The target area of the comet that measures some 4 kilometres long by 3.5 kilometres wide and 3.5 kilometres thick - some people liken its shape to a rubber duck - is to be at the "head" of the rocky mass.
The spot is to be next to a 130-metre-wide basin. ESA engineers believe the Rosetta will touch down 1-2 kilometres from the lander Philae.
The end of a mission is a time for taking stock, and scientists say that the secrets of Churi that were unveiled were of enormous significance.
Comets are thought to contain the oldest, largely unchanged, matter from the time when our solar system was formed some 4.6 billion years ago.
Altogether, Rosetta and Philae carried around 20 instruments to give Churi a close-up examination.
The Rosetta mission, costing some 1.3 billion euros (1.5 billion dollars) is one of the most ambitious projects undertaken by ESA.
The probe was launched March 2, 2004, from the European launching facility in Kourou, then spent years in an elliptical journey of several billion kilometres before finally catching up with the comet.
During its journey, the probe's systems were shut down in order to save up the energy needed for the final approach.
History was made November 12, 2014, when Philae was detached from the Rosetta and landed on the comet.
"This day is a historic one," proclaimed then-ESA general director Jean-Jacques Dordain of the first-ever landing on an asteroid.
Asked about his most special memories of the Rosetta mission, Ferri talks about the discoveries made.
"For me, the most spectacular result was a very early one, soon after the (Philae) landing. Namely, that the water on the comet had anything at all to do with the water here on Earth."
Nicolas Altobelli, ESA's expert for robotic exploration, noted that "never before could gas and dust be measured to close to a comet as we did with Rosetta."
He also cited the photographs sent back to Earth. "The pictures that Rosetta had already made of Churi in July 2014 were an absolute novelty. We recognized the comet core and its outlines. The similarity with a duck was there."
Stephan Ulamec, of the German space centre DLR and the project leader for the lander Philae, saw things similarly, stressing the camera system on board.
"The main findings from Philae were gained through the high-resolution photos taken by the Rolis and Civa cameras, revealing for the first time how a comet surface looks from right up close."
There were suggestions that Rosetta could be kept going, its systems shut down to save energy, to make yet another rendezvous with the comet and take further measurements, but Ferri dismissed the idea.
"Such a hibernation would last nearly four years," he said. But the decisive factor against prolonging the mission is that the Rosetta could not really do all that much more work.
The probe still has fuel for perhaps another six months. "It wouldn't be worth the risk," Ferri said.
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