As it happened: Rosetta comet landing

tweets: That's right.... I have a bounce in my step. #YouWouldToo #HarpoonGate

From the web comic xkcd:

XKCDCopyright: XKCD

Stephan Ulamec went on to explain that the early data suggests Philae may have lifted and turned slightly after touching down for the first time.

There was a signal suggestive of turning - which stopped after two hours.

"Maybe today, we didn't even just land once - maybe we landed twice," he told the room, to laughter and applause.

"We will know a lot more tomorrow."

EsaCopyright: Esa

Philae lander manager, Stephan Ulamec, says: "It's complicated to land on a comet it's also complicated to understand what has happened during this landing.

"What we know is we touched down, we landed at the comet at the time when you all saw us cheering and when it was announced. We had a very clear signal there, we received data from the landing - housekeeping and science data - that's the good news."

Then he told us the bad news. The anchoring harpoons did not fire, and so they are not anchoring the lander to the surface.

"We still do not fully understand what has happened."

Jean-Jacques Dordain, Director-General of Esa, said that Philae's radio link and power are functioning.

"When you have radio and power, you collect data."

The live stream has begun for the final press briefing - watch using the "live coverage" tab above.

And here's an update from the Open University's Prof Monica Grady, speaking moments ago on BBC News:

"There is data in the can, we have to wait for it to be transmitted. We will know by midday tomorrow how much data we've got."

Any minute now... The final briefing of a day when history was made.

tweets: Ptolemy on board @Philae2014 has data from a mass spectrometer safely down on the ground; exciting to have chemistry from a comet! #cometlanding

A mass spectrometer is a device which uses the mass of atoms to understand their chemical composition.

You could pass the time with this remarkable 3D tool that shows you exactly where Rosetta is, and how it got there - courtesy of the European Space Agency themselves:

http://sci.esa.int/where_is_rosetta/

The briefing has now been postponed until 19:00 GMT - Esa suggests you fetch a glass of wine before settling down to see the latest data from comet 67P, 300 million miles away.

Watch a moment of pure joy in our "key video" tab above: excitement and tears from the Open University's Prof Monica Grady.

"It's landed - I've waited years for this", she says, as she hugs our very own science editor David Shukman.

This illustration shows the many instruments on Philae - including Rolis (centre right) which has already sent us an postcard of the looming comet surface.

ESA/ATG medialabCopyright: ESA/ATG medialab

The Sky at Night team tweets that they are hoping to see surface images at the imminent briefing.

They are not alone!

Esa TV will run a live briefing in five minutes' time (18:30 GMT).

Watch using the "live coverage" tab above.

John Grunsfeld, astronaut and associate administrator for Nasa's Science Mission Directorate says:

"This achievement represents a breakthrough moment in the exploration of our solar system and a milestone for international cooperation. We are proud to be a part of this historic day and look forward to receiving valuable data from the three Nasa instruments on board Rosetta that will map the comet's nucleus and examine it for signs of water.

"Small bodies in our solar system like comets and asteroids help us understand how the solar system formed, and provide opportunities to advance exploration.

"It's a great day for space exploration."

A new video is now up on our key videos tab above, with Esa project scientist Dr Matt Taylor. You may remember him from his tattoos, or have seen his name trending on Twitter earlier today...

He says people have invested their entire lives on the mission.

"I can't put words to it - it was beautiful".

Rebecca tweets: Team investigating what no harpoons and unexpected telemetry means - awaiting surface picture to see what's happened #CometLanding #Rosetta

To clarify, that is the first image taken by Philae's camera ROLIS as it dropped towards its target landing site on the comet.

Emily is among many planetary scientists who are extremely excited to see it.

ROLIS is positioned on Philae's "balcony" pointed downward. It should now be about 31cm from the comet's surface.

tweets: This is a ROLIS image of the comet!!! Right on target!!!!!

ESACopyright: ESA

Landing on the small, strange world of a comet ranks as one of the greatest achievements in space exploration. Neil Armstrong and Buzz Aldrin would obviously take pride of place.

People might debate the relative prowess of robotic rovers driving on Mars or the Voyager spacecraft edging out of the solar system. But touching down on a primordial lump of rock and ice that dates from the earliest days of the Solar System - and which is hurtling through space at 34,000 mph - is a genuine triumph by any standards.

tweets: Last night I held in my hands original works of Galileo & Newton. Today we landed on a comet beyond Mars. Incredible.

Chris HadfieldCopyright: Chris Hadfield

Scientists at mission control are now processing the first images from the surface of the comet.

But they are also getting intermittent drop-out in the communication between the lander Philae and the "mothership" Rosetta, still in orbit.

Paolo Ferri, head of operations at Esa, told BBC News: "We need to stabilise this situation over the next three hours."

We've heard a lot of comments describing what we might learn from the comet, but this is the first time we've heard it described as soup. (Note - we do not expect it to be edible.)

The Ptolemy instrument's lead investigator, Prof Ian Wright from Open University, says:

"The comet is very, very old. Analysing the material in a comet is like looking back in history; it's like a time capsule.

