Landing on a comet was never going to be easy and the European Space Agency never said it was going to be a sure bet, but today, at 17:03 CET (11:03 a.m. EDT), Rosetta's plucky little lander defied all the odds and grabbed onto Comet 67P/Churyumov-Gerasimenko.
"We landed and Philae survived the landing. We have landed at the right place. It's also the right comet, don't worry," said a relived and jubilant Jean-Jacques Dordain, Director General of ESA, during a post-landing press conference on Wednesday.
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Dordain also confirmed that Philae had transmitted signals to the orbiting Rosetta spacecraft and that all systems were powered up.
"When you have the link and power, you can collect data," he said.
But the landing certainly wasn't plain sailing and it's going to take some time to decipher the data to understand what happened during Philae's dramatic comet rendezvous.
"It's complicated to land on a comet ... it's also, as it appears, very complicated to understand what has happened during this landing or after this landing," said Philae Lander Manager Stephan Ulamec.
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"What we know is that we touched down, so we landed at the comet ... we had a very clear signal there and we also received data from the lander, housekeeping data and also science data. That's the very good news," Ulamec continued.
"Not so good news is that the anchoring harpoons apparently did not fire, so the lander is not anchored to the surface. Now we've started to think about what could be the situation. Did we just land in a soft sand box and everything is fine although we are not anchored? Or is there something else happening?"
On Earth, Philae weighs 100 kg, but on Comet 67P, in its extremely weak gravitational field, the lander weighs less than a penny. This factor led to mission planners designing an elaborate landing system that acts more like a grabbing mechanism. Philae couldn't simply ‘land' on this comet, it had to grab and anchor itself in place. Hit the comet too fast, and the robot could have bounced back into space. And, it appears, the lander did bounce, but Philae's shock-absorbing landing system may have saved the day.
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"Some of these data indicates that the lander may have lifted off again, we touched down and we bounced very slowly as the landing gear worked perfectly well and it was designed to damp the majority of the impact energy," said Ulamec.
Through early analysis of radio signals and data relayed from the lander's solar generator, Ulamec speculated that the lander "lifted off and started to turn itself" after bouncing off the surface. The turning was likely caused by the flywheel (also known as a reaction wheel that is used in spacecraft to maintain stability in flight) which was switched off when it initially landed. "About 2 hours later, this information of turning on the power generator stopped."
It therefore seems possible that the lander touched down, lifted off again as the anchoring harpoons did not fire to lock the lander on the comet's surface, and then landed 2 hours later away from the initial landing site.
"Maybe today we didn't just land once, we even landed twice!" he added.
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Now that the lander has (apparently) settled, Rosetta has gone behind the horizon relative to Philae, so the signal has been lost, which is to be expected. However, Paolo Ferri, ESA's Head of Mission Operations, pointed out that the signal was lost slightly earlier than predicted, but that was likely caused by hills and boulders on the comet's craggy surface getting in the way of telemetry data sooner than calculated.
Ferri also said that the ‘first' landing was very accurate and that Philae was "very, very close to the center of the landing ellipse."
Although Philae's mission has only just begun, it has sent back valuable data and more soon-to-be-released photos of the comet's surface. But as for the lander's future, we'll have to wait for Rosetta to regain contact Philae, the results of which probably won't be known until Thursday.