Marking the climax of a ten-year interplanetary journey the Philae lander, previously carried by comet-chasing Rosetta spacecraft, has successfully completed its seven-hour descent, providing scientists with the first ever close-up view of comet 67P.
Controllers at the Space Operations Centre of the European Space Agency (Esa), as well as their colleagues at the Lander Control Centre operated by the German Aerospace Agency, anxiously waited for the signal to arrive confirming the mission success.
First intercepted by Esa’s ground station in Malargüe, Argentina, and Nasa's station in Madrid, Spain, the eagerly awaited confirmation arrived shortly after 4pm GMT on Wednesday, about 28 minutes after the actual touchdown due to a communications delay caused by the enormous distance between the Earth and Comet 67P/Churyumov-Gerasimenko.
"Esa and its Rosetta mission partners achieved something extraordinary today," said Esa’s director general Jean-Jacques Dordain. "Our ambitious Rosetta mission has secured another place in the history books: not only is it the first to rendezvous with and orbit a comet, but it is now also the first to deliver a probe to a comet's surface."
The first data from the lander's instruments were transmitted to the Philae Science, Operations and Navigation Centre at France's CNES space agency in Toulouse to scientists who will finally get the unprecedented opportunity to examine in detail the comet's composition.
"After more than 10 years travelling through space, we're now making the best ever scientific analysis of one of the oldest remnants of our Solar System," said Alvaro Giménez, Esa's director of Science and Robotic Exploration. "Decades of preparation have paved the way for today's success, ensuring that Rosetta continues to be a game-changer in cometary science and space exploration."
Despite the successful landing, the hard work will now start for the engineering and scientific teams at Esa, DLR and CNES, as well as the other teams from around Europe and the world who helped design and build the instruments aboard both the Philae lander and the Rosetta spacecraft.
“After the landing, the first scientific sequence will be launched, lasting about two and a half days until the primary battery is depleted,” said Gerhard Schwehm, Rosetta scientist at Esa. “After that, we will try to start the secondary sequence, using the secondary battery. If it works according to our plan, the secondary sequence will last for a couple of weeks, maximum months, until it gets too hot on the surface of the comet as it gets closer to the Sun."
The landing site, named Agilkia, was chosen from four options, each of which presented specific challenges due to the rugged and complicated terrain. The scientists admitted they only had a very limited control over the lander as it descended and had to hope it wouldn't fall from a cliff and would land on all of its three legs.
“The landing ellipse, the area where the lander could have touched down, was not a single point, it was 500 metres across,” Schwehm said. “And in this area, if you look at the nucleus, there are still craters and cliffs so we needed a little bit of luck for the lander not to have fallen off from a cliff and to come down on the three legs.”
Over the next hours the scientists will start evaluating the data, which they hope will help them look far into the history of the Solar System into the time when planets formed.
Rosetta was launched on 2 March 2004 and travelled 6.4 billion kilometres through the Solar System before arriving at the comet on 6 August 2014. At the time when the engineers were designing both the Rosetta orbiter and the Philae lander they didn’t know anything about the distant comet, which made the whole venture all the more challenging.
"Rosetta is trying to answer the very big questions about the history of our Solar System. What were the conditions like at its infancy and how did it evolve? What role did comets play in this evolution? How do comets work?" says Matt Taylor, ESA Rosetta project scientist.
"Today's successful landing is undoubtedly the cherry on the icing of a 4km-wide cake, but we're also looking further ahead and onto the next stage of this groundbreaking mission, as we continue to follow the comet around the Sun for 13 months, watching as its activity changes and its surface evolves."
The mission is by far not over for the Rosetta orbiter, which will only now be able to start performing most of its science.
“Prior to the landing, the orbiter was mostly configured to support the lander,” Schwehm said. “Now it will be free to perform all the exciting science it was designed to do.”
While Philae begins its close-up study of the comet, Rosetta must manoeuvre from its post-separation path back into an orbit around the comet, eventually returning to a 20km orbit on 6 December.
Next year, as the comet grows more active, Rosetta will need to step further back and fly unbound 'orbits', but dipping in briefly with daring flybys, some of which will bring it within just 8km of the comet centre.
The comet will reach its closest distance to the Sun on 13 August 2015 at about 185 million kilometres, roughly between the orbits of Earth and Mars. Rosetta will follow it throughout the remainder of 2015, as they head away from the Sun and activity begins to subside.