"I like to think of it as frozen primordial soup, and this is the stuff that rained down on the early Earth. The idea that comets may have brought the building blocks of life to Earth is one of the reasons why we want to study them."

Emily also reports that Mark McCaughrean, Esa's senior science adviser, has confirmed the lander's screws - if not its harpoons - have dug into the comet's surface.

Emily tweets from Darmstadt:

"Not knowing about the stability is clearly worrisome, but by any reasonable measure Philae is successful, and we can expect good data.

"It's going to take Philae mission controllers some time to understand how stable they are, but in the meantime everything is working great. Yay!"

Arturo Opaso: "Wow, very impressive and exciting! We are all proud, these are the events that make us feel as one. Awaiting for images here in Chile."

We're all waiting, Arturo! Impatience born of enthusiasm...

Rebecca tweets from Darmstadt:

"Have been hearing how close the #CometLanding was to being pulled last night - they've only told us now it landed! #Rosetta"

Gloria Jefferson: So very well done to all involved at the European Space Agency. Proud to be a member of humanity on such a day.

tweets: "Hollywood is good, but Rosetta is better" - great quote from our CEO Dr David Parker #CometLanding

Our colleagues at #BBCtrending write:

The sound coming from comet 67P/Churyumov-Gerasimenko has caught the imagination of hundreds of thousands on social media.

The scientists are just as surprised as social media users.

"This is exciting because it is completely new to us. We did not expect this and we are still working to understand the physics of what is happening," Karl-Heinz Glaßmeier, head of Space Physics and Space Sensorics at the Technische Universität Braunschweig, Germany, explained on the ESA Rosetta blog.

tweets: Deciding whether to re-fire the harpoons is tricky - without the hold-down thruster, presumably @Philae2014 could recoil? #CometLanding

The Esa Operations team confirms that the harpoons designed to attach Philae to the comet did not fire.

But they say the lander is in great shape, and the team is "looking at refire options".

tweets: Philae sank about 4cm... but the harpoons didn't fire and the thruster problem was real and so it also didn't fire? Yow, the drama continues! #CometLanding

tweets: It looks like @Philae2014 made a fairly gentle touch down on #67P based on amount of landing gear damping #CometLanding

Watch the celebrations in a to the "key video" tab above.

"We're down on the surface", said Esa's Prof Mark McCaughrean.

tweets: "No rest for the wicked..."

BBCCopyright: BBC

They appear to be referring to presenter and University of Oxford astronomer Prof Chris Lintott.

tweets: I've never seen a leading planetary scientist leap for joy before - Prof Monica Grady at #CometLanding - coming up on News at Six

Further checks are now needed to ascertain the state of the lander, but the fact that it is resting on the surface of the speeding comet is already a huge success.

It marks the highlight of the decade-long Rosetta mission to study comets and learn more about the origins of these celestial bodies.

The head of the European Space Agency underlined Europe's pride:

"We are the first to have done that, and that will stay forever,'' said Esa director-general Jean-Jacques Dordain.

EsaCopyright: Esa

Dan Jakubowski: This is mankind's greatest feat in Space Exploration since landing on the Moon in 1969. Congratulations to all from Cleveland, Ohio USA.

@NancySteinman tweets: Federation of planets, here we come. Congrats to @Philae2014 and its team. #CometLanding. @That_MarcC tweets: My 95-year-old mother-in-law watched the #CometLanding on a laptop. @Philae2014 @ESA_Rosetta

Jonathan tweets: One of the "fathers" of the @ESA_Rosetta mission, Gerhard Schwehm, says he intends to smoke a cigar.

"Today's successful landing by Rosetta's Philae lander, a major contribution to this mission by the German Space Agency (DLR), at Comet 67P/Churyumov-Gerasimenko is a tremendous accomplishment," says Space Foundation's Elliot Pulham.

"We congratulate our partners at DLR and ESA for achieving yet another milestone during this ambitious mission."

David Shukman, BBC Science Editor:

"Wow, they did it - @Philae2014 is now sitting on a comet. Harpoons fired. Incredible atmosphere. People quite can't believe it - nor can I."

Rebecca Morelle, BBC Global Science Correspondent:

"Can't believe they've done it - @esa's Mark McCaughrean very emotional at success - a huge moment #CometLanding #Rosetta"

Monica says the landing happened "bang on time".

She tweets: "Break out the bubbly....no, got to wait to hear from @Philae_Ptolemy #CometLanding"

Ptolemy is an instrument that will do chemical analysis, "sniffing" the surface of the comet.

The mission was almost postponed in the early hours of the morning, says Esa's very happy Prof Mark McCaughrean, live on BBC News as he heard the news.

tweets: Stephan Ulamec: It has done its job, we are on the comet. *mic drop* #CometLanding

ESACopyright: ESA

tweets: I just saw @Philae2014 co-PI Jean-Pierre Bibring carrying two bottles of champagne

"Philae is talking to us," Stephan Ulamec has told the cameras.

