| 12. November 2015
Anniversary of Philae comet landing – expecting the unexpected
By Karin Ranero Celius
One year ago today, the Philae lander touched down on Comet 67P/Churyumov-Gerasimenko. This was no easy task, of course. With the Rosetta mission, it was demonstrated that it was not only possible to travel to a comet more than 500 million kilometres from Earth, but also to follow it in its orbit around the Sun and land on it.##markend##
Philae was equipped with 10 instruments to carry out in-situ measurements never before conducted on the surface of a comet and complement observations by the Rosetta orbiter. Because some things, such as measuring the hardness of a comet's surface, cannot be done from a distance – one actually has to be there. So, on this day, and to commemorate this turning point in the exploration of the Solar System, let's take a look at how some of Philae's pioneering measurements of the surface of 67P have changed our view of comets and how the little Philae lander has managed to surprise us – more than once.
Philae separated from its 10-year companion Rosetta, on 12 November 2014 at 08:35 UTC when the spacecraft were at an altitude of approximately 20.5 kilometres above the comet's surface. After a seven-hour descent, finally travelling at one metre per second, Philae touched down on the comet surface at exactly 15:34:04 UTC, at the planned Agilkia landing site. But it wouldn't stop just there. As its anchoring harpoons failed to deploy and the active descent system (a cold-gas thruster intended to push Philae onto the surface) did not work, Philae bounced twice, travelling approximately one kilometre across the surface of 67P.
Despite this, Philae finally landed at Abydos, at 17:31:17 UTC, demonstrating that its design was robust enough to survive all that and more. This is extraordinary, as Philae and its suite of instruments had been designed, years before, essentially 'in the dark', because no one at the time had any idea of what comets would be like – and 67P is quite surprising, there is no doubt about that.
During the descent to Agilkia, various instruments were already at work. SESAME/DIM registered a millimetre-sized, fluffy particle at an altitude of 2.4 kilometres above the surface – this was the closest particle ever detected at a cometary nucleus.
The images acquired by ROLIS at an unprecedented resolution – down to one centimetre per pixel – did not show what was expected – dust deposits. Instead, it revealed that the surface of the comet in the proximity of the originally planned landing site, Agilkia, was dominated by coarse debris, pebbles and rocks with dimensions ranging from a few centimetres up to five metres.
ROMAP instrument registered that 67P did not have a measurable magnetic field, which confirms that when comets formed from the solar nebula, the existing magnetic fields were not strong enough to align the individual dust particles magnetically and permanently magnetise the cometary material – an important finding for developing formation models.
The COSAC instrument detected a total of 16 organic compounds, four of which were not known to exist on comets. Some of these are precursors of molecules important for the emergence of life. In the case of the Ptolemy instrument, it all worked fine (and to plan) on the day. Philae sent data back during the bounce. Not only that, but it was instantly clear that the results were good and would tell a story.
The measurements made at Agilkia are known as the 'scratch 'n' sniff' results – the impact must have kicked up some fine dust particles that made it into the instrument and were subsequently analysed. Organic compounds rich in carbon, hydrogen and oxygen were detected. It is believed that some of these come from the formaldehyde polymer 'polyoxymethylene' – if correct, this would strengthen the connection between results made by Giotto (Halley's comet). And there’s still more data to be analysed.
Already on the surface, Philae's MUPUS hammering at Abydos provided the scientists with quite a surprise, revealing that the surface was a lot harder than expected. The ground at Abydos is very hard, with a measured value of four megapascals. A hard material indicates some degree of 'evolution'. And so today, thanks to the landing, we know that comets are far more evolved than previously thought.
As MUPUS hammered its penetrator into the comet's surface, the SESAME/CASSE instrument recorded the resulting vibrations using accelerometers in all Philae's 'feet'. This data will allow scientists to learn more about the mechanical properties and surface structure of 67P.
But what did Philae's final landing place look like? CIVA, the panoramic camera system on board not only gave us the first ever image from the surface of a comet, but also characterised the lander's immediate, and unexpected, surroundings. Philae had landed at a location that exhibits a complex structure and varying particle sizes, as well as a soil with varying compositions.
Comet 67P's active areas are more fantastic and beautiful than was expected – a great deal of the activity comes from localised areas, typically cliff or ledge faces, and erosion processes so fast that even plain water ice is exposed. This is something nobody was expecting or thinking could be physically feasible, although the flyby of comet Hartley in 2010 had hinted to this. Another surprising find of the mission thus far is that comets lose a substantial amount of surface material due to erosion close to perihelion, but not uniformly. Instead, they create the bizarre and absolutely fascinating structured areas and features that we see all over 67P.
Philae successfully conducted all of its experiments. As the insolation of the solar arrays was too low to continue operating the lander, when the batteries were exhausted, the lander went into hibernation. In the months that followed, the comet got closer and closer to the Sun. Thanks to the gradual increase of the insolation, contact was re-established with the lander on 13 June 2015. This could mean that Philae could conduct more measurements on 67P, something that was never anticipated. But, then again, when it comes to Philae, we have learned to expect the unexpected.