This article is from the source 'bbc' and was first published or seen on . It last changed over 40 days ago and won't be checked again for changes.

You can find the current article at its original source at http://www.bbc.co.uk/news/science-environment-29380448

The article has changed 2 times. There is an RSS feed of changes available.

Version 0 Version 1
Rosetta: Date fixed for historic comet landing attempt Rosetta: Date fixed for historic comet landing attempt
(35 minutes later)
The date has been fixed for Europe's daring attempt to land on a comet: Wednesday 12 November.The date has been fixed for Europe's daring attempt to land on a comet: Wednesday 12 November.
It will see the Rosetta satellite, which is currently orbiting the huge "ice mountain" known as 67P, drop a small robot from a height of 20km.It will see the Rosetta satellite, which is currently orbiting the huge "ice mountain" known as 67P, drop a small robot from a height of 20km.
If all goes well, the lander will free-fall towards the comet, making contact with the surface somewhere in a 1km-wide zone at roughly 15:35 GMT.If all goes well, the lander will free-fall towards the comet, making contact with the surface somewhere in a 1km-wide zone at roughly 15:35 GMT.
The European Space Agency (Esa) says the challenges ahead are immense.The European Space Agency (Esa) says the challenges ahead are immense.
Imagine pushing a washing machine out the back of an airliner at twice cruising altitude and expecting it to hit Regent's Park in London - all while the ground is moving underneath.Imagine pushing a washing machine out the back of an airliner at twice cruising altitude and expecting it to hit Regent's Park in London - all while the ground is moving underneath.
Although not really analogous for many reasons, this scenario does give a sense of the difficulties involved. The chances of failure are high.Although not really analogous for many reasons, this scenario does give a sense of the difficulties involved. The chances of failure are high.
Esa's confirmed date is actually a day later than the one that had been discussed in provisional planning in recent months.Esa's confirmed date is actually a day later than the one that had been discussed in provisional planning in recent months.
The extra time will give flight controllers a bit more latitude as they try to get Rosetta into just the right position to deliver the lander, which goes by the name of Philae. The extra time will give flight controllers a bit more latitude as they try to get Rosetta into just the right position to deliver the 100kg lander, which goes by the name of Philae.
This requires careful "phasing" of Rosetta's path around the 4km-wide 67P so that the satellite turns up at the precise, pre-determined ejection point 22.5 km from the centre of the comet at 08:35 GMT. This requires careful "phasing" of Rosetta's path around 4km-wide 67P so that the satellite turns up at the precise, pre-determined ejection point, 22.5km from the centre of the comet at 08:35 GMT.
Because the whole event will be taking place 509 million km from Earth, any radio signal will take 28 minutes and 20 seconds to reach Esa's ground station network.Because the whole event will be taking place 509 million km from Earth, any radio signal will take 28 minutes and 20 seconds to reach Esa's ground station network.
It means confirmation of success or failure will not come until about 16:00 GMT. It means confirmation of success or failure will not come until perhaps just after 16:00 GMT.
The chosen landing site is on the "head" of the rubber-duck-shaped comet - a broadly 1km-wide zone currently referred to simply as "J", the designation it was given in a list of possible destinations in the selection process. The chosen landing site is on the "head" of the rubber-duck-shaped comet and is currently referred to simply as "J", the designation it was given in a list of possible destinations in the selection process.
It is far from ideal. It contains some terrifying cliffs, but is the flattest, most boulder-free location the mission team could find in its survey of the icy object.It is far from ideal. It contains some terrifying cliffs, but is the flattest, most boulder-free location the mission team could find in its survey of the icy object.
Mapping of J and a back-up site known as "C" is ongoing.Mapping of J and a back-up site known as "C" is ongoing.
This past week, Rosetta manoeuvred into an orbit just 20km from 67P, enabling its camera system to see details that can be measured on the sub-metre scale.This past week, Rosetta manoeuvred into an orbit just 20km from 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 automated touchdown can only be targeted with a best precision that will likely run to hundreds of metres.For landing, such information only has a certain usefulness, however, as the automated 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 in the J zone.And that error is larger than any of the apparently smooth terrains in the J zone.
The whole separation, descent and landing (SDL) procedure is expected to take seven hours.The whole separation, descent and landing (SDL) procedure is expected to take seven hours.
Philae will take a picture of Rosetta as it leaves its "parent".Philae will take a picture of Rosetta as it leaves its "parent".
It will also point a camera downwards so that it can see the approaching comet. Not that this information can change anything; Philae has no thrusters to control or alter its descent trajectory. It will land where it will land.It will also point a camera downwards so that it can see the approaching comet. Not that this information can change anything; Philae has no thrusters to control or alter its descent trajectory. It will land where it will land.
But the images will help controllers determine where the robot ended up after the event.But the images will help controllers determine where the robot ended up after the event.
If Philae gets down successfully into a stable, operable configuration, it will fire harpoons and deploy screws to try to hang on to the surface.If Philae gets down successfully into a stable, operable configuration, it will fire harpoons and deploy screws to try to hang on to the surface.
The action of these devices will tell Esa mission control in Darmstadt, Germany, that the surface was engaged.The action of these devices will tell Esa mission control in Darmstadt, Germany, that the surface was engaged.
Will it hang on? Part of the problem here is that no-one really knows what the surface conditions will be.Will it hang on? Part of the problem here is that no-one really knows what the surface conditions will be.
Philae could sink into a soft powder or impact ice as hard as rock. A major worry is that it could simply bounce off into space.Philae could sink into a soft powder or impact ice as hard as rock. A major worry is that it could simply bounce off into space.
Whatever the outcome, the Rosetta mission will continue.Whatever the outcome, the Rosetta mission will continue.
Already the main satellite has returned some astonishing pictures of Comet 67P and the close-quarters observations it will conduct over the next year will transform our understanding of these remarkable objects.Already the main satellite has returned some astonishing pictures of Comet 67P and the close-quarters observations it will conduct over the next year will transform our understanding of these remarkable objects.
The timings mentioned on this page will change if subsequent mapping shows the J site to have a major problem and Esa shifts its attention to the back-up destination. The timings mentioned on this page carry some uncertainty and would change if subsequent mapping shows the J site to have a major problem, with Esa forced to shift its attention to the back-up destination, C.