It may not have been as graceful as this illustration, but the message has reached Earth - and spread around the world on Twitter:

Philae just landed on a comet!

ESACopyright: ESA

tweets: Well, I guess that means tears of joy. I think we just bagged comet. @Philae2014 checks now ongoing but it appears to be alive on the comet.

tweets: Touchdown! My new address: 67P! #CometLanding

There seems to be some movement in the control room... And cheers. Many cheers!!

tweets: Argh!!! This wait is horrible #CometLanding - watching mission control and waiting for #rosetta

"The wait is nearly over," she tells us from Darmstadt.

BBCCopyright: BBC

tweets: It's weird to think Philae is down, and the signal from it is just now passing by Mars on its way to Earth.

This already seems like an old triumph - but here is Holger Sierks, the chief investigator on the OSIRIS part of the project, sharing a moment with BBC Sky at Night's Prof Chris Lintott.

They had just sighted the first image of Philae's descent.

BBCCopyright: BBC

tweets: This gives me the shivers. 500 million km from Earth, a human-made robot is attempting a #cometlanding. Right now.

What's happening now on Philae as it approaches the surface?

Well, its onboard commands have told it that the landing is imminent. For 40 minutes prior to the expected touchdown, all data and images that it is collecting are being stored onboard, hopefully for retrieval later.

Much like a voice-data recorder on an aeroplane, this storage system works on a loop. If the preparation periods lasts longer than 40 minutes, any data recorded at the beginning of the landing window will be over-written.

This ensures that the very latest data, acquired just as Philae touches down, is the information that gets preserved.

tweets: GO @Philae2014 good luck for touchdown! @ESA_Rosetta, myself and the world are with you! #CometLanding

Don't forget, whatever the outcome you can join me for a special Frontiers programme about Rosetta and Philae on BBC Radio 4 tonight at 21:00 GMT

tweets: Breathlessly waiting to hear back from @Philae2014! Curse you, 28 min light travel time! #CometLanding

tweets: Packed and tense waiting for #CometLanding: reminds me of the last shuttle launch but without a countdown

BBCCopyright: BBC

As a quick reminder - Philae could already have touched down, but we won't know for 28 minutes because of the sheer distance the radio waves have to travel from the comet back to earth.

@uglyshirts_ tweets: I'm not getting a single thing done at work until @Philae2014 lands or doesn't #cometlanding

Stephan Ulamec, head of the lander team, offered the final comments before crossing to the control room to await confirmation - or not - of the landing:

"Our touchdown window has started, the lander in principal is now prepared, every second now we could hit ground!

"It's getting very tense and very exciting now."

tweets: The touchdown "window" for @Philae2014 opened at 16:22 - so could already be on the ground! Have to wait to hear... #CometLanding

tweets: How might the detected slow spin of @Philae2014 might affect the landing? It seems to have taken some of the team by surprise #cometlanding

tweets: Piece of cake @Philae2014 ! Just target the green spot ! Go Philae ! #CometLanding

EsaCopyright: Esa

tweets: Nearly there... Landing window is from 1540 GMT to 1620 GMT... Nerve wracking! Follow live on @BBCNews @BBCWorld #Rosetta #CometLanding

More speculation - we won't know the facts until later - but lander manager Stephan Ulamec says:

"We will have to rely fully on the harpoons at touchdown," because the cold gas thruster on top of the lander is not working.

"We'll need some luck not to land on a boulder or a steep slope."

David tweets from Esa mission control in Darmstadt:

"Astronauts on moon, Mars rovers, Voyagers edging out of solar system, GPS - where would you rank #CometLanding? Just wondering as we wait..."

Vic Grout: Thoroughly enjoying this. Was fortunate enough to be in that mission control room in the summer on a visit to Darmstadt. Fingers crossed!

tweets: Data coming back from those @Philae2014 instruments that are switched on. The probe is very slowly rotating as it goes down. #CometLanding

Dr Holger Sierks is the principle investigator for Rosetta's cameras and explained just how important the lander was.

"It will sample the surface, it will drill into it and analyse the material it sees.

"It even carries it into an oven and bakes the material to 'sniff' the volatiles [chemical elements], to find out if there are organics in there, pre-biotics or building blocks. For us, we'll find volatiles, the water, and even link the water to the water we have here on earth."

tweets: "Ohh, I look gooooood! #LetsBeHonest"

ESACopyright: ESA

Grant David: Thank you, thank you. This is my 'moon landing'. To all the people involved in this mission, you are creating major history and you should be so proud. The cherry on the cake.. [soon] I am like a five year old on Christmas Eve! Good luck everybody.

Llewellyn Jones: This is absolutely incredible, stuck in the office, but snuck my laptop in with one headphone on to keep an eye on this historic event unfolding. Just amazing what you guys have achieved. Finger crossed it all goes to plan. Looking forward to some amazing photos!

Here we go. Our colleagues tweet a first screengrab: Breathtaking "@NASASpaceflight: Epic view of Philae on the way to the comet!

