In the first part we talked about the fantastic journey lasted ten years along the solar system, in pursuit of the comet 67P / Churyumov-Gerasimenko, the ultimate goal of the mission. We had left the moment the probe went into orbit around the comet, a crazy maneuver performed with unprecedented perfection. Think about it for a moment: thanks to our scientific progress, we are able to calculate in the smallest details a journey of ten years and billions of kilometers until we are in orbit to a celestial body in turn moving.
The surface of the comet was in fact unknown before the arrival of Rosetta, so first we began to study the morphology and to identify possible landing sites for the Philae lander. Finally, on 15 September 2014, ESA announced that it had selected the landing site J, renamed Agilkia, located in the head of the comet, for the historic enterprise: to land a lander on the surface cometaria to study closely the composition.
Philae’s landing on the comet
Then began one of the most delicate phases of the mission. The lander Philae broke away on 12 November 2014 from the Rosetta probe and began the descent at a speed of about 1 m/s. Landing would have been a very complicated maneuver. The comet, having a much lower mass, also presents an acceleration of gravity equal to 1/10,000 of that present on Earth. The escape velocity from the comet 67P / Churyumov-Gerasimenko was therefore very small, and this meant that even a small rebound would have risked sending the lander back to where it had come from. It was therefore planned to shoot two harpoons towards the surface, to retain the lander anchored to the surface. At the same time, a retrorhort would have cushioned the recoil of harpoons by pushing the lander down.
Unfortunately, a problem peculiar to these components, prevented an easy landing to the lander, which, after a descent lasted about eight hours, rebounded twice before stopping in the shade of a large boulder. The first rebound, reported Philae to about 1 km altitude at a speed of 38 cm/s. The second rebound instead occurred with a lower speed, about 3 cm/s.
At this point, the location of the lander was unknown, but the data and images from the surface were received correctly. From the first images it was immediately clear that Philae had landed in an area partially in the shade, and this would have made it impossible to recharge the secondary battery to ensure sufficient electrical power for the experiments. All that Philae had, then, was the main battery, which would have guaranteed about 60 hours of operation. It was therefore decided not to waste time and carry out more scientific experiments, selectively activating the necessary tools in order to guarantee maximum energy efficiency to the lander. The MUPUS indenter and the SD2 drilling machine were activated. MUPUS managed to penetrate only a few millimeters into the ground, being as hard as ice and not as soft as initially thought. SD2, on the other hand, failed to bring soil samples for analysis on board. A final maneuver was then attempted to orientate the lander a little better, with the possibility of a future reactivation. Immediately after the maneuver, the energy dropped abruptly and Philae went out.
The following summer, 13 June 2015, the comet had approached the Sun enough to partially recharge the Philae batteries, and contact was restored. The lander had been in operation for some days but it was not possible at least at the beginning to establish contact. Some data were collected that were successfully sent to Earth. However, it was not possible to give new instructions to Philae and there were no further contacts after July 2015. For this reason, ESA shut down Rosetta’s system for communicating with the lander, to save energy, putting an end to any possibility of recovery. .
The scientific results of the Rosetta mission
he scientific and technological results of the Rosetta mission have been remarkable. The technologies developed specifically for this mission will prove to be very useful in the future. Rosetta measured a magnetic field that oscillated with a frequency of about 45-50 mHz. However, Philae has not detected any magnetic field from the surface and therefore scientists hypothesize that solar wind was generated.
The mission was also an opportunity to measure the isotopic signature, that is the ratio between deuterium and hydrogen, of the water present on the comet. If it coincided with that of water on Earth, it would have been a very important proof of the origin of water on our planet. The detected isotopic signature, however, turned out to be different from that terrestrial water and this led scientists to infer that the water on our planet would not come from comets of this type.
Surely however the greatest scientific result of the Rosetta mission was the study of the organic molecules present on the comet. From spectrographic analyzes, it was already known that the comet contained organic compounds of various kinds. Thanks to the instruments on board both Rosetta and Philae, several very complex organic compounds have been detected, such as acetone, acetamide, methyl isocyanate, propionaldehyde and another twelve different organic molecules. Finally, for the happiness of astrobiologists, the presence of one of the amino acids that make up the proteins of all the living organisms of the Earth, glycine, was detected.
The end of the Rosetta mission
Moving away from the sun more and more, the solar panels aboard Rosetta would no longer provide the power needed to keep the probe alive. Scientists evaluated hibernation until the next perihelion, but there was no guarantee that on-board systems would be able to reactivate after long sleep. So it was decided to crash Rosetta in a controlled manner on the surface of the comet, collecting as much data as possible during the descent.
First though, there was one last outstanding question to be solved. Where was Philae? Scientists had a rough position, obtained with radar triangulation. But they still did not know the exact position. So images were taken at very high resolution and were screened one by one until, finally, the lander was identified and finally understood because he could not get enough light to recharge the battery. He had ended up in the shadow of a big boulder! We might even think that this was an unfortunate event but, probably, without that boulder stopping Philae’s race, the lander would have continued to bounce and risk ending up in space.
On 29 September 2016, about 14 km away from the surface of the comet. the mission technicians made a 208 seconds ignition and Rosetta began her descent, which would last 14.5 hours. During this time, he continued to take photographs and transmit them to the mission center on Earth. The latest data package was received by the European Space Operations Center in Darmstadt, Germany, at 11.19am. Then there was only silence.
These are the words of Sylvain Lodiot, Operation Manager for the Rosetta probe:
All stations and the briefing room, we’ve just had loss of signal at the expected time. This is another outstanding performance by flight dynamics. So we’ll be listening for the signal from Rosetta for another 24 hours, but we don’t expect any. This is the end of the Rosetta mission. Thank you, and goodbye.