Rosetta: Audacious comet landing site chosen

Europe's Rosetta mission, which aims to land on a comet later this year, has identified what it thinks is the safest place to touch down.

The extracted square covers about 1km, which is approximately the extent of the uncertainty in Philae's landing precision

Scientists and engineers have spent weeks studying the 4km-wide "ice mountain" known as 67P, looking for a location they can place a small robot.

They have chosen what they hope is a relatively smooth region on the smaller of the comet's two lobes.

But the team is under no illusions as to how difficult the task will be.

Comet 67P/Churyumov-Gerasimenko, currently sweeping through space some 440 million km from Earth, is highly irregular in shape.

Its surface terrain is marked by deep depressions and towering cliffs.

Even the apparently flat surfaces contain potentially hazardous boulders and fractures.

Avoiding all of these dangers will require a good slice of luck as well as careful planning.

Pre-mission analysis suggested the chances of a successful landing on a roughly spherical body were 70-75%.

With 67P's rubber duck shape, those odds have surely lengthened, but European Space Agency (Esa) project manager Fred Jansen is excited at the prospect of trying.

"At the end of the day, you'll only know when you land. Then it will have been either 100% or zero. That's the way it is," he told BBC News.

The plan still is to make the landing attempt on 11 November.

The Rosetta probe will despatch its piggybacked Philae robot from a distance of about 10km to 67P.

This spider-like device will then hope to engage the surface at "walking pace", deploying screws and harpoons in an effort to lock itself down to an object that has very little gravitational attraction.

Esa says it will be a one-shot opportunity. Events will be taking place so far away that real-time radio control will be impossible.

Instead, the process will have to be fully automated with the final commands uploaded to Rosetta and Philae many hours in advance.

The choice of landing site follows a weekend of deliberations in Toulouse, France.

Mission team-members gathered to assess the latest imagery downlinked from Rosetta, which has been closely tracking 67P now since early August.

Five potential landing locations were on the table, and these have now been reduced to just two - a primary and a back-up.

Both will be studied further in the coming weeks before a final go/no-go decision is made in mid-October.

The favoured location is identified for the moment simply by the letter "J". (A public naming competition will run in due course).

On 67P's smaller lobe, it has good lighting conditions, which for Philae means having some periods of darkness also to cool its systems.

"It's relatively flat, but there are still some cliffs in this terrain; there are still boulders. So, it's not easy to land on 'J'," explained Philae project manager Stephan Ulamec from the German Space Agency.

"We're getting very close now, and it is fascinating but I have to say also quite frightening to some degree - that 20 years of work boils down now to just a few hours. Are we going to be successful, or will we be unlucky, hitting a boulder that just happens to be under the lander?"

The back-up site is situated on the larger of 67P's lobes. Its designation through the selection process has been the letter "C".

Getting Philae down safely to any location on the comet is going to be an immense challenge

It hosts a range of surface features, including depressions, cliffs and hills, but - crucially - many smooth plains, also.

More detailed mapping of J and C is ongoing.

This past week, Rosetta manoeuvred into an orbit just 30km from the 67P, enabling its camera system to see details that can be measured on the sub-metre scale.

For landing, such information only has a certain usefulness, however, as the "hands-off" touchdown can only be targeted with a best precision that will likely run to hundreds of metres.

And that error is larger than any of the apparently smooth terrains on the reachable parts of the comet

The whole separation, descent and landing procedure is expected to take seven hours.

If Philae gets down successfully into a stable, operable configuration, it would represent a historic first in space exploration.

But Esa cautions that this high-risk venture should really be seen as an "exciting extra" on the Rosetta mission.

The major objective from the outset has been to catch the comet with the Rosetta probe and to study it from orbit.

This is happening right now. The spacecraft's array of remote-sensing instruments are currently investigating the comet's properties, endeavouring to find out how the object is constructed and from what materials.

The 10-billion-tonne target: 67P is roughly 4km wide with a bulk density of about 400kg per cubic metre - similar to many types of wood

But, of course, an in-situ analysis of the surface chemistry would be a huge boon to the mission overall, and this is what Philae aims to provide.

It will carry a drill to pull up comet samples into an onboard laboratory.

And, indeed, any surface information gathered by Philae will provide important "ground truth" for Rosetta's remote-sensing observations.

"If we get only a few measurements and a few samples, we will have been successful," said Jean-Pierre Bibring, the co-principal investigator on Philae.

Site C will only be used if further study shows up previously unseen problems with site J

"We'd like to complete the first science sequence, which is two days on the comet. But for understanding activity on the comet, we also need the long-term science. That would be a matter of a few weeks, not necessarily a few months."

In any case, engineers do not expect Philae to survive beyond about March, when it will likely succumb to overheating.

But irrespective of the outcome on 11 November, Rosetta will continue to follow 67P for at least a year.

The probe will get a grandstand view of the comet as it warms on a swing around the Sun.

67P's ices will vaporise, throwing jets of gas and an immense cloud of dust out into space.

Holger Sierks, the principal investigator on Rosetta's Osiris camera system, will soon be acquiring pictures of the surface of 67P that resolve features down to just 20cm across.

He expects the mission to yield some profound knowledge.

"I feel this is really a historic moment in science. It's unprecedented; it's a quantum step in cometary science," he told BBC News.

"We will achieve centimetre resolution, getting us closer to understanding the origins of the Solar System and its building blocks 4.5 billion years ago."

Site "J": The cross marks the spot, but in reality the errors in the landing mean only a broad area can be targeted with confidence