EsaCopyright: Esa

More images!!

Glyn Jones: Having lived through all of the space age, I'm so impressed with this achievement. I remember Sputnik, dogs, monkeys then Yuri Gagarin, Shepherd and the moon landing. Great day for space exploration.

Klim Ivanovych Churyumov, one of the discoverers of 67P says: "Astronomy is the most important science for civilisation."

Co-discoverer Svetlana Ivanovna Gerasimenko said she likes the comet "very much" now that she can see it up close.

"It reminds me of a boot," she said.

This was the moment the long-awaited first image from Philae's descent was presented to journalists in Darmstadt.

It was greeted with excited applause. More to come!

BBCCopyright: BBC

Picture comes from the BBC Sky at Night team: @BBCStargazing

tweets: Flight director Accomazzo: We're receiving good signals from both spacecraft. @Philae2014 trajectory looks good #cometlandingESA Operations @esaoperations

Now it's a waiting game. In less than an hour we're expecting the Philae landing, but remember this is space time. The comet is so far away that it will take 28 minutes for the signal to reach us.

Some good news though: the Esa team says the lander is on the right track, based on remote measurements.

tweets: Yup. We're soft landing on a comet today. Normally my "We" means @NASA. But in this case it's @esa, the European Space Agency

tweets: Nice one! I've never seen you from this angle before, @ESA_Rosetta! #CometLanding

John Iles: Fantastic. Full of admiration for the team - the second giant leap for mankind is about to be taken. A second step on our journey to the stars!

tweets: The first CIVA image is raw (not processed), and shows blurring from the rotation of the @Philae2014 lander #CometLanding

The first image taken by Philae after separation - looking back to the Rosetta probe:

ESACopyright: ESA

"We are trying to do what nobody else has done before," says Esa director general Jean-Jacques Dordain

The next broadcast from ESA TV is now in progress - watch using the "live coverage" tab above.

While we're waiting for the ESA TV stream to return...

Ever wondered why Rosetta Mission's various bits have Egyptian names: Philae, Agilkia, Ptolemy, Osiris?

Open University's Dr Jessica Hughes writes in The Conversation: Using hieroglyphs to frame the mission presents space as an entity that can - and eventually will - be deciphered. And while most of us have trouble grasping the colossal distances involved in space travel (Rosetta has travelled a cumulative distance of over 6.4 billion km), the names of ancient places, pharaohs and gods can help us to mentally reach the physical remoteness of celestial bodies.

Other symbolic resonances include the Ptolemaic system of astronomy and the infamous conspiracy theories about the alien origins of the pyramids.

BBCCopyright: BBC

A superimposed Cheops pyramid gives a sense of scale to the terrain on Comet 67P

Martin Bradley: This is fantastic, takes me back to staying up all night watching the Apollo 11 landing. I can't believe how exciting this whole thing is. I am in awe of these scientists.

tweets: A thistledown the size of a washing machine is falling to a gentle crash-landing on a comet. My finger are crossed.

We're hoping for some new pictures soon, please don't keep us hanging Esa!

tweets: MUPUS anchor temperature gone up about 10°C since separation #CometLanding

This is a very big day for many UK engineers, who designed and built the Rosetta spacecraft. We should be very proud of the achievements of the UK in this remarkable mission.

My boss is beginning to get very excited - he was the lead Electrical Engineer on Rosetta. All students out there - you can do this sort of work here in the UK and Europe - just imagine, it could be you watching a mission like this in a few years' time knowing that you were responsible for part of it!

Eamon: I'm astounded at the ambition - fair play to all who have been involved... just wish I wasn't stuck in work for the biggest astronautical event of my lifetime!

tweets: ESA TV webcast will resume shortly, at 14:00 GMT / 15:00 CET #CometLanding

Head of the European Space Operations Centre, former astronaut Thomas Reiter, said the cost of the billion-dollar mission was a small price to pay for such an important undertaking.

"It's to find answers to very fundamental questions about the history of our own planet, how it evolved.

"The overall costs were in the order of 1.4 billion euros. If you divide it by the 20 years that the development and the mission has cost, it's a matter of cents per European citizen per year... contributing to this new knowledge that we get about the evolution of our own planet and the fundamental question: if life really emerged on Earth, or if it might have been brought to Earth by such comets many billions of years in the past.

"And I think this is really worth it."

What a great image - not real, of course...

The New York Times science desk tweeted: Imagine comet #67p was falling toward Manhattan rather than @Philae2014 falling toward it (for scale)

New York TimesCopyright: New York Times

tweets: We all saw a photo flash very briefly on the webcast stream! Want photo! #CometLanding

The brief Esa TV broadcast ended "without a clear look" at the images from CIVA, she adds.

CIVA is a set of cameras on Philae that can analyse both visible and infra-red light.

The next burst of live video from Esa headquarters is expected soon, to announce the delivery of new images.

If you've just joined us: everything has gone according to plan today, except for a malfunction with the lander Philae's gas thruster.

