Europe's audacious bid to put a robot on the surface of a comet isunder way.
At 08:35 GMT, the Rosetta satellite released its Philae lander towards Comet 67P/Churyumov-Gerasimenko, a large mass of ice and dust some 510 million km from Earth.
The descent should take seven hours, with a signal confirming touchdown received at Earth at around 16:00 GMT.
Success would be a first for space exploration - no mission has previously made a soft landing on a comet.
Part of the difficulty is the very low gravity on the 4km-wide ice mountain.
Philae needs to be wary of simply bouncing back into space.
As a consequence, on contact it will deploy foot screws and harpoons to try to fasten its position.
It will then take a picture of its surroundings - a strange landscape containing deep pits and tall ice spires.
This is, though, an event with a highly uncertain outcome.
Early on Wednesday (GMT), the third "go" signal was delayed due to concerns over the health of the Philae lander."We almost didn't get the third 'go'," said Paolo Ferri, head of operations at Esa.
The thruster system used to push the robot into the surface of the comet when it touches down could not be primed. This means Philae will now have nothing to push it into the surface of the comet.
"We will just have to rely now on the harpoons, the screws in the feet, or the softness of the surface. It doesn't make it any easier, that's for sure," said lander chief Stephan Ulamec, from the German Space Agency. But the landing attempt goes ahead.
The terrain that has been chosen for the landing on the rubber-duck-shaped object is far from flat.
Philae could bash into cliffs, topple down a steep slope, or even disappear into a fissure.
Esa's Rosetta mission manager Fred Jansen said that despite these challenges, he was very hopeful of a positive outcome.
"We've analysed the comet, we've analysed the terrain, and we're confident that the risks we have are still in the area of the 75% success ratio that we always felt," he told reporters here at Esa's mission control in Darmstadt, Germany.
And Prof Ian Wright, a leading British scientist working on the lander, said he was determined to be upbeat: "We realise this is a risky venture. In a sense that is part of the excitement of the whole thing. Exploration is like that: you go into the unknown, you're unsure of what you're going to face," he told BBC News.
The prize that awaits a successful landing is immense - the opportunity to sample directly a cosmic wonder.
Comets almost certainly hold vital clues about the original materials that went into building the Solar System more than 4.5 billion years ago.