History beckons for the European Space Agency as it attempts to put a small probe on a comet later.

Previous missions have flown by these "ice mountains" but no-one has ever emplaced a surface lander before.

Esa's Rosetta satellite will drop the probe on to Comet 67P/Churyumov-Gerasimenko from an altitude of 20km.

The mission has passed last-minute checks, and a final "go" signal is due at 0735 GMT. Confirmation of touchdown is expected at 1600 GMT.

If all goes to plan, the little robot, called Philae, will deploy screws and harpoons to secure its position on the comet after a seven-hour flight.

The first thing Philae will do on landing is send back 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.

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.

(BBC)