This makes the landing trickier but not impossible.

The delivery manoeuvre, separation from the "mothership" Rosetta, re-establishment of radio contact, and switch-on of various components on Philae have all happened on schedule.

ESA/Rosetta/NAVCAMCopyright: ESA/Rosetta/NAVCAM

Jacqui Grant: This is absolutely fantastic, got everything crossed that little Philae misses the cliff and lands safely. Thank you BBC for allowing us to be a part of this amazing day.

Assuming Philae can avoid the cliffs, the boulders, the fissures and the steepest slopes, it has a good chance of getting down in a stable configuration. But how do we know it's down? The action of the feet and legs touching the surface is to move a central pole running up the middle of the robot's main housing.

This will generate a signal that activates the screws in the feet and the harpoons on Philae's underside. It should also have activated the small gas thruster on the roof of the housing, pushing the probe into the surface. But, as we heard earlier today, we're no long sure this will work. So for Philae to succeed at landing, a soft surface will be preferable - something like a "snowdrift".

We're not sure what the strength of the surface materials is, but, if Philae encounters a consistency like "cigarette ash" or "champagne snow" - as some scientists have speculated - there's every hope it will become embedded in the surface.

The confirming of touchdown will come from here at Esa's mission control in Darmstadt, but it will be the German space agency's lander control centre in Cologne that will determine whether Philae is stable on the surface. This could take a few minutes. The news - good or bad - will then be relayed to Darmstadt for the world to hear.

EsaCopyright: Esa

Michael Willby: As one old enough to remember the grainy black and white TV from the Apollo project this endeavour deserves our greatest admiration for all those involved. If we, as fascinated observers, are on tenterhooks think how those who have given years of their lives feel. Good luck.

There is a new interview in the "key video" tab above: What is the comet's structure like?

Prof Mark McCaughrean speaks to BBC global science correspondent Rebecca Morelle.

He says the failure of the Philae lander's gas thruster, which delayed one of the decisions overnight, has "added another risk" to the already ambitious mission.

Tom Shaw: Our sixth form computing class is watching, completely gripped by the events unfolding. Amazing.

As a reminder: this is the target landing site, known as "Agilkia".

Not what you might call an easy spot to park... Read more about the choice here.

ESA/Rosetta/NAVCAM/MPS for OSIRISCopyright: ESA/Rosetta/NAVCAM/MPS for OSIRIS

tweets: Taking bets now on how deep we'll sink when @Philae2014 lands (that is, if it doesn't hit the cliff at the edge of the landing ellipse).

One of the researchers working on the mission, Dr Marina Galand from Imperial College of London, said that landing the spacecraft on the comet could be a precarious exercise:

"We don't know exactly what the surface is made of, we don't know if it's hard ice, and then the probe could bounce back; or it could be fluffy snow and it could sink. So in order to prevent the bounce-back, there are two harpoons which are going to be fired. There are mechanical screws which hopefully will lock it to the surface.

"The lander is as big as a washing machine, so hopefully that's big enough to prevent it from sinking. It could also topple. So a lot of challenges. I'm very anxious, it's very risky - but on the other hand if it goes through, it will really make history by being the first probe to ever touch down on a comet."

@japmfut tweets: Right now, someone is landing a robot on a comet 4 billion miles away at 40,000 mph. What the hell are you doing with your life? #Rosetta

tweets: Here are those @twitter stats shown in the broadcast, #cometlanding trending worldwide!

EsaCopyright: Esa

tweeted at 11:47: Not much will happen for the next hour. After that, some time in the subsequent hour we should see the CIVA photos. #CometLanding

Adrian Macey, Needham Market Middle School, Suffolk: My year 6s excited following bbc live updates, are coming back at lunch break to see if radio contact reestablished, and aim to follow when they get home! Thank you BBC!

tweets: @esa @ESA_Rosetta Following @Philae2014 progress with eager anticipation.Wish you all great success with landing later today! #CometLanding

@ldelaperrelle tweets: Congratulations to the scientists on the #CometLanding @ESA_Rosetta @Philae2014 awaiting landing of a craft on a comet - amazing!

tweets: While waiting for the latest announcements @chrislintott quizzes @mggtTaylor for #skyatnight on Sunday at 9pm BBC4

BBCCopyright: BBC

The signal from the Philae lander was a critical moment in the mission. It confirms that the probe will be able to send pictures and scientific data back from the comet should it land successfully in a few hours time. But that is far from certain. The landing site is strewn with boulders and there are large cracks in which the probe could fall into.

The task has been made harder by the fact that a thruster that should have pushed Philae, gently on to the comet is not working. But engineers are optimistic that two harpoons and ice screws on Philae will be enough to anchor it successfully.

John Lucas: Maybe if we did more things like this we would forget what divides us in terms of religion, nationality, language etc. and realise what we can achieve as a species.

