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Touchdown! Rosetta probe Philae lands on comet

Paul Sutherland, Feature writer
Nov 13, 2014, 1:33 UTC, Updated Nov 13, 2014, 5:29 UTC

Sen—Mission Control, Darmstadt, Germany—The European Space Agency's daring mission to land on a comet became a magnificent success today when Philae phoned home to say it touched down on 67P/Churyumov-Gerasimenko. 

There were hugs and cheers at mission control in Darmstadt, Germany, as the mini probe beat all the odds after separating from mothership Rosetta earlier in the day.

But amazingly, it appeared last night that the probe had landed not once but twice because it bounced on the first attempt.

With almost no gravity to help it land, Philae was meant to harpoon the comet and then screw its three legs into the surface after leaving the British-built mothership Rosetta. ESA said the harpoons did not work, but the probe was on the surface.

The probe’s landing manager Stephan Ulamec said that the harpoons failed to fire but that ESA knew that the probe had touched down on the comet.

ESA’s space experts are still analysing the data. But Stephan said: We know we touched down. We had a very clear signal and we know we landed OK.

“The not-so-good news is that the harpoons did not fire so the probe is not anchored. We don’t fully understand what happened. We had fluctuations in the RF (radio) link but the signal always came back again. 

“We have very good data already and the science people are very happy. We’ve got nice images too. 

“Some of the data suggests the probe landed then lifted off again, in a very slow rebound. We saw a flucuation, not just in the signal but also from the solar generator which could be interpreted that the probe lifted off then started to turn itself.

“Two hours later this information of turning stopped. We still got signals and science data which suggests we didn’t just land once but twice.”

The two spacecraft had travelled for ten years on a 6.4 billion km (4 billion mile) journey through the solar system following launch in 2004.

The landing attracted huge media interest with reporters and TV crews from all over the world descending on mission control at Darmstadt in Germany. Head of ESA Jean-Jacques Dordain told them: “This is a big step for human civilisation.”

Earlier in the day, mission controllers had given the Go for separation despite a technical problem with part of Philae's landing gear. Overnight a gas thruster on Philae was found not to be working. It was meant to help push the comet down to the surface to help avoid a recoil as the harpoons fire. 

The piggy-back craft, which is the size of a washing machine, separated from Rosetta at 8.35 UT. But it took 28 minutes and 20 seconds for mission control to receive the signal saying it had happened.

Later, Rosetta sent back an image of Philae disappearing into the distance with its landing legs extended. Philae sent back a picture too of Rosetta.

Gravity was so weak at the comet that there was a danger that Philae could just bounce off the comet, a crumbly surface of rock and ice.

ESA picked a relatively flat region on the head of the 6km (4.5 mile) long comet, and named the site Agilkia.

Comets excite scientists because they are packed with water and organic materials and so could have delivered the seeds of life to the Earth.

Monica Grady, Professor of Planetary and Space Sciences at the Open University, is looking forward to learning the meaning of life from the mission.

She told Sen: "Comets were made right at the start of the solar system and they haven't much changed since then, so they're bits of the jigsaw that we're trying to put together to understand the solar system. These are like the corner pieces.

"One corner piece is the water, another is the dust, another is the carbon and another could be something else that we don't yet know.

"The ingredients of life might have come from comets, and the water that fills the world's oceans. We're trying to find out."

The mission’s senior science advisor, Professor Mark McCaughrean, told Sen: “Next year is by far the most exciting for comet scientists who want to understand the global picture. There is huge amount of real estate on the comet that needs to be understood. 

“It’s not unchanging like an asteroid, it is going to change dramatically in the next 12 months. Philae won’t see that because it will be dead in three months.

“We’re seeing about 5 kilos of water a second coming off the comet now, but that will increase by about a factor of 100, to half a ton of water a second as it gets closer to the sun. That is going to dramatically change the what we see close up and what is going on on the surface.”

Rosetta's own mission is expected to end in December 2015 as the comet begins to head back into the outer reaches of the Solar System

The lander Philae will have a shorter lifetime. It will carry out its first scientific measurements in just two and a half days, powered by its main batteries.

After that, for up to three months, it will attempt to conduct further tests and measurments using backup batteries recharged by solar cells. But no one knows for sure how long it will survive.

Update: It was not clear tonight exactly how Philae landed. Communications are intermittent and scientists are studying data to see how stable the landing was.

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