Rosetta mission: Countdown to historic comet landing

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

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.

If all goes to plan, the little robot, called Philae, will touch down about seven hours later, deploying screws and harpoons to secure its position.

Confirmation is expected at Earth around 1600 GMT.

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.

Key timings for landing effort (GMT) Rosetta delivery manoeuvre - shortly after 06:00 Latest Go/No-go decision - before 07:35 Philae separates from Rosetta - 08:35 Confirmation signal at Earth of separation - 09:03 Rosetta's post-delivery manoeuvre - 09:15 Radio connection established - 10:30 First data from descending Philae after 12:00 Landing of Philae on 67P - after 15:30 Confirmation signal at Earth - around 16:00

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.

Analysis by Science editor David Shukman

The handshakes are warm but the smiles are brittle here at the European Space Agency's mission control in Darmstadt. As the clock ticks down to the unprecedented attempt at landing on a comet, there's real nervousness in the air.

From the days of the earliest astronauts displaying the Right Stuff, space exploration has always required an ability to remain calm in the face of extraordinary pressure. But managing an audacious operation more than 300 million miles away, with so much uncertainty about the gravitational pull and a potentially treacherous surface, clearly demands more steel than normal.

A sudden influx of reporters and camera teams, and the arrival of VIPs, has added to the sense of drama. It reminds me of Europe's last great venture to touch down on an alien body: the Huygens mission that descended to the surface of Saturn's moon Titan in 2005. That was a triumph, and astounding images were sent back.

Now, with fingers crossed, there's an open admission that the mission's fate is far too hard to call. No-one will say it, but failure is an option, and success almost too exciting to contemplate.

One theory holds that they may have been responsible for delivering water to the planets.

Another idea is that they could even have 'seeded' the Earth with the chemistry needed to help kick-started biology.

Philae's onboard instrumentation will test some of this thinking at 67P - if it can get down safely, and keep working long enough to run its experiments.

Mission control will closely monitor the mothership and lander as they move towards separation.

The vast distance between the comet and the Earth - 510 million km - means radio commands take almost half-an-hour to reach the spacecraft.

Nonetheless, the flight team must put Rosetta on a very precise path, to make sure Philae has the best opportunity of arriving squarely in the chosen landing zone.

These navigation instructions were due to be sent up late on Tuesday.

Once the 100kg robot is let go at 0835, it has no means of adjusting its descent; Philae will go where the comet's gravity pulls it.

Controllers in Darmstadt will want to hear not only that Philae landed in one piece but that it is securely fastened to the comet.

The nature and strength of the surface materials are unknown, however.

Philae could alight upon terrain whose constitution is anything between rock hard and puff-powder soft.

If it can, the robot will endeavour to lock itself in place with screws in its feet and harpoons that fire from its underside.

Esa has cautioned that success may require a large slice of luck in addition to the skill of all the teams involved.

Not only is landing on a comet an untried technique, but Wednesday's effort is also having to rely on some relatively old technologies.

Rosetta was despatched from Earth to catch 67P in 2004. That means it and Philae were designed and built in the 1990s.

And given the conservatism of space engineering, a number of its onboard systems will therefore undoubtedly be 1980s vintage.

But even if the landing attempt fails, the pictures and measurements of 67P acquired by the Rosetta mothership in recent weeks will be enough to re-write the textbooks.

'The real scientific value of this mission is spread all over Rosetta and its instruments, and the lander is just a part of that,' explained Esa flight director Andrea Accomazzo.

'The lander is obviously spectacular; it's the thing the public recognise. But already, even before the landing, the scientific return of Rosetta is orders or magnitude above what we knew about comets previously.'

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos

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