@flight_14 tweets: @ESA_Rosetta is landing @Philae2014 on the comet over next few hours. Good luck to them, but reminds me of the alltime worst "space" movie

tweets:

Finally! I'm stretching my legs after more than 10 years. Landing gear deployed! #CometLanding"

To which @Esa_Rosetta replies:

@Philae2014 :) My back is chilly now you've left, but I'm in a better position to watch you now. Send me a postcard! #CometLanding

tweets a picture of "handshakes and relief - again" from the moment that @ESA_Rosetta mission control re-established its radio link with @Philae2014:

BBCCopyright: BBC

tweets: This is still on. @Philae2014 calls mum (@ESA_Rosetta), who calls us, Earth. Now we can pull pics of the lander. #CometLanding

@Trillionblue tweets: @daraobriain @ESA_Rosetta really exciting! It's crazy! Thousands of miles into space, a robot is landing on a comet controlled from Germany!

Stuart Booth: This is a really exciting mission. The stuff of science fiction comes to life. If they can successfully land a robot on an asteroid, then there are several possibilities opening up.

More anxious words from the landing team:

"It's on its own now," said Stephan Ulamec, Philae Lander Manager at the DLR German Aerospace Center.

"We'll need some luck not to land on a boulder or a steep slope."

ESACopyright: ESA

Helena: Words can't describe the excitement! Hoping for the best. Good luck all!

tweets: Phew! Connection with both Rosetta and lander re-established after post delivery manoeuvres. Touch down in just under 5 hours.

This is what MUPUS should look like in two days' time, when it is ready to do some measurements.

MUPUSCopyright: MUPUS

The team behind the MUPUS probe, which will take the comet's temperature, tweets that Philae's landing gear has been deployed.

Prof Monica Grady from the Open University says that space is a risky business.

"We don't do it because it's easy.

"The landing is going to be a real nightmare, we've had beautiful images of the surface of the comet - which is a real problem. It's shown the landscape is strewn with rocks and boulders."

It will be a nerve racking wait for her team, who have built one of the lander's instruments.

tweets:

"Phew! Back in contact with Earth after separation. #CometLanding"

"Also now back in contact with @philae2014! Good to hear you again buddy :) #CometLanding"

Meanwhile, the @esaoperations account tweets that "science data download" from the Philae lander is expected to start at 12:05 GMT.

Esa's Dr Paolo Ferri, Head of Mission Operations Department says: "It's established, the link is there, now we can follow it on its descent. We are happy with the signal we have."

Philae should hopefully land in about five hours' time.

"It's still alive," said one of the scientists with relief...

Cheers in the control room!

We're still waiting for a signal from Philae, via Rosetta... Anxious waiting for the scientists in Darmstadt. Watch using the "live coverage" tab above.

tweets: We're now anxiously awaiting both @mggtTaylor for an interview and the first signals from @Philae2014, both expected shortly...

Pete: Jonathan, nobody said it was going to be easy. But I for one hope that taking these risks will inspire many other future missions to other comets and asteroids and maybe other worlds. I think we ALL (the human race) need this kind of boost to our confidence.

Pierre Harter: I live in the Seychelles... watching and reading the live updates... I am 54... I remember listening together with my parents to Neil Armstrong on the BBCWService, there was no TV here in those days... technology has changed and with it our frontiers... The "selfie" Rosetta is one of the astonishing pics this century. Goose pimples. Ground Control to Major Tom.

BBC's Sky at Night presenter Prof Chris Lintott sends this image from the briefing room at Esa headquarters.

"Comets may be closer than they appear..."

BBCCopyright: BBC

So why is the radio reconnection two hours after separation so important? Because it means whatever happens at the surface, we will at least get some pictures and environmental data acquired during the descent.

I'm really looking forward to the "goodbye" pictures that Philae was supposed to have taken of Rosetta. They'll be iconic - images for the history books; but they have an engineering importance as well because they will show us whether the lander came off the mothership in a stable configuration.

Stand-by.

Peter King, Scunthorpe: @esa, It should be us who should be thanking you guys, letting us share this monumental challenge live. Especially after so many of you have dedicated years to this project. Good luck to you ALL.

Peter Johnstone, East Sussex: What a mind-bending project! Given that the technology within Rosetta is at least 11 years old, can anyone tell me how different the project would be if current-day technology could be deployed?

This would give a great insight into technological development over this period and assist in understanding the extent to which the scientists are currently constrained.

There's now an interview with enthusiastic, tattooed Rosetta project scientist Matt Taylor in the "key video" tab above. He says he is so confident Philae will successfully land that he has got himself a new tattoo to celebrate.

I bet you didn't know that the Comet 67P can sing... Listen to it here.

Rosetta's Plasma Consortium (RPC) has uncovered a mysterious "song" that Comet 67P/Churyumov-Gerasimenko is singing into space.

The comet emits its song in the form of oscillations in the magnetic field in the comet's environment. It is being sung at 40-50 millihertz, far below human hearing, which typically picks up sound between 20 Hz and 20 kHz.

To make the music audible to the human ear, the frequencies have been increased in this recording. Thanks Esa!

