(CNN) Here's the challenge: Land a spacecraft that was designed to be an orbiter on the surface of a speeding comet that is millions of miles away and carry out pioneering science on the way down.
It's difficult -- but the Rosetta mission has continued to surprise and delight those who have followed its epic 12-year, comet-chasing journey.
Mission controllers from the European Space Agency (ESA), which is leading the Rosetta mission consortium, are aiming to find out more about the lumpy structures they are calling "goosebumps."
Collected in the wall of pits on the comet's surface, scientists believe they may be the original pieces of material that bonded together to form the body and date to when the solar system was young.
ESA describes the latest phase as the "biggest challenge yet" as it tries to maneuver the orbiter into position for the descent and impact on the comet on September 30.
The agency says Rosetta will try to gather images and other data on the gas, dust and plasma very close to the comet.
Rosetta has already returned spectacular images and science discoveries from Comet 67P/Churyumov--Gerasimenko as it followed its orbit around the sun.
And millions followed the drama when the landing craft Philae separated from Rosetta and bounced across the comet after its thruster and grappling harpoons designed to anchor it to the surface failed in November 2014. The accident proved to be something of a blessing though as Philae was able to carry out some of its objectives in two locations.
The instruments discovered 16 organic compounds, including four that had been never detected on comets, before Philae's batteries drained. Scientists were excited by the results because some of those chemicals form the building blocks of the ingredients for life.
Mission controllers believed Philae was trapped under a cliff but were unable to find exactly where it ended up after going silent in its resting place.
Then just weeks ago Philae returned from the cosmic dead when scientists had a stroke of luck. After months of searching they discovered the probe with their last opportunity to photograph from Rosetta a possible location for the craft.
It's important too because as lander system engineer Laurence O'Rourke explained, they now have "perfect contextual data" for the measurements Philae made.
"It couldn't have been in a more difficult location -- quite literally between a rock and a hard place," he told CNN.
But the engineer said he was thrilled to find out the fate of Philae after working on the campaign for so long.
"It was like drinking a gallon of adrenaline," O'Rourke said. "I was astounded by the quality of the picture. It was magnificent."
Now mission controllers are preparing for the last act at 67P.
"We are not trying to gain science from hitting the comet," said O'Rourke. "We are trying to gain science before we hit it.
"This is a controlled descent and impact."
Now that the comet and the orbiter have traveled so far away from the sun there will come a point where there isn't enough solar power to run the heaters and computers.
O'Rourke told CNN that as the probe would die anyway the decision was made to try to capture more data including very high resolution images of the pit structures.
"We don't have enough power to keep it alive. We wanted to take control of the end," he said.
The engineer said he would miss the mission, especially the public enthusiasm for Rosetta but added: "The legacy will be there for many generations."
Astronomer Dan Brown, who lectures at Nottingham Trent University in the UK, said the Rosetta mission had been an "astonishing" engineering feat but also inspirational.
"The Rosetta mission has helped us gain an insight into the activity of comets, how comets were created and to some extent if they could have been the source for water on our early Earth," he told CNN.
"The presence of complex molecules, some of which 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.
"The image that does it for me is seeing the surface of the comet and the lander together. Such imagery inspires as much as the Moon landings and drives us to go and discover, achieve and face challenges on our never-ending quest for knowledge."
The end of the mission also brings to a close the Twitter chat that has endeared followers to Rosetta and Philae, and their surreal and strangely touching conversations, especially when the landing craft bounced and disappeared.
Asked if he would change anything, O'Rourke said: "Philae couldn't have been designed better than it was ... but I think I would have added LEDs that flash so it could have been detected wherever it landed."