London, Nov 12 (IANS): The European Space Agency (ESA) moved closer to putting a robot on the surface of a comet Wednesday, as its Rosetta satellite released the Philae lander towards its destination.

The lander separated from the satellite at 8.35 a.m. GMT and was headed for Comet 67P/Churyumov-Gerasimenko, a large mass of ice and dust some 510 million km away, the BBC reported.

The descent should take seven hours, the report said.

The mission's success would mark a "first" in the field of space exploration, with no mission having made a soft landing on a comet before.

"It's all down to Issac Newton and the laws of physics now. Philae is on its way down to the surface," said Mark McCaughrean, senior science advisor at the ESA.

However, the mission is tricky. Part of the difficulty is the very low gravity on the comet, four km-wide ice mountain.

Philae needs to be wary of simply bouncing back into space, according to the report.

Therefore, it will deploy foot screws and harpoons on contact with the comet's surface to try to fasten its position.

It will then take a picture of its surroundings -- a landscape made up of deep pits and tall ice spires.

Early Wednesday, the third "go" signal was delayed. The thruster system used to push the robot into the surface of the comet when it touches down could not be primed.

This robs Philae of the ability to push itself into the surface of the comet.

"We will just have to rely now on the harpoons, the screws in the (lander's) 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 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.

Ian Wright, a leading British scientist working on the lander, also sounded upbeat.

"We realise this is a risky venture. In a sense that is part of the excitement of the whole thing," he was quoted by BBC as saying.

Comets almost certainly hold vital clues about the original materials that went into building the solar system more than 4.5 billion years ago and if the mission is successful, it will provide an opportunity to sample a comet directly.