ESA/Rosetta/NAVCAMCopyright: ESA/Rosetta/NAVCAM

Andrew Hodson: To say this mission has created more interest than the first moon landing has left me in despair - Apollo Eleven absolutely gripped nations the world over. I can remember getting up in the middle of the night to watch Armstrong's first steps on the moon.

Don't forget you can also follow developments on the BBC's TV news channels.

BBCCopyright: BBC

Rebecca Morelle stands by to interview Prof Mark McCaughrean, ESA's senior scientific advisor.

Daniel Varnam: And today's topics with my 6th formers? Kepler's Laws, rocket dynamics, and gravitational fields. Thanks ESA!

Gary Watt, Aberdeen, Scotland: This is quite fantastic. Safe landing Philae... and hurry up with the photographs :)

One of the instruments on the Rosetta Philae Lander - Multi Purpose Sensor One showed us its anchor harpoons in a tweet. "My last sensors are mounted inside the @Philae2014 lander anchor harpoons. An accelerometer and a temperature sensor."

EsaCopyright: Esa

ESA flight controller Andrea Accomazzo talks through the separation moment with Philae co-principal investigator Jean-Pierre Bibring.

Accomazzo's team has done everything it can to give Philae the best opportunity of landing.

Will Prof Bibring now get to realise his dream of doing science at the surface of a comet?

BBCCopyright: BBC

Phil Chadwick: This is truly momentous... The technology involved to be able to do what is being attempted is mind blowing... I hope they are successful, because the pictures from the surface will be unlike anything we have ever seen before..

I also think that based on the technology being used here, we would be able to defend our world against a collision from one of these comets...

Here's an impression of what Philae looks like on its journey to the ice and dust of comet 67P.

EsaCopyright: Esa

The next key developments will come from about 11:00 GMT, when Rosetta and Philae will hopefully be in radio contact.

Here's hoping for some nice "goodbye" images of the separation.

tweets: Thanks for all your great messages of support, too many to answer but it is great to have you all with us on this adventure #cometlanding

George Brooke, Cambridge: Somehow this has generated as much interest, if not more, than the original moon landings. If we can sustain the interest it will significantly help the upcoming generations understand that there are still challenging careers out there.

@Super_Vi_ tweets: 10 Years between decision and launch, and again between launch and landing. Long-termism #Rosetta #CometLanding

@BriKi tweets: Huh? Oh no we're not up to much as a species, just landing a spacecraft on a distant comet is all. @esaoperations @ESA_Rosetta #CometLanding

Michael de Halpert: All of us here in Zurich are on the edge of our seats! Go Philae! Go Rosetta!

EsaCopyright: Esa

This sounds fun...

@AstroKatie tweets: Walking on Comet #67P would be like: Step (carefully). Hurl through space. Flail. Slowly arc back toward comet. Land. Bounce. Skid. [Repeat]

G Wilson: Hi, just so amazing, I remember watching the moon landing in '69. This feat just leaves me speechless - even to have got as far as they have is staggering. "May the wind be behind you, and the road rise up before you."

tweets: Philae Timeline: Lander should now have completed post-separation activities. We won't know for hours, though.

Matt Popple: I have been explaining the importance of this to my 8 year old on the school run! We both can't wait for touchdown (fingers crossed) at 1600!

John Huntington commented: So audacious, what an incredible day for mankind. Look what we can attempt by working together. Another step to the galaxy.

@KatyPrestedge tweets: An exciting day to be working at @OpenUniversity as #rosetta attempts the first landing on a comet, will be watching live!

tweets: Telemetry confirms that CONSERT is on! It will help the lander team know altitude and speed of @Philae2014 during descent. #CometLanding

tweets: Boulders, cliffs and canyons await #Philae lander on the surface of the duck-shaped comet 67P #Rosetta

EsaCopyright: Esa

tweets: Expected loss of signal w/ @ESA_Rosetta, which is now slewing for escape manoeuvre. We expect to regain contact at 11:58CET #CometLanding

tweets: Good Luck today Guys, #Rosetta Craft Landing on Comet, a journey of over 10 years. For Greater understanding of our #Universe!

tweets: Ow...owww...owwww... Leg cramp! My leg fell asleep after 10 yrs of being in that crazy Yoga pretzel position! #mmMMMmmYogurtCoveredPretzels

"It's all down to Isaac Newton and the laws of physics now. Philae is on its way down to the surface," says Esa's senior scientific advisor Prof Mark McCaughrean.

"If Isaac's friendly to us we'll have a great landing later today."

For the next couple of hours Rosetta and Philae will be out of radio contact. The umbilical that has joined them for 10 years was dropped at separation. Rosetta must now slew to get into a better position to follow the descent.

Philae, on the other side, should be taking some goodbye pictures of "mummy". The little robot also needs to think soon of putting out its legs ready for the landing. 1053 GMT is the time when Rosetta and Philae should make radio contact with each other. Assuming that happens, we will get to see those goodbye pictures here on Earth.

