Despite bouncing around the surface and coming to rest in the shadow of a cliff, the Rosetta mission to sample a comet has transmitted vital scientific data back to Earth.
The European Space Agency (ESA) spacecraft called Rosetta launched in March 2004 (two years before Twitter!). For the next 10 years it travelled to the comet Churyumov-Gerasimenko, or 67P for short, near the orbit of Jupiter. It reached the comet this August, and after taking measurements a landing site was selected for its detachable probe named Philae.
Answering big questions
Philae is the first man-made object ever to intentionally land on a comet (a previous mission had been deliberately crashed into one, but not landed). Its mission was to collect information about the composition of 67P. It is thought that comets formed when the solar system was young, 4.6 billion years ago, and have remained relatively unchanged and undisturbed ever since.
According to Museum extraterrestrial materials researcher Dr Penelope Wozniakiewicz:
Comets contain the most pristine examples of the materials available during the early history of the solar system. Studying cometary samples provides us with the opportunity to learn about the conditions and processes that operated back then.
The landing was the most complex part of the mission and successfully took place on Wednesday 12 November, but it wasn't without complications.
The News in brief team captured the milestones leading up to Philae's bumpy touchdown in a Storify, and you can catch up on the events of the day as they happened.
Off to a good start
It started overnight on 11 November with a series of Go/NoGo decisions - a final checklist of whether all systems were operating and ready to go. All decisions were Go, despite a problem detected with the cold gas thruster around 7.00 GMT. The thruster is part of the apparatus designed to help Philae latch on to the comet. The extremely weak gravity of the comet meant Philae would have to work to secure itself to the surface using just its harpoons and foot screws.
Philae's landing apparatus © ESA
The hundreds of millions of kilometres between comet 67P and Earth mean that any signals coming from Rosetta or Philae took about 30 minutes to reach Earth. Philae separated from Rosetta just after 8.30 GMT on 12 November and confirmation was received at ESA just after 9.00 GMT.
It then took Philae seven hours to travel to the comet's surface, all the while switching on scientific instruments and taking measurements and photos. While everyone waited expectantly for news of Philae's arrival, the Nature Live team at the Museum interviewed ESA Rosetta scientist Leah-Nani Alconcel about what it takes to catch a comet. She described how big the comet was, as shown by an ESA provided graphic of how it would look when hovering over London, and the aims of the mission:
Comet 67P compared to London © ESA
When Philae finally got back in contact it looked like it was safely on the surface, and celebrations and congratulations started to pour in.
Inspirational tweets from the Chief Scientific Adviser to the President of the European Commission
However, it soon became clear that Philae's harpoons did not fire upon landing. Without harpoons or the cold jet thruster, the lander was vulnerable to bouncing across the surface of the comet.
After several hours of confusion, it turned out that Philae had indeed bounced across the surface, touching down three times in total before coming to rest. The first bounce caused Philae to drift hundreds of metres above the surface for nearly two hours. A second bounce lasted only six minutes.
Finally, more than 24 hours after Philae separated from Rosetta, we got to see the view from the surface of a comet:
Philae's final landing site on the comet © ESA
Although Philae was down safely and transmitting back to Earth, it was not at its intended landing site, and the race was on to use Rosetta's cameras to locate its exact position on the comet. From the photos Philae sent back, however, it was clear that it had come to rest at the base of a cliff, and in considerable shadow for most of the day.
This was a problem for its power supply, which comes from solar arrays. Comet 67P was always intended as Philae's final resting place after it ran out of solar power, but the shade meant it would have an even shorter lifespan.
Mission (nearly) accomplished
Philae shut down early on Saturday 15 November, but not before drilling into the comet and performing 80% of its planned scientific readings. The data beamed back to Earth will take months to analyse, but already scientists have one conclusion: the material making up the comet beneath its dusty covering is far harder than anticipated. It's more akin to rock than ice, breaking some instruments. It also explains Philae's first high bounce; the impact of landing was not absorbed by a soft surface as had been hoped.
Philae may yet be able to recharge and come back to life, but even if not, Museum planetary scientist Prof Sara Russell thinks the mission has been a huge success:
I think it is exciting and amazing that some data has been acquired and transferred to Earth by Philae. These data are now being processed by the mission scientists, so there is no further news about what has been sent back, and there may not be for a while. I am hoping that the lander was able to make chemical analyses of the cometary material.
Rosetta as a mission is already a success, as the orbiter has been successfully acquiring lots of information about the comet. The lander part of the mission was always the most risky but it has worked remarkably well too, sending us beautiful and intriguing images of the comet surface.
At the time of writing, the hunt is still on for the lander. The latest news is that Rosetta was able to capture images of Philae as it approached the comet's surface, and also during the first bounce where the lander can be seen heading 'east' and away from the planned landing site:
Rosetta’s Philae lander as it approached and then rebounded from its first touchdown on Comet 67P/Churyumov-Gerasimenko on 12 November 2014. The final inset image (top right) in the series shows the lander during its bounce, not its final resting place that is still be found. © ESA