Rosetta mission: Robot heads towards comet surface

Europe's audacious bid to put a robot on the surface of a comet is under 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 BBC reported.

The mission will shine a light on some mysteries surrounding these icy relics from the formation of our Solar System.

Success would be a first for space exploration - no mission has previously made a soft landing on a comet.

The descent should take seven hours, with a signal confirming touchdown received at Earth at around 16:00 GMT.

"It's all down to Isaac Newton and the laws of physics now. Philae is on its way down to the surface," said Prof Mark McCaughrean, senior science adviser at the European Space Agency (Esa).

"If Isaac's friendly to us, we'll have a great landing later today."

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/no-go" decision was delayed. The thruster system used to push the robot into the surface of the comet at the moment of touchdown could not be primed.

"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.

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.