Tweets: No turning back for #cometlanding now

Part of the difficulty is the very low gravity on the 4km-wide ice mountain. Philae needs to be wary of simply bouncing back into space. As a consequence, on contact it will deploy foot screws and harpoons to try to fasten its position.

It will then take 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.

Paul Merchant: Been saying for weeks that I wasn't interested in the landing, but I have been up since 6.30am in front of the pc waiting on every word. Good luck to all the ladies and gentlemen involved in the project.

@ValMellon tweets: Excited about comet landing! Come on mankind, you can do it! #Rosetta

tweets: SEPARATION CONFIRMED! Safe journey @Philae2014!

ESACopyright: ESA

Confirmation! Philae has separated from the Rosetta "mothership", and the signal has been received at mission HQ in Darmstadt.

So, the lander has begun its 7-hour drop.

Let's hope it finds a nice soft landing spot like this one, which is actually on the other end of the comet:

ESA/Rosetta/NAVCAMCopyright: ESA/Rosetta/NAVCAM

This is another of Esa's "top 10 images from 10km" released yesterday.

It's still silent in the Esa control room...

Tense moments here - waiting for confirmation in the control room....

Dr Daniel Brown from Nottingham Trent University writes in to say:

"Although we have landed on planets, moons and asteroids, it has never been attempted for a nucleus of a comet - and with good reason. These objects have a very low gravity, are loosely composed of ice, dust and rocks, and are very irregular in shape. They are temperamental in their behaviour and notoriously difficult to predict.

"During the landing, and on the surface, the lander - about the size of a washing machine - will begin to analyse the comet with ten instruments. Apart from the amazing scientific results, the sheer challenge and ambition of such a mission is outstanding and illustrates how our space exploration of the solar system has become more advanced and successful.

"It gives us much to hope for in future missions."

tweets: Matt Taylor, @ESA_Rosetta project scientist on #CometLanding has the coolest tattoo (pic @BBCBreakfast):

BBCCopyright: BBC

‏

tweets: We are now in separation window, expecting signal confirming that @Philae2014 has departed @ESA_Rosetta #CometLanding

Three spindles driven by a common belt will push Philae away from Rosetta with a velocity of 18cm per second.

Think of the spindle that moves the inkjet head up and down inside your printer - Philae's ejection spindles are just like that.

If for some reason this system fails to operate, a back-up spring mechanism is automatically primed to fire 10 seconds later, to ensure the robot is booted off come what may.

A radio confirmation is expected at Earth just after 09:00 GMT. Remember the vast distance between us and Comet 67P means radio messages take 28 minutes to arrive.

tweets: There's a backup system - a spring - which should eject @Philae2014 with a short delay (10 secs) if the main system fails... #CometLanding

@GamerZeeSS tweets: @esa Best of luck on the #Philae landing to the Rosetta team. You've already achieved so much, this would be one hell of a cherry on top!

Philae should be separating from the lander any time now. But it will take about 28 minutes for any information to reach us here on Earth!

Good luck Philae....

@imbrianj tweets: Want to go to sleep, but we're landing on a freakin' comet right now. #rosetta #esa #space

The target landing site was chosen on 15 September and later christened "Agilkia" following a public competition.

Yesterday Esa released 10 amazing "favourite pictures" taken by Rosetta from 10km above the comet's surface, including this one.

ESA/Rosetta/NAVCAMCopyright: ESA/Rosetta/NAVCAM

Will the surface be hard or soft? Will Philae stick the landing?

Get in touch and let us know what you think.

A little more on the cold-gas thruster issue on Philae:

Esa mission manager Fred Jansen says the "go" was given despite the failure because there was nothing to be gained by delaying the landing. In other words, the thruster would not be fixed by waiting another couple of weeks. Philae will just have to live with it.

Have faith in those screws and harpoons, though. And if parts of the surface are as fluffy as some suggest, the robot could nestle nicely in a "snow drift".

tweets: Yes! My @Philae_MUPUS instrument has been switched on. With its 3 sensors, it will study #67P surface material #CometLanding

So, we start landing day with an interesting development, and an explanation as to why the third Go/No-go decision was delayed in the night. It concerned the health of the Philae lander.

Engineers report that all is good apart from the cold gas thruster that is used to push the robot into the surface of the comet when it touches down. It is designed to stop Philae bouncing back into space in the low-gravity environment.

This system could not be primed. It appears a critical valve would not open. What does it mean?

Stephan Ulamec, the lander chief from the German space agency, told me: "We will have nothing now pushing us into the surface. We will just have to rely now on the harpoons, the screws in the feet, or the softness of the surface. It doesn't make it any easier, that's for sure."

But the landing attempt goes ahead.

It's a crisp 10C in London, 7C in Darmstadt and as usual, about -70C on comet 67P. A good day for the European Space Agency (Esa) to try and make history.

A video feed from the agency's headquarters will appear in the "live coverage" tab above and we'll keep you up to date as the landing